Why would the outer tanks fuel on the A320 be colder than the inner tanks?

Short answer:

This is very normal because the outer wing is thinner.
The warm fuel (which is warmed by the IDG) is not always returned to outer/inner tanks.

Some explanation is needed wrt the circulation function on the A320. Excerpts below will be denoted (M) and (F), for maintenance and flight manuals, respectively.

  The FADEC controls the fuel return valve.
  If the outer tank is already full, the fuel overflows to the inner tank through a spill pipe. On ground, the fuel recirculation is not inhibited if there is an overflow in the surge tanks. (F)

The main takeaway from this is the first line. The fuel isn't always returned. The return is managed by the engine's FADEC, not the fuel system. The second thing is that the circulation also works on ground.

  The fuel system:

  Stores fuel in the tanks.
  Supplies fuel, in the correct quantities, to the fuel tanks during refueling.
  Supplies fuel to the engines and the Auxiliary Power Unit (APU).
  Circulates fuel to cool the Integrated Drive Generator (IDG).
  Keeps fuel in the outer tanks for wing bending and flutter relief. (F)

The function of the circulation is cooling the IDG, not warming the fuel. But the fuel is indeed warmed due to the process, but the engine fuel system mixes that warm fuel (inside the fuel IDG cooling system) with fresh supply of cold fuel, to be used again by the engine (combustion) and for the IDG cooling.

In other words, the engine burns the same fuel that was used for cooling the IDG (not necessary to return it to the tank).

The FADEC only returns the fuel to the outer tank in 1 condition, and it does that by opening the FRV (fuel return valve). That condition:

  The return valve opens at low engine fuel flow if the engine oil temperature is above 93°C (199°F). (M)

Low engine FF (e.g., idle) will not be encountered in cruise. Because of the low FF, there is no high demand for the warm fuel (inside the fuel IDG cooling system), i.e., the engine is not using it quickly for combustion. And it will only keep getting warmer. So the FADEC mixes it with fresh fuel, and reintroduces it to the outer tank (schematic below). The mixing ensures it will be reintroduced at less than 100°C.

In other words, when the engine is not burning enough fuel, the fuel inside the IDG cooling system is returned to the tank, because it will keep getting hotter since it is not being consumed at a fast enough rate.

  The return valve mixes the hot fuel with cold fuel from the LP pump to maintain the temperature of the returned fuel below 100°C (212°F). (M)

This means, for example, if the plane is sitting on the ground with the engines running for too long, the outer tanks will become warmer than the inner tanks.

In cruise, because the outer wing is thinner, the surface area to volume ratio is higher than the inner wing, and thus the fuel will lose temperature faster than the inner tanks. This is highlighted in Airbus' FAST magazine issue 36:

As you can see, the outer tanks cool down at a faster rate. Note: the magazine discusses the long-range operation of the A340, which unlike the A320, does have a fuel transfer procedure when a LO TEMP message is shown.

And below is the A320 IDG cooling schematic. Close the FRV, and you'll see how the fuel is mixed and reused by the engine:

The above explains why when the crew gets a confirmed FUEL L (R) OUTER (INNER) TK HI TEMP warning, the actions could be:

Turning off the galley (reduces load on IDG).
Delaying takeoff and shuting down affected engine (side).
Disabling auto thrust and increasing FF of affected engine (side).
Using APU IDG instead of engine IDG (taking it offline).

(Source: A320 QRH)

All this would reduce the hot fuel being fed back into the affected tank.

A slightly more detailed operation of the FRV is found in the engine technical training manual (CFM56):

  The purpose of the Fuel Return Valve (FRV) is to bypass warm fuel from the Hydromechanical Unit (HMU) to maintain the efficiency of the Integrated Drive Generator (IDG) Oil Cooler and engine oil/fuel heat exchanger.

  Consequently this prevents engine and IDG oil over temperature.

  FRV logic: the FRV is fuel pressure operated, and electrically controlled by the Electronic Control Unit (ECU).

  The ECU control logic of the FRV is mainly based on the engine oil temperature.

  Above a certain engine oil temperature, the ECU orders a low fuel flow return to the A/C fuel tanks.

  When the engine oil temperature increases, the ECU orders a high fuel flow return to the A/C fuel tanks.

  The two return fuel flow levels are 500 kg/h and 1000 kg/h, or 1100 lb/h and 2200 lb/h. The hot fuel is mixed with the cold fuel to limit its temperature, before it is returned to the A/C fuel tanks.


What if there are no volunteers to sit in the emergency exit row?

The government requires any passengers seated in an exit row to be 15 years of age or older, be willing to assist in an evacuation and have no limitations that would prohibit their assistance.

Source: United

I.e. you won't be given that seat if you can't assist or unwilling.

Personally I always request an aisle emergency [exit] seat near the window. This way the bulky slide is out of my way and I get plenty of legroom. I'm very tall, legroom matters. You'll always find someone willing to switch if they're tall. What I'm saying is, this seat that you hate, others love.

But in your very specific hypothetical scenario of a full flight and wrong seat and unwilling passengers, the government says someone must switch. If the issue causes too much trouble, then someone (who is causing the trouble) will be escorted off the plane more likely, freeing up a space...

... to be immediately filled by someone from the standby list who doesn't mind sitting in the exit row. voretaq7

Have a nice flight.


Why do pilots do walkaround inspections for regularly scheduled commercial flights instead of aircraft mechanics?

Because it's the Captain's plane, for all intents and purposes. This is exactly the same for many other industries - in particularly shipping (as in boats) and haulage (as in trucks).
Similar to planes, ships and boats have captains and they are all fundamentally and legally responsible for everything that happens on that mission. And, likewise, a truck driver is expected to maintain responsibility for their truck - indeed, over here in the UK this is a legal requirement and truck drivers have to do a mandatory walk-around check.
It's also the same for car drivers - you, as a driver, are fundamentally responsible for your car and its safety.
Focussing on aircraft, the pilots probably have the single most holistic view of the aircraft, its mission and the requirements. In addition, the pilot is always going to be there - whether that's Heathrow at 2pm on a Tuesday, or a deserted dirt strip in the outback at 10pm.
The better question might be - what good reason would there be to skip a walk around in one of the most safety-critical industries? At best, for some airports in some circumstances, you can argue somebody else might be able to conduct the walkaround - but it's not guaranteed and it introduces another potential point of failure or miscommunication.


There's no legal limit of how high you're allowed to fly by law: there's no law that says

  It is illegal to fly above X,000ft

In some juristictions there are more specific laws, eg

  It is illegal to operate an unpressurised aircraft above 25,000ft

However, each aircraft has a service ceiling when it is certified, and it would be generally illegal to operate the aircraft above this altitude (because it's unsafe).

I'm fairly confident that you weren't at 55,000ft. No currently-operating commercial airliner I know of has a service ceiling higher than 45,000ft (the newer -800 version of the 747) and they almost never operate above 41,000ft. A few private jets can go a little higher, up to around 50,000ft, but I know of no non-military/experimental aircraft which is currently capable of going much above 50,000ft, even if you were daft enough to try.


Do "air ferries" for civilian vehicles still exist?

There are a number of suitable airplanes - C-130 Hercules, C-17 Globemaster, C-5 Galaxy, AN-124, up to the rather monstrous An-225 Mriya. Cargo versions of airliners don't usually handle vehicles - they are designed for palletized cargo while the above cargo planes have ground-level ramps.

If you want to fly with your car somewhere it's just a matter of calling someone who owns one of these airplanes and putting down your no-limit credit card.

As you surmised, the reasons they don't operate scheduled service is financial - boats are much cheaper, and not enough people want to go where a boat is impractical.


Is the weather radar on and active during the whole flight?

Not necessarily; it is the pilot's prerogative to use the weather radar as required. If the weather is clear and the pilot decides that the weather radar display is going to take up valuable space in display, he/she can fly without the weather radar on. Of course, in cases like night flying, it is better to have it on during the entire flight, but it is the pilot's decision.

The regulatory requirements are for the weather radar to be operational only if potentially dangerous conditions are expected en route or for night flying. From CFR §135.175   Airborne weather radar equipment requirements:

(b) No person may begin a flight under IFR or night VFR conditions when current weather reports indicate that thunderstorms, or other potentially hazardous weather conditions that can be detected with airborne weather radar equipment, may reasonably be expected along the route to be flown, unless the airborne weather radar equipment required by paragraph (a) of this section is in satisfactory operating condition.

European regulations are also similar. From JAR-OPS 1.670 Airborne weather radar equipment:

(a) An operator shall not operate:
(i) (1) A pressurised aeroplane; ...
...
unless it is equipped with airborne weather radar equipment whenever  such an aeroplane is being operated at night or in instrument meteorological  conditions in areas where thunderstorms or other potentially hazardous  weather conditions, regarded as detectable with airborne weather radar,  may be expected to exist along the route.


What were the advantages of the L-1011 AoA sensor design?

I found several patents that resemble the L-1011's, dating back to at least the 50s. One such patent remarks that there are two main types, the vane, and the rotating pneumatic tube (also comes in conical shape).

The advantages of the latter when compared to the vane-type are:

  [B]etter balancing characteristics, lower air friction, greater sensitivity, and less tendency to flutter or otherwise yield erroneous readings under adverse flying conditions. Also, it requires less power to prevent ice accumulations on a probe than a vane.

A disadvantage is keeping the moisture/dirt out:$^1$

  There is one difficulty in making this a rugged instrument. The movable cylinder must be mounted to move as freely as possible so as to have good sensitivity. This requirement makes it difficult to make a good seal around the moving cylinder to keep out dust and moisture.

A motor would solve this seal issue. But the advantages of the form should hold whether motorized or not. A more recent patent uses the tube design to combine the AoA and static/dynamic pressure sensors into one probe.

Was it used on any other aircraft besides the L-1011?

The F-14, for example, uses the pneumatic (conical) type:$^2$

(primeportal.net) The left (port) side of the F-14's nose.

$^1$ https://apps.dtic.mil/dtic/tr/fulltext/u2/102442.pdf page 50.
$^2$ The location is confirmed by the F-14D manual (only 1 probe), and it's referred to as probe (not vane).


Why hasn't anyone tried making two propellers facing upwards and the other two facing downwards?

People have done that.

And it works.

They do it because it's one of the logical configuration for a quadcopter based VTOL plane.

Example:


Why and how would I land straight-in from a circle-to-land approach?

The sentence before the one you bolded, with my bolding applied, says:

  When either the normal rate of descent or the runway alignment factor of 30 degrees (15 degrees for GPS IAPs) is exceeded, a straight-in minimum is not published and a circling minimum applies.

So you can have a "circling approach" that is lined up straight in to a runway, but with only circling mins, not straight-in minimums, published.  This could be driven by the need for a high FAF altitude, high enough that you can't get from that FAF at that altitude down to the runway, with a descent rate that's within the TERPS criteria.  So they build an approach that assumes you maneuver to lose the excess altitude, and give you circling mins so that you can.

However -- and this is what the exception that you bolded in the question is about -- if you're in, let's say a STOL airplane, you may very well be able to lose the excess altitude and be established on a normal visual glidepath to the runway with ample time to make a "normal approach and landing" without any S-turns or other maneuvering required.  And in that case you're absolutely allowed to fly the approach straight in.  The guy behind you in a Lear Jet may not be able to do the same thing that you did, and the TERPS descent profiles criteria are written to guarantee that if he has straight-in mins, he CAN get from the FAF at its crossing altitude to the runway without having to use what would be, for him, a dangerously high descent rate (angle).  But in the STOL aircraft, a steep descent is perfectly normal, so you can go ahead.  The Lear shouldn't try to mimic the Maule's descent profile, but that doesn't mean that the Maule has to mimic the Lear's descent profile.


Can 80hp Rotax 912s be upgraded to 100hp?

It looks like there are several kit upgrade options from third parties, e.g.:

120HP turbo kit
125HP kit
95HP kit

I have no idea about the pros and cons or costs. The 125HP kit "requires machining of the crank case and heads", the others seem to be additional components.


It's an F-18 Hornet:

Source: Wikipedia

Here is a view a little closer to your angle:

Source: Wikipedia

The one in your picture has extended range fuel tanks under the wings, the one above shows the hardpoint mounting for those fuel tanks.


What causes a helicopter to autorotate farther?

In general, regardless of the number of blades, the more blade inertia, relative to blade area, the better for autorotation safety because the inertia buys you more time to establish the autorotative glide before blade stall when power is removed, and more time to settle into a landing at the end of the glide.  The downside is more sluggish response to power application and maneuvering inputs on top of the added weight itself, so there are the usual tradeoffs.

The Robinson R22 started out with very low blade inertia for maneuverability (Robinson did not initially intend the R22 to be a trainer), but blade weight was increased later in production to make the aircraft more forgiving, to help with the blade-stall-following-power-loss accident rate.  In the earlier R22's you had under 2 seconds to lower collective pitch following a power loss before rotor RPM decayed enough to get blade stall, and if you weren't fast enough, that was it - you were trapped in a falling object (that's not as bad as it seems - if a car stops or swerves in front of you, you have a hell of a lot less than 2 seconds to react to avoid being killed, don't you).

In large helicopters or smaller machines with high inertia rotors like 2 blade Bell machines, you have oodles of time to establish the glide and settle after the flare.  A Huey can be lifted into a hover, held for a couple seconds, and settled on the ground just on rotor inertia alone.

You see a somewhat similar issue between regular airplanes and flying lawn chair ultralights.  When an engine quits on an ultralight, because the thing is big with lower mass, it slows down pretty fast and you have to aggressively shove the nose down right away to maintain flying speed compared to something like a Cessna 150. And a helicopter rotor is just an airplane going in a circle.


Is it possible to build a honeycombed vacuum balloon?

You can load a decent titanium alloy to about $500 \mathrm{MPa}$ (the ultimate compression strength is quoted at least $850 MPa$, but you need to stay below the elastic limit and have some safety margin). That is $5000$ times more than the pressure at sea level. Therefore, the supporting elements have to take about $\frac{1}{5000}$ of the volume. At density about $4500 \frac{\mathrm{kg}}{\mathrm{m}^3}$, the average density comes at least $0.9 \frac{\mathrm{kg}}{\mathrm{m}^3}$. That is just slightly better than the $1.225 \frac{\mathrm{kg}}{\mathrm{m}^3}$ of air, but way worse than the $0.09 \frac{\mathrm{kg}}{\mathrm{m}^3}$ of hydrogen or the $~0.17 \frac{\mathrm{kg}}{\mathrm{m}^3}$ or helium. And this is only weight of the support—for containing vacuum, the skin will be much heavier as well.

Therefore there is no way this could be better than traditional hydrogen or helium-filled aerostats and it's unlikely to get off the ground at all in practice though the above back-of-the-envelope estimate suggests it just barely might.


Where can I find the best site to learn aircraft design on a small scale?

It's not a website, but it's totally worth buying: Dan Raymer's Simplified Aircraft Design for Homebuilders (ISBN 10: 0972239707  ISBN 13: 9780972239707) is applicable to RC aircraft and has the best ratio of comprehensive coverage to beginner-friendliness of any resource I know.


How long can an ELT run before the battery is dead?

The reference you quote is correct as it defines the "useful life" of the battery.  that is: if you use it for more than 1 hour (that would include the cumulative test time) it must be recharged or replaced before being returned to service.  But that isn't what you are asking about.  You are asking for the duration of operation.

The requirement for Duration of Operation is 24 hrs.

RTCA DO-204A, MOPS for 406 MHz ELTs, states

  1.5 Operational Goals

  b. Operability

  (1) The technical requirements of the 406 MHz beacon radiated signal
  are based on the COSPAS-SARSAT system.

  (a) The 24-hour operating life requirement (§2.2.1) is based upon the
  maximum COSPAS-SARSAT system waiting time, the number of satellite
  overpasses required to optimize the beacon location solution and
  battery capacity limitation at low temperatures.

and

  2.2.1 Duration of Operation

  Refer to COSPAS-SARSAT document C/S T.001.

and from C/S T.001

  4.5.1 Duration of Continuous Operation

  The minimum duration of continuous operation shall be at least 24
  hours* at any temperature throughout the specified operating
  temperature range.

  The minimum duration of continuous operation for an ELT(DT) to meet
  the ICAO GADSS requirement at any temperature throughout the specified
  operating temperature range shall be 20† hours.


After decades of solid research, mostly led by Robert J Englar, why is Circulation Control, the Circulation Control Wing not in use at all?

Circulation may be controlled via either air blown through spanwise slots or horizontal-axis rotors, alone or in combination.
Experiments on wing systems go back at least as far as 1902 and since then almost every variation imaginable has been tried.
The problems come in the engineering implementation. Where investigations have reached the stage of flight testing, and in many cases sooner than that, the conclusion is always the same. Either you have a system of large air ducts threading through the wing structure and weakening it, or you have a spinning rotor to support along the span of the wing. Either way, the wing must be strengthened and that means more weight. Both approaches require a supplementary power source, more weight again. The mechanical complexity compromises development time, manufacturing and maintenance costs, and reliability. The thing may work, but the downsides of the weight and complexity are greater than the benefits; the technology is not cost-effective.
So I am not sure what evidence base there might be for claims of actual weight reduction.
If you really want super-slow flight technology, get a helicopter with VTOL thrown in for free or, if you are rich and desperate for somewhat higher speeds, a V-22 Osprey.
Who knows, the balance may change as technology advances, stranger things have happened.


Which avoidance maneuver is best when seeing crossing traffic on approach to a runway?

When you are on short final and something is on, is entering, or even seems to be entering the runway, you execute a go around.

As for the second part of your question, given the loose parameters I'm afraid there is an infinite amount of different scenarios, and therefor I can't provide a comprehensive answer.


Why was the Vulcan bomber used for the Falklands raid?

This is borderline as an aviation question but I will answer it anyway.

They didn't use those weapons because they did not have them at the time:

Conventional cruise missiles did exist but most were nuclear, and the UK didn't possess any conventional ones.

ICBMs with conventional warheads did not and probably still do not exist because they are not a good idea:

Their capacity is low: A sub launched ICBM has somewhere around 2.5 to 3 metric tons' throw weight, much of that would be taken up by warhead guidance, thermal protection and other necessities so your actual warhead would be maybe 1000kg, or 2200lb. That's not that big a conventional bomb, and it could only launch one. A fighter bomber could drop 2 of those. The Vulcan bombers on the raid carried 21 1000 pound bombs.
They aren't accurate enough: a nuclear weapon can be a bit off and still destroy its target utterly, a conventional weapon needs a great deal of precision. The Polaris missile had an accuracy of 3000ft (900m). That's okay for a nuke, it' still ruin someone's day, but completely inadequate for a 2000lb bomb. More modern ICMBs have more like a 100 meter accuracy, which is a lot closer but still not good enough for a conventional bomb
They are expensive: that's a lot of cash for a 2000lb bomb that probably will miss
Launching one could start WW3: If you launch an ICBM someone is going to see it and if they think it might be directed at them a retaliatory strike could be the answer. The world has come perilously close to this more than once, it simply isn't worth the risk!


At what position or distance from an airport are you considered to be in the "traffic pattern"

This AC describes a fair bit of what/where a traffic pattern is. ultimately it comes down to definition but the answer seems to be what ever the defined traffic pattern says

  The airport traffic patterns described in part 93 relate primarily to
  those airports where there is a need for unique traffic pattern
  procedures not provided for in part 91. Part 97 addresses instrument
  approach procedures (IAP). At airports without operating control
  towers, part 91 requires only that pilots of airplanes approaching to
  land make all turns to the left, unless light signals or visual
  markings indicate that turns should be made to the right

It seems they are somewhat careful to not explicitly define anything and instead allow some definitions to define it for explicit cases and others to be left to interpretations. The FAA advises that calls be made as far as 10 miles out.

The FAA technically allows airports to define their traffic pattern themselves:

  Traffic Pattern Design. Airport owners and operators, in coordination
  with the FAA, are responsible for establishing traffic patterns. The
  FAA encourages airport owners and operators to establish traffic
  patterns as recommended in this AC. Further, left traffic patterns
  should be established, except where obstacles, terrain, and
  noise-sensitive areas dictate otherwise

They do provide some guidance on how to do so in regards to altitude they even allow the pilot to decide

  Traffic Pattern Altitudes. It is recommended that airplanes observe a
  1,000 foot above ground level (AGL) traffic pattern altitude. Large
  and turbine-powered airplanes should enter the traffic pattern at an
  altitude of 1,500 feet AGL or 500 feet above the established pattern
  altitude. Ultralight aircraft should operate no higher than 500 feet
  below the powered aircraft pattern altitude. A pilot may vary the size
  of the traffic pattern depending on the aircraft’s performance
  characteristics

Going straight to regulations taken from here

  §91.127   Operating on or in the vicinity of an airport in Class E airspace.

  (b) Departures. Each pilot of an aircraft must comply with
  any traffic patterns established for that airport in part 93 of this
  chapter.

and for class D

  §91.129   Operations in Class D airspace.

  (1) Unless required by the applicable distance-from-cloud criteria, each pilot operating a
  large or turbine-powered airplane must enter the traffic pattern at an
  altitude of at least 1,500 feet above the elevation of the airport and
  maintain at least 1,500 feet until further descent is required for a
  safe landing.

and Class C

  §91.130   Operations in Class C airspace.

  (b) Traffic patterns. No person may take off or land an aircraft at a satellite airport within
  a Class C airspace area except in compliance with FAA arrival and
  departure traffic patterns.

If the tower is in operation they can vector you as they see fit (at least here under the FAA) as such they may use pattern terms and organization but I cant tell if the FAA considers it a pattern or not if there is a tower.


Do flight recorders need more efficient antennas?

The short version of the answer is 37.5KHz is not "radio frequency", it's "audio frequency", so an antenna is not necessary - just a (very loud) speaker, which is basically what the pinger is.

Specifically, the 37.5KHz "pinger" tones used in the Underwater Locator Beacons that are bolted onto Cockpit Voice Recorder / Flight Data Recorder "black boxes" are in the low end of what we call ultrasonic frequencies. Water is generally a good conductor of sound (with at least one notable exception), so it is usually possible for sonar detection equipment to "hear" the pinger tones at reasonable distances (with the interpretation of "reasonable" varying based on the depth of the area being searched - much like in air the sound intensity falls off as distance from the source increases, so if we're listening at the surface of the ocean a pinger in 500 feet of water will have a larger coverage footprint than the same pinger in 5000 feet of water).


Why would short-haul flights be pressurised at a higher cabin pressure?

For the structural loading, the absolute cabin pressure is not as important as the pressure differential with respect to the outside pressure.

Short-haul flights typically operate at lower altitudes; either because they are operated by turboprop aircraft which have a lower service ceiling, or because there is simply not enough distance to climb to a high altitude and glide back to land. At lower altitudes, the outside air pressure is higher, and thus the cabin pressure can be higher without exceeding the maximum pressure differential the fuselage can handle.

Your other concerns are not really a factor. Only the pressurisation cycle count really matters to aluminium, not the total pressurised time duration. The fuel required for pressurising the aircraft is also not really a factor compared to the energy required to either heat or cool the air for passenger comfort.


How long does it take for a contrail to hit the ground?

A contrail is a cirrus cloud composed from water vapor nucleating on soot in the engine exhaust and the contrail itself doesn't fall to the ground.  You will occasionally see cirrus uncinus "mare's tails" form from contrails, which are heavier ice particles in the contrail precipitating from the contrail.  These particles evaporate long before reaching the ground.

(image courtesy of Wikipedia, CC licensed)

The fate of the contrail cloud itself is death via entrainment.  Watch some over time as they distort from the winds aloft and then mix with the surrounding dry air and evaporate.

Once the contrail is gone and only vapor remains it moves with the air it resides in.  For synoptic scale flows, vertical motion of the atmosphere is on the scale of centimeters per second and as such until that vapor finds its way into subsiding air (e.g. a high pressure system, the dry conveyor belt of a midlatitude cyclone), it will tend to remain aloft.


Who is responsible for getting approval of cabin layout?

I can answer for the FAA as that is the jurisdiction I am in and familiar with. The FAA (and I assume the other similar organizations) have a variety of guide lines for airplane interiors and its generally on the owner/operator to get them certified. However manufacturers may elect to get an interior configuration certified so that they can sell complete aircraft. As for regulations there are plenty of them, you may want to check this out as well as this and also this. The FAA generally looks at evactuation times (how long it takes every one to get out of the plane in the event of a crash) as well as certain aspects of the seats and restraints. Generally speaking they also look at the materials for their flamability.

One of the biggest interior policies of recent years was the move to "16G Seats" over the older seats. However as long as the seat is an approved 16G seat it can be used.

This is a nice summary of some of the research the FAA has done on cabin safety and its components.

As far as I know the FAA will let you submit just about any layout as long as it passes their flame, evac, and safety regulations. With the popularization of large private planes interior options are pretty much limitless.

On larger planes there are some things that may be fixed like the bathroom and galley since that requires plumbing and electric to be run but its more than likely they can be moved if requested.

There have even been talks of standing room only sections of airplanes but the regulators put a hard no to that as of now.


What is the difference between a non-directional signal and an omidirectional signal from a radio aid?

"Omnidirectional" may not be the most descriptive word for the actual radio signal emitted by a VOR, as contrasted with the signal from a NDB.  We could note that a VOR and a NDB actually both share the characteristic of broadcasting in all directions, as opposed to the narrow beam emitted by a landing approach aid, or the characteristic pattern emitted by the "four course" radio ranges of long ago.

But as for the difference between the radio signals emitted by a VOR and a NDB--

Think of the signal from a VOR as a collection of beams radiating out from the transmitter.  Each beam carries data: it is encoded with the azimuth from the station.  You can easily find more information about how this done: in actual practice the azimuth information is encoded in a phase shift between two different signals.

With a NDB, we also have beams radiating in all directions from a transmitter, but all the beams are the same.  That's why you can only get a relative bearing from a NDB.


Are the items in newbie "kits" actually used during or after training?

Well going by Sportys own price lists

Flight Gear HP Crosswind bag - 50
PHAK - 16
AFH - 16
AWTB - 16
FAR/AIM - 16
Fuel tester - 11
E6B - 40
Logbook - 16
Plotter - 16
Private Pilot Maneuvers Guide - 16

TOTAL: 213

If the list price is 150, that represents a savings of 63.

Of all that equipment, save for the maneuvers guide, you will need the rest of it to complete your training knowledge test and checkride.  The E6B and the chart plotter are a little anachronistic these days but can still be put to good use for time or fuel sensitive flights.   You will make good use of the flight bag for the rest of your flying career.  All in all it’s a pretty good kit.

One thing they left out and I highly advise you buy is a flashlight and spare batteries.  It’s a $30 item and it could save your life if you ever have an electrical failure at night.  I would recommend the Smith and Wesson Captain’s Flashlight for this.  Avoid tactical flashlights with high output white lights; these are counterproductive when flying and will wreck your night vision.  Another good option are the head mounted lights popular with cavers and climber.  Select one with white/red output and, again, not too bright.

Another useful item is a headset.  High end ANR headsets cost over 1000 USD but ther are economical and good quality one in the 200-300 USD range.  I used the David Clark H10 series headset during my primary flight training and still have the headset handy for passengers.

I would recommend spending the money on the following apps

Either ForeFlight or Garmin Pilot as an EFB.  Best thing since sliced bread for charts, maps, weather products, flight planning, filing, etc.  much lighter and a hell of a lot cheaper than paper products for this.

ASAs FAR/AIM - Heck of a lot lighter than a paper FAR/AIM, can search regulations faster and it automatically updates annually, instead of having to buy a new paper one every year.


Can a view limiting device be used in an aircraft requiring two pilots?

There is no regulation against doing so provided that the third person in the cockpit is type rated for PIC privileges on the aircraft and has a waiver from the FAA to operate the aircraft as a single pilot (they are available with some additional training.)  In practice, this would almost never occur both for safety reasons and cost.  In addition full motion simulators are available for most aircraft type rated for a two person flight crew, so it would just be more feasible to carry out the training in one of these using simulated IMC, which is arguably more realistic and accurate than flying the real thing under the hood.


Why do you need to provide your arrival airport when you close a VFR flight plan?

My understanding is that providing multiple pieces of information related to your flight plan when communicating is to ensure that the correct flight plan is being manipulated.

It seems odd that it would be required on an electronic format, but what if your N-number is N1234RF and a person closing the flight plan types N1234EF?

There is potential to close the wrong flight plan.  Now the likelihood of both of those N-numbers having an active flight plan to the same destination and having someone fat-finger it at the same time is much less likely, obviously.

Also, "if you are flying to multiple airports this function provides a chance to confirm which of the flight plans you are using. Ex. a student pilot flying from their Home airport to airport A, taking a break then flying onto airport B before flying back home." - Dawn Breaker

So it is just a cross-reference for accuracy.


What are these accelerating flare countermeasures called?

These are generally called "propelled flares", or sometimes "aerodynamic propelled flares". Some antiaircraft systems are able to distinguish between a normal flare and the aircraft, as the flare is not travelling along the expected path of the aircraft. Aerodynamic flares were first developed to allow the flare to continue along the path, but as antiaircraft systems developed further, they added a method of propulsion (which uses the same fuel source as the flare itself), to ensure that the flare continued alongside the aircraft, slightly angled away, at a similar speed. This does a better job of confusing these aircraft systems, but it's a fairly "niche" type of flare that is meant to combat conical scanning.

I believe the ARM-031, MJU-47/B, and MJU-71/B are all forms of propelled flares.


What is this white rectangle painted on runway 23 at KCRW?

Seems to be touchdown zone markings for assault landing practice by C-17's. For assault landings, the standard is to touch down in the first 500' of the Landing Zone (LZ), and to go around if your mains aren't on the ground by the 500' point. On a dirt LZ, there will typically be panels set out to mark the threshold, the 500' point, and the far threshold -- that way the combat control team which is marking the LZ doesn't have to put out lots of panels, but there are enough for the crew to get what they need. The 500' markings on the main runway would give enough information to the crew to simulate landing on an LZ, even though the actual runway is far longer & wider.

This frame below shows examples of C-130's practicing assault landings, and you can see orange panels at the threshold and 500' points in them:

Source: YouTube

Some places, KLRF (Little Rock Air Force Base) for example, had an actual 3500' long paved strip for crews to use for assault landing practice, with permanent markings at the threshold and 500' points (2013 imagery shown below):

Source: Google Earth

Most airports don't have the budget to create a separate runway for this specialized requirement, so some sort of markings on the main runway are used instead.


Where exactly can I find the FAA's UAS pilot training course (ALC-451)?

Even when faasafety.gov is working without issue, the course can be difficult to find with the site's search feature. The Part 107 Small Unmanned Aircraft Systems (small UAS) course is presently available from this direct link: Part 107 Small UAS Course.

Note that you will have to be logged in to do the course. If you follow that link and are not logged in to faasafety.gov, log in and then go to that link again.

Note also that faasafety.gov currently seems to be experienceing occasional technical issues, either due to maintenance or traffic volume. The link may not work reliably at present.


What airfoils are used for the current generation of general aviation VTOL aircraft?

Those airfoils never see angles of attack of more than a few degrees. Since both wings of the Blackfly carry propellers along their full span, their prop wash ensures that airflow is mostly along the wing's chord. Slow flight is not much different from the hover of quadcopters, in both cases the propellers create most if not all of the lift.

Blackfly ultralight (picture source)

So to answer your question directly: There are no VTOL ultralights where the wing is exposed to flow at 60-70° AoA. The airfoils used are most likely about 15% thick with moderate camber because they operate at single-digit angles of attack.


What is the purpose of blades on a compressor?

The statement about the rotor blades ("the rotors increase the speed of the air and descrease the pressure") is not correct, because Bernoulli's Equation does not apply in this situation. Hence, the statement "as the flow of a fluid increases, the pressure decreases, and vice versa" isn't appropriate for the rotating compresser blades.

Bernouilli's equation (below) assumes no energy is being added or removed from the fluid. This is true when the fluid is flowing through the stationary stator vanes, but not the rotating blades.

The rotating blades increase the velocity, but do not decrease the static pressure, becuase they are adding mechanical work (energy) to the fluid. This energy (or power) comes from the turbine.

When no energy is being added to the fluid:

$$
total\space pressure = static\space pressure + 1/2 \rho v^2
$$

where
$\rho$ is the density and $v$ is the velocity.

This is the mathematical equivalent of your statement "as the flow of the fluid increase, the static pressure decreases".

But, the rotating blades add total energy. So this equation does not apply, so as the velocity increases, static pressure does not go down. But, total energy has increased, by the mechanical work done by the blades. So the value on the left of the equation goes up as the fluid flows through the rotating blades. Next, the fluid flows through the stationary stator vanes. Because they are static, they add no energy, so Bernoulli's equation now does apply. Hence, as the velocity is decreased, the static pressure goes up (i.e. there is no change in total pressure).

If you just have the rotor blades, and no stator vanes, the compressor would just make the fluid move (i.e an increase in the total pressure) but with no increase in static pressure. The stator vanes slow the fluid down, and convert the velocity into an increase in static pressure.

You need to understand the difference between total and static pressures.


How to represent graphically airways and routes?

ICAO Annex 4 contains guidelines for creating aeronautical charts.

The type of information to be shown and required accuracy of information is defined in the different chapters, depending on the type of chart. You will probably be interested in chapter 2 and chapter 7/8.

Symbology and standard colors are explained in appendix 2 and 3.


Is the A380 ailerons' kinematic modified in alternate law?

Valse Des Ailerons (Waltz of Ailerons) or VDA is a nickname for the Load Alleviation Function (LAF) which is part of the flight control system. LAF uses all ailerons and spoiler 6 to 8 to alleviate the fatigue and static loads on the wings by reducing the wing bending moment.

It consist of various sub functions dealing with passive load alleviation (based on commanded load factor from normal law) and active load control based on measured accelerations in pylons under the wing.

From this it already follows that passive load alleviation requires normal law for load factor inputs.

There is a document circulating on the internet which seems to be an official Airbus Pilot Briefing for A380 pilots called "Flight Deck and Systems Briefing for pilots, issue 2". In the section about the Flight Control Systems under auxiliary system the following is said about the LAF:

  Load Alleviation Function (LAF) The objective of the LAF is to reduce structure fatigue and static loads on the wing during maneuvers
  and turbulence. Spoilers 6 to 8 and all the ailerons are involved
  in the LAF.
  The LAF is available in normal law only.

(Emphasis added by me)

So your feeling that Valse des Ailerons is only active in Normal Law is correct.

Update: Based on the A380 flight crew manual, it depends—


Would it be a viable option to make the engine generate lift?

Good thought, and it does happen. Tilt the jet exhaust or propeller downwards at a shallow angle $\phi$, and there is lift created at sin $\phi$ while thrust is reduced by cos $\phi$. If we take small angles, let's say 3 degrees:

$\Delta L$ = T $\cdot$ sin(3°) = 0.052 T
$\Delta T$ = T $\cdot$ cos(3°) = 0.9986 T

So 5.2% of engine thrust is converted into lift, for a loss of 0.14% of horizontal thrust. Free lift! Slight angles like this are found in aircraft installations, for instance in tail mounted jet engines which are angled horizontally to reduce yawing angle with a failed engine.

With engines mounted underneath the wing,the downwards pointed thrust would help a tiny bit in lift and reduce the take-off length. As @mins points out, with tail mounted engines the nose down pitching moment may counteract the lift benefits.


How do I distinguish 2 fixes with the same name?

The answer is embedded within the question:  The pilot is able to distinguish between the two fixes because they are in different regions.


Can a normal PASSENGER pilot a plane with no training?

Without any help, and no prior introduction, it is highly unlikely in a transport or biz jet.  Too much complexity, too much a priori knowledge needed, and too many flight configuration changes to land the plane.  Just an opinion.

There have been lots of attempts to take the uninitiated and have them pilot a simulator to a landing. The outcomes are not favorable, unless there is some previous experience or there is very very active coaching. Every now and then, someone writes a paper or magazine article on this. There have been no large scale studies where more than a handful of lay people have been "tested."

So bottom line, somewhat informed opinion, no it is not likely that the uninitiated would be able to fly to a selected destination and land a plane that is in the transport or bizjet class.


How does inlet spillage drag produce a lip suction effect?

Yes, your last sentence gets it about right. Just one detail: Air does not move "forward" to flow out again once it has entered the intake - which air goes where is all settled ahead of the intake. Never forget, though, that this effect is small and spillage leaves a net drag component.

When compared to a sharp-lipped intake as used on supersonic planes, subsonic intakes are more forgiving for a mismatch between capture area (area of the stream tube which goes into the engine) and the actual intake lip area. Below I tried to fake the flow around an intake lip using Inkscape and added blue and red shading to indicate pressure. The core flow gets compressed ahead of the intake (blue shade, diverging streamlines) while the outer flow is pushed to the outside of the intake lip by the increasing pressure and now has to follow the contour of the intake. Like with air flowing around the upper side of an airfoil, the air is accelerated (converging flow lines) and a suction peak forms on the intake lip (red shading). This suction peak sits on the forward-facing part of the intake lip and provides some suction.

The sharp intake lip of a surpersonic intake would produce separated flow under the same conditions with a considerable increase in drag.


It is a two position tumbler switch on the overhead panel to allow/disable the activation of the Evacuation alert from the cabin (explained here page 13). The view can also be found here for A320 and also here, with the following comments:

CAPT and PURS - the alert may be activated from either cockpit or cabin.
CAPT - the alert may be activated from cockpit only.

The A320 pictures seem containing the safety wire to keep the switch in the CAPT and PURS position, so it most likely another position is only selected for the reason.

The manual says Allows activation of an Evacuation alert from
the cockpit or cabin but from explanation above and the notation on the panel looks like the alert activation from the cocpit cannot be disabled by this switch.

This electric component is also called "toggle switch" and may be called "flush switch". These components were patented as early as in 1917.


Is the maximum power the Merlin 61 reciprocating engine occuring at its maximum RPM?

Power output of a reciprocating engine goes up with speed, and yes, maximum power is reached at maximum RPM. Considering the size and age of the Merlin 61 (27 liters displacement) it had a rather high maximum speed. Some contemporary engines ran even slower:

Daimler-Benz 601E: 2700 RPM for take-off, 2326 RPM continuous power
Daimler-Benz 605A: 2800 RPM for take-off, 2300 RPM continuous power
Allison V-1710-A: 2800 RPM
Klimov VK-107: 3200 RPM for take-off, 2800 RPM continuous power
Jumo 213A: 3250 RPM for take-off, 2900 RPM continuous power

Note that the considerably smaller GA engines still only allow a maximum speed of 2700 RPM. The smaller the engine becomes, the higher the maximum RPM will be. Model engines for RC racing easily top 50.000 RPM.

Age is also a factor. Modern engines run much faster, and the early rotary engines of WW I would run 1350 RPM, max. The radial engine of the Boeing Stearman which I once flew was limited at 1700 RPM, requiring me to throttle back when accelerating for a loop-the-loop so I would not overspeed the engine in the dive.


What are the consequences of pumping the throttle in an carburetor piston engine airplane during start?

Pumping the carb during a cold start, in an updraft engine such as the 152 has, is the #1 cause of engine fires, and can be eliminated by the proper and factory recommended starting procedure.

The correct procedure is to prime, then open the throtte about 1/4" and then crank the engine.  All planes vary, especially if the primers are varnished up to one degree or another, but here is some guidance for a 152 and 172:

10C  - 1 full pump of primer

0C   - 2 pumps of primer

-10C - 3 pumps of primer

-20C - 4 pumps of primer, however the engine should be preheated prior to starting at this and colder temperatures.

Using the primer, puts a charge in either one or all cylinders, which is adequate to compensate for carb fuel/air mix being condensed on the cylinder walls at the ambient temperature.

Using the throttle pump to enrichen the mixture sprays a little squirt of fuel into the carburetor. If the engine is not running, much of that fuel drips downward out of the "bottom" or inlet side of the carburetor.  Too much (easy to do) will drip out, and load the inlet box, and can also drip to the ground.  If the engine has even a small backfire, it may ignite the additional fuel there, which will rapidly heat up the carb which has an ample supply of more fuel.

If you can't start with the primer, it is likely that the primers are varnished up and need to be cleaned by a mechanic.  They come out, similar to spark plugs.

Also in the 152, you may have a primer on #1 or on all four cylinders. It largly depends on where the plane was initially delivered.  If a colder climate, the plane would likely have come with four primers.


When did pilots start making announcements to passengers?

This varies by jurisdiction and air frame in question as the brief is usually specific to the air frame. Ill answer for the FAA.

What the pilot needs to say (by law) is governed by (follow link for full text),

  § 121.571 Briefing passengers before takeoff.

  (a) Each certificate holder operating a passenger-carrying airplane shall
  insure that all passengers are orally briefed by the appropriate
  crewmember as follows:

  (1) Before each takeoff, on each of the following:

  (i)Smoking. Each passenger shall be briefed on when, where, and under
  what conditions smoking is prohibited including...

  (ii) The location of emergency exits.

  (iii) The use of safety belts...

  (iv) The location and use of any required emergency flotation means.

  (v) On operations that do not use a flight attendant, the following
  additional information:

  (A) The placement of seat backs in an upright position before takeoff
  and landing.

  (B) Location of survival equipment.

  (C) If the flight involves operations above 12,000 MSL, the normal and
  emergency use of oxygen.

  (D) Location and operation of fire extinguisher.

  (2) After each takeoff, immediately before or immediately after
  turning the seat belt sign off, an announcement shall be made that
  passengers should keep their seat belts fastened, while seated, even
  when the seat belt sign is off.

  ....

FAR 121.571 comes off Doc. No. 2033, 30 FR 3206, Mar. 9, 1965 and was presumably added to the regulations in 1965.

Yes there is a standard. You can find more details in this Advisory Circular AC121-24C which covers what the oral brief needs to contain in more depth and outlines the standards. This AC has been in place since 2003 and replaces AC121-24B which took effect in 1999 I cant find the full text of that one so Im not sure what it replaced (presumably AC121-24A).

There is no direct regulation in the FAR's (or current AC) that require announcing things like route, altitude or temp.


What is a demonstrated crosswind component?

A demonstrated crosswind component is highest crosswind (corrected to make it 90°) which has been shown to be possible to safely land by a test pilot. It shall not require exceptional skill by an ordinary pilot, however it does not mean every pilot will be able to do so. It is also NOT a limit (contrary to what some say) - if the pilot does decide to land higher crosswinds, they can do so (but please take caution).

This number may not necessarily be the highest crosswind component that it's possible to land in -- it just has to be above the minimum capabilities required in the regulations. For example, say an aircraft, flown perfectly (yes, we're dreaming), was capable of handling a 30 knot crosswind component (no gust, ISA conditions), the manufacturer only needs to demonstrate safe landing with a crosswind component of one-fifth of the reference stall speed in the landing configuration. This would be:

10 knots @ 50KIAS VS0
12 knots @ 60KIAS VS0
14 knots @ 70KIAS VS0
15 knots @ 75KIAS VS0

Reference:

  (a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VS0.


Why is the planned Kingdom Tower Skydeck unsuitable for use as a helipad?

A quote from The Guardian:

  A Starship Enterprise-like circular helipad, 30m across, protrudes out at level 157. This was originally designed to provide fast access to the penthouse, but the wind makes it a dangerous place to land, so instead it has become an observation deck – the world’s highest at 630m.

link (emphasis by me)

Helicopter prefers steady wind condition in order to land safely and comfortably. The nature of wind at such height is gusty and turbulent due to proximity to the building itself. It would be unsafe and very uncomfortable to the rich man who rides the chopper for 'quick access' to the penthouse on that floor. The highest helipad now is at 438 meters high at that is at the TOP of the building.


How do I determine the VFR pattern altitude at an airport?

In April 2013, the FAA's Charting Group met to discuss this question.  The standard pattern used to be 800 ft AGL, and the Chart Supplement (formerly called Airport/Facility Directory or A/FD) was inconsistent in listing them.  Ultimately they decided:

  Chris Criswell, AJV-22, reported that, per ACF recommendation, all traffic pattern altitudes, standard and non-standard, will be added into NASR (the AFD) for all airports. This will be a day forward implementation beginning in July 2014.

Some other places the TPA is recommended to be 1000 feet AGL:

Advisory_Circular AC90-66A

  c. It is recommended that airplanes observe a 1000 foot above ground
  level (AGL) traffic pattern altitude. Large and turbine powered
  airplanes should enter the traffic pattern at an altitude of 1,500
  feet AGL or 500 feet above the established pattern altitude. A pilot
  may vary the size of the traffic pattern depending on the aircraft's
  performance characteristics.

The Chart Supplement (A/FD) will typically list the Traffic Pattern Altitude (TPA) if it is non-standard (not 1000 ft/1500 ft AGL)

The FAA's Airplane Flying Handbook says

  The traffic pattern altitude is usually 1,000 feet above the elevation of the airport.

Now the Chart Supplement (A/FD) will be the definitive guide!


Could abrupt flight controls input cause damage in these circumstances?

Now you know what a terrible idea it is to do "fool around" stuff with a passenger.

The AA-5's maneuvering speed is 122 MPH so you can't overstress it at 105; it will stall first (and you'd have likely snap rolled it) so you should be fine.

When in doubt, the rule of thumb is, when it's metal, if it's not permanently bent, it's good to go.  The metal structure is a big spring (elastic deformation under load and returns to its original shape below the "yield" threshold) and if you don't go past its elastic limit into the plastic range (the yield limit - permanent set to a new shape after bending) it just springs back and Bob's your uncle.  A single application of a near overload to a metal structure has no significant effect fatigue wise if you stayed in the elastic range.

If you think you might have over stressed it past the metal's yield point so there is a permanent set, there will be telltale wrinkles in the skins of the wings or tail that weren't there before. If you know what to look for, I wouldn't call for a mechanic, but if not sure what you're looking at, you should have an expert take a look.

But, like I said, a pull or push on the AA-5 at 105 mph can't overstress the airframe in the first place, so if you were really going that speed, I wouldn't worry about it.  I would just resolve to be more careful about passenger briefings and "horseplay".


May I fly from the right seat due to a temporary disability?

Generally speaking if your carpal tunnel syndrome is so bad as to preclude normal operation of an aircraft you probably shouldn't be flying from either seat: If something goes wrong you may well need both hands to operate the aircraft (we do have one-armed, and even no-armed pilots, but they generally have medical exceptions, and possibly additional restrictions).

If you cannot complete your training before your surgery your best bet would probably be to defer your checkride until after the surgery - yes this will probably mean additional training time after your surgery to knock off the rust, but you will likely need this anyway (you're going to get rusty during your recovery whether you're a certificated pilot or a student).
Additionally it's probably a good idea to have a safety pilot with you for your first few flights after recovery to ensure you're physically able to perform all the tasks required to safely fly an airplane, and an instructor accustomed to flying from the right seat makes an excellent safety pilot so you could knock off the rust and do your checkride prep at the same time you're making sure you can safely operate an airplane.

Re: FAA paperwork, I'm not an AME but looking at the FAA's Guide for AMEs carpal tunnel syndrome doesn't appear to be disqualifying unless it presents a "medical deficiency" (broadly "interferes with you operating the flight safely").

When you have your surgery you'll be effectively grounded until you recover adequate normal function in your hand to operate the flight controls without risk to the safety of the flight, and more importantly without risk of doing further damage and hindering your recovery.

Having had surgery is something you'll need to report on your next medical application (Question 18, item x: "Other illness, disability, or surgery "). Speak to your AME for details on what you may need to bring to the exam, if anything: Some may want a note from your treating physician stating you've regained normal grip and sensation, others might be OK just checking out your hand during the exam per 67.313 and saying "Your hand seems fine to me."
If you're enrolled in AOPA's Pilot Protection Services you can contact them for information on what you should have available for the exam.


How secure is an airplane on the ground whilst passengers are boarding the aircraft?

Those guys would gain no more than they'd gain if they hijacked the bus taking you to the airport.

If fact they'd gain less as they'd have nowhere to go and there's a lot more (armed) police at the airport to take them down than there is in some remote place they could drive that bus to.

The greatest risk to airliners isn't people forcing their way into the cockpit on the ground who are among the passengers waiting to board.
It's in the ground crew who can (at least in theory) bring aboard almost anything they wish (including bombs) with very little oversight (in fact this has happened in the past).
And there security has been seriously tightened (and it needed to be) over the decades, making that scenario far harder.
I won't say impossible, because nothing is impossible as long as there is a human factor involved anywhere in the chain, but bloody hard.


What makes V1 one or two knots less than Vr on this performance table?

$V_R$ is:

FAR 23 (e.g. multi-engine small planes):

  $\geq 1.05\;V_{MC}$ (105% of minimum control in flight)

  $\geq 1.1\;V_{S_1}$ (110% of V-stall for a specific configuration)

FAR 25 (e.g. transport jets):

  $\geq V_1$

  $\geq 1.05\;V_{MC}$

  Must result in $V_2$ before 35 ft height

  Must result in $V_{LOF}$ [when the airplane is rotated at its maximum practicable rate]

    $\geq 1.1\;V_{MU}$ (all engines operative; AEO)

    $\geq 1.05\;V_{MU}$ (one engine out)

    $\geq 1.08\;V_{MU}$ (AEO, geometry limited)

Geometry limited is when there is a greater risk of a tail-strike, e.g., a stretched variant of an airplane (think A321 or 737-900).

So the 1-2 knot disparity in your example can be down to the difference between $V_{MCG}$ and the 105% of $V_{MC}$ or the different $V_{MU}$ speeds. (See below for full text on points 2 and 4.)

To clear any confusion about the Weight:V1 tables such as the one in the question, those tables are not for 1 runway length, those weights are the field limit weight, i.e., each higher weight in the table is a longer runway.

(Twitter) $V_{MU}$ test.

Source: http://wpage.unina.it/fabrnico/DIDATTICA/SPER_VOLO/APPUNTI_3CFU/11.2-TO&LAND.pdf

FAA AC No: 25-7C text:

  VR may not be less than V1; however, it can be equal to V1 in some cases.
  VR may not be less than 105 percent of the air minimum control speed (VMCA).
  VR must be a speed that will allow the airplane to reach V2 at or before reaching a height of 35 ft. above the takeoff surface.
  VR must be a speed that will result in liftoff at a speed not less than 110 percent of VMU (unless geometry limited) for the all-engines-operating condition and not less than 105 percent of the VMU (unless geometry limited) determined at the thrust/weight ratio corresponding to the one-engine-inoperative condition for each set of conditions such as weight, altitude, temperature, and configuration when the airplane is rotated at its maximum practicable rate.

(Emphasis mine.)


Does “trimmed for current speed” mean that the plane will do whatever to maintain the current airspeed?

The pitch trim setting is based on the angle of attack.  The angle of attack is affected by indicated airspeed, center of gravity and any unbalanced forces.

In a steady state aircraft where the airplanes forces are all balanced then we are left with airspeed and center of gravity.  Under normal operations where fuel is not being transferred from an aft tank to the wing tanks or vice versa, center of gravity for a certain period of time remains relatively constant.  This leaves airspeed as the primary change in angle of attack.

Your analysis would be correct.  If you are trimmed for 290 KIAS (not KTAS like you stated) and you increase thrust the following sequence will occur.

Airplane will begin to accelerate
The lift formula comes into play and additional lift is created on the wings causing the airplane to pitch up and start to climb.
The airplane experiences an increase in induced drag which opposed thrust.
The airplane will now begin to decelerate while still climbing.
The decreased airspeed causes lift and induced drag to decrease.
The aircraft will pitch down and start to descend.
Aircraft will begin to accelerate as it descends.
The cycle continues until the phugoid oscillation dissipates (provided the airplane demonstrates positive dynamic stability)
In the end, the airplane will be at a higher pitch attitude (in relation to the horizon), climbing, and at 290 KIAS.

The same type of analysis can be accomplished if airspeed is reduced.  The airplane, at the end of the phugoid oscillation, will be at a lower pitch attitude and the same airspeed.


Position interpolation between readings based on last positions & speeds calculated?  Plenty of 10Hz update GPS units exist, so it doesn't seem be rocket science to get there.

Units can track 12 or more satellites.  If calculations are being made from several sets of readings, then multiple positions can be calcuted at any one time as well.


What happens when the eigenvalues of the longitudinal A matrix in dynamic stability analysis, are NOT two sets of complex conjugate pairs?

what happens when the eigenvalues consist of 2 real roots and 1 complex conjugate pair?

This means that the one complex conjugate root is still an oscillation but the other has split into two aperiodic movements. One of them should be a positive and the other a negative real value. While the negative root signifies a damped, aperiodic movement, the positive root shows that the airplane has one instable, divergent mode.

Can this happen?

Yes, for example when the center of gravity is moved aft of the neutral point, the phygoid will split into two real roots. The positive one describes the divergent behavior when static stability has been lost.
Backgound:
The equations of motion describe how acceleration, speed and pitch rate of the airplane react to small changes of the control parameters (elevator angle and throttle setting). They are second order differential equations and can be solved similar to systems of quadratic equations where the coefficients form the elements of a square matrix. The eigenvalues of the matrix describe frequency and damping of the movement which is caused by changes to the control parameters while the eigenvectors describe how acceleration, speed and pitch rate are affected in detail.
Note that this only concerns longitudinal movement (forward-back, up-down and pitch). The equations can be expanded to cover roll, yaw and left-right movement as well, but the question only concerns the simpler longitudinal part.


what would happen if we use needle shaped nose in blended wing?

I assume you talk about the airfoil's nose, not the fuselage nose.

A pointed nose will cause flow separation in subsonic flow if the angle of attack is only slightly different from the angle of the leading edge. This flow separation will mightily increase drag because now no suction peak at the nose will develop. This suction peak is causing what is sometimes called "leading edge thrust" (PDF!). Without it, the aerodynamic forces will act in a direction orthogonal to the wing's surface and not (almost) orthogonal to the flow direction.

In pure supersonic flow the pointed leading edge is superior, because it causes less wave drag. However, it is preferable to sweep the wing such that the flow around the leading edge will stay subsonic even in supersonic flight, precisely because it still allows leading edge thrust to manifest itself.


Only if you know the laser wavelength.

If the windshield uses a color filter, it can attenuate the light intensity at one wavelength while being transparent for other wavelengths, so the pilot can still see what is going on outside. The colors will look strange, but laser beams of the filter wavelength would have less intensity when viewed through this screen. This only works, however, if the laser wavelength is known beforehand, so the right filter can be used. All other light signals, approach lights and what not would need to use sufficiently different wavelengths so they are still visible.

Think of an approach in fog, rain or haze. Sometimes all the pilot sees is a faint glow - if the filter takes this away, the pilot will notice the lights only later, when they are closer and there is less time to react.

The other alternative is synthetic vision. The cockpit has a row of screens which show the images of cameras in the nose of the aircraft. This was already investigated by NASA for supersonic passenger aircraft, but met resistance from pilots. The reduced contrast of screens will make lasers merely a nuisance, but not harm the pilot. Clever software could even filter out too bright light sources.


How much time does it take to calculate the leveling and weighing of big and small aircraft?

Having recently weighed my plane I can tell you what's involved for determining the Basic Empty Weight for a small aircraft:

The aircraft equipment list is checked and updated.
This is kind of important as the equipment list includes the weight and arm of everything installed on the airframe. If this list is not accurate future weight and balance updates based on adding/removing equipment using the weights and arms on that list will be incorrect, and your computed weight and balance will start to diverge from the actual weight and balance until the plane is weighed again.
The aircraft interior is cleaned to "factory empty" status.
Pens, pencils, 3-year-old charts, note pads, and other things not on the equipment list are all removed, and things like tow bars are placed in their appropriate location/holder.
The aircraft is serviced with "fluids" per the manufacturer's procedures for weighing.
For most small planes this means de-fueled, full engine oil, & full hydraulic/brake fluid.
The shop sets up and zeros their scales.
You tow/push the aircraft onto the scales.
(Or lift it on jack stands with integrated scales designed for this purpose.)
The aircraft is leveled on the scales.
The plane must be leveled laterally (left-to-right) and longitudinally (nose-to-tail), otherwise the computations made later will be inaccurate.
The numbers on the scales are recorded.
The Weight and Balance is determined:

Add the numbers from all scales used. This is your Empty Weight.
You do some math on the scale numbers to determine the Center of Gravity.
(Since you know the station/arm of each scale you can figure out the CG based on the weights)

As Terry mentioned, setting up took more time than the actual weighing and calculations. In my case I spent about a day straightening out the equipment list, and the shop that did the weighing spent about 20 minutes verifying that I hadn't missed anything. Defueling the aircraft adds a significant amount of time to the process as well. Moving the plane onto the scales and leveling it took approximately 15 minutes, and doing the math took maybe another 5 minutes.

On larger aircraft the same general process can be used with larger scales (or jacks) - load cells integrated into a taxiway or ramp area are another option for doing this.

I'm not sure if aircraft that can determine their operating weight (by load cells installed in the gear assembly) can also determine their own empty weight - I know the weighing report I received includes calibration information for the scales used, so presumably weighing a large aircraft using its built-in load cells would require those cells to be properly (traceably) calibrated prior to using them for this purpose.


Is my understanding of magnetic variation and declination (for a VOR) correct?

Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

You are assuming that the VOR alignment will always be kept bang-up-to-date with the actual measured magnetic variation. Sadly, this is not the case! There is actually a considerable amount of work involved in re-aligning a VOR (it is not just a matter of tweaking a knob!) and re-alignment is only performed rather rarely.

Also note that the terms 'variation' and 'declination' are NOT synonymous in this context - 'variation' refers to the actual measured difference between true north and magnetic north; 'declination' refers to the difference between VOR zero-bearing and true north. These are NOT generally exactly the same.

For example: UK AIP ENR 4.1 (PDF)


Can diplomats be allowed on the flight deck of a commercial European airline?

Generally, airlines are required to have content in their ops manual clearly stating who is entitled to flight deck access from the point the engines are started until shutdown. In some countries, such as the UK, airlines are required to limit access only to essential personnel, and foreign airlines must follow the rule while in UK airspace. I would find it rather disturbing that an airline would allow access to the flight deck during flight ops to someone just because they have a diplomatic passport, given that some countries play it fast and loose with such documentation, and flight crews are not equipped to validate a passport.

SKYbrary is a wiki created by the European Organisation for the Safety of Air Navigation, International Civil Aviation Organization, and the Flight Safety Foundation to create a comprehensive source of aviation safety information freely available online. Its guidance on this subject is crystal clear:

  In the air, procedures vary from airline to airline and from region to region, but the principle that only those with legitimate need must be permitted flight deck access must be paramount. Beyond the pilots at the controls, this will include relief, check and training pilots. Exceptionally, such flight deck occupancy may exceptionally be extended to include other specifically-authorised employees of the operator - pilots, cabin crew and maintenance personnel. Flight Operations Inspectors carrying out observations on behalf of the State Regulatory Body may also be permitted access at the discretion of the aircraft commander subject to valid identification being presented prior to entry approval.


What are those sensors on the forward A321neo fuselage?

The probe on the left is a TAT (Total Air Temperature) probe and the device on the right is an Ice Detector.

From this website: UTC Aerospace Systems (Total Air Temp)

From this website:UTC Aerospace Systems (Ice Detector)


How do wide-bodied commercial aircraft achieve pressure equivalent to 6000 feet altitude?

The killer in fuselage pressurisation is the cycling of the structural loads. Aluminium is very unforgiving of repeated load changes: While steel has a load limit which can be applied infinite times, no such limit exists for aluminium. This means an aluminium aircraft will eventually fail if it flies enough cycles. It also means the plane can be built lighter or operated for more cycles if the pressurisation loads are lower.

Fatigue curves of steel and aluminium (By Andrew Dressel at English Wikipedia, CC BY-SA 3.0, source)

The consequences of this nasty characteristic were illustrated in Aloha Airlines Flight 243 in 1988, when the upper part of the forward fuselage failed at the suction peak aft of the cockpit and more than 5 meters of fuselage skin were ripped away.

Aloha Airlines Flight 243 after landing (picture source)

A fuselage made from carbon fiber reinforced epoxy shows very little fatigue and is much more tolerant of repeated loading changes. Here, thermal and impact loads are the most critical issues. However, the bad historic experience with aluminium structures means that not only the engineers, but even more the certification authorities are very careful to clear any new material for high, repeated loads. As a consequence, the graphite fuselage structure of the Boeing 787 is much stronger than a regular fuselage made from aluminium and can be loaded to a higher pressure without much added risk.

Whether this means less headaches and constrictions for you is less clear. The added pressure does feel better, but the major improvement in creature comforts in the cabin of the most modern airliners results from  higher humidity in the cabin air.

Most single-engined aircraft have no pressurisation; only the high end types like the Piper Malibu or the TBM 700 do. On the other hand, some business jets can keep the cabin at sea level pressure up to 12000 m altitude. In airliners, however, holding the cabin pressure at or above the equivalent pressure at 8000 ft of altitude is mandatory. Citing Wikipedia:

  Keeping the cabin altitude below 8,000 ft (2,400 m) generally prevents
  significant hypoxia, altitude sickness, decompression sickness, and
  barotrauma. Federal Aviation Administration (FAA)
  regulations in the U.S. mandate that under normal operating
  conditions, the cabin altitude may not exceed this limit at the
  maximum operating altitude of the aircraft.


Which kind of display are used on commercial aircraft?

Traditionally, it was almost only movie projectors in the cabin, since Cathode Ray Tubes (CRT) were too bulky and impractical to fit. It was also very expensive. I think only with the arrival of LCD displays seatback displays that we know now were made possible.

As for cockpit displays, the CRTs were widely used until LCDs were introduced and offered better performance.

You can compare the 737 Classic with retrofitted CRT displays with those of the 737 Next Generation.
Compare the 747-400 to the 747-8 which had LCD in place of CRT.

I can't find pictures, but have been told that the old CRT modules were huge, very deep (which you can't see) and heavy, compare to the thin panel that now is available.

You can still backwards order CRT for aircraft. I was flying with one airline that had done on a very new A320 to maintain commonality with the rest of the fleet. The pilots didn't appreciate it though, since it was bad for the eyes, needed to be dimmed and had (I believe) inferior resolution.

Page 9 of this document gives some details on the display type tradeoff.


Efficiency of the Kuznetsov NK-88 or NK-89 engine burning hydrogen

Based on my reading, the biggest disadvantage of hydrogen as an engine fuel is its low density.  Even when stored as a cryogenic liquid, it take up more than ten times the space as the same mass of kerosene, which means the same size tanks will hold only about a third as much energy (even though hydrogen has about three times the energy content per kilogram as kerosene).
Given these qualities, an engine suitably redesigned to burn hydrogen ought to be able to run more efficiently in terms of specific power output (energy produced per mass of fuel) or specific fuel consumption (same thing only upside down -- fuel consumed per energy produced), but even ignoring the weight of insulation and the fuel cost of boil-off in flight (or before takeoff), an aircraft the same size would have roundly a third of the range.  This is usually taken to mean that hydrogen fueled aircraft would have the same issues as hydrogen fueled rockets: they have to be very large to carry an economically useful amount of fuel.
There are other methods of storing hydrogen (developed for automobile prototypes); as I recall, the highest density of storage (kilograms of usable hydrogen per volume) has been with metal hydrides; these require neither very high or very low temperatures, and operate at reasonable pressures (rather than hundreds of atmospheres as common pressure bottles do) -- but they barely exceed the density of liquid hydrogen, and there's a penalty in the weight of an "empty" tank, which is still filled with a solid material when no more hydrogen can be drawn from it.
Beyond qustions of range, hydrogen is difficult to work with in terms of materials (it causes most metals to become brittle just from being in contact with hydrogen), it leaks through almost everything, and is so flammable that these unavoidable leaks are very prone to become invisible conflagrations (because a hydrogen fire contains nothing that glows from heat, the flame is practically invisible -- the bright flames of the Hindenburg were due to the envelope and its coating, not the hydrogen).  This is obviously irrelevant to engine efficiency, but very relevant to whether such an engine can last long enough in service to be economical to operate.


What is the parasite drag for Airbus A320?

For numerical usage (in some question) you can use the above formula, but for exact drag (and lift) calculation, the industry standard is to use Computer Solvers to compute this drag because of the following reasons:

The formula used above is correct but the value of Cd varies for every point on the wing, as things like wing twist, sweep back, wing tip sharklets contribute to a different Cd throughout the wing.
Other effects like drag due to fuselage etc can not be calculated by using a generalized equation for the whole plane, as usually local Cd for every point on the airplane surface are known.
Application of force on the surface of an airplane (Lift/Drag force) causes deformation in the geometry of the surface and this leads to recursive change in the value of Cd/Cl, which hence needs further set of calculation to again arrive at a value of Cd from the Solver.

For approximate usage though an averaged out Cd, Cd0 and AR if known, then the above formula can be applied to arrive at the results.


Why Western fighters' afterburner glow is reddish orange while Eastern Bloc fighters' is blue?

It all depends. I don’t think there’s a distinctive color between the two.   I would submit that the photos of the Flanker may be in a early stage of the afterburners lighting versus the photos of the Raptor or the F-35, which appear to be in full afterburner.   The color of the exhaust gas is a function of the stoichiometric mixture of fuel and air ratio. It full burner the mixture is quite rich, resulting in a orange colored flame.   Lower stages of afterburner are probably  much closer to peak stoichiometric mixture.  Incidentally these blue colored flames are indicative of a much hotter exhaust gas temperature than an orange colored exhaust.   I’m not sure of the reason why one is selected over the other.   I can submit that there are plenty of photos of Russian aircraft in afterburner with orange colored exhaust and other pics of western military airplane with blue colored exhaust as seen below.


What is a 'Wing Loop Fault' on an A320 series Aircraft?

My guess is that there was some circuitry that was playing up. While I'm not sure about 'Wing Loops' specifically, for what I think is a similar example the Airbus A320 series has fire loops in the engines.

There are two fire loops. If I'm right, it's got little thermal sensors along way, which disrupt the current if they get too hot (or if the wire is broken). Logic gates ensure that both loops must be 'high' for the fire warning to go off. If one is not working, it works from a single input.

Assuming both of these were working from the beginning, I think it throws a FIRE DET FAULT warning on the pilot information display if one triggers without setting off the other one. I'd imagine that maintenance either disabled the faulty loop within those allowed by the Minimum Equipment List or by changing the part. If it was quick, it was probably the first, but they're not going to go telling passengers that they disabled safety equipment :)


The purpose of the anhedral is to get the tail surfaces away from the wing wake. The distance between the wing's trailing edge and the horizontal tail is low enough so that the tail needs a vertical offset in order to have full dynamic pressure available locally.

Earlier designs moved the tail up for the same reason (Lockheed F-104, Grumman F-9F) until it became obvious that at some angle of attack some airplanes could get into a deep stall. Especially with the wider angle of attack range possible with swept, low aspect ratio wings no position above the wing's plane is safe anymore.

For that reason all modern fighter aircraft position the horizontal tail as low as practical. This in turn has led to situations where ground effect is so strong that other tricks had to be employed to get the airplane to perform as intended.

Stability is not affected, aside from a small positive contribution to the sideslip-induced rolling moment. This is easily compensated by adjusting wing dihedral properly. Or, in other words, the F-16 wing can have zero dihedral because the horizontal tail in combination with the ventral fins reduces the rolling moment of the vertical tail in a sideslip to the desired value.


Airships are much more susceptible to bad weather. With an airplane you can pretty much count on it taking off on time and arriving within a couple of minutes of the scheduled time. This allows operators to employ their aircraft with a reliability which an airship will never be capable of. This means that the investment made into an airliner will lead to a predictable stream of revenue and (hopefully) profit, whereas the airship might need to wait several days until it can take off again, or headwind  might delay it by more than a day on a long trip. In this time, it will accumulate interest and fixed cost, but no additional revenue.

Scheduled air services were indeed first done with airships, but as soon as dependable aircraft were available, only a few niches like transatlantic travel were operated with airships. As soon as airplane performance covered those niches, airships became obsolete.

Today the most credible use of airships would be transportation of heavy equipment into remote areas. Tourism could be a second use, but the number of airships needed to cover this market is small. Airships do have some benefits:

no need to build roads or runways
carrying of outsize cargo is no problem
if the airship is big enough, payload can be immense
destinations can be anywhere, not only on a coast or a big river (where regular ships cover this niche more than adequately)

The few cases where the airship could be employed gainfully would not cover the expense of developing and building one. Development cost alone would be similar to the development cost of a new airliner!

But if we look back in history, even at a time when airships were available, big transportation tasks were done by aircraft. Papua has impenetrable jungles, and to transport dredging equipment and personnel to the Morobe goldfield from 1932 on, aircraft were used rather than airships. The huge dredges were dismantled for air transport and flown into Bulolo, where they were re-assembled and put to work. With a Zeppelin, less dismantling would have been needed, but just the complication of keeping the airship balanced when the load is released made this option less attractive.

For the history buffs: To transport very long items like the dredge shafts, a single-engined Junkers Ju-52 (Ju-52/1m) was used. It had a detachable rear fuselage with  automatic couplings for the control rods. Spherical collar screws ensured precise positioning of the two fuselage sections.


Why is the top left corner cut off on the cg location envelope for small airplanes?

With "top left corner" you mean the combination of a heavy airplane and a forward cg location.

This is to limit maximum stick forces. Manoeuvring (= flight at a specified load factor ≠ 1)  needs force proportional to the distance between the cg and the neutral point ("static margin") and the mass of the airplane. There are several situations in which certification requires not to exceed specific maximum forces. For example, FAR Part 23.145 says:

  (b) Unless otherwise required, it must be possible to carry out the
  following maneuvers without requiring the application of single-handed
  control forces exceeding those specified in §23.143(c). The trimming
  controls must not be adjusted during the maneuvers:

[...]

  (c) At speeds above VMO/MMO, and up to the maximum speed shown under
  §23.251, a maneuvering capability of 1.5 g must be demonstrated to
  provide a margin to recover from upset or inadvertent speed increase.

Another would be out of trim situations as those in Part 23.255:

  (f) In the out-of-trim condition specified in paragraph (a) of this
  section, it must be possible from an overspeed condition at VDF/MDF to
  produce at least 1.5 g for recovery by applying not more than 125
  pounds of longitudinal control force using either the primary
  longitudinal control alone or the primary longitudinal control and the
  longitudinal trim system.


Are all EGPWS-equipped aircraft using the same underlying system?

Yes and no.  When you say EGPWS that is a trademark of Honeywell, so all EGPWS systems are made by Honeywell.  The fine print is the the requirement TSO-C151c is for a Terrain Awareness and Warning System (TAWS).

The TSO functionally describes the Honeywell system. (Honeywell actually designed it before the FAA decided it was a necessary addition to the rules.)  Since implementing the standard would most likely require the use of Honeywell proprietary information (patents), Honeywell is legally required to provide the product it to all customers on a non-discriminatory basis.

As a result, even though Rockwell Collins provides the Integrated Surveillance System (ISS) for the B787, the TAWS component of the ISS is sourced by Collins from Honeywell.

That said, Thales was able to patent a similar system in France about the same time.  There were lawsuits, and in the end they agreed to let each other exist.  ACSS (a joint venture between L-3 Comm and Thales) sells TAWS units in the US that is based on the Thales system, so there is an alternative to Honeywell. Thales does sell their system outside the US.

Some of the primary Honeywell patents for the EGPWS have recently expired or are about to expire. So there may be some new alternatives soon.  Of course Honeywell has been making 'upgrades' beyond the regulatory requirements which they have been patenting.  These may help them hold their market share.

So no, there is not a single system to meet the requirement.  OTOH, there's not much competition either.


What classification of aerial vehicle would a manned quadcopter or octocopter be?

From 14 CFR 1.1:

Helicopter means a rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors.
Rotorcraft means a heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors.
Aircraft means a device that is used or intended to be used for flight in the air.

A manned quad- or octocopter seems to fit the definition for “helicopter”, assuming you mean the kind that have no fixed wing.


How does the forward speed of an auto-gyro relate to the rotation speed of the rotor during auto-rotation?

An autogyro flies at constant rotor RPMs. In vertical autorotation, that is a particular case, the rotor revs may be 10-15% less, but in straight and level flight, the rotor revs are higher, and always the same, independently of the speed of the gyro. It cant be otherwise, since, if the rotor revs went higher with the speed, the rotor thrust would also rise, and the gyro will gain altitude, no longer flying straight and level...

It's also true that if, flying s/l, the pilot opens the throttle, he also pushes the stick slightly forward in order to reduce the AoA of the disk, and thus keep flying s/l...


Can hydrogen gas be an alternative fuel source for a gas turbine engine?

Yes, and it has been demonstrated 30 years ago on the Tupolev 155. This is/was a hydrogen-powered version of the Russian Tu-154B tri-jet. Only one has been built and has since been retired after demonstrating the use of liquid hydrogen in 5 experimental flights. In total, the Tu-155 performed about 100 flights with several fuels, among them hydrogen and liquefied natural gas.

Tu-155 (top), cut-out (center) and layout (bottom). Source: https://www.aviaru.net/pr/?id=11633

Hydrogen is actually better for a jet engine: It is gaseous at normal temperature, so there is no delay for the evaporation step to happen as there is for liquid fuels before mixing and combustion can start. Also, hydrogen burns in a wide variety of mixing ratios with oxygen, so flame-outs are much less likely, making a smaller combustion chamber possible.

The downsides, of course, are storage and the small molecular size of hydrogen. It is very hard to contain, and it needs big volumes to store a given amount of chemical energy. Pressurized storage at 200 bars holds only 18 kg/m³ or 45 times less than kerosene. With 142 MJ/kg, hydrogen holds three times as much chemical energy than kerosene, but then the volumetric efficiency of kerosene is still a factor of 13.3 better.

Cryogenic storage swaps pressure for low temperature: Below 33 K and above 13 bar, hydrogen becomes a liquid and storage density increases to 30 kg/m³. Still, cryogenic hydrogen storage needs 4 times the volume of the same amount of energy stored as kerosene, plus the isolation and the energy to cool it down and compress it.


Why is there an altitude limitation on operating the slats/flaps on the A320?

Why would you want to deploy high lift devices at any juncture except for departure and arrival?
Holding is the predominant scenario.
Above 20,000ft ICAO holding speeds are higher thus allowing clean configuration.
Above 20,000ft, at slower speeds, the likelihood of encountering icing on the wings when moisture exists is also a very real consideration (upto about 28,000ft) and ice accretion is not what you want with high lift devices hanging out.
Extending slats/flaps in typical cruise or when we are flying in the Mach regime of flight, say about 29,000ft and above, is something that would cause much grief to the average Airliner, a possible jet upset scenario.
If other reasons already cited such as aircraft certification/cost of certification are a factor - most operators would not need a change from the 20,000ft figure.


What are good apps to help with VFR navigation on my first solo?

No, there is no "app" you should be seeking at this stage of your training to do what you're asking about.

First, if you have questions about what you'll be doing while you're flying solo and how you'll be navigating, you should be discussing this with your instructor, not on an internet message board.

Second, whatever tools you'll use in flight need to be ones that you've used under supervision of an instructor, so that you understand how to use them, and what their limitations are.  Student solo flying is NOT the time to go exploring technology and apps you've not used before, in addition to the full-time tasks of maintaining aircraft control, aviating, navigating, and communicating.  Too many dead pilots got focused on some technological toy and, without realizing it, lost situational awareness and/or control of their aircraft.

Third, before you are navigating anywhere solo, you need to be able to (and demonstrate to your instructor that you can) navigate successfully with the tools you have -- a map, compass, stopwatch, and the pre-flight planning you've done.  If you don't feel up to that task (which has been all of us early on in our training -- nobody is born with a built-in INS), work on those skills with an instructor.  Navigation apps are fine, but they supplement, rather than substitute for, your own skills when it comes to VFR navigation.

Fourth, the fact that you would come here to pose such a question, rather than to your instructor, is troublesome: you want an app that can enable you to go off on adventures sightseeing all of Barcelona, but you aren't discussing these plans with your instructor... perhaps he might not approve of all that you're planning?  Perhaps, maybe, you should limit your expectations of how much adventuring & exploring you'll undertake initially, and have a more complete discussion with your instructor of what you are, and are not, expected to do when, or if, you are signed off to solo.  (As stated in comments, the initial solo is typically pattern work only, rather than navigation.)

Flying is about far more than aircraft control; it is a matter of judgement as well, and the difference between sound judgement and the lack of same can make the difference between a successful or a catastrophic outcome of a flight, at least as much as the difference between great and mediocre "stick & rudder" skills.


Where can I find an accurate list of IATA meal codes?

SPML is Special Meal, differs from airline to airline.
VGML is vegan, i.e. not even dairy products are used. Unlike vegetarian were animal products can be used, say eggs or milk.
Source: International Air Transport Association. "Cabin Operations Safety Best Practices Guide." (2017).


Descending while turning inbound on a non-precision approach

It's because 2500 feet is the minimum altitude prior to starting the inbound descent at 8 DME and you can descend to that altitude pretty much anywhere between ABV and 8 DME inbound, within the protected airspace of the procedure turn.

When you started the outbound leg at 3000, you can pretty much descend to 2500 sooner or later, as long as you don't descend below 2500 until you are 8 DME inbound on final.  So if you find yourself at 2700 at 8 DME outbound and you're ready to turn in, you can keep descending to 2500, leveling at 2500 until you cross 8 DME inbound.

Basically, the airspace between ABV, the 10 mile circle, and 30 degrees of the approach path is the protected airspace with a floor of 2500, and technically, you can do whatever you want within that zone.  Also the altitude is a minimum.. you don't have to descend to exactly 2500, technically (I'm assuming SA IFR rules are similar to NA)


How can I calculate CLmax for a wing with multiple airfoils?

The result depends on the wing twist and its taper ratio. You will never get the full 2D lift with a 3D planform, and how much you get depends on the details of your wing. The first step would be to apply Hans Multhopp's methods for calculating the lift distribution of wings (Subsonic Lifting Line Theory), and a search on Github shows a variety of codes to try. Factor in the different airfoils with local twist according to their zero-lift angle of attack.

Lifting line theory does not deal with flow separation and stall, so you need to interpret the results. Ideally, your airfoils will reach their stall angle of attack with a straight lift curve slope, and this stall angle of attack is reached at the same time over the whole span. This should maximise the lift from that wing. Adjust taper and twist until you reach this result (and subtract the twist from the differences in the airfoil's zero-lift angle to arrive at your geometric twist distribution).

However, if you also care about handling, leave some margin at the outer wing in order to have stall start first at the root. Also, you might want to add the effects of aeroelastic deformation in order to avoid unpleasant surprises.


Why not always take off with 10 degree flaps in a Cessna 172?

Flaps out will reduce the ground run, but you're forgetting that they also increase drag. This is why you don't climb all the way to cruise altitude with flap extended. A 172 will climb better without flaps.
With a take-off, you have to consider both the ground run and initial climb. After all, the take-off distance required is defined as the distance required to climb to screen height (50ft in Australia, sorry if the US is different). The aircraft also has to meet a legal minimum climb gradient to that height.
Without looking at the data, it is possible that in certain conditions, taking off with flaps 10 will get you off the ground quicker, but will actually take longer to reach 50ft compared to a takeoff with no flaps. In any case, retracting flaps in a busy time after takeoff is a potential hazard, and in a training aircraft like the 172 simple procedures are best.


Can a commercial pilot get paid by friend?

If your friend approaches you and asks for a one-time ride, you're fine to charge for it. If you decide to advertise, or your friend tells a friend who tells a friend and word of mouth is that you offer an air taxi service, you are holding yourself out as a carrier and need to get a part 135 operating certificate.

Same thing with ferrying, banner towing, etc. If you are offering it to the general public as a service you need to get an operating certificate.