How much horsepower to weight is required for a 1:1 thrust ratio?

Horsepower cannot be converted into thrust without knowing the speed at which this is done. In your case this is zero, so the special equation for static thrust applies. With $T$ for thrust, $P$ for power, $\rho$ for air density and $d_P$ and $\eta_P$ the propeller diameter and efficiency, respectively, this is $$T_0 = \sqrt[\LARGE{3\:}]{P^2\cdot\eta_{P}^2\cdot\pi\cdot d_P^2\cdot\frac{\rho}{2}}$$
With this equation you cannot simply say how many HP are required to lift that many pounds; instead, you need to add the propeller geometry. If we assume a very efficient propeller with a large diameter, we can combine blade size and mass into disk loading – here 8.5 kg/m² or 41.5 lbs/ft² is a good value. Now we can write, using standard atmospheric density at sea level:
$$\frac{T_0^2}{P^2\cdot\eta_{P}^2} = \frac{2\cdot1.225\,\frac{\text{kg}}{\text{m}^3}}{8.5\,\frac{\text{kg}}{\text{m}^2}\cdot g} = 0.0294\,\frac{\text{s}^2}{\text{m}^2}$$
assuming for simplicity a 1 m propeller diameter. If we now use 70% for propeller efficiency, as you did in the question, the power to mass ratio becomes 8.33 Watt power per N of weight force or 81.7 Watt per kg. Converted into funny units this is 0.05 HP per pound of mass. 1 HP can hold 20 lbs in the air, but that will not be enough to even spin up the propeller unless it can adjust blade pitch. For practical usability I recommend to double that number. If the propeller diameter shrinks and disk loading increases, efficiency will drop and the power required will rise.


In general terms, is a Constant Speed Propellor equiped aircraft more expensive to maintain?

Yes, they are more expensive because they are more complex.  I would not avoid buying a plane simply because it has a CS prop because, in the grand scheme of airplane ownership, CS prop maintenance has a small impact on your hourly cost.  Maybe ~$1/hr for a piston single with repetitive ADs.

Now, the maintenance of a CS prop can range from a simple visual inspection of about \$100 USD to a complete replacement of the prop (up to \$15,000 USD, maybe more for large props).  An owner will likely not do an "overhaul" when the prop reaches TBO but will instead opt for an IRAN (Inspect, Repair As Necessary).  This is typically a little cheaper than a full blown overhaul and, from a practical perspective, extends the life of the propeller as an overhaul would.  At some point you'll end up grinding blades and balancing the prop but that's not annual maintenance.  I've seen the cost quoted at around \$1,500 USD for a high performance piston single IRAN which comes to less than $1/hr for a private owner.

All in, a CS prop only adds a small amount to the hourly operating cost.  In my opinion, the performance gains are worth the cost in most cases.


Why is "heavy braking" recommended in an aircraft with carbon fiber brakes in order to prevent brake wear? That seems backwards!

I know nothing about this topic but it's quite an interesting one so I did some Googling and found a detailed Airbus document called "Proper Operation of Carbon Brakes". It includes the following information:

  All brake manufacturers highlight the fact that carbon wear is heavily
  affected by brake temperature

This is completely different from steel brakes:

  ...energy is theoretically not the primary parameter for carbon wear,
  whereas it is the  most important one for steel brakes.

The document goes on to say this:

  Generally speaking, to increase carbon brake life, brakes should
  either be operated cold or hot but not at intermediate warm
  temperatures

And this:

  All recommended braking techniques should aim at reducing the number
  of brake applications and optimizing the carbon brake temperature

The specific temperatures mentioned in the document are different for different manufacturers, but generally the intermediate range to avoid seems to be roughly 100-300C. That would explain why 'heavy' braking might be recommended: it's better to use one single brake action that generates more heat and gets into the higher temperatures than to brake multiple times more lightly. Nevertheless, the document also says that overheating is still unsafe and the use of max autobrake is not recommended.

I found a related discussion of exactly this question elsewhere that goes into the physical explanation (although it doesn't quote any sources): at lower temperatures, the carbon becomes a powder which abrades the brake pads, lowering its life; as the temperature rises, the carbon starts behaving like a fluid so the wear is greatly reduced. Most discussions of carbon brakes that I found say that their physical properties and behavior are well documented, but not well understood.


What are the layers of a pressurized fuselage (cross-section and construction)?

The pressure hull is the aluminum tube formed by the outer skin and supporting frames which serve to stiffen it.  Within that is a layer of insulation which is fibreglass in a ripstop plastic material envelope made into custom fitted sections clipped into place, then the facia panels that create the interior surfaces.  At max pressure differential the hull swells a little bit, maybe a quarter inch in a large airliner.  The primary structure is the outer tube.

A big problem with airliners is warm and very humid interior air (from the passengers) is able to circulate between the facia panels and outer hull and some of this air gets between the insulation batts and the outer skin, which at altitude is really cold so the moisture condenses as frost/drops.  Over time water starts to get absorbed into the insulation.  Airliners can have hundreds of pounds of water in the insulation especially down at the bottom, and at heavy checks the insulation has to be removed and dried out.


What military aircraft has six propeller engines?

You've probably seen a tanker variant of the C-130, like this KC-130J:

Source: https://www.thedrive.com/the-war-zone

On each wing there is an extra fuel tank and the pod containing the air-refueling hose.


When is International Procedures Training required?

I have never heard of a specific "International Procedures"  program/rating. However, this was taken from one of the websites offering this training:

  "it will satisfy the training requirements needed for MNPS, RVSM,
  RNP-4, RNP-10, B-RNAV and P-RNAV Special Use Airspace approvals"

My guess is this course is offered as a bundle of MNPS/RVSM/RNAV training programs to the operators.

For the operator to be certified to fly in MNPS airspace, they need to have a specific authorization from the regulating authority (in FAA they specifically need OpSpecB039) which in turn, requires specific training programs for the pilots. The same is valid about operation in RVSM,RNAV,etc.

There is no specific recurency training period; however, each approval certificate, for each specific operator, must include an approved 'initial and recurrent training program' which will specify the training frequency. At most, these periods must abide
by the operator's program for proficiency check and line checks.

8900.1 4.1.4 98(d)

  NAT/MINIMUM NAVIGATION PERFORMANCE SPECIFICATIONS (MNPS) AIRSPACE.

  Each  aircraft is suitably equipped and capable of meeting the MNPS standards.
  The operator has established operating procedures that
  ensure that MNPS standards are met.
  The flightcrews are trained and capable of operating to MNPS requirements.

  [...]

  All NAT/MNPS approvals are granted by issuing OpSpec paragraph B039,

ICAO NAT Doc 007 § 1.2.1 (I'm really not an FAA guy, somebody might find the Part 135 / 91 equivalent)

  All flights within NAT MNPS Airspace must have the approval of either
  the State of Registry of the aircraft, or the State of the
  Operator[...] These aspects include: the navigation equipment used,
  together with its installation and maintenance procedures; plus the
  crew navigation procedures employed and the crew training
  requirements.

  The ICAO doc goes on to further detail what that training should include (1.3.8 - > 1.3.12), but does not specify a training frequency or the need for recurrent training.

Appendix G to Part 91: Operations in Reduced Vertical Separation Minimum (RVSM) Airspace
Section 3 Operator Authorization

  (b)An applicant for authorization to operate within RVSM airspace
  shall apply in a form and manner prescribed by the Administrator. The
  application must include the following:

  (2) For an applicant who operates under part 121 or 135 of this chapter or under subpart K of this part, initial and recurring pilot
  training requirements.

AMC1 SPA.RVSM.105 RVSM operational approval

  (c) Training programmes, operating practices and procedures :
      The operator should submit training syllabi for initial and recurrent training  programmes together with other relevant material.


How to find total climbing time based on Rate of Climb?

The fastest way is an integration. If you plot the inverted climb speed over altitude like in the plot below, the area under the curve will give you the time to climb. I would suggest you calculate the climb speed at every 1000 m by rule of proportion — at sea level it is 20.87 m/s and at 11.000 m it is 0 m/s. Then you get a plot like the one below. The red circles are the calculated points; the lines between them are straight interpolations.

Since the unit on the Y-axis is seconds per meter and the unit on the X-axis is meters, the area is in seconds. Just calculate the area of each trapezoid between the start and the end altitudes and add them. In the figure below I have crosshatched the trapezoids between 3000 m and 8000 m.

Inverted climb speed over altitude plot. Caution — this is a generic plot I did for this purpose earlier, so it uses different numbers. I crosshatched the area between 3000 m and 8000 m, because I made the plot for calculating the climb time from 3000 m to 8000 m.


Is a VHF NAV receiver inherently capable of processing a LOC signal?

Any VHF radio can receive the carrier signal of a LOC signal but to use it, you need dedicated circuitry.

The LOC transmitter transmits the carrier wave, frequency modulated with two side lobes of 90Hz and 150Hz.  In order to use the LOC signal, circuitry needs to exist which will filter out the 90 and 150Hz then measure the difference between them to determine where in the beam the receiver is located.

This circuitry is dedicated to the ILS receiver.

It's a similar story for GS which simply uses lobes above each other, rather than left and right.

So

  Is LOC signal processing an inherent capability of a NAV receiver

No.

  Are there NAV receivers incapable of LOC signal processing.

Yes.  All of them which do not have an ILS function (e.g. the Rockwell Collins VOR-900).

[ BEGIN Mythbusters Territory]

I've just tuned my hand held airband receiver to the ILS of my local airport.  I think I can make out the beat frequencies from the 90 and 150Hz although I doubt that the low pass filter is letting much 90Hz through.  I guess it's optimised for voice reception so probably cuts off at about 100 or 120Hz.

The next time I drive past the end of the runway, I'm going to test whether I can detect any change in the beat frequencies as I drive through the lobes.  There is a convenient hill about a mile from the threshold which will put me a couple of hundred feet above so I should be in the beam.

I guess it might just be plausible to hear when you are going left and right of the centre line.

I'll report back.

[END Mythbusters Territory]


Why are on-board computers allowed to change controls without notifying the pilots?

There is a general design principle, that some, but not all, of the behavior of the flight guidance system or autopilot should be visible to the pilot.  Usually automatic engagement or disengagement of a control system is indicated, but sub-modes of these controls or manual changes might be unindicated. That sounds simple and logical, but in reality it's a complex and tricky human factors issue. Displaying too much information to the pilots is just as bad as displaying too little, as too much information can cause them to ignore the most important indicators, like a stick shaker warning about a impending stall.

I know this isn't a very satisfying answer, but it's hard for anyone other than a human factors expert to answer a very general question like this with hard-and-fast rules.

For example, many autopilots will skip the aural warning about disengagement if the disengagement was directly caused by a pilot action. The pilot in this situation only gets a visual indication that AP has changed behavior. This sounds perfect, but on Aeroflot Flight 593 a pilot let his child into the cockpit, who then applied enough pressure on the yoke to disengage the automatic aileron control. There was no warning chime because the disengagement was considered intentional by the autopilot. This was unlike previous planes the pilot had flown, which contributed to the pilot's confusion when the plane began to turn and then go into a sharp bank.

The indicator light you mention for the 737 Max Lion Air crash is for "AoA disagreement", not MCAS engagment as you suggest.  While these kinds of AoA indicators are generally considered good ideas, it's still an area of debate how much they'd help confused pilots like in the Lion Air crash or the Air France 447 crash.

Similarly, the quality of indication was not the biggest cause to the accidents you've listed. For example, the Scandinavian Airlines Flight 751 could probably have returned to the airport successfully, but a compressor stall due to ice striking both engines at low altitudes is a bad situation no matter the auto-thrust behavior. Bad sensors and confused pilots can crash a plane with perfectly designed indicators.

Edit:  since this answer is getting a lot of attention, I think it's important to note that I don't really cover any special considerations for the edge of the flight envelope, auto-trim vs auto-disengagement vs changing sub-modes, or idiot-proofing of flight systems. I'd love to see this covered in more detail by people with experience.


Why is there no alarm when the transponder is turned off?

Because it's not worth it.
Quick, name me another accident where a transponder-off alarm would have been exactly what was necessary to save the crew, the passengers, or the airframe?
Hint: there aren't many.
The number of false positives would drastically outweigh the number of actual events where a transponder alarm would have been able to do some good.  For example, air traffic control might ask an aircraft to turn off their transponder.  Nothing dangerous there, but alarms would be going off back at dispatch.  Good idea on paper, but not practical enough to make it worthwhile in the real world.


Is it allowed and/or safe to place objects on the dash in a PA28?

Next time you go flying just stick your hand up there and see if you can feel any suction at those holes when the engine is running. I doubt it. The holes in the coaming panel (the flat panel on top of the dash) near the base of the windshield are defroster outlets.

The vacuum pump on the engine accessory case sucks air through the air driven gyros on the instrument panel through rubber hoses interconnected to the vacuum instruments (on the Cherokee there is a regulating valve/manifold between the pump and the instruments).  On the upstream side of the instruments there is an intake hose with a cylindrical filter to filter the cabin air before the pump pulls it through the gyros.

The filter is mounted in the open below the instrument panel somewhere in the jungle of wires and tubes, on a bracket of some kind.  If you slide on your back and get your head under the panel, you should be able to spot a cylinder about the size of a small can of beans with a wire mesh around it and pleats like a car air filter.  That's the vacuum system filter.

The main thing to be wary about when putting electronic devices on top the panel is they may or may not alter the wet compass when turned on.  Better to have an external antenna you can stick there and keep the GPS unit on a control wheel clip or something like that.


Do nose cones work well at high angles of attack?

No, they don't.
The slanted intake is slanted because the top part produces a compression shock wave that hits the lower intake lip so all of the compressed air is ingested and nothing is wasted. In order to do this reliably up to Mach 2.5 (where the Mach cone angle is 73.8° from the vertical), the whole intake needs to be slanted by those 73.8° (actually a bit less due to the intake wedge). In order to adjust the intake for different Mach numbers, one or both parts are adjustable, as shown in the F-14 example below.

Cross sections of the F-14 intake at subsonic, transsonic and supersonic speed. Note that the shocks of the supersonic example do not fit well with the intake geometry – the geometry is optimized for a much higher Mach number than shown. See here for a better example.
Here is (admitted a quite messy) cross section of the F-15 intake. The adjustable upper wedge is clearly visible (picture source):

In order to minimize losses, the upper intake uses adjustable ramps in order to force additional shocks so the compression happens over a sequence of weak, oblique shocks.
Regarding spike intakes: They were the lightest and simplest way to produce efficient supersonic intakes and were ill suited to large sideslip angles or angles of attack. Note that they were a characteristic of early supersonic flight when speed was the main objective and have since been replaced by ramp or even pitot intakes.


I am neither builder nor airport manager and can't provide details, but as there is no answer yet, I'll try to summarize the main differences between the materials:

Concrete can support highest loads and lasts long. It is however most expensive to build and when it eventually gets damaged, it can't be patched and has to be rebuilt in large slabs. It also requires stable base, because it is rigid. It is built in slabs with spaces between them that are required due to account for thermal expansions and give somewhat bouncy ride.
Asphalt. There is a big variation between different kinds of asphalt concrete, but it is generally cheaper than concrete, but can't support as big loads and is susceptible to damage by frost. Asphalt is never completely rigid and slowly deforms under load. This allows using it on unstable base, but it means bumps form by prolonged use reducing the total lifetime. It also easily cracks and when water freezes in the cracks, potholes quickly form. They can however be relatively easily patched. It does not require any thermal expansion gaps and provides smooth ride.
Asphalt and concrete can be combined. A concrete with a layer of asphalt on top provides the strength of concrete and smooth ride of asphalt. The asphalt also protects the concrete base from freezing water while itself can be easily patched. This is lately common on roads (where concrete alone is really uncomfortable), not sure how much on runways.
Gravel is simple to build; all you need is some stone from nearby quarry and a steamroller. The main disadvantages are risk of foreign object damage by loose stones thrown up by the wheels and need of regular maintenance. So it is used mainly in remote cold places where concrete would be too expensive and asphalt would not last long in prevailing weather conditions.
Rolled earth with or without grass is even simpler then gravel, used for small airports where big loads are not needed.
When runway is listed as grass, it means it is not even rolled. Usually enough for small airplanes and obviously cheapest.
Other surfaces usually as provided by local conditions (snow, salt etc.)

For every particular airport, the investor evaluates the available technologies whether they can handle the expected traffic (to handle large jets like 747 or A380 concrete or concrete-asphalt combination is preferred), whether they are buildable in given location (the ground has to support the weight too) and how much building them will cost and choose the option that satisfies the requirements and they have money for.


If an aircraft cannot fly above 300ft, is it an aircraft?

There is actually a legal Australian definition for aircraft, in Part 1 section 3 of the Civil Aviation Act:

  aircraft means any machine or craft that can derive support in the atmosphere from the reactions of the air, other than the reactions of the air against the earth’s surface.

So those drones you can buy from the store are aircraft (and if you knock one out of the sky because it annoys you, you are technically bringing down an aircraft and could be punished). However the latter half of that definition suggests hovercrafts are not aircraft.

Also, the minimum height over populated areas in Australia is 1000ft, and 500ft over non-populated areas (CAR157)


Why do airplanes have complex air conditioning systems?

It's because for the Air Cycle process, to chill ambient air without refrigerant, to work, you have to take really hot high pressure air and and remove most of the heat while keeping the pressure up, so that when you let the pressure drop at the end of the process it ends up way below ambient temperature.

So you take air off the engine compressor at some pressure like 80 psi and, say, 400F, pass it through a radiator (precooler) to cool it down, pass it though another compressor (the Air Cycle Machine's compressor) to boost the pressure, and therefore the temperature, again, run that through more air to air radiators or heat exchangers to cool that down, then, when you have the pressure still really high but have removed most of the heat, you let it expand and drop the pressure closer to ambient.  While doing that, you pass it by a turbine, which drives the compressor.  (An Air Cycle Machine is more or less a big turbocharger like device, pipes, radiators, and valves).

You now end up with air that entered the engine at 70 deg F coming out the Air Cycle Machine at just above freezing when it's properly regulated.  That air is mixed with raw hot bleed downstream to create the final inflow to the cabin.

An ACM can in theory put out extremely cold sub zero air if all the bleed flow available is run through it.  That is to be avoided because ice formation on the expansion turbine and inlet nozzles becomes a problem and this is one of the main factors in damaging them.  The other headache is dealing with all the condensate water, which causes turbine erosion problems.


How does hovering in ground effect (IGE) reduce drag?

The inflow becomes less vertical and also reduces in velocity since the pressure under the disk increases.  The rotor airspeed is therefore reduced and induced drag reduces in proportion to the square of the reduced airspeed.

The tip vortices (recirculation) are also reduced meaning that the outer portion of the blade, where most of the lift is generated, becomes more efficient.

Since the angle of attack has reduced, because the inflow is more horizontal and the lift vector is now more vertical, you get more lift for the same pitch angle and therefore require less power.

Less power into the rotor = reduced induced drag.

This image, from Copters.com, may help.

To add some clarifications.

Why does the rotor effective airspeed reduce?

Imagine a helicopter in a zero wind, ideal conditions out of ground effect hover.  The rotor, to produce lift, accelerates air downwards. Since there is no wind and the helicopter is not moving, the air is drawn into the disc from around it and mainly from above it.  Ignoring side effects, the air is drawn down equally all around the disc.  The airflow drawn into the disc has a horizontal component and a vertical component.  The vertical component is large as there is little to stop the disc from pulling air down and accelerating it downwards.  The speed of the rotor plus the horizontal component of the inflow co-incident with the average chord of the blade can be thought of as the "relative airflow".

Now imagine the same except hovering in ground effect.  The cushion (increased pressure) below the disc reduces the inflow (imagine a fan partially blocked, the fan draws in less air) and thus the horizontal component of the inflow is reduced.  Therefore, the relative wind airspeed is reduced.  As you can see from the diagram solving the triangle of vectors, the angle of attack is also reduced.  The collective could actually be reduced to accelerate the same amount of air.  This power reduction results in a decrease in the induced drag.

Why does the same pitch angle produce more lift when hovering in ground effect?

The biggest "drain" of power when hovering, and the main reason why hovering uses more power than any other flight phase, is the vortices around the rotor tips and the outer third of the rotor where most of the lift is produced.

When hovering out of ground effect, there is nothing to stop the accelerating column of air under the disc from "escaping" and being drawn back up into the in-flowing air leading to re circulation and significant vortices.  When hovering in ground effect, the air is forced to move out laterally from under the helicopter which effectively increases the radius of the vortices.  There is a significant reduction in the turbulence around the tips and the outer third with a corresponding large increase in efficiency.  Therefore, more lift is generated in ground effect than out of ground effect for the same angle of attack.

This can be seen practically by hovering over a hard surface and measuring power required then comparing this when hovering over long grass or worse, hovering over a dense standing crop.  Since the long grass or crops reduce the ability of the air to escape further out laterally, the vortex radii are not increased as much as when hovering out of ground effect and more power is required.

It is possible, when there is little power in reserve, to be unable to take off vertically from long grass but be able to do so from the adjacent hard standing.

I think this video helps to visualise the vortices.


It seems to me that it is an  Fairchild Republic A-10 Thunderbolt II, commonly know as "Warthog".

(Source: wikipedia)

The "stubs at the front of the wings" are pods in which the main landing gear is stowed after retraction.

If you are in doubt that it was military, it might have been a business jet like the Cessna Citation II / Bravo / Mustang, which has a similar planform albeit without the stubs in front of the wing.

Using the replay function of Flightradar 24 you may be able to find out which exact aircraft it was. The coverage at low levels is not so good for all aircraft though.


No it does not. Simple oval shaped tube.

{Image from Wikimedia}


What is the primary means of IFR navigation for private pilots?

There are numerous options, and it depends highly on your route. But there are generally enough VORs scattered across the country such that you should be in range of a couple of them pretty much everywhere. As mentioned in another answer, there are NDBs, but in the continental US, generally, the ones not on an approach are getting fewer, as the FAA has followed a path of not repair them once they break. Other parts of the world, still use them and there are also NDB based airways(AIM has examples for how they can be charted on US charts).

If you had to plan a  flight from a field that doesn't have a navigational fix, and you don't have GPS, you'd start by looking for airways near where you are. Once you find some that go in the general direction you want to go, you start figuring out intersections near that or how you plan to intercept the airway. Then you end up most likely planning a zig-zag course following airways towards your destination.


How do you search for all incidents of a Canadian airline online?

Try http://www.tsb.gc.ca/eng/stats/aviation/index-ff.asp on TSB Canada's website. Download the Aviation Occurrence data set and open it in your favourite spreadsheet. It has approximately 60 incidents for WestJet and WestJet Encore.


Can an amateur-built aircraft ever be used for commercial activities?

According to the Experimental Aircraft Association (EAA), aircraft with Experimental/Amateur-Built airworthiness certificates cannot be used for commercial purposes. However, I cannot find an FAA reg that specifically says that. (Anybody else know of a reg that says that?)

There are other grey areas as well. For example, what if your insurance is requiring dual instruction in the type before they will issue you a policy? The CFI would be using your aircraft for commercial purposes, even if you can legally claim you're the Pilot-in-Command.

However, the FAA talks about what things you cannot do without a commercial pilot license -- fly passengers or cargo for hire. My suggestion is that if you want to use your amateur-built aircraft for commercial photography, call your local FSDO and ask about that. Be nice to them and you'll be impressed how nice they can be to you.


Can air to air refueling can be done in the rocket engines as done in the Aircraft jet engines?

It depends if you are asking about rocket ships or rocket planes. On any note, its unlikely in the lower atmosphere, but in orbit, quite possibly.

There are a few issues you may encounter,

Some rockets use solid fuel which cant be pumped from one tank to the next so you would not be able to refuel on the go.
As @TomMcW mentions in the comments most rocket engines also carry oxidizer which you would need to pass over as well, doubling the volume you normally need to pass
Most rockets use liquid oxygen which is stored at very high pressures, you would need to not only have another craft carrying a specialized tank but a way to transfer the oxygen in the proper state.
You would need to transfer the fuel with out causing a pressure issue that lead to cavitation in the engine. During the design of the Saturn V Engines pressure variances in fuel flow caused serious cavitation issues and even lead to the loss of a test bed engine. While this problem was inevitably solved, the solution (as far as I know) does not account for pumping into the tank during operation.
Rockets move fairly fast and also roll on takeoff so some sort of connection mechanism would need be quite complex. Matching speeds is less of an issue than having something revolve around another object that is rotating on an axis.
Most rocket planes operate above the celling of practical refueling aircraft.

Frankly there is little practical reason as most rocket planes don't really need to refuel. Historically rocket planes like the Bell X-1 or the X-15 were built as test beds to push speed and altitude limits they carried the fuel they needed, burned it, broke the record, and glided home using histories oldest engine, gravity.

Refueling spacecraft has not proved to be a need yet as we have been able to successfully escape earth orbit on a single tank of gas. Building a rocket in space and sending it off has been theorized but there no examples of that yet and in that case you could ferry fuel up in small loads and deal with the docking in orbit.

Once in orbit docking two craft is fairly straight forward, then just connect the fuel lines and pump (presumably with out the engines running)...


The FAA has a Flight Inspection Manual which describes the checks that must be done and how often. Table 4.1 shows that an ILS must be flight checked every 540 days. Section 15 describes what happens during one of these checks. It's quite an extensive inspection!


Where can I find a worldwide General Aviation incident/accident ocurrence reports database?

No such official worldwide consolidated general aviation accident database exists. Most countries have their own, many smaller countries have none.

The closest you will find to a consolidated aircraft accident database is the one maintained at the Aviation Safety Network's web site. However, this database does not qualify as an answer because it only covers airliner, military transport category aircraft and corporate jet aircraft safety occurrences since 1919, and then again only if the aircraft had 12 seats or more. Most other sites advertising such databases also have similar exclusions.


Why are Leading Edge Flaps (LEFs) create lift while down at slow speeds but decrease lift while down at higher speeds?

Leading edge faps also known as slats don't really create lift. The effect on lift coefficient is minor (less than 10% in general) but they help increase the stall angle of attack.
As a summary, flaps creates lift, by increasing surface area and profil camber whereas slats postpone the stall and allow for higher AOA by reducing pressure drop around the leading edge. This increases the theoretical maximal lift by increasing the stall AOA but doesn't change much the lift coefficient.
Slats are ment to work at high AOA by design, at higher speed your AOA is very small, the slat is thus not working properly, disrupting the flow on the suction side of the wind and reducing actual AOA, thus reducing lift of that particular wing while simultaneously increasing drag.


Can the 787 mini navigation display be controlled independently from the ND?

The MiniMap is not an option, it's there as standard.

It's fixed at 20-miles range; the optional items are Wxr, Terrain and Traffic. The classic STA, WPT and ARPT from the ND are not available.

The MiniMap shows ADF needles, whereas the ND shows VOR needles.

The MiniMap looks like a development of the previous compass-rose at the bottom of the PFD, but it's mostly of little practical benefit. The major use of the MiniMap is to show Terrain while having Wxr on the ND, a dual-view that wasn't possible before.


What is the effect of Mach number on efficiency, thrust and specific fuel consumption of a ramjet engine?

The thrust of a ramjet depends on the dynamic pressure of the airframe since this determines how high the pressure inside the ramjet can become. Thrust grows therefore initially with the square of speed.
The efficiency of heat engines depends on the temperature and pressure ratio of their cycle, and again speed helps to improve that. The specific thermodynamic process of ramjets is the Joule cycle (in the US called Brayton cycle).
Why this doesn't grow indefinitely has to do with nonlinearities at high temperature. Once gas temperatures exceed 2000 to 4000K, Oxygen starts to dissociate, depending on pressure. Nitrogen follows beyond 8000K. Now the combustion heat will not be converted into a further temperature increase but will be wasted in the dissociation of air. As this limit is approached, less margin for heating is left and the SFC drops. The optimum with current technology is at approximately Mach 2.4.
A possible remedy is to slow down the air less, so the maximum heat inside the combustion chamber can be restricted. However, the flame propagation speed of conventional hydrocarbons puts a firm and low limit to that. The higher speed simply means that mixing, evaporation and burning will happen mostly once the air-fuel mixture has left the engine. Here you need to switch to hydrogen and a supersonic ram jet, but that will bring a host of new problems.


Why do planes make landing approaches at significantly varying heights?

Unfortunately, since I don't know your particular location relative to the airport, I can only give some general answers as to why this might be.

Contrary to popular belief, the availability of precision approaches does not mean they are used 100% of the time, nor are aircraft always routed the same way. Depending on the airport, the use of precision approaches can drastically reduce the number of aircraft that can be handled in a given period of time.  As such, during visual meteorological conditions, flights will almost always be cleared for a visual approach. Busy airports will typically have a number of standard arrival routes (STARs) that make the approach routing consistent and predictable (which could explain why noise is not an issue 99% of the time), but pilots are not obligated to use them.  If a STAR is not used during a visual approach to an airport with approach control services (like JFK):

The approach controller (using radar) will direct the plane in such a way to allow the pilot to visually acquire the runway, at which point the controller will clear the pilot for a visual approach. Additionally, under radar vectoring, the aircraft may be told to fly a particular heading for sequencing and/or spacing which can vary due to the amount and type of traffic.
Once cleared for the visual approach, the particular approach course and altitude is at the pilot's discretion.

The situation is similar for an instrument approach, except that the approach controller will arrange to have the aircraft either navigate to an initial approach fix on a published approach procedure or vectored to intercept the localizer before clearing the pilot for the approach.

Sometimes, unusual flight paths can also occur when a runway that is not normally used for landings is made active.  This can happen when wind favors a particular runway, or the one that is normally used is taken out of service (e.g. maintenance or an accident).


Need to add elevator downforce to total lift when sizing wing area?

No.

First, your horizontal tail will need some control margin. A trimmed state where it flies close to c$_{l_{min}}$ is a grave design error. Next, your horizontal tail sees the same angle of attack change as the wing, reduced by downwash. This means that the strongest downforce is trimmed in fast flight, when the angle of attack is small. Fly slower and angle of attack rises not only on the wing, but also on the tail. Chances are that your tail produces positive lift in slow flight.

Flaps will change this picture slightly. Setting flaps will need you to re-trim the aircraft because now the center of the wing's lift has moved slightly backwards, so the load on the tail becomes more negative.

Fun fact: If you have a symmetrical wing airfoil, the trimmed tail load will be constant over the whole envelope. In order to achieve the best control power, this tail load should ideally be zero.

Now for the tandem wing: You assume that now both surfaces create lift, so overall drag should be lowest. But adding area on the tail will reduce the needed wing area. Given that the planform shape stays constant, less area means less wing span. However, induced drag is proportional to the span loading, and a lower span will mean more induced drag. Therefore, a conventional design with an unloaded tail of minimum size will have the lowest drag.


Do lighter-than-air aircraft communicate with ATC?

Short Version:

No aircraft is required to talk to ATC, except in certain situations.  Whether or not they have a radio, all pilots are required to continuously watch outside the airplane for other aircraft and to avoid them visually, much like someone driving a car avoids other cars by looking for them.

Long Version:

Aircraft are required to communicate with ATC only in certain situations.  14 CFR 91 contains the requirements:

91.126 - Within 4 miles and 2,500 ft of an airport in Class G Airspace
91.127 - Within 4 miles and 2,500 ft of an airport in Class E Airspace
91.129 - Within Class D Airspace
91.130 - Within Class C Airspace
91.131 - Within Class B Airspace
91.135 - Within Class A Airspace
91.155 - In weather conditions less than VFR
91.183 - While operating IFR
91.511 - More than 1 hour flight time or 100 miles from shore (overwater)

Many of these can be  waived with prior coordination with ATC.

Outside of the above situations, no radio is required.  In fact, there are still many airplanes that don't even have an electrical system, much less a radio, and they are perfectly legal to go fly around.

On the other hand, if they do have a radio (even a hand-held radio) it can greatly add to the safety of flight if they remain in contact with ATC since they can point out traffic that they might not have seen as quickly otherwise.

Now, whether or not the pilot is talking to ATC, they are required to "see and avoid" other aircraft, and certain aircraft have the right of way over other aircraft.  As you can imagine, an unpowered balloon can't really get out of way of a 747, so it is the responsibility of the more maneuverable aircraft to turn to avoid hitting the less maneuverable one (see 91.113 for more details).

They are also required to remain a certain distance away from clouds (91.155).  This is so that one aircraft doesn't come out of a cloud, possibly at a very high speed, and run right into another aircraft because they didn't have time to avoid them.  If they are far enough away, then the pilots will have time to see and avoid each other.


Freezing cold cabin housing just under a window

It's not a structural problem, and nothing to worry about. The cabin of an airliner is a pressure vessel, the pressure outside of the airplane is far lower,  if there was a leak air would be going out, not in. The air outside is very cold, which makes the skin of the airplane cold, there's insulation between the skin and the cabin fixtures but if that comes off a bit you get cold spots.

By all means tell the cabin crew so they can log it for the next time the airplane's in for maintenance, it's important for passenger comfort.


What is that missing instrument on a Beechcraft 1900C?

Originally there was no instrument at this place, except for UE series with an electrically driven artificial horizon, where there was a fast erect pushbutton/indicator for the copilot gyro.

According to Wikipedia, UD series is for the military model and UE series is for 1900D, so I assume UC series is for 1900C model. Instrument panel for series UA/UB/UC:

Instrument panel from training manual, models UA/UB/UC (source)

Anything visible here has been added after the aircraft left the plant, when the panel was retrofitted. When browsing pictures of this cockpit online, we can see various additions:

Sources: Left, center, right

Looking at details of your image, we can see there was likely a sticker and a button associated with the instrument. This could fit with a battery temperature indicator and its test button:

Source: Right

This configuration was actually found on F-GPYY. Comparison:

Source: Right

Full cockpit of F-GPYY when it was an aircraft of the now defunct French Air Littoral operator:

Photo by Thierry Izzi, source


If I fly a balanced turn long enough, will the AI show I'm level?

A gyroscopic Attitude Indicator will not build up long-term error simply because of its erection mechanism in an extended coordinated turn. However, all gyroscopes will accumulate error over time, which must be corrected in some fashion.

A gyroscopic AI will display a turning error of 3-5 degrees in a 180 degree turn. However, this precession is reversed in the second 180 degrees of a turn, so after 360 degrees, the precession is eliminated and the AI is correct again.

To explain why this is, first consider the design of the attitude indicator: the pendulous vanes exert an erecting force when the gyro is not vertical with respect to downward acceleration.

(FAA AC 65-15A)

This causes the gyro to erect itself over time, and would in fact do so incorrectly if the aircraft were kept on a constant heading (if, for example, flown uncoordinated on a constant heading.)

However, in a coordinated turn, the case of the AI (including the pendulous vanes) rotates around the gyro. For the first half of the turn, the gyro is erecting in a particular direction. However, as the pendulous vanes move more than 180 degrees around the gyro, they add error opposite the original direction - removing the original error.

Note that while this fixes turning error in the long term, other sources of error are still in place. Precession caused by the fact that the gyroscope gimbals are not perfectly friction-free, which will vary in each individual instrument, will still happen - and the erection system will be unable to correct for it, as it's too busy causing and removing its own error.

A ring-laser gyro would be subject to the same problems, because it must still have a correction process of some sort. All gyroscopes, even perfect ones, are subject to apparent precession:

  A freely spinning gyro tends to maintain its axis in a constant direction in space, a property known as rigidity in space or gyroscopic inertia. Thus, if the spin axis of a gyro were pointed toward a star, it would keep pointing at the star. Actually, the gyro does not move, but the earth moving
  beneath it gives it an apparent motion. This apparent motion is called apparent precession. The magnitude of apparent precession is dependent upon latitude. The horizontal component, drift, is equal to 15° per hour times the sine of the latitude, and the vertical component, topple, is equal to 15° per hour times the cosine of the latitude.

(FAA Flight Navigator Handbook, p. 3-7)

Any gyro, whether a ring-laser, MEMS, or gyroscopic, in a coordinated turn, will exhibit apparent precession and will have to be corrected over time if their only source of correction is acceleration.


How do the blades on a propeller generate thrust?

The picture of the turboprop is probably confusing you because you are looking at a shutdown engine.  For a multi-engine propeller airplane a desirable feature is that the propeller with auto-feather when shutdown, so that an in-flight engine failure will cause the prop to be in a minimal drag configuration.  This minimal lift, minimal drag configuration is what you posted, and this is not producing thrust.  When the engine is started and system pressure (oil, hydraulic, whatever controls the adjustable pitch prop for that particular engine) will drive the propellor pitch angle into lift-producing regimes.  Turboprops also typically feature "Beta" propellor pitch, which allows the propellor pitch to be adjustable past zero-lift into negative-lift and is used for landing rollout and (in some airplanes, perhaps not many) slowing down in-flight or steep descents.  During normal flight the turboprop propellor is producing lift just like the propeller on the cessna you are familiar with.

The N1 fan in a turbofan (the big one you see when you look at the front of the engine) is a ducted fan.  The individual blades of the N1 fan are angled to push air backwards.  This works the same way as a big box fan you can buy at walmart.  The key difference is the N1 fan rotates an order of magnitude faster than a box fan.  The bypass air is accelerated by this fan and exits through the rear of the engine to produce thrust.


Is there a standard install direction of the abradable shroud honeycomb on a CF6-80c2 engine?

As noticed by Daniel Kiracofe, this applies to the turbine, but as far as orientation is concerned the logic is the same

The abradable material in the honeycomb cells can readily withstand the high temperatures encountered in the turbine stages of the engine and protects the annular steel backing from high temperatures as well as providing a smooth gas flow surface. However, under extreme operating temperatures, the turbine blades may expand sufficiently to actually rub the surface of the abradable alloy overfilling the honeycomb material, whereupon the abradable sealant material will yield so that the turbine blades will not be damaged

However, the abradable material filling a given cell of the honeycomb can become loosened and fall out due to thermal cycling, rubbing of the abradable filler against the turbine blades, engine vibrations, or can otherwise be blown out by action of the hot gas flowing across the seal.

The orientation is the one, the manufacturer found the least probabilistically susceptible to allow the abradable material to separate; internally the cells walls are not strictly perpendicular to their outer flat surface, and this angular orientation prevents to the best the falling of the abradable material. Thus indeed, as you write, the repairs are understandably performed with the same orientation, using identical material.


Why is "LINING UP" used as the proper response to a "conditional" line-up instruction?

"Lining up" is not a proper response to a conditional lineup instruction;

"(condition) lining up (brief reiteration of condition)" is correct.

To use an actual example:

ATC: "RYR123 behind landing Airbus 320, line up runway 28L behind"

Pilot: "Behind landing Airbus 320, lining up runway 28L behind"

In my opinion, that leaves very little room for confusion.


Was the "Air Berlin" lap of honor over Düsseldorf problematic?

The maneuver was safe.
A spokesperson of the DFS (German Air Traffic Control) told the Berliner Morgenpost that the pilot requested and was given clearance to fly a left curve in case of a go-around. The necessary distance to other aircraft was ensured and a specific height for the maneuver was not determined.
The spokesperson also said that the DFS believes that this maneuver was safe.
However the LBA (Federal Aviation Office) started examining the case and requested Air Berlin to submit a statement which provides a reason for the go-around. The editorial staff of that newspaper believes this to be a routine procedure.
source (in German)
A rough translation of the relevant part in english:

Christian Hoppe, spokesperson of the German Air Traffic Control confirmed that: "The pilot previously requested if he is allowed to fly a left-curve in case of a go-around and was given clearance." The necessary distance to other aircraft was ensured, the specific height of the maneuver was not determined. "From the DSF's point of view the maneuver was safe." The Federal Office for Air Navigation corrected earlier information, which claimed that the pilot had reported problems with the landing-gear. "That was our assumption."
The Federal Aviation Office will nonetheless examine the case, Air Berlin should submit a statement on the reason of the go-around. Our editorial staff believes, after multiple requests, that this is most likely routine operation.


How to compute the change in static temperature using only altitude?

I found the answer, it's literally looking at ISA temperature table of the initial FL. The lapse rate is 2degrees per 1000 feet of climb. You would see what the initial temperature is for your FL and then you minus the lapse rate for the climb that you are given. So in this case, the lapse rate would be 4degrees as it climbs 2000 feet.


What does Airspace 24 mean in this chart supplement?

It looks like a mistake of some kind. The next release of the Chart Supplement is valid from 2021-04-22 and the airspace line has been removed completely from the KJYO entry.


As you said, they used a monorail for launches. The aircraft was pulled forward by a pulley which was connected to a weight which hung from a derrick. First the weight had to be pulled up, the pulley was run to the tip of the rail and back to the Flyer, and once the engine was running and the pilot in place, a helper released the weight and the pulley pulled the aircraft forward.

The pictures here are taken from this site; I recommend it for further reading!

Below is a picture of the rail, note the uptick at the end which gave the airplane a gentle push upwards, similar to the ski jump technique of British aircraft carriers.

For landing the Flyer used two skids. Depending on the ground, this could mean a gentle slide to standstill. Rougher landings would mean bruises for the pilot, like the ones suffered by Orville on August 23, 1904. But in most cases of a rough landing, the elevator assembly absorbed most of the impact energy. Elevator repairs were quite frequent.

I know less about the glider launching technique. Most glider tests were done with an unmanned glider held by strings into the wind. Glider flights would be downhill; thermalling was discovered much later (in 1913). However, even Otto Lilienthal already mentioned the uplifting effect of gusts, but neither he nor the Wright Brothers could circle tightly enough to stay inside a thermal.


What twin engine is this USAF-painted plane?

(wikimedia.org) A T1 variant.

It appears to be a British Aerospace C-10A or Jetstream 3M.

  In order to improve sales prospects in the US, the fifth prototype was fitted with the US-built Garrett TPE-331 in place of the French Astazou. Changing to the US-built engine was enough to allow the United States Air Force (USAF) to consider it for cargo use. It eventually placed an order for 11, fitted with a cargo door and accommodation for 12 passengers or six stretcher cases, to be known as the C-10A, or Jetstream 3M. The USAF cancelled its order in October 1969 citing late delivery.

From https://en.wikipedia.org/wiki/Handley_Page_Jetstream

and it sounds like none may have been actually built.

See http://www.aerospace-technology.com/projects/turboprop/ for more info.

In the 1980s I remember them being a successful civilian commuter design. However, they were slow compared to the Fairchild Swearingen SA-227 Metroliner, a major competitor, and suffered from a lack of baggage capacity. They somewhat mitigated that by having an optional belly cargo carrier on them, but that slowed them down even more.

I was flying the SA-227 at the time for Horizon Air. Over the route from Redmond, Oregon to Portland, Oregon, the Jetstream (I can't remember the carrier's name—they eventually ceased operation), would leave a few minutes before us, but we would always overtake them and land in Portland before them.

For a time, over routes they shared with us, they would send the bags they couldn't carry over to us under a reciprocity agreement since we had ample baggage capacity. Eventually Horizon Air required them to pay since it was turning out to be a one way agreement, they never having any spare baggage room.


PPL 40 hours requirement, are you really ready?

With the caveat that this is based on experience mostly in the US:

Most people take 60-80 hours to get their private pilot license:
http://www.aviationcareerguide.com/private_pilot_license.asp

The harder you work, and the more often you fly, the fewer hours you'll need.  The average hours are inflated somewhat by the many pilots who run out of time or money partway through, have to stop, and must then relearn what they forgot when they start back up again.  If you have the time, money, and dedication to fly two or three times a week, you'll get your license in fewer hours (not to mention fewer days!).

Don't worry too much about the safety of it though.  Your instructor won't let you take the exam, and your examiner won't pass you, if you're not ready.

As far as flying out of a major airport, my opinion is that it doesn't much affect the time or amount of experience required.  In the US, it's usually more expensive because costs are higher at large airports, but you may not have the option.  For most pilots, the hardest part of flying into a large airport is getting lost on the taxiways - but it will not take long to learn your way around your home airport.  If anything, it will make you a better pilot.  Today much of flying is dealing with controllers and airspace rules, and if you learn at a major airport, you'll have plenty of practice with that.


What is the use of this object in the cockpit of a Bombardier Dash 8 Q400?

It is a mechanical checklist where each item on the list can be flipped to indicate that is has been checked. You can see it in operation in this YouTube video (which I believe is where you got the image from). Each of the 6 stacks can be extended to check the items and then collapsed again in order to not obstruct the pilot's view.

It is not a default part of the Dash-8 Q400. As far as I know, Qantas is the only airline using it.


Pilots Vs Maintenance Engineers
After every flight, pilots fill out a form, called a “gripe sheet” which tells mechanics about problems with the aircraft. The mechanics correct the problems; document their repairs on the form, and then pilots review the gripe sheets before the next flight.
Never let it be said that ground crews lack a sense of humor. Here are some actual maintenance complaints submitted by pilots (marked with a P) and the solutions recorded (marked with an S) by maintenance engineers.
P: Left inside main tire almost needs replacement.
S: Almost replaced left inside main tire.
P: Test flight OK, except auto-land very rough.
S: Auto-land not installed on this aircraft.
P: Something loose in cockpit.
S: Something tightened in cockpit.
P: Dead bugs on windshield.
S: Live bugs on back-order.
P: Autopilot in altitude-hold mode produces a 200 feet per minute descent.
S: Cannot reproduce problem on ground.
P: Evidence of leak on right main landing gear.
S: Evidence removed.
P: #2 Propeller seeping prop fluid
S: #2 Propeller seepage normal – #1 #3 and #4 propellers lack normal seepage
P: DME volume unbelievably loud.
S: DME volume set to more believable level.
P: Friction locks cause throttle levers to stick.
S: That’s what they’re for
P: IFF inoperative.
S: IFF always inoperative in OFF mode.
P: Suspected crack in windshield.
S: Suspect you’re right.
P: Number 3 engine missing.
S: Engine found on right wing after brief search.
P: Aircraft handles funny.
S: Aircraft warned to straighten up, fly right, and be serious.
P: Target radar hums.
S: Reprogrammed target radar with lyrics
P: Mouse in cockpit.
S: Cat installed.
And the best one for last.
P: Noise coming from under instrument panel. Sounds like a midget pounding on something with a hammer.
S: Took hammer away from midget.
(Source)


What is "Hotel Mode" on a turboprop engine?

Hotel mode is a feature on ATR 42/72 turboprops replacing an APU by locking the right-hand #2 engine (PW100) with a 'propeller brake' while allowing the turbine, and therefore also the generator, to run, providing electrical power and bleed air.
ATR describes this as the following:

The propeller brake is fitted on a countershaft on the RH engine reduction gearbox in order to stop the propeller (and the power turbine)

HP (High Pressure) spool drives the DC generator.

Bleed pressure is available downstream the HP compressor and supplies both packs.

This picture should give you sort-of an idea of how it works:

If I gather correctly, the propeller drive shaft is locked, and the (HP) compressor shaft is free to run.
Here's another pic of a PW100 engine:

Hotel Mode seems to be generally disliked for a number of reasons:

It's very loud, especially if you're lifting baggage next to it
It consumes a lot of fuel, since you're running a full engine.
It could be dangerous if the propeller brake is in poor condition and suddenly loses its grip.

This configuration is rare, if not unique. The rival Dash-8 has a separate APU, as does the Saab 2000. It was an optional feature on the Fokker 50. The smaller Jetstream 41 only has a battery.
Source: SmartCockpit ATR-72 documents.


Can a turbofan engine have less noise so that it doesn't disturb our ears?

Turbofans already produce much less noise than the turbojets used in the 1960s. In general, the higher the bypass ratio, the lower the noise level.

It's difficult to silence a jet engine: any silencer impedes the airflow which means a significant power loss.

Noise levels are usually expressed in dB rather than %, a reduction by 90% is a 20 dB difference, which is huge.
To give an impression: from 1992 to 2000 NASA ran a technology development program aimed at reducing noise by 6 dB.

This chart shows various noise sources in an engine:

Here is an overview of the noise regulations currently in effect:

So Stage 3 is a reduction by 10 dB compared to Stage 2, Stage 4 is a reduction by 7 dB. The quietest aircraft are already at 17 dB below the Stage 4 limit.

So an engine that 'has the noise levels reduced to about 10%' seems plausible (depending on what you're comparing to), and the graphic identifies a few candidates.


Does EASA have access to medical records outside of the EU?

No, EASA cannot access anything.

You will be asked to bring your medical records to the exams. Neither EASA nor your examining doctor is able to access any of your medical records without your consent. It's restricted personal information.

Some countries already have medical records in digital (computerized) form, but many still rely on written data. In any case, EU institutions have no jurisdiction outside of EU.

You will also be given a questionnaire that will include a checkbox for any known medical conditions you have (or had in the past). I don't know what your condition is/was, but it might be a good idea to consult a private examiner first before checking anything on the list that might prevent you from getting a certificate issued.


There's no benefit to burning dumped fuel, it actually introduces risk to the airplane which is dumping fuel and any other aircraft in the vicinity and possibly people on the ground depending on the altitude at which is done. Building a system to burn it is not worth the time, effort and money given there's no good reason to do it.

Dumped fuel will vaporize and mix in with the air, and then break down over time from exposure to air, heat and UV light. This is not ideal for sure, but burning it isn't environmentally friendly either as it produces CO2, CO and soot. Evaporation of dumped fuel is a miniscule problem when looked at in context as it's such a rare occurrence. Canada measured 58 million liters of gasoline and diesel lost to evaporation from retail outlets and pump in 2009, if you extrapolate that to the US it's got to be over 100 million US gallons of fuel evaporating into the atmosphere every year. An airbus A380 contains 320,000 liters of fuel, if you dumped all that at once it's not going to make a statistical difference.


What could cause a rear twin-jet's throttle to oscillate on descent?

I think your note about this taking place in the Atlanta area with heavy Cumulus clouds in the area is the best clue as to why such large throttle inputs were necessary.  It sounds like there were thunderstorms in the area in the cumulus and mature stages.  The pilot was most likely encountering heavy updrafts and downdrafts passing through the air and had to compensate accordingly with the power levers to maintain approach airspeed and glidepath during the approach.


It's a design standard to set minimum wing area for the airplane's intended speed range using "unassisted" Clmax as the basic parameter. And most low speed wings have a Clmax without enhancement of somewhere around 1.5/1.6.

The 701 gets its low speed range with small wings using both slats and slotted full span flaps to get its 3+ Clmax, so it doesn't really fall within the definition.


Cessna started to apply marketing names to their line in the early 60s (along with goofy marketing gimmicks like the "Land-O-matic" name for the spring steel gear).  Up to then they were just known by model numbers.  The 150 was the "Commuter". For some reason the name wasn't applied to the 152 (which is a 150 with a Lycoming engine), which just has the model number.


Why do some military jets have needle-like structures at the nose and others don't?

What you see on the SR-71 is an Alpha-Beta probe, with a pitot tube branching off the side of it. The nose cone of the SR-71 (and its variants) were changeable depending on its loadout.

From the A12 Configuration:

  The sharply tapered nose section was pressurized and contained navigational and communications equipment, a
  remote compass transmitter, periscope optics, air inlet computer and angle transducer, and other radio equipment.
  A combination pitot-static and alpha-beta probe was installed at the forward tip to capture airspeed and altitude data.

About the SR-71 versatility

  The SR-71 also served as a testbed for an Optical Air Data System (OADS), a fiber optic device using laser
  technology to replace the pitot tube (airspeed probe) on high-performance aircraft. It used laser light instead of air
  pressure to produce airspeed and attitude reference data such as angle of attack and sideslip normally obtained with
  small tubes and vanes extending into the air stream or from tubes

Both sources above: Design and Development of the Blackbird

The F22 serves quite a different purpose to the variants of the Blackbird, and thus probably has less need for sensitive sensor equipment of the SR-71. It still has a pitot tube which can be seen in the photo you posted

And it, no doubt gets other avionic information from a traditional pitot-static system rather than rely on an alpha-beta probe as the Blackbird did.


What are good strategies for the "impossible turn"?

The already linked question has a link to this study. Its authors seem to have done the calculations and concluded that turn with 45° bank flown on the edge of stall lets you do it with smallest loss of altitude. Which is the point here. Don't forget you still need to glide back to the runway.

As rbp mentions below, flying the turn at edge of stall is extremely dangerous, so in practice you should plan turning around only from altitude from which the turn can be made with limit above stall (and don't forget in 45° bank the stall speed is ~20% higher and quickly increases with more bank) you are confident you can manage to maintain. This increases the altitude above which you should only attempt it but the optimal bank angle remain 45°.

If you used higher bank angle, you'd have to fly faster so you'd still have enough vertical lift to balance your weight. That would mean you'd need more lateral lift (even for the same turn rate the lateral acceleration is proportional to speed) to complete the turn and this lift would come with it's additional induced drag. Drag equals energy loss and the only energy you have at that point is potential energy due to altitude. So such turn would spit you out lower, which is the last thing you need. And note, that this is exactly your second option.

Also note, that due to reasons mentioned by rbp below your margin between stall warning and severe stall is even lower in higher bank, so that's another reason against that option.


How fast can pushback tugs actually go? (maximum possible speed)

This AC covers a lot of the regulations around towing aircraft and according to the regulations

  Towing Speed. Towing speed should not exceed that of walking team
  members. Towing speed should not exceed the safe operating speed for
  the conditions or the published speeds established by the TLTV
  operators published procedures.

In other words you can do either as fast as your operator says you can or as fast as the tug manufacturer says you "safely" can. The AC goes on to discuss a lot of other things but also mentions this tug in their section on tow-bar-less tugs. According to their website:

  Unique hydrostatic drive for moving aircraft on the ground at up to 32
  km/h [~20 MPH] – without compromising safety

This is about the same speed that Terry discusses is the maximum taxi speed in this answer so you don't really gain any speed. Depending on the aircraft and airport in question you don't want to cut down taxi time to much as that may be an important time during which the engines come up to temp.


If pilot use rudder to roll the aircraft at high AOA in the F4 Phantom then one aileron from one wing and one spoiler from other will be deflected?

A Barrel Roll is a maneuver where the aircraft flight path follows what would be the surface of a huge horizontal barrel in the sky. To do this you of course need to roll the aircraft. To roll the Phantom, no matter what maneuver you are attempting, requires that you create differential lift on the wings. (more lift on the up-moving wing than on the down-moving wing). At low angle of Attack (AOA), in any model of F-4, you use the ailerons/spoilers to do this. At higher AOA, the ailerons would induce adverse yaw, and therefore you need to create this differential lift by yawing the aircraft with the rudder (since it's a swept wing aircraft). As AOA increases above 10-12 units, the pilot needs to blend in more and more rudder, and less aileron, until at high AOA (19 units in early non-slatted aircraft like F-4B/C/D), he is using rudder exclusively. In later slatted aircraft (USAF F-4Es, F-4Js, USN F-4J/S, etc.) this effect was somewhat mitigated by the slats (they energized the airflow over the top of the wing), and the pilot could use some aileron throughout the AOA range, (but still less and less as AOA increased), all the way to 25 units AOA.
As to your other question about slat deployment, this happened automagically, based on AOA, at about 12-14 units AOA as I recall. And then would retract when AOA fell below 10-12 units... No pilot input was necessary.
So, in the execution of a Barrel Roll in the F-4, starting at 450 KIAS and low AOA at the entry point, the aircraft would climb and descend 4-5000 feet, and  slow to somewhere between 200-250 KIAS and higher AOA at the high point, and then accelerate back to 450 KIAS and low AOA at the completion of the maneuver. As a result, the need for aileron and rudder to effect the roll continuously and gradually changed throughout the maneuver.
NOTE. (to address comment made in the question relating to photograph). The F-4's ailerons did move up slightly when the stick was deflected towards that wing, but at high AOA, the aerodynamic effect was negligible due to the wing blanking the airflow back there. SO the F-4 also had spoilers on the top of the wing that would deflect upwards as well, both to decrease the lift on the wing and cause a roll in that direction, and to add drag on that wing to balance the drag from the down aileron on the opposite wing. The control surface deflected on the upper wing in the photo is the spoiler, caused by the pilot pushing the stick to the left.
The following is from the USAF F-4E flight Manual:


How does helicopter mustering make financial sense?

Two reasons:

It is way faster and allows one person with a relatively cheap-to-run R-22 to do the work of several cowboys.  You're paying for the cowboy 24-7 if they are on staff, which is usually the case on a sprawling and remote ranch/station where you have to keep them on full time (and the horse if using them); you're only paying for the machine (direct hourly operating costs at least) while it's running.
It almost eliminates losses from stragglers that get missed and later become dinner for predators.  This feature was a welcome surprise when gyrocopters and helicopters were first tried out. Originally, gyrocopters were used to help out with the cowboys, mainly to hunt for strays that couldn't be seen from ground level. When the R22 came out, being able to hover and maneuver in confined areas, it became feasible to do both the mustering and overwatch role from the air.

Between the two, the business case starts to favour the helicopter.  If it didn't, they wouldn't do it. Granted, it is pretty dangerous flying, not for the faint of heart, but there are lots of young pilots keen for a thrill.


What is the purpose of the "Mirror-Thing" over the Mig-29 Cockpit?

Additional details to Marco's selected answer.

On the two-seater version, the instructor seat (aft) is not high enough, contrary to other training aircraft, to see the runway. A forward view mirror replaces the rear view mirror and allows the instructor to see ahead. The runway is projected on a second mirror inside the cockpit, creating a full periscope system:

Photo by Pavel Vanka. Modified.

(Source)

Note how the projected part of the image fits very well into the direct view of the exterior.

An opening is created in the canopy when the top mirror is deployed. From below this is what it looks like (not exactly the same model):

Photo by Burkhard Domk. Modified.

The bottom mirror unfolds down to instructor eyes height:

(Source)


Could Henri Farman have steered his plane with ailerons on January 13, 1908?

I would first check to see if the car in the background was actually pulling him along in 1908, or maybe the aircraft engine was a bit better.

Banking does indeed reduce lift, but not as much as we may think:

If vertical lift at 0 degrees bank was 1000 lbs, a 20 degree bank at the same AOA and speed still produces: $$cosine 20 degrees × 1000 = 940 lbs$$ of vertical lift, easily corrected by slightly increasing the Angle of Attack of the wing.

The reason the plane previously could not make the turn can be explained by the relationship of thrust and drag.  Turning increases drag, which will slow the plane down.  These early aircraft were very draggy and underpowered, barely getting off the ground.

Deflecting the ailerons (really 2 halves of a speed brake) increases drag.  The bank additionally decreases lift.  With no reserve power to add, the plane goes down.
What an accomplishment the first circle must have been at that time!

But "skidding" the plane around the circle, with rudder only, presents the whole side of the plane as a dragging surface, there for is not a better solution.  The "coordinated turn" using  both rudder and ailerons produces the least amount of drag, and remains the recommended aircraft turning method to this day.

But even the best method of turning requires more power (thrust) than straight and level flight.  A few more horse power, and/or a lower drag design wins the prize.

Reducing weight and a better prop design would also be helpful.


When Boeing designed the 737NG, what factors influenced the change in the flap system?

One of the biggest differences that you will see between the flaps on the two is that the NG (photo on the right) has continuous span flaps while the classic has split inboard and outboard flaps.

Combined with the longer wing on the NG, this gives the new flaps more surface area, so they don't need to be quite as effective.  Overall, the NG actually has better performance than the classic, mainly because of the redesigned wing which is 25% larger and has winglets; both of which help out.

Even though the performance is better with comparable triple-slotted flaps, the double-slotted flaps are lighter (which helps with overall aircraft performance) and have fewer moving parts.  This makes them more reliable and they require less maintenance.  Most new aircraft designs are moving to either double or even single slotted flaps because of this simpler design and the fact that today's runways don't require the extra performance that would be gained by the more complicated flaps.

Photo Credits:
737 Classic Flaps
737NG Flaps


Is there an issue with an acceleration followed by a deceleration when flying a SID?

There is another limitation apart from the 250/10,000 one, according to ICAO Annex 11 Chapter 5 Appendix 4:

  Speed limitations applicable to both IFR and VFR aircraft:
  — 250 knots below 10 000 ft ASL; and
  — 200 knots below 3 000 ft AGL within 10 NM of a controlled airport.

So within 10 NM and below 3,000 ft AGL of the controlled airport, you are not supposed to exceed 200 knots.

Note: In the Rome example before the 210 knots PRA turn, the leg is 4+10+6 (20) NM, but all of it is still within (or very close to) 10 NM of the airport. Also note the 'at 3,000 ft' point, which is still below 3,000 feet AGL, so it's hard to justify trying to accelerate after that only to slow down again in 6 NM while climbing to 'at or above 6,000 ft'.

I checked the Italian AIP for any differences from the above ICAO limitation. AIP GEN 1.7 refers to Commission Implementing Regulation (EU) No 923/2012, in which no differences from Chapter 5 are listed, which is to say the quotation above applies in Italy.

What if it's more than 10 NM from the airport?

In this case the procedure design group (PDG) that designs the procedures will typically make any speed restriction clear.


Is the Harbin Y-11 a fully indigenous Chinese design?

Though Harbin Aircraft Manufacturing Corporation (HAMC) said Y-11 is a fully indigenous design, no other direct evidences prove/reject what it says. However, some indirect evidence shall be considered:

1) Except Y11, the only brand new design from HAMC is Z15 in 2008(Coperation with Airbus), other designs from HAMC are either authorised production or modification from existed model.

2) Y11 was designed in the period of the Cultural Revolution. There is insufficient engineers and research process is often influenced by political movement. However, Y-11 started first flight 8 months after the requirement is issued.

Based on the evidences i dont think it is a fully indigenous Chinese design, as they dont have experience for fully indigenous design in such a short period.


Cabins of fighter jets are in fact pressurized, but not same as the passenger airplanes. Cabin pressure in a passenger airplane will increase (almost in a linear fashion) as the airplane climbs. However, fighters keep ambient air pressure to a certain altitude. It maintains this pressure until a certain pressure differential is met, and then cabin altitude increases to maintain this differential.

Therefore at higher altitudes, cabin pressure in a fighter jet is low enough that supplemental oxygen is required for the pilot(s). This is supplied via On-Board Oxygen Generation Systems.

By not pressurizing the cabin completely reduces stress on air frame and is beneficial during an unplanned decompression.

By the way, your friend (military fighter pilot) is correct.


How do I have a gravel kit certified as an STC?

The FAA has a nice checklist here.

You should read through the FAA's GUIDE FOR OBTAINING A SUPPLEMENTAL TYPE CERTIFICATE. I assume you are a civilian but,

  This advisory circular (AC) is a certification guide for obtaining a
  supplemental type certificate (STC). It is intended for civilian
  applicants only. We have written it to help designers, manufacturers
  and installers applying for STCs to better understand the STC process.
  The guidance in this AC is general in nature and intended to help an
  applicant gain a basic understanding of the STC process and our
  respective roles and responsibilities.

You will need to fill out this application form and submit it to the FAA ACO office in your area.


How can I get 'lead point' for aircraft to intercept final course?

I was taught the following rule of thumb for providing a final vector to final, in the USA, using the STARS radar system which (when using ADS-B inputs) has a one-second update time:
Set the Predicted Track Line to 30 seconds. Vector the aircraft to a 90º base-to-final. When the PTL reaches the extended runway final, issue the Position-Turn-Altitude-Clearance. The aircraft should roll out on a good heading to intercept final approach course.
This is obviously at low altitudes and relatively low speeds, 230ish knots or below. Theoretically (because the method uses the aircraft's speed instead of a fixed distance) this method should work for any aircraft going any speed, but that may not always pan out. In particular, some military aircraft will be flying rather quickly on the base leg and slow down as soon as they get their dogleg turn-to-final, which could end up leaving them too far from final for a legal intercept.


Is it legal to land when an airfield is reporting low IFR due to fog with a visual approach?

A visual approach can not be authorized by ATC during true IMC.

  5–4–23 Visual Approach
  a. A visual approach is conducted on an IFR flight plan and authorizes a pilot to proceed visually and clear of clouds to the airport. The pilot must have either the airport or the preceding identified aircraft in sight. This approach must be authorized and controlled by the appropriate air traffic control facility. Reported weather at the airport must have a ceiling at or above 1,000 feet and visibility 3 miles or greater. ATC may authorize this type approach when it will be operationally beneficial. Visual approaches are an IFR procedure conducted under IFR in visual meteorological conditions. Cloud clearance requirements of 14 CFR Section 91.155 are not applicable, unless required by operation specifications.


How can speed and power be made independent in an airplane?

The problem is your assumption of

speed = rpm * coef

This is only true as long as there is a rigid connection between the RPM and the medium against which the speed is measured.
Let us consider a car for a moment. In a fixed gear, there is a fixed relationship between the engine RPM and the RPM at which the tires rotate. As long as the tires are not skidding, this will directly translate to a motion w.r.t. the road (against which speed is typically measured for a car). This results in the linear equation you show.
This is however not true for an airplane, where the propeller RPM does not directly translate to a speed w.r.t. the air because air is not a solid (otherwise it would be true and you can think of the propeller as a a screw that drives into the solid). Air can flow through the propeller without moving the plane. Consider e.g. a plane that is tied down on the ground and therefore cannot move w.r.t. the ground. You can still start the engine and run the propeller, which creates an airflow behind the propeller, but does not cause any movement. The same could not work with a car, as long as the tires are not allowed to skid.
In flight, the accelerated airflow behind the propeller creates thrust, which is a force that can accelerate the aircraft forwards. The actual motion will also depend on the aerodynamic forces (lift and drag) as well as gravity of course. A higher propeller RPM (at the same blade angle) will give you more thrust, but there is no rigid connection to speed.


The second segment of the takeoff is defined as (source: skybrary.aero):

  Second Segment - begins when the landing gear is fully retracted. Engines are at takeoff thrust and the flaps/slats are in the takeoff configuration. This segment ends at the higher of 400' or specified acceleration altitude. In most cases, the second segment is the performance limiting segment of the climb.

The minimum climb gradient is defined for each segment of the takeoff (same source):

  Each segment of the one engine inoperative takeoff flight path has a mandated climb gradient requirement. For example, a gross second segment climb gradient capability of 2.4%, 2.7% or 3.0% is required for two, three and four engine aircraft respectively. Similarly, the required gross gradients for the fourth segment are 1.2%, 1.5% and 1.7% respectively.

I'm not familiar with the performance tool you show, but I would assume that the given value is the maximum TOW, which still ensures this required climb gradient can be met with one engine inoperative, so in your example a maximum of 71.0t allows 2.4% gross climb gradient for the second segment.


What are the disadvantages of a tailless ultralight design?

When we designed the SB-13, we also thought that it's not so much different from a regular layout. Boy, were we wrong.

It starts with ground handling: Regular aircraft have some distance between front and main wheels, and even more between main wheels and tail skid in taildraggers. This makes them fairly stable in pitch on uneven surfaces. When both wheels are close together because there is little of a fuselage to space them apart, every bump on a grass field translates into heavy pitching. This is annoying.

It gets worse once you lift off: The little pitch damping (now I am talking of the aerodynamic sort) means that every gust makes the tailless aircraft enter into a noticeable short-period mode. This is normally so much damped that the pilot doesn't even notice. In a tailless plane the damping is just enough to stop the motion once a considerable change in pitch has happened. Of course, the pilot will try to steer against it, but with the unavoidable delay of a human in the control loop. That means the counter action will now make the aircraft pitch against the initial motion just when natural stability does the same, so an overshoot follows. Either the pilot suppresses the learned reflex of actively stopping the pitch motion or a pilot induced oscillation follows. This is even more annoying.

So much for the annoying stuff. The outright dangerous would be:

coupling between fast period mode and wing bending. Flutter at low speed is the consequence.
Nasty stall characteristics once the combination of sweepback and aspect ratio surpasses a certain threshold. For 20° sweep this would be an aspect ratio bigger than 7.

You are right about the larger wing area, but this actually helps to make the spar lighter since there is more height to place it in. Unless you need to stiffen the spar in order to shift its bending eigenfrequency above that of the fast period mode in your allowed speed range (plus 20% safety margin).


Where did the SOE agents sit in the 161 squadron Short Stirlings?

IIRC, the agents dropped (with parachute) through a hole in the belly of the aircraft, just aft of the bomb-bays. So they would sit in that general area, so that they wouldn't have to move far with the 'chute attached. The position of the pilots is immaterial to this.


How can I work out Sea Level Pressure from QNH?

Yes the QNH value is based on the atmospheric pressure taken at the measurement site, then adjusted to represent that same atmospheric condition at seal level based on the normal expected pressure change between sea level and the measurement site, and at the standard temperature lapse rate.  In other words, if the measurement site was 1000 ft above sea level physically, it's what the atmospheric pressure at the measurement site would be if you were to dig a 1000 ft deep pit at that spot and take the pressure measurement at the bottom of the pit which is physically at true sea level.
The adjustment is based on the lapse rate of the standard atmosphere, so an error is introduced in doing this adjustment if temperatures are significantly different from standard temperatures (59F at SL).  Normally not a big deal because everybody in the vicinity is using the same "erroneous altitude", so no harm no foul, but in cold weather this has to be taken into account for IFR operations because the error (altitude reading higher than it really is when temperatures are below standard) becomes significant for the safety of IFR approach operations, and correction tables have to be used, generally when doing approaches in below freezing weather, where the potential high reading error starts to become dangerous in terms of obstacle clearance and decision altitudes.


Why is there no numbering system for more than three parallel runways?

"Because those are rules."

ICAO Annex 14 (volume 1) provides the following guidance:

  5.2.2.4 A runway designation marking shall consist of a two-digit number and on parallel runways shall be supplemented with a letter. On a single runway, dual parallel runways and triple parallel runways the two-digit number shall be the whole number nearest the one-tenth of the magnetic North when viewed from the direction of approach. On four or more parallel runways, one set of adjacent runways shall be numbered to the nearest one-tenth magnetic azimuth and the other set of adjacent runways numbered to the next nearest one-tenth of the magnetic azimuth. When the above rule would give a single digit number, it shall be preceded by a zero.

and

  5.2.2.5 In the case of parallel runways, each runway designation number shall be supplemented by a letter as follows, in the order shown from left to right when viewed from the direction of approach:

  for two parallel runways: “L” “R”;
  for three parallel runways: “L” “C” “R”;
  for four parallel runways: “L” “R” “L” “R”;
  for five parallel runways: “L” “C” “R” “L” “R” or “L” “R” “L” “C” “R”; and
  for six parallel runways: “L” “C” “R” “L” “C” “R”.

Presumably, the rules for runway designation were created before anyone had the fantasy to imagine mega airports with 4 or 5 parallel runways like we have today. They have then later been adopted to accommodate such airports, without changing the core principles, since doing so would potentially require thousands of airports worldwide to rename their runways and change associated procedures. Not to mention the changes that would be required to aircraft and ATC software, meteorological reporting formats, training and certification of various personnel and so on. Simply put, the disadvantages of using a slightly flawed system are significantly smaller than the cost of redesigning it completely.


Did some early jet engines catch on fire at higher power settings?

Yes indeed. For the Jumo 004:

  [A] shortcoming of the engine, common to all early turbojets, was its sluggish throttle response. Worse, it was fairly easy to inject too much fuel into the engine by throttling up too quickly, allowing heat to build up before the cooling air could remove it. This led to softening of the turbine blades, and was a major cause for engine failures.

To avoid rapid movements the levers of that era (and still being used on some types) had a friction setting that can be increased on decreased. High friction results in more force required to move the thrust lever.

Advancements in the hydromechanical units and the introduction of engine control units made controlling a jet engine much easier.

I remember reading (but I can't seem to find it now) that Frank Whittle had problems shutting down one of the early jet engines he worked on. It would just keep running and speeding up.


Why would a pilot on final to a towered airport not speak up when ATC clears another aircraft to land on the same runway, without mentioning them?

why wouldn't the pilot of '1DA speak up?

The simple answer is that you are not trained to "speak up" at a towered airport, you are trained to follow instructions from the tower and there is an assumption the tower will provide separation and appropriate clearances. As an ultimate fall back you are always supposed to "see and avoid".

For the official report you should reference the NTSB report but if you listen to the ATC audio in the linked video as well as their explanation of the link the pilots had no reason to think there was an issue and the clearances given were rushed but explicit. As I note above AOPA also notes

  "it is a pilots responsibility to visually confirm final approach"

You can also hear that the pilot of '4SR was giving confusing instructions about base extension and clearance to land. The pilots was not incorrect in his approach actions but made a critical go-around error and may have been a low altitude, dirty config, stall.

Ultimately the lower aircraft on approach had the right of way so '1DA had no reason to speak up as they were in the right. AOPA states that the pilots may have thought "he was overtaking the aircraft from above" which is incorrect, or at least procedurally incorrect as '4SR would have had to yield to '1DA anyway.

In the end the controller should have simply issued

  '4SR Go Around

Any time, you as a pilot hear that you should build the habit to just go around. As PIC you can ALWAYS execute a go around, controllers should be prepared for it and more importantly you as the PIC should be prepared for it.


In what direction(s) is any debris typically ejected from a jet engine in the case of failure during flight?

For debris ejection out of the engine, the failure of the engine and the failures that follow that create debris; the design criteria is that the failures must be contained in the cowling and the debris exits through the exhaust. Here is the link to the FAA Advisory Circular for designing for catastrophic failure: AC 20-128A

Here is another link that is more devoted to prior uncontained engine failure.


How does a pilot tell passengers to prepare for severe turbulence without causing panic?

Turbulence is reported on a scale of light, moderate, severe, and extreme.  The full definitions are available in the FAA Aeronautical Information Manual. As experienced in the cabin,

For Light Turbulence, "Occupants may feel a slight strain against belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking."
For Moderate Turbulence, "Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult."
For Severe Turbulence, "Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible."
Extreme Turbulence is "turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may causes structural damage."

As a pilot, once the ride gets to the point of light turbulence (and not just light chop), I'll probably ask the flight attendants to go ahead & sit down.  Even though they might be able to serve, I don't want them out in the middle of the aircraft if things get worse & then they'd have nowhere to sit.  That may not be a PA announcement; I may just call them on the interphone & say something to the effect of, "ATC says this is probably going to last for a while, so why don't you guys go ahead & take your seats for a bit."  If we find out that the ride is about to get bad soon & suddenly, that's when I'll typically use the PA and be more directive: "Flight attendants, take your seats; flight attendants please take your seats."

In moderate turbulence, there's no question but that everybody needs to be sitting down.  And I'll be looking for better options (typically, changing altitude) in order to get a better ride.  Sometimes a better altitude is available, and some times you just have to ride it out.

I won't intentionally fly into severe turbulence.  If I know before takeoff that the only way to complete the flight is to transit an area of severe turbulence, I won't take off.  If it's possible to work out a different routing and/or altitude(s) that avoid all the areas of known & forecast severe turbulence, then we can plan on that, but you won't find me ever explaining to passengers before the flight that we'll be flying through severe turbulence today.  Not unless we're the last flight out of Saigon before it gets overrun by the murderous hordes, but I don't really foresee that scenario.

If you do fly into severe turbulence despite the best precautions, you've (hopefully!) already got everybody seated since you were probably expecting moderate turbulence anyway.  After landing, the aircraft will need to be inspected, because any encounter with turbulence classified as severe requires an inspection before the aircraft can fly again.  Typically, no problems are found (airliners are built pretty sturdy these days), but a mechanic will check all the things that might be over-stressed or damaged in that sort of an encounter in order to ensure that everything is still good to go fly again.

As far as preflight PA's when we're expecting light or moderate turbulence, that depends a little bit on the situation.  If it's a short flight with enough time in some sort of turbulence, I'll coordinate with the flight attendants ahead of time to not bother trying to serve, and then on the "welcome aboard" PA I'll explain that situation to the passengers.  The ride is forecast to be pretty bumpy, and so to keep our flight attendants safe, and since I'm sure nobody wants hot coffee spilled in their laps, I've asked our flight attendants to remain seated for the flight and not attempt any beverage service. So sorry, it's all about safety, thank you for your understanding, blah blah blah, and since they'll be staying seated, we do ask that you also observe the Fasten Seatbelt sign and remain seated for the short duration of the flight today as well."

If only a small part of the flight is expected to be turbulent, I probably won't say much about it at the gate (the Flight Attendants cover "stay seated while the seatbelt sign in on" at length), and then maybe as we're getting close, I'll mention something just to reinforce what they say on the PA.  But if there's nothing special to communicate, I don't see great value in creating apprehension before the flight about an interval of turbulence that we may end up avoiding altogether.

Some pilots to make it part of their preflight PA's to re-emphasize the Fasten Seatbelt sign and stress again that we want you seated when it's on.  That's an individual pilot's preference (at least at my company).  Our guidelines recommend against using the word "turbulence" because it does cause apprehension, and instead talk about a choppy ride or rough air or bumps in the road or whatever.  Conveys the same meaning to the passengers, while hopefully scaring the nervous fliers a little less.