Reason for limited engine fuselage on Sukhoi fighter aircraft?

In a fighter you generally want to optimize fuel and ordinance capacity as well as have competitive performance. This use case generally leads to a design that is different than your average transport category aircraft. The SU-27 has some similar characteristics to the F-14 Tomcat and it should be noted

  The aircraft had a large wing, clipped, with two separate podded
  engines and a twin tail. The ‘tunnel’ between the two engines, as on
  the F-14 Tomcat, acts both as an additional lifting surface and hides
  armament from radar.

Here is a good picture of the engines from the back

(source)

You can see that they could connect the engines but that would increase cross sectional area, disturb the center portion which helps add lift and add weight. If there is nothing they need to stick in there, there is no reason to have such an area on the aircraft. Its also worth noting this is not terribly different from the back of an F-14.

There is not necessarily a whole lot of metal surrounding any aircraft engine.

In the case of the SU-27 it also looks like the engine coverings are unpainted, this may be for heat related reasons.


Where can FAA Letters of Interpretation be found?

The FAA Regulations Division maintains a webpage which includes a searchable database of "Legal Interpretations & Chief Counsel's Opinions".  It includes the following disclaimer:

  Legal Interpretations and the Chief Counsel's opinions are now
  available at this site.  Please note that not all interpretations or
  Chief Counsel's opinions are available at this time.  This database
  consists of legal interpretations issued from 1990 to the present and
  will be updated on a regular basis.

The letters can be searched by year and/or keyword.


In a 777, what would happen if the auto-pilot and the altitude hold failed at the same time?

Regardless of whether it's a 777 or any other airplane where the altitude is being held by the autopilot and the altitude hold function stops, whether because of general autopilot failure, just altitude hold failure, or the altitude hold function is inadvertently or purposely switched off; the airplane will seek whatever pitch attitude it is trimmed for.

For example, using the 747-100 and -200 autopilot setup, let's say you are in cruise at 35,000 ft, the autopilot is navigating and altitude hold is on. Further, let's say we're beyond 1990, and many operators of those aircraft had disabled autothrottles because they were a maintenance hog. Thus the airplane is maintaining whatever speed has resulted from where you have positioned the power levers. The altitude hold function is maintaining altitude by using the elevator. You can look down at an indicator on the center console between the pilots to see the degree of up or down elevator being used.

As the flight progresses and fuel is burned, the c.g. moves forward and the altitude hold function adds more up elevator to maintain altitude. When a certain amount of up elevator amount is reached, the system automatically retrims so that no up elevator force is required.  You hear and see it happen because the large trim wheel on the right side of the center console rotates.

If you're a good captain or a good flight engineer, you can monitor the amount of up elevator force being held and trim it out manually before it reaches enough to trigger the automatic retrim, and save the company a little fuel money (or if you're JFK to Tel Aviv in a -100, possibly avoid a fuel stop in Athens).

Now let's say that you reach up and disengage the altitude hold sometime between retrimmings. The airplane will nose down, and actually quite abruptly if it's been awhile since it last retrimmed. So abruptly that normal practice was to always have the yoke in hand before disengaging the autopilot.

Let's say that you that you let the condition persist. The nose is down, so the airplane would descend, of course. The speed would oscillate during the descent. If there was a severe nose down trim ... well, you wouldn't want to go there.

For a humorous account (at least meant to be humorous) of the altitude hold function being inadvertently disengaged, go to http://terryliittschwager.com/hajj4contest.php.

Now for the case of total autopilot failure, I defer to the other answer already here when I posted this. Suffice it to say that the pilots would have to take control in short order to avoid an extremely serious condition, very possibly the loss of the aircraft. You can go to http://community.seattletimes.nwsource.com/archive/?date=19911219&slug=1323954 for information on an incident that involves such. The interesting thing is this airplane had started to do this a number of times (including once to me coming out of Hong Kong), but in previous instances the pilots quickly recognized that the autopilot had failed and had taken control before things got serious.

Following the incident described, the airplane was repaired and reentered service, and we called her Christine.


How would you use the mental clock code to solve this dead reckoning problem?

Step 1: Calculate maximum drift.
Max drift $ = \frac{wind speed}{TAS (NM/min)} = \frac{24}{120/60} = \frac{24}{2} = 12$
That means if you have a direct crosswind, you'll need to correct 12$^{\circ}$.
Step 2: Determine how much of the wind is crosswind
The mental clock code is a way of estimating crosswind component from total wind. There are 60 minutes on a clock face.  If the wind direction is 60$^{\circ}$ or more, it's considered all crosswind, just as 60 minutes is considered the whole clock face.
By extension, a wind angle of 30$^{\circ}$ means 30 minutes, which is half a clock, so half the wind would be considered crosswind.  15 degrees would be a quarter of a clock, therefore a quarter of the wind, and so on.
Your maximum drift is 12 degrees with a full crosswind, but you don't have a full crosswind. Instead you have a 45$^{\circ}$ angle, so 45 minutes of a clock, which is $\frac{3}{4}$.
$ 12 * \frac{3}{4} = 9$ so you'll need to apply 9 degrees of crosswind correction toward the wind.  $270 - 9 = 261^{\circ}$
Step 3: Correct for variation
"East is least, West is best".  If variation is west, add to true to get magnetic.
$261 + 4 = 265^{\circ}$


In the US, in the last 50 years, have there been any accidents known to involve a student pilot intentionally practicing stalls in solo flight?

Yes.

I did a search of the NTSB database, limiting my search to final reports with a search term of "practicing stalls." This lead to about 50 reports. I scanned the reports where the aircraft was a typical trainer, such as a Cessna 172 or Piper Cherokee. I found three reports this way that involved a solo student pilot practicing stalls. Interestingly, only one of these seems to be due to a failure to recover from the stall.

12/2/1990 A student pilot experiences engine roughness and eventual failure during stall practice. Cause: failure to apply carb heat during low power settings, leading to carb ice.

3/22/1989 A student pilot fails to recover from a stall, while practicing too low to the ground. A factor involved was the featureless, snow covered terrain that made it difficult to gauge altitude above the surface.

11/5/1982 A student pilot experiences engine failure due to a stuck valve during stall practice.


Does a turboshaft's exhaust provide any thrust?

Yes, the exhaust of any engine does provide some thrust (very little for piston engines).

The amount of jet thrust for turboprops is in the range of 4%-15% of propeller-produced thrust. For a helicopter, it's only 2%-4% of rotor lift, but it's directed backwards. The helicopter in your picture would get 100-150 lbf of thrust from the engine exhaust, compared to 3,000-5,000 lbf of lift provided by the rotor.

This thrust is not controlled in any way. It helps a little in forward flight, and is compensated for by cyclic pitch in hover or maneuvering.

Residual thrust can be calculated, but is rarely specified in most sources. You can find some analyses of residual thrust for common engines online, such as for the PW-100 series, or for the Allison T-56.

The T-56's residual thrust, per above analysis, is 750 lbf, with 3442 kW on the shaft. That allows the T-56 engine to produce up to 10,000 lbf of thrust when used as a turboprop with a propeller. If used in a helicopter, such an engine would be able to deliver 25,000-40,000 lbf of lift with a rotor.


What are the advantages and disadvantages of elevons?

Image source

The advantage is: if there is limited real estate available for flight control surfaces, you don't have to divide it up into elevators and ailerons and only use half the available surface for pitch or roll.

The disadvantage is: you cannot go past the surface stop, and there will be situations where a combined pitch and roll cannot be commanded fully by the elevons because one of the surfaces will run into its stop. Resulting in smaller roll & pitch responses than commanded, in only certain situations.


Is there anything better than LiveATC to get accustomed with RT phraseology?

The loud whistling and popping has to do with the equipment that the person monitoring the frequency is using in addition to the range they are from the airport.

Depending on local law you could purchase a receiver capable of receiving those frequencies yourself directly. Under some conditions however transmissions may sound like that because they are AM and subject to whatever interference is on the air at the time.

When you are in an aircraft you are subject to less interference because there are no objects between you and the transmitter. The frequencies  you need to be able to receive are 118.000 Mhz to 136.975 Mhz AM mode for civil aviation.

Even if you are not directly near a major airport there is likely controlled airspace near you that you can check by looking at a sectional for your area. That will have the frequencies in use by your local airports. Then you can listen to controllers with your local accent. Once you know the words you'll be able to pick them out of the LiveATC feed better.


What is the differance between gliding for maximum range and gliding for maximum endurance?

No they are different.

Gliding for endurance, maximum time in the air, is at minimum sink rate, which is usually close to stall because it's where maximum lift is being generated. In a glider this is the speed you thermal at.  In a power plane it might be the speed you'd glide at if you wanted to delay the inevitable as long as possible for some reason, say if your engine quit while sightseeing over a lava field during a volcanic eruption.

Gliding for range, flattest glide, is at max L/D speed for the weight, which also affected by the wind's effect on "effective" L/D or glide angle relative to ground covered.   If you were in the volcanic eruption scenario, it's the speed you'd wished you'd flown at as you realize you could have cleared the lava field if you'd sped up to best glide angle.

I'm only partly kidding around; inexperienced glider pilots can fall into a trap, when they get in sinking air, of slowing to min sink to minimize their rate of descent, when the proper action is to speed up to best L/D, or faster, to get through the sinking air as fast as possible, even though their sink rate goes up even more.  I tell students to think of sink as a rain shower while strolling; you run to spend as little time in the rain as possible.


Why do airliners still have a switch for the non-smoking sign?

Smoking was outright banned on flights in the US in 1998/2000 which was in reality not all that long ago. It should be noted that smoking on domestic flights under 2 hours was banned in 1988 and under 6 hours in 1990. With that in mind we must remember that there are plenty of planes that are still in service and older than that, many were designed long before that time as well. In the view of the airlines its simply cheaper to leave the lights and switches there than put in a new interior panel in many cases.

Also as mentioned in the comments planes are built for the global aircraft market and there are still countries that allow smoking on flights. This question is similar and has some good info as well.

But here in the US the main reason is because the FAA mandates the signs,

For charter (135) operations

  § 135.127 Passenger information requirements and smoking prohibitions.

  (a) No person may conduct a scheduled flight on which smoking is
  prohibited by part 252 of this title unless the “No Smoking” passenger
  information signs are lighted during the entire flight, or one or more
  “No Smoking” placards meeting the requirements of § 25.1541 of this
  chapter are posted during the entire flight. If both the lighted signs
  and the placards are used, the signs must remain lighted during the
  entire flight segment.

For scheduled commercial (121) operations

  § 121.317 Passenger information requirements, smoking prohibitions,
  and additional seat belt requirements.

  ...

  (c) No person may operate
  an airplane on a flight on which smoking is prohibited by part 252 of
  this title unless either the “No Smoking” passenger information signs
  are lighted during the entire flight, or one or more “No Smoking”
  placards meeting the requirements of § 25.1541 of this chapter are
  posted during the entire flight segment. If both the lighted signs and
  the placards are used, the signs must remain lighted during the entire
  flight segment.

The reason they are not lighted the entire time most likely stems from history. Since there is a switch to turn them on chances are there is a check list item to flip the switch. You will most likely see them go on and off when the pilot hits that item on the pre flight checklist that may occur at the gate or on the taxiway heading out for take off.

The Piper Archer I fly still has ash trays (there is even one in the pilots seat arm rest)...


Can you tell ATC you are a "student pilot" when you are not?

It's a bad idea to give false information to a controller as a matter of practice. Instead of calling yourself a student when you're not, it would be better to say you're unfamiliar with the area or that your English is not perfect.

Controllers are almost always accommodating to less-than-proficient pilots and will usually make allowances whenever time and traffic permits.

If your English is not good, be sure and use simple, direct phraseology. Read the AIM to become familiar with the common phrases used with ATC. Even something as simple as hearing "radar identified" in Canada instead of the usual U.S. phrase "radar contact" has caused many a native English speaker to reply "say again?"


Would installing 6 engines on an Airbus A380 and or Boeing 747-800
  achieve a shorter take off distance?

Yes in theory, more thrust is faster acceleration.

No, because engine/wing interaction is not simple:

For the 747: the flaps will need an additional thrust gate, which will reduce the flaps capability.
For the A380: since it doesn't have a thrust gate, the additional thrust will create extra drag and pitch down moment, making it harder to lift the nose.
Bolting on engines is unrealistic, as it will require a new wing to take into account the extra forces on the wing.
The extra weight of the engines and their associated systems will increase the weight, which in turn will reduce the acceleration due to the increased normal force on the tires.
The extra engines will intake some of the air meant for the wings, so alongside the reduced capability of the flaps, there goes the full potential of the wing.
After a successful higher-speed takeoff, the extra engines will draw more fuel than needed (jet engines run less efficiently at lower-power). Which will result in a diminished range capability.

  Would that result in excessive acceleration and harmful or uncomfortable G-forces?

No, you can use the engines at low thrust if needed.

  Would the extra thrust and leverage exert excessive force on the wings?

Idem.

  How about braking, would the braking be shorter if all engines were
  fitted with thrust reversers.

Yes, in theory, this could be shorter.

No, with the decreased lift, the airplane will land at a faster speed, so no, not necessarily.


Is there an easy way to determine where a part goes on an airplane?

What you need is a parts catalog/diagram.

Here is one for the 767 that shows parts and where they go. Interestingly all the pictures seem to be of the BBJ (business jet) version of the 767, the other parts should remain the same.


How to accurately determine Navigation Accuracy from FAA's PBN Criteria (Order 8260.58A)?

Im more familiar with EASA then FAA but im sure there are similarities:

why a desired ground track would require a smaller navigation accuracy value?

RNP 2: the aircraft must be within ± 2 nm of the indicated position 95% of the flying time (Required for Basic RNAV).

RNP 1: the aircraft must be within ± 1 nm of the indicated position 95% of the flying time (Required for Precision RNAV).

So flying in a congested airspace (for example the NAT-OTS routes) ATC can accommodate more aircraft with smaller separations.

the goal is to provide an adequate Obstacle Clearance Area

Yes, as well as separation and spacing of aircraft.

doesn't this mean that smaller accuracy value would lead to a smaller obstacle clearance area?

In theory yes. But Obstacle clearance requirements around Airports and high terrain take into account all aircrafts including general aviation and older aircraft without PBN, RNP equipment so while you are right with more accuracy the approach and departure corridors lead to smaller clearance areas due to the regulations the actual clearance areas could stay the same.

More likely there will be changes at the approach and departure procedures (points, fixes and routes).

Hopefully you found something helpful.


Can propeller driven aircraft sustain speeds at or above Mach 1?

Supersonic level flight?  No.

The fastest propeller-driven aircraft is either the turboprop-powered TU-114 with a measured top speed of Mach 0.73 or the turboprop-powered XF-84H "Thunderscreech" with a design top speed of Mach 0.9 and an actual top speed of either Mach 0.83 or Mach 0.7 (sources disagree).  The latter is an indication of why supersonic propeller-driven aircraft are unlikely: the continuous sonic boom coming off the propeller tips made the Thunderscreech the loudest airplane in the world.

The record for piston-engine aircraft is slightly lower, at Mach 0.71.


Remote access to Area Forecast Discussion

I have an old Garmin 496 & XM which works great for this. XM lets you suspend your subscription in winter if you want to. Tom M


Why does only one hydraulic intake line from the reservoir have a filter on the Arrow II?

Why is it designed like that?

This design combines two advantages:

The system works reliably with one filter instead of two, saving a bit of cost, weight, and complexity
In case of a blocked filter, the gears can still be lowered (thanks @GgD for the hint!).

How it works

Gear down:

  When the flight deck gear selection handle is put in the gear-down
  position, a switch is made that turns on the electric motor in the
  power pack. The motor turns in the direction to rotate the hydraulic
  gear pump so that it pumps fluid to the gear-down side of the
  actuating cylinders. Pump pressure moves the spring-loaded shuttle
  valve to the left to allow fluid to reach all three actuators.
  Restrictors are used in the nose wheel actuator inlet and outlet ports
  to slow down the motion of this lighter gear. While hydraulic fluid is
  pumped to extend the gear, fluid from the upside of the actuators
  returns to the reservoir through the gear-up check valve. When the
  gear reach the down and locked position, pressure builds in the
  gear-down line from the pump and the low-pressure control valve
  unseats to return the fluid to the reservoir. Electric limit switches
  turn off the pump when all three gears are down and locked.

Gear up:

  To raise the gear, the flight deck gear handle is moved to the gear-up
  position. This sends current to the electric motor, which drives the
  hydraulic gear pump in the opposite direction causing fluid to be
  pumped to the gear-up side of the actuators. In this direction, pump
  inlet fluid flows through the filter. Fluid from the pump flows
  thought the gear-up check valve to the gear-up sides of the actuating
  cylinders. As the cylinders begin to move, the pistons release the
  mechanical down locks that hold the gear rigid for ground operations.
  Fluid from the gear-down side of the actuators returns to the
  reservoir through the shuttle valve. When the three gears are fully
  retracted, pressure builds in the system, and a pressure switch is
  opened that cuts power to the electric pump motor. The gear are held
  in the retracted position with hydraulic pressure. If pressure
  declines, the pressure switch closes to run the pump and raise the
  pressure until the pressure switch opens again.

Source: FAA

Aircraft Handbooks & Manuals: FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 2

chapter 13: Aircraft Landing Gear Systems, pages 13-18 to 13-20

Hydraulic fluid reservoir

Ideally, a hydraulic system could be a closed circuit without a reservoir. The actuators have two cavities, one to push the piston out, and one two pull the piston in. The total volume is constant, except for the volume of the piston rods.

A practical hydraulic system has to cope with the fluid displacement of the piston rods, leakage, geometric imperfections, and the thermal expansion of both, the equipment and the hydraulic fluid. A reservoir of hydraulic fluid is needed to allow fluid to enter or leave the system. This compensation flow could be very minimal. The fluid also needs to get filtered, because of possible impurities in new oil as well as possible contamination from the hydraulic equipment and conducts.

Shuttle valve

The shuttle valve is switching the routing of hydraulic fluid:

When the gears are pulled up, the return flow from the actuators goes to the reservoir, and fluid from the reservoir goes through the filter and the right valve to the pump.
When the gears are pushed down, the system is mostly a closed circuit. There might be some influx of fluid from the right valve, but that would be a small amount (unless the system is leaking badly).

Without the shuttle valve, there would be no point in the circuit where the main stream of fluid goes only one way. In such a design, there would be no place to put a filter out the contamination from within the system.


Why do most GA aircraft not have vortex generators?

Vortex generators are a kludge - a good design does not need them.

They are mostly added at later stages of the design process when some deficiencies need to be cured, but re-designing the aircraft is too expensive. They add a lot of drag, and do so always, for the whole duration of a flight, when they are needed only in some corner of the flight envelope (say, when flying close to stall).

A clever way to install vortex generators can be found on the Short Belfast:

Tail of the Short Belfast from below (own work). Note the vortex generators hidden in the hinge gap - they only move out into the airflow when they are needed: When the elevator is moved to large negative deflections.


Is there a minimum weight requirement for an commercial aircraft?

While there is a minimum weight to be defined as Transport Category, 12500 lbs for jets and 19 seats/19000 lbs for propeller driven airplanes, that's a "minimum maximum weight" you could say, to meet a category definition, which determines the certification standards you have to meet.
There is no minimum weight for certification in any category.  You define the mission you are designing for, payload, range, speed etc., and design it to be as light as you can possibly make it and still meet certification requirements for structural strength and performance (while allowing fudge factors for mistakes and flaws), while meeting your design goals.
So if you could design a 747 with new miracle materials, so its empty weight is 300,000 lbs instead of 400,000 lbs, and it meets all the Transport Category requirements for strength and performance, and it can still do what you want it to do in terms of speed/payload/range, you're good to go.


Did the X-15 ever conduct a point-to-point flight?

my question is whether the X-15 ever landed on another airport than the one it launched from

That depends on what you mean by "landed", "airport", and "another".
On its free flights, it never landed on another airport than the one it started from. However, it also never landed at the airport it started from. In fact, it didn't land at an airport at all.
It always landed on a dry lakebed near the airport. Whether you consider them part of the airport or not, is a matter of definition. As is the question whether you consider a dry lakebed an airport. I believe that the lakebeds were prepared with temporary runway markings, and they also may have cleared the "runway" from rocks and debris.
However, on aborted flights or captive carry flights, when it was never released from the carrier aircraft, it obviously landed attached to the carrier aircraft back at Edwards AFB.


Why did this old Japan Nakajima J1N airplane use a Yagi rather than a parabolic antenna?

Note that the assertion "It can only receive signal if the transmitting antenna is not exactly in front of it" is wrong. The most sensitive direction for receiving is in fact exactly in front of the antenna, so if the incoming beam is directly in line with the yagi shafts, the maximum signal strength will be received.

A parabolic antenna would be bigger across than the width of the plane's fuselage, causing a huge amount of (unnecessary) drag.


Is there a rule of thumb for aircraft design?

No, there isn't any single rule of thumb for airframe shape.  Just look at the huge variety of airframes that have been built and flown over the years.

One way to scratch-build is to start with a cardboard glider that's roughly the shape you want.  Tape the wings and tail to the fuselage in different places, tape on some noseweight, give it a toss, deflect the elevator, give it another toss, vary the dihedral, sweep, decalage.


What is the history of the aircraft logbook?

Similar to several other terms (e.g. port and starboard), the logging formats came from the shipping industry. Ship used to have detailed logs about their voyages.

Smithsonian states in an article:

  In a logbook from 1913, a pilot penned on one page the date of a
  flight, airplane used, passenger’s name, time aloft, and course flown.
  On the opposite page, he recorded height achieved, distance, weather,
  and remarks.

The above-mentioned article also tells:

  The Wright brothers kept detailed notebooks, recording dispassionately
  the results of their experiments in building the first airplane.

In the US, pilots are required to document their flights in logbooks (14 CFR 61.51). I doubt that logbooks would be optional anywhere in the world. In fact, if you do not log your flight, it is your loss more than anyone else's.


Do I need to call Clearance Delivery for IFR Practices when taking off from a D airport?

When flying VFR from a Class D airport, there is no need to contact clearance delivery to practice IFR approaches. You may, on the other hand, have to call clearance delivery if that is the local procedure for all departures at that airport. This is not very common at a Class D airport. If a clearance is necessary for VFR flights at a Class D airport, most often it is given through ground control. The US Chart Supplement and/or the ATIS will indicate if it is otherwise.
Normally, a simple indication to ground control of your intentions will suffice. This lets them know how to prepare for your return with the rest of their traffic flow and provide separation. This would be a demonstration of common and professional courtesy. It is not mandatory.
You can request a practice approach once airborne and inbound back to the airport without notifying ATC before your departure. As a matter of fact, it is very common to request a practice approach at any class of airport at a distant destination when making your initial call and position report prior to entering the airspace. Be prepared to be denied the approach if the airport is extremely busy during VMC.


Can a helicopter carry a large airplane?

If you look more closely, this "airplane" is only the husk of a Tupolev Tu-134, a rather small airliner, and the helicopter is a Mil-26, the heaviest helicopter ever to go into production. Not only are the empennage and the outer wings missing, but also the engines, which helps to reduce the load considerably.

Mil-26 lifting a Tu-134 carcass, seen from below (source)

The empty weight of a flightworthy Tu-134 is 28 tons, but the engines alone weigh 2.3 tons each. Now remove the outer wing, empennage and cabin interior, and the remaining weight is within the 20 ton load limit of the Mil-26.


How high are you when you are three miles away from the threshold of the landing RWY and get 3 white and 1 red on PAPI?

According to AIM 2-1-4, three white one red would equal 3.2 degrees above the horizontal. Doing some math, and assuming you are 3 NM from the TDZ (ie the PAPI) and ignoring slant range error, 3 NM would put you 18,228 ft from the threshold. Assuming you were on the 3 degree PAPI glideslope, you would then be 955.3 ft above TDZE. If .2 degrees high, or 3 white one red, you would be at 1019.1 ft above TDZE, or about 64 ft above the correct glideslope.

Math if you care: [tan(3.2)*18,228]-[tan(3)*18,228].


Why are parking brakes not used for parking jetliners?

Parking brakes are used by commercial aircraft at the gate if the ground crew can not set the chocks. The most common time for this to happen is during a lightning storm when ground crews are not allowed on the ramp. Large airports like DFW have automatic parking lights that will guide the aircraft to the proper alignment and stopping point. The crew will then set the parking brake before deplaning the passengers. Some crews will also use the parking brake for ramp holds, de-icing, etc.

The reason why chocks are preferred over the parking brake, is due to the way parking brakes work on aircraft. On commercial aircraft, the pilots apply the brake manually, then turn on the parking brake, which closes a hydraulic valve trapping the pressurized fluid in the brakes. Due to internal leakage of the valve, the brake pressure will slowly bleed down over a few hours. Therefore, after a few hours the brakes will no longer hold the aircraft. Further, most aircraft ramps are slightly sloped away from the terminal, so the aircraft would end up rolling backwards. Due to the bleed down issues, most airlines require pilots to stay in the cockpit ready to apply brakes until the wheel are chocked.

I have personally seen cases of large aircraft being parked with a parking brake, that rolled after the brake bleed down. In one case, a 777 rolled backwards and the jet-bridge tore off its door. Link to News Article

Car parking brakes work differently. Parking brakes in cars have a direct mechanical linkage (typically a cable) that is held in place. Therefore, they do not have the bleed down issue and will maintain the same amount of braking force at all times.


How are STCs granted for similar aircraft? Archer vs Arrow

Having been involved with several FAA STC's for GA and airliners I can pass on my experience. Additional FAA information is available in FAR part 21, 23, 125 and here.

First, transport, airliner's, etc are not held to the same rules. The FAA generally considers the engineering department of aircraft manufactures such as Cessna, Boeing or Aerobus to know what they are doing and pretty much rubber stamps anything they come up with. What they are mostly looking for is performance and operational documentation.

[ For 5yrs I was part of a Boeing team that primarily modified doors such as cargo doors for special purposes. We installed and verified or corrected engineering drawings for the modification and approval process. FYI: when I wasn't doing that, I did the final ramp, taxi and opps check of aircraft before their first flight out of Everett WA.]

GA aircraft STC's are administered by the regional FAA office (FSDO) and the process is highly dependent on the region and personal inspector. (The FAA says it's standardized, but it is not.) For example many float planes are flown all the way to Alaska to get approvals because the Anchorage regional office is experienced and generally more permissive than other offices.

Typically the FAA reviews the applicant against new product standards and expects the applicant to provide the "same as" level of safety found in FAR part 21, 23, and 125. With the new "industry self policing" policy passed last year, all bets are off on how it will be done in the future.

I have seen STC's approved with as little explanation as "performs equal to or better than original equipment..." on high performance Alaska Borer props, vortex generators, lights, antennas, gauges, etc with very little or no other data. In 2000, the Alaska FSDO changed from pencil whipping strobes (i.e. ...it works fine on 3000 other Cessna 172's..), to requiring 3 page form that includes a illumination profile chart to be filled out! The A&P's started a revolt and I don't know what happened, I moved three years later to work for Boeing in Seattle.

EXAMPLE: I assisted an Alaska company in a fuselage battery mount. I took a damaged C-180 and removed the tail surfaces and separated the fuselage aft of the cabin. I then hung a 25"G" horz weight (later 7G vertical) from the battery CG for 5min and took a picture. I certified the test and submitted it and received approval. I then took that approval and resubmitted approval for all applicable aircraft with the same skin thickness (individual W&B was calculated for each installation). It took about 6mo for the first approval and an additional year for about 15 additional aircraft to became STC approved. The company estimated it cost around $20-30k.

It often depends on the whim of you local FSDO. One of the reasons Wichita Kansas is a popular place for manufactures is the FSDO office is notorious for ignoring FAR regs. A good example is one FAR says "...every part large enough to be identified must have a unique part or serial number". Try to find a serial or part number on control columns/sticks, rudder peddles, flaps, wings, ribs, bell-cranks, etc. There are AD's on Cessna parts such as gauges but they don't have a part or serial number! An A&P can't write, I complied with the AD by replacing part "x" with new part "y"! (what some A&P's do is use the parts manual number, but the parts manual is not "approved" data by the administrator.)


What factors are involved in determining the spanwise pitching moment distribution?

In order to answer this question you need to know the detailed construction of the wing. In particular, the elastic center of each wing section is important. A simple way to find it would be to cut a cardboard into the cross section of the wing. Adjust local thickness according to skin panel thickness. Now find the center of gravity of this piece of cardboard, and you know where the elastic center is. Note that this only works for a closed torsion box; open sections tend to have their elastic center outside of their circumference.

The Free Dictionary has this definition for the elastic center:

  A point within a wing section at which application of a concentrated
  load will cause the wing to deflect without rotation. This is the
  point in the wing section at which rotation will occur when the wing
  is subjected to a twist.

Now you need to calculate the distance between the elastic center and the quarter point, around which the pitch moment coefficient is defined. This, multiplied by the local lift coefficient, will give you the offset moment which you need to add to the pitch moment in order to calculate the total moment twisting the wing.

  Which point on the section of the aerofoil would I take moments about?

That depends what moments you want to know. Aerodynamic moments are defined around the quarter point. If you want to calculate the aeroelastic twist of the wing, you need to use the elastic center.

  Would the weight of an engine produce a pitching moment and hence
  affect the pitching moment distribution?

Masses produce moments which depend on load factor while aerodynamic forces depend on dynamic pressure. Yes, you need to add masses and their moments around the elastic center, and you need to do this separately for each speed at which you do the calculation.


Do airlines require aptitude tests or other similar tests for new pilots?

Yes they do, and so do some independent training organisations. For instance EPST markets an aptitude test called Compass, developed to test the suitability of candidates before they engage in pilot training.

Some airlines only hire self funded students (who have paid for their own CPL, type rating etc) and they can be flight tested in the simulator.


What does the correlation between weather and midair collisions tell us?

In poor weather there are apt to be less VFR aircraft and more IFR. IFR are required to participate with ATC and automatically receive separation by time, altitude, and radar (when available). VFR do not require any separation and are not required to participate with ATC. There are also more flights in VFR weather than there are with IFR.

I might get flamed for this, but in general a IFR pilot is more proficient than a VFR pilot. The medical (commercial opps), testing, and currency requirements for a IFR license are usually considerably more complex than VFR - meaning (admittedly there are exceptions i.e. aerobatic pilots) IFR pilots are generally more proficient.

The following added in response to comments below

According to the FAA, "...  the majority happen within five miles of an airport" https://www.faasafety.gov/gslac/alc/libview_normal.aspx?id=6851. So, cruise environment is not a factor, and IFR have ATC separation at all times, making this predominantly a VFR landing/takeoff problem. Most IFR flights are commercial requiring a 6mo medical where as private VFR pilots require 2yrs or LSA and Ultralight do not require a aviation medical. (The private medical requirements just changed, but this discussion is based on prior years data.)

In addition to excersize IFR privileges, a pilot must remain current every 6mo where as a VFR pilot does not (except for night landings).

IFR flights will have a higher percentage of two man crews compared to most VFR flights with a single pilot. The extra set of eyes is extremely significant particularly during the landing/takeoff phase of flight.


Is there a backup to fuel inerting systems?

Short Answer

No, it does not have a back-up, there are some fail-safe components in the system(see below), but in general the system itself is not duplicated. The system, together with a design that avoid ignition sources within the tanks, is just a part that contributes to prevent fuel tanks to ignite. An Inerting System for commercial planes have been considered since 1996 TWA accident and first regulation arrived in may 2006.

Longe Answer

The certification CS25.981 Fuel tank ignition prevention states:

  (a) No ignition source may be present at each point
  in the fuel tank or fuel tank system where
  catastrophic failure could occur due to ignition of
  fuel or vapors.

  (c) Design precautions must be taken to achieve
  conditions within the fuel tanks which reduce the
  likelihood of flammable vapors.

So the function that should be achieved is prevent tank ignition and there are two fronts two work on, avoid ignition source, and reduce fuel vapors (similar to the two slices of cheese that rbp was mentioning in the comments).

So the fuel ignition system is a system designed to avoid fuel vapors. As can be seen in the image below, within the systems there are some redundancies (e.g.: Dual Flow Shut Off Valve), but the system itself is not duplicated.

Moreover the system is not on the Minimum Equipment List (MEL), so the aircraft can still be dispatched with the system inoperative for 10 days.


Do headrests considerably increase safety on aircraft?

You aren't going to get rear ended in an airplane, which is the primary safety benefit of headrests.  On the other hand, there is still some benefit to having them because in a hard forward crash, if you have shoulder harnesses on, you will probably get rebounded back into the seat to some degree or another, and the headrest will keep your head from snapping back too far during the rebound.  So better one there than not.

I've been following GA since the late 60s (I still have a FLYING magazine issue I bought at a corner store in 1969) and would say that they were mostly adopted in the 70s for "styling" purposes (keeping up with car interiors; there was a big effort to make GA airplanes as car-like as possible back then), but they aren't totally useless for protecting your neck in the right kind of crash scenario.

The really big sea-change in crash safety for GA aircraft was the adoption of shoulder harnesses, also in the 70s.  They made many crashes walk-away events that previously left you with fatal head injuries, or if you were lucky, you survived with a hamburger face.


It used a set of gyroscopes and pendulums that would correct the plane's attitude when it deviates. They were usually enhanced with pneumatic or hydraulic power to actually move the control surfaces.

For example the V-1 flying bomb (WW2 German UAV) could fly across the Channel and initiate a dive after a preset distance. It remained on course using a pendulum for pitch control and a gyroscope for yaw control. It used its inherent stability for roll control (no ailerons).


Why does the B777-200 freighter not use the letters I, N, O and Q for deck positions?

Those letters are commonly unused in aviation in markings, registrations etc. They might be easily confused with similar letters (I with J, N with M) and are therefore left out.


Why is thrust available constant with speed for turbojet engines, when it varies with speed for turboprop engines?

Turboprops and turbojets - or, more broadly, jets - produce thrust in somewhat different ways.

First of all, let's address the way thrust is produced. Per Newton's 2nd and 3rd laws, force equals acceleration times mass, and an action (accelerating the air) produces an opposite reaction. After canceling out the variables (the math is easy to find), thrust is proportional to T=v*m' (m'=mass flow rate), and power transferred to the air is proportional to P=v^2*m'/2. All velocities are  in the airplane's frame of reference.

Now let's go to how engines produce this thrust. A jet engine first decelerates the incoming air to a near-zero velocity, generating drag, then accelerates it to a constant velocity, higher than the initial one, producing thrust. Both v and m' for a jet engine vary across the envelope, but they change much slower than the plane's speed. The engine spends roughly the same amount of power per unit thrust at any velocity.

A propeller doesn't decelerate the air at all. It only accelerates the incoming air by some amount. This converts power P into velocity v. Now, per above, P~v^2. If you encounter air moving at 0 m/s, you need 5 kJ/kg to accelerate it to 100 m/s. If the air is already moving at 100 m/s, you need 15 kJ/kg to get it to 200 m/s.

The result is that propellers encounter less resistance at low airspeed, so they get more thrust per horsepower the slower the plane is. This allows them to push more air or push it faster, producing more thrust. For fixed-pitch propellers, this works by producing more air delta-V and less drag. High-performance turboprops tend to have variable pitch propellers, which will be adjusted to push more air (slower) at low airspeed, or less (but faster) at high airspeed.

Turbofans, combining a fixed-pitch ducted propeller with a turbojet core, are somewhat in between. They lose some efficiency and some thrust as they gain more speed, like propellers, but their curve is much smoother and closer to turbojets in this regard.

The above is an extreme simplification (as is the jet's thrust curve in the question), just enough to get the idea across. The jet's actual thrust is also non-linear, which has been addressed in another question: How (and why) does engine thrust changes with airspeed?


Are there any aircraft with a nose wheel braking system?

There are quite a few 727 that actually had nose wheel braking, however they utilized it only for maximum braking power. (Also they were proven to be ineffective and unreliable, so a service bulletin was written to remove the system).

Saab Gripens use nose wheel brakes to stop as well, the main gear sits farther aft than most planes, so there is quite a bit of weight on the nose gear, which makes it effective for stopping.


What is the difference between a Stand and Gate in Airports? How do they differ ?

An aircraft stand is a place where an aircraft can be parked.

A gate is a designated area inside the terminal building where passengers will assemble and wait prior to boarding. It includes facilities for checking boarding passes, last-minute customer service such as seating changes and so forth.

In the typical setup most passengers meet, each gate serves just one stand located right outside it. But this is not always the case: airports often have remote stands too, and there will be a smaller number of gates from where passengers are bused (or walk across the apron!) to the appropriate stands. Such a gate is not dedicated to a particular aircraft stand.

In the former case, where a stand is served by a dedicated gate, there is little point in distinguishing between the two. They will often (but not always) share one identifying number, and people happily speak about, for example, taxiing to the gate after landing. But for remote stands a distinction needs to be made.


How are aircraft wings protected against flutter - aerodynamic oscillations that can break bridges?

Flutter happens when the eigenfrequencies of two oscillations move close together so their motions can reinforce themselves mutually. When that happens, then the amplitude will increase with each oscillation, up to a point where the amplitude is big enough to break things. That is how bridges are destroyed: Ever more energy is accumulated in the structure, a tiny bit with every oscillation, until the total overwhelms the strength of the weakest part.

If the two eigenfrequencies are not close enough together, both motions will still exist but will normally be damped so they die down quickly after being excited, e.g. by a gust. A typical example is the wing bending motion: When the aircraft flies into an updraft, the wingtips flex up, elastic energy is accumulated and set free once the extra lift from the updraft dies down. The wingtips move down and will very briefly oscillate around their long-term vertical position.

Now add a free-flying aileron (mechanical control system with stick free). This also can oscillate around its long-term deflection angle. As long as the eigenfrequencies of both motions are sufficiently apart, they will not interact. This should be ensured throughout the normal flight envelope. However, flying faster will increase the aileron eigenfrequency, and in this case we should focus on the first antimetric bending motion of the wing. At one point the aileron frequency will come close to the wing bending eigenfrequency, and only then this particular flutter mode will happen.

The glider in the second picture of your question is the SB-9, a longer-span version of the SB-8 (which itself is flutter-free). The added span moved the wing eigenfrequencies down, so they met with the aileron frequency at a lower speed. Now flutter could be observed when flying at the right speed, but it could be stopped simply by gripping the stick.

Therefore, the possible protections against flutter are:

Limit the flight envelope. Prohibit flight speeds at which flutter is possible.
Increase damping. This can be as simple as adding some friction. The rudder cables in gliders are run in PE tubes which add some friction, eliminating rudder flutter.
Increase eigenfrequencies by increasing stiffness. In the SB-13 flying wing glider high modulus carbon fiber was used in the spar to shift the wing's eigenfrequency sufficiently up to avoid a coupling of the wing bending mode with the short period mode of the airplane
Increase eigenfrequencies by mass reduction: When a wing is made lighter especially towards the tips, e.g. by clipping the wing tips, its eigenfrequency will move up, shifting flutter speeds up. Conversely, by adding wingspan to the sound SB-8 design, the eigenfrequency of the SB-9 wing went down.
Change eigenmodes: By running the aileron pushrods near the wing shell, wing bending can effect an aileron deflection which helps to dampen the wing's bending mode.
Cross-coupling of eigenmodes: By reducing the wing spar sweep of the SB-13 by 3° relative to that of the whole wing, the bending motion causes wing torsion which reduces incidence when the wingtips are bent up and vice versa. This is similar to the pushrod trick and is an application of aeroelastic tailoring.

In a similar way, the forward location of the engines on an airliner wing creates an inertial torsion moment when the wing flexes. This increases damping, so this example could had equally appeared in the first point.

To ensure a design is free of flutter, a range of tests is performed prior to first flight and during flight testing. From this answer:

  Since maybe fifty years every new design must go through a static
  vibration test to check which elastic eigenfrequencies exist.
  Analytical methods are used to predict the (speed dependent)
  aerodynamic frequencies and the structure must be modified to remove
  any possibility of resonance. Next, the airframe is tested in flight
  at ever increasing speeds and with exciters at the tips of all
  surfaces (those can be like the vibration motor in your smartphone,
  only bigger, or, in the simplest of cases, the pilot moves the control
  surfaces). At increasing speeds, the exciters are run through a
  frequency sweep (typically from 5 to 60 Hz) and the resulting
  amplitude is measured by strain gages or accelerometers.


Does extension of flaps 1, 2, 5 or 10 increase descent rates?

It depends what you intend to do with the speed. If all you are looking for is absolute descent rate:  a high-speed, spoilers out and high speed dive will get you the best rate.

What you are usually interested in a normal approach however is not descent rate (vertical speed) as much as descent angle. In this respect, all/any flaps will decrease the L/D ratio, which is going to increase you descent angle. Each stage of flaps will have increasingly higher descent angles, but mind you at a lower speed.


Are there typical values of pitot-static and TAT probe measurement uncertainties?

The pressure and temperature measurement devices are analog instruments, extracting parameters that are of value to pilots from physical phenomena. Unfortunately, the physical world throws in a variety of effects, linear and non-linear, that make it difficult to extract the parameter of value. Noise is one of them, but in a way that is exactly what we want to measure since pressure is just the average value of noise in the speed of gas molecules in air.

In the pitot tube there are a couple of effects that make the measurements non-trivial:

Friction between the air and the tube causes a velocity distribution that varies with flow radius relative to tube radius.
Fluid mechanics causes different behaviour at different conditions, such as laminar or turbulent flow, or supersonic shock waves.
Inertia of the particles creates a time delay in pressure measurements as a function of airspeed and could can cause back pressure surges.
Old fashioned thermodynamics such as the ideal gas law impose non-linear functions on the relationship between pressure, volume and temperature.

From Wiki

All these effects need to be considered in order to measure just the total pressure and the static pressure, and then to extract the airspeed, altitude etc from these.

If it is calibration data of the sensors that you are after, that science is fortunately over 100 years old.


Are there any safety mechanisms to prevent emergency slides deploying inside an aircraft?

Yes, they can inflate into the cabin. The FAA has been criticized for not taking better action regarding this and several other escape issues. Consider what happened on July 6, 2013 when Flight 214 struck the sea wall while landing...

Here is a direct quote:

  During the crash two of the inflatable chutes deployed into the cabin. The first chute, which blocked the forward right exit, nearly suffocated a flight attendant and was deflated by a pilot with a fire axe from the cockpit. The second chute expanded toward the center of the aircraft near the fire. It trapped a second flight attendant until a co-pilot deflated it with a dinner knife.

When I worked for Continental Airlines, a slide would deploy unexpectedly either while the aircraft was stationary at the gate or while the attendants were opening the door (no fault of theirs). I have never heard of a slide deploying while in flight.


How do the wings connect to the centre wing box?

Transport airplanes, in general, use one-piece wings with no joints between sections. The "wing box" is the central structural element for the entire wing, made up of front and back spar webs, and upper and lower planks, which make the 4 sides of the box.  Leading edges and trailing edges/control surfaces are tacked on to the front and back walls of the box.

The upper and lower planks (machined skins) are the primary structural members in bending, supported by the spar webs to resist shear loads and the wing ribs to support the planks themselves against buckling.  You could say that the entire upper and lower skins or planks are the spar caps.

The "center wing box" is the middle section of the overall wing box, but that just describes the center part of a very long box.  The fuselage will simply sit on top of it, with a square cutout in the bottom to fit over the wing, with fairings all around the connection to make an nice external transition for the air flow.

The fuselage will be integrated into the wing box structurally with some sort of connecting links joining fuselage frames to the box, and may also have fuselage skins attached directly to the wing box.  Sometimes the cutout in the fuselage has its own pressure bulkhead forming a box outside the wing box and you can simply unbolt the fuselage from the wing and lift it off, and sometimes the wing box itself is made part of the pressure hull, requiring lots of rivets to be drilled out to separate them.

If the airplane has wings that attach to each side of the fuselage with big spar fittings and bolts, the center wing box will be an integrated part of the fuselage, what you would call a structural pass-through.


Yes, if in VMC a pilot can cancel IFR, then request VFR flight following for the remainder of the trip.


Does an airplane has to take off from beginning of the runway or it can take off from displaced threshold?

Can the displaced threshold be used for takeoff?
The part of the runway in front of the displaced threshold must not be used for landing (it can be used from the other direction during roll-out), but it may be used for takeoff:

Displaced Threshold. A displaced threshold is a threshold located at a point on the runway other than the designated beginning of the runway. Displacement of a threshold reduces the length of runway available for landings. The portion of runway behind a displaced threshold is available for takeoffs in either direction and landings from the opposite direction.

(FAA AIM Chapter 2 - Section 3 - Airport Marking Aids and Signs)
This also follows the ICAO definitions in Annex 14, Volume 1, Attachment A, Volume 1, 3. Calculation of declared distances:

The declared distances to be calculated for each runway direction comprise: the take-off run available (TORA), take-off distance available (TODA), accelerate-stop distance available (ASDA), and landing distance available (LDA).

Where a runway has a displaced threshold, then the LDA will be reduced by the distance the threshold is displaced, as shown in Figure A-1 (D). A displaced threshold affects only the LDA for approaches made to that threshold; all declared distances for operations in the reciprocal direction are unaffected.

(Landing Distances on SKYbrary)
Note that the TODA (TakeOff Distance Available) is unchanged in D, but the LDA (Landing Distance Available) is reduced.
Does it have to be used for takeoff?
No, a takeoff can be performed starting from anywhere along the runway. This is typically called an intersection takeoff. Some airports even have taxiway signs displaying the remaining TORA at some taxiways:

(ICAO Standards - Airfield Information Signs)
Such an intersection takeoff requires ATC clearance and of course that the aircraft takeoff performance calculations are done based on this intersection. See also Are intersection takeoffs less common in the USA than in other countries?


How common are diversions to high-latitude airports for intercontinental flights?

Very very uncommon. Goose Bay, Canada, is a diversion airport for US-Europe flights. If you search Aviation Herald for "goose", you will find about 2 to 3 flights divert there each year.

3 flights diverted in 2017:

Delta A330 Oct 20, engine problem
SAS A340 April 16, low cabin pressure
Air France A380 Sep 30, engine structural failure

2 flights diverted in 2016:

Virgin B787 Nov 22, engine oil leak  United B767 Sep 15, generator/APU failure

3 flights diverted in 2015:

American B757 Sep 30, electrical problem
Delta B767 July 31, generator failure
United B767 June 12, flight controls

2 flights diverted in 2014:

Delta B767 Aug 9, engine problem
Delta B767 May 1, engine shut down

In all these incidents except for 2, the report in Aviation Herald states a replacement aircraft was required. Sometimes the delay was 6 hours, sometimes the next day. In the 2 other incidents, it's unclear if a replacement aircraft was required or not, but in both cases maintenance was required, so at least a delay occurred.

This compares with a bird strike, or fumes/smoke in the cabin, almost daily somewhere. It's much rare than even lightning strikes, of which 25 were reported in 2017.


In a constant speed propeller, why does the RPM drop with a prop pitch increase?

The propeller blades indeed have least resistance when aligned with the inflowing airstream, however the inflowing airstream is not the free stream at infinity, but the vector sum of free stream velocity $V$ and rotational velocity of the propeller blade $\omega \cdot r$.

In the picture the local Angle of Attack is indicated as $\alpha$. Increasing blade pitch results in increasing $\alpha$, which means more air accelerated backwards: more thrust delivered, and more torque required.


Are any airlines actually flying the Comac ARJ21?

You are correct in that the FlightRadar24 site doesn't show any flights for the ARJ by aircraft. But going to the routes tab and searching through the routes there are ARJ assigned to some routes. So it appears that Chengdu Airlines is operating the aircraft at least on a limited basis.

I would also say that while FlightRadar24 data is generally very good, it isn't perfect. They do rely on data from external sources and considering the Chinese government's information and internet policies, it's entirely possible that the data availability is not the most reliable.


That is a Douglas O-38E, with sliding cockpit windows and rounded tail.
The tail on the right of the picture is also a Douglas O-38, but another version (standard, B,C & F had such tails)

Source: wikimedia Photographer: USAF
It seems that the picture in the question is damaged; there are some stains covering the area below the horizontal tailplane and also the arms crest on the fuselage. It seems that the arms crest is a circle with a cross in it. This is the insignia of the 33rd division,  Illinois National Guard.
Some of their other O-38's are pictured here
A picture of possibly the same aircraft with the insignia clearly visible can be found on alamy.com


What is an “aeronautically qualified witness”?

It means a witness with credentials and knowledge that makes the witness competent to give a reasonably accurate description of an event, or an informed opinion, to an investigating authority or a jury.  Likely a pilot witness to a crash as in this case, who knows more about weather than the average person, or a mechanic witness in an investigation or lawsuit over maintenance.


Can an aircraft be turned without rudder input?

It depends on the aspect ratio of the wing and the lift coefficient. Short, stubby wings at low angle of attack will not create much adverse yaw with aileron input, especially when the ailerons have differential linkage (more deflection on the trailing-edge-up side and less on the opposite side). Differential gearing is also beneficial in reducing stick forces, so many small aircraft with manual controls employ it. Therefore, turning with ailerons alone is possible with low aspect ratio wings.

Airliner wings have aspect ratios between 7 and 11, so here it is helpful to reduce the lift coefficient by flying fast and lower than at maximum cruise altitude if you want to turn with the ailerons alone. If you allow rudder movements but just want the pilot to rest his/her feet on the floor: Almost all jets have yaw dampers, that is a device which will deflect the rudder automatically such that yaw is minimized. Therefore, airliners can be flown with the feet off the pedals and still ailerons and rudder will keep the plane in a coordinated turn.

With gliders and their high aspect ratio wings, however, using only ailerons will mostly yaw the aircraft, especially at low speed when the lift coefficient is high. The wing also has high roll damping from its high aspect ratio and low flight altitude, so the roll angle achievable before yaw becomes dominant is small. The yawing condition will now create its own rolling moment due to the dihedral effect of the wing. Unfortunately, this effect counters the intended rolling motion and prevents the aircraft from turning with ailerons alone. If adverse yaw is large enough, the aircraft will not even begin to roll but will only yaw with aileron input.  Some glider pilots even apply cross controls when entering a thermal in order to maximize their rolling speed (rudder into the turn and aileron against to provoke more yaw and let the dihedral effect roll the glider).

If you find yourself in a high-performance glider with a jammed rudder, speed up in order to fly turns with the ailerons (and elevator) only. The lower lift coefficient will reduce adverse yaw so direction can be controlled with ailerons and elevator only.


What is the operating temperature of a turbo bypass engine, and how is it kept at that temperature?

Yes, if you could increase the combustion temperature in the hot section of a gas turbine, you could dramatically increase the efficiency of the engine.  The problem of course is finding materials which can sustain structural integrity in these regimes for long service lives and still contain the hot gas; we are currently at the thermal limits of existing materials.

The key problems here are:

Temperatures - gas turbine hot sections operate at 1600°F all the way up to 2600° F for high performance military engines in order to yield high thrust or high useful work for a unit of fuel burned.  No known metals can survive these upper limits and still maintain high yield strengths.  It also causes great thermal expansion problems between non homogenous materials and coatings as well as oxidation and galvanic corrosion.

Stresses - the spools of a gas turbine can spin as fast as 40,000 rpm in the cases of high pressure compressors and high pressure turbines.  This creates tremendous centripetal loads on the blades and other components which they must be able to endure and not exceed the yield strength of the material.

Extended operation between servicing and reliability - the engine must survive these loads for several hundred cycles over a period of several thousand hours between major overhauls.  Redundancy as well as resistance to damage must be built into the design as well.  The consequences are grave; the trustworthiness and reliability of two gas turbines engines are all that separates a successful Seattle to Moscow polar flight in a 777 and 320 people freezing to death in the Arctic Ocean.

Historically gas turbine hot sections have been constructed from alloys of metals with high melting points and good yield strengths like nickel and cobalt of a very high purity.  Titanium and Molybdenum are also beginning to be used in this as well.  Additionally forming techniques such as single crystal casting are used to remove crystal grains from the material to improve its strength.  The blades ('buckets') of a high pressure turbine are also cast with channels in them and have fine holes laser drilled in their surface in order to take compressor bleed air and 'bathe' the boundary layer of the bucket with cooler air, effectively wrapping it in a cool gas blanket to protect it from the high temperature exhaust.

More exotic materials and techniques are being considered for this.  GEAE and P&W both are exploring the use of composite and ceramic blades in hot sections.  These offer high yield strengths at high temperatures but again pose challenges in terms of oxidation and thermal expansion problems.


What is the location of AC for typical modern airplanes?

AC of the whole aircraft must be located behind CG, so that AC was naturally statically stable [in pitch, or more accurately, by angle of attack]. This pretty much follows from the definition of AC, and there is no way around it.

However, this is not the whole story.

First, it is possible to design an airplane with 'positive cambered tailplane'. As such, there is no requirement that the tailplane produced negative lift (for a classic design); there is merely a requirement that the tailplane had a lower lift coefficient than the wing.

Second, a statically unstable airframe can still be made stable by using computer control. From the point of view of the pilot, F-16 or Su-27 are stable, even though their airframes are not. In some marginal cases, when the aircraft has a lot of pitch damping, even average pilot can cope with mild instability.

As for airliners, all of them have AC behind CG, but I don't have specific numbers.


Do any aircraft possess washed-in wing characteristics?

According to Wikipedia, the Grumman X-29 had washed-in wings, as its considerable forward wing sweep gave it a strong tendency to stall at the roots.  As the wing roots on a forward-swept-winged aircraft are located aft of the wingtips, a root stall on such an aircraft would cause a strong pitch-up tendency, so the X-29's wings were washed in to mitigate the root-stall tendency.


What does the altimeter indicate if it's at a certain elevation

An altimeter is just an adjustable barometer with extra hands.

There is a barometric correction you dial in so that the altitude shown is always feet above sea level.  The correction is from the current barometric pressure reference you get with the "altimeter setting" or the weather report.

You dial in the altimeter setting, the current barometric pressure reference for your location, and shows your true height above sea level.  In your example, if the altimeter setting is correct, it will show 10 ft above 0 ft.

Or, you can work it the other way.  If you know the elevation you are at, you can dial the altimeter so its altitude indication matches your known elevation.  Voila!; the current barometric pressure is shown in the setting window on the instrument's face.


How can I use XFLR5 on a shape with no maximum thickness at one point?

XFLR5 uses XFOIL to perform airfoil analysis. From the User Guide:

  XFOIL is a (...) program for the design and analysis of subsonic isolated airfoils.

  The XFOIL code is not foolproof, and requires some level of aerodynamic expertise and common sense on the part of the user.

The most general equations to describe the motion of fluids are the Navier-Stokes equations. XFOIL doesn't solve the NS equations directly but uses panel methods instead. This makes it much faster than other CFD simulation tools at the cost of restricting the geometries it can analyze.

Normally an airfoil has the following properties:

Simple, smooth geometry
Smaller thickness than length
Sharp trailing edge

The problem with your shape is not the maximum thickness (a relative thickness of 20% is still OK) or the constant distance between the upper and lower side but the blunt trailing edge. To some extent the really large thickness at over 75% of the chord affects too.

Whitcomb's supercritical airfoil for example has a nearly constant thickness from 20 to 50% chord but can be analyzed without errors.

The panel method splits the airfoil in many small panels and tries to find a circulation for each one of them so that the air follows the airfoil geometry (normal speed to each panel equals zero). To get physically more realistic results other equations like the Kutta-condition have to be used. One of the assumptions for this equation is that only shapes with a sharp trailing edge can produce lift.

Trying to solve equations when the basic assumptions are not met normally doesn't work and that's why your calculations do not converge.

Note: I'm not an XFOIL power user and perhaps someone can tweak it to get good results, but I doubt it. While it is possible to analyze airfoils with a blunt trailing edge the shape above is very far away from being an airfoil.

TL;DR: XFLR5 can only analyze airfoils. Your shape has not a sharp trailing edge and it is therefore not an airfoil.


Why does a 3D wing's CL keep increasing even after 50° AOA in XFLR5?

XFLR5 is not made for this kind of calculation. Do not trust the results.
Stall is not the end of lift. See below for 2D wind tunnel data collected by S. Hörner in his book Fluid Dynamic Lift. 3D results would look similar, but with a less steep gradient around 0° and lower peaks.

180° polar for several airfoils


Where and when would a 737-700 be taking off at an altitude of 19 kft?

As Ron points out in the comments the question is not so much where as it is when. Aircrafts see the density altitude of the air around it not the true altitude they are at. On a very hot day at a very high altitude airport the density altitude may very well be higher than the true altitude by a significant amount which affects takeoff performance. The FAA provides a nice overview of density altitude here. SO even though there are no airports at 19,000 MSL a lower altitude airport that routinely sees very high temperatures can yield the same situation.


When the 737 is a type, the 737-700 (IATA: 73G) and 737-800 (IATA: 738) are sub-types. They have different capacities and differing weights. Other documents will use the word "variants" where your document uses "subtypes".
Related info here.


How do bearings in a jet engine keep water out when it's been cleaned?

To keep dirt out, bearings with exposed faces have rotary seals on them. Those seals will also keep water out, as long as the bearing is not rotating when wetted and not completely immersed. If immersed for days or weeks, even well-sealed bearings will eventually allow water in.


Why don't the noses have the same de-icing metal surface like that of the wing's leading edge, engines, tail, etc.?

Areas on the airplane with ice protection have it for important reasons:

Ice on the wing/empennage disrupts airflow, adding drag and making it more susceptible to stalling.
Ice on an engine cowl lip can disrupt flow into the engine, causing stability issues. If it dislodges it can damage the engine.
Ice in the pitot/static system prevents the air data system from reading airspeed or altitude properly.
Ice on the windshield can make it harder to see through.
Ice in drain lines can cause them to clog and not drain properly.

The nose is not as important. Ice will add drag, and has some risk of breaking off and damaging something. But this is not as critical as with other areas. There is also an added challenge in adding anti-ice measures, as it must interfere as little as possible with the weather radar inside. It's made from composite material, which doesn't conduct or cope with heat very well, so using either bleed air or electric heating would be an issue.


Why are aircraft external lights round, not tear-drop shaped?

That beacon lens (and most of the strobe lenses shaped like it) are actually Fresnel lenses.
They are optically designed to produce a specific plane & intensity of coverage, and tinted for a specific color. (The color, intensity, and plane are specified in the FARs & applicable Technical Standard Orders if you want to go hunt down the requirements.)

The lens optics are easier to see on the split (Red/White) lenses:

A Fresnel lens is not the only way to meet these requirements, some anti-collision lights do it by sheer intensity (while retaining the "jar" shape, mainly for installation compatibility), and as GdD pointed out there are more aerodynamic options available. The Fresnel lens method is usually adequate for smaller/slower aircraft however, and the drag penalty at relatively low speeds isn't significant enough for designers to worry about.


What is "Carson cruise" and can I determine it myself?

From this highly reliable web page http://www.eaa1000.av.org

source

The maximum range airspeed may be the most economical way to fly, but it's quite slow... Hence, we may consider the speed for which the ratio fuel flow/knot of airspeed reaches a minimum. That speed, marked by the point where a line from the origin is tangent to the thrust required curve, is called 'Carson's speed'.

source

In s/l flight, drag = thrust. Hence, the lower graph shows the 'thrust required' curve. There is a fixed ratio (1,316) between the maximum endurance speed, maximum range speed and Carson's speed, so that:

Maximum range speed = 1,316 x Maximum endurance speed

Carson's speed = 1,316 x Maximum range speed

That's a simple way to find Carson's speed for your plane. You can get the value of the Max. range speed from the POH


Does night takeoff and landing currency also count for daytime currency?

Yes, night landings satisfy the requirements of §61.57(a)(1) for both nighttime and daytime currency.

The wording of the regulation is not crystal clear, and some (including me) have thought that the wording of (a)(2) required daytime landings for daytime currency. This is not the case, per the following interpretation from the FAA (Springfield, IL FSDO, dated 16 April 2014):

  Please note, in this section, there is no mention of a day or night differences in the regulation.

  In 14 CFR 61.57(b) the regulation specifically mentions the requirement for Night Takeoffs and Landings and the time period they can be completed in.   If the passenger carrying will be done at night, the additional requirements of 14 CFR 61.57(b) must be met.

  The night takeoffs and landings, as required by 14 CFR 61.57(b), will qualify for both the day and night requirement, provided all other requirements are met.   The day takeoffs and landings will NOT qualify for the night (section (b)) requirement though.

  Stanley E. Swank II
  Aviation Safety Inspector
  Springfield, IL FSDO, AGL-19

Other sources on the Internet agree with this interpretation:

Eric Gideon at askacfi.com:

  ... your three takeoffs and landings to a full stop at night will also fulfill the recent flight experience requirements for day currency.

A thread at pilotsofamerica.com.


What is the busiest route of Turkish Airlines in 2017?

According to Turkish Airlines VP Ziya Taskent, they are adding a ninth weekly flight between Tel Aviv and Istanbul. He mentioned that:

  695,000 [passengers] flew roundtrip with the airline in 2014 [between
  Tel Aviv and Istanbul], 4.86% more than in 2013, making it the leading
  foreign airline in Israel

The largest number of airports served by the carrier is Germany with 14 destinations.

For other European carriers as requested, the Wikipedia article 'List of busiest passenger air routes' is a good starting point.


How do airlines plan their payload for multiple flights in a day?

The only real answer we can give to this is it depends

It depends on the airline, the nature of the legs (long/short), the specific airports/countries involved (re-fuelling costs can vary significantly) and route profiles used.

If the aircraft is doing 4 very short legs, the cost of carrying the fuel may be less than the cost of refuelling three times. Equally it's certainly faster to fill the plane once, rather than 1/4 fill it 4 times, because so much time is spent in setting up etc, and you can't usually re-fuel with passengers on board.

On the other hand, if legs 1 and 3 are nearly at the endurance of the aircraft, you physically can't carry fuel for legs 2 and 4: your fuel tanks aren't big enough. Equally on longer routes, the cost of carrying extra fuel increases dramatically.

Flip that around and say that legs 1 and 3 are short, but legs 2 and 4 are long (without being at the aircraft's endurance) and you have a problem that during legs 1 and 3 you're carrying a lot of un-neccessary fuel.

All airlines will have different policies and procedures, and those policies will vary wildly depending on the fuel costs at different airports. But I'll try to summarise at least some of the options here

For lots of short routes, fuelling once for multiple legs can be viable, but may not be the best option depending on fuel costs at each location. You wouldn't necessarily do a full day on one re-fuelling instance, but you may fuel twice for 4 legs, for example
For a short leg followed by a long one, it's rarer but the above will still reply
For a long leg followed by a short one, it's unlikely to be cost effective, although in certain situations your second leg's fuel load could be close to the reserve needed for the first leg: as long as you don't have to eat into your reserve on the first leg, you don't necessarily need to refuel.
For multiple long legs, you almost certainly don't have the fuel capacity, and carrying so much extra fuel for so long makes little sense: that's expensive.

In short, you usually want to fly with as little fuel as possible - a light aircraft is cheaper to run, can hold more cargo/luggage etc.

In a few situations, the cost of refuelling can make it worthwhile to fuel up in one location, where the cost of fuelling there makes up for the cost of carrying that extra fuel around. That's a niche scenario though, and wouldn't be the "usual" mode of operation for most airlines


Is Ducted fan or Propeller better to maximize forward speed of RC aircraft with a given power?

A propeller is better. Just look at any pylon race model to see what they use.
DKNguyen put it very nicely:

A duct doesn't beat a larger prop, it only helps make up for a smaller one.

You're right that "EDFs induce a greater delta V on the air they move" but EDF models generally use twice as much power as a similar aircraft with a prop. Moving half as much air twice as fast generates the same thrust but requires twice as much power. This is because momentum is mass x speed, but energy is mass x speed squared.
Ducts (and multi-blade propellers) are used when you don't have space for the right sized prop for the engine power.


How do contemporary Russian jet engines compare to Western ones?

200 kN range

They should not be necessarily behind. To compare apples to apples, the nearest similar thrust to the Aviadvigatel PS-90A1 (171 kN) is the General Electric CF6-6 (185 kN), but the CF6-6 is 20 years older, so it's not fair.

Comparing those two, the PA-90 has better specific fuel consumption in cruise, 0.595 vs. 0.646 lb/hr/lbf. Also the bypass ratios (BPR) are 4.4 and 5.76 respectively. So even with the lower BPR, which generally would imply a higher SFC, that's not the case.

                       PS-90A1     CF6-6       PW2040

Year                   1992        1971        1981
Thrust                 171 kN      185 kN      178 kN
SFC (cruise)           0.595       0.646       0.582
BPR                    4.4         5.76        5.9
Power-to-weight        5.9:1       5.08:1      5.57:1
OPR                    35.50       25.2        27.6

The low BPR but also low SFC, high overall pressure ratio (OPR), and high power-to-weight ratio, all hint at an advanced engine.

The A2 variant for instance was developed "in co-operation with Pratt & Whitney." If I had to guess, it's because the PS-90 fills a gap in the thrust range, gone since ETOPS put the tris and quads in decline. Nowadays the most common thrusts are 100-120 kN for the narrow-bodies, and the latest big jets are 350+ kN.

Big ones

We haven't seen a twin-jet wide-body from Russia, say like the 777 or A330. If that day comes, and they decide to build an engine and not out-source it, then we can have a better comparison with what's available from the big three—GE, P&W, and RR—in the high thrust range.

100 kN range

The smaller Aviadvigatel PD-14 is still in development, and it's in the 120 kN range. Wikipedia says its SFC is 0.526, which is comparable to the CFM56 (not the new LEAP model), I'm not sure how reliable that figure is, because the references linked don't show that figure, instead they say:

  10-15% [improvement] relative to other contemporary engines of the similar thrust range and application.

And a geared-turbofan offshoot is planned, so they want to compete with the PW1000G.

Military (trivia)

To this day the Kuznetsov NK-32 remains the "largest and most powerful engine ever fitted on a combat aircraft."

RE comment for TBO:

PS-90A:

  In 1997 the Supplement to the engine Type Certificate was received to approve the aircraft engine field operation based on technical condition without fixed mean time between overhauls (MTBO) rates.

  The PS-90A became the first Russian aircraft engine that accumulated over 9,000 hours without any removal - the engine installed on Aeroflot's Il-96-300 accumulated 9,936 hours (source).

CF6-80 (what I could find):

  ... many CF6-80s make it to two or more shop visits, with overhaul intervals ranging from 10,000 hr. for those in short-haul operations to 40,000 hr. for those in long-haul (source).

Again, comparable.

RE comment for Wikipedia SFC figures:

That's an issue with Wikipedia, they mix and match sea level and cruise SFC. The links/figures I already included in the answer are both for cruise SFC, which you can compare. There's a question here about how the SFC changes with altitude and speed. So, to compare, you need to compare figures from the same situation.

Related: Why don't Russian aircraft sell as much as their European/American counterparts?


How do I calculate this question? mass and balance

The basic equation is $Weight \cdot Arm = Moment$, which also means that $Arm = \frac{Moment}{Weight}$.That is all you need to work through these problems.

Calculate your starting moment:
$2,850\ lbs \cdot 101.67\ in = 289,759.50\ lbs/in$

After you add 400 lbs, the new Moment is:$(2,850 \cdot 101.67) + (400 \cdot 97.8) = 328,879.50\ lbs/in$

And the new total weight is:$ 2,850 + 400 = 3,250\ lbs $

This produces a new CG of:
$\frac{328,879.50}{3,250} = 101.193\ in$

The difference in CG is:
$101.193 - 101.67 = -0.477\ in$

Or, doing this all in one big equation:
$$
\frac{(2850 \cdot 101.67) + (400 \cdot 97.8)}{(2850 + 400)} - 101.67\ in = -0.477\ in
$$


As I understand it, aileron reversal can occur in two regimes of flight, high speed and near stall.

Near stall, the wing is very near it's critical angle of attack. When the pilot inputs let's say a left turn, the right aileron will deflect downward. This downward deflection increases the AOA of the outboard portion of the right wing such that the lift generated by that outboard portion is lower (drag higher as well) than the lift generated on the opposite wing.

You can kind-of visualize this by thinking of a wing operating on the backside of the lift curve.

Aileron reversal at high speed, (again, as I understand it) can occur when the wing isn't stiff enough tortionally to prevent twisting about the aircraft's lateral axis when a control input is made. Some old warplanes had this characteristic at high speeds.

For example, the pilot makes a control input to the left. The right aileron deflects downward, and instead of raising the overall AOA of the wing, it lowers it because the aileron acts like a trim tab, twisting the wing's leading edge downward.

Your question is asking: If the control reversal velocity is calculated to be higher than the divergence velocity, (When the wing breaks apart), how then can control reversal take place in flight?

The answer would be when it occurs near a stall.


What does it mean when an airplane has the "super" call sign?

According to ICAO wake turbulence categories, it is a super-heavy aircraft capable of generating correspondingly super-strong wake vortices. Currently the only "super" aircraft are the Airbus A380 and Antonov An-225 (FAA).


What is this fuselage-side aerial used for?

One of the features on this American Airlines Airbus A319 is the Gogo Inflight Internet.

  The A319 will be the first aircraft type in American’s fleet to use Gogo’s ATG-4 Wi-Fi solution. ATG-4 uses more antennas and better data compression technology to provide more robust and reliable service (August 15, 2013).

There's another one on the other side. It's part of the air-to-ground (ATG) internet setup that utilizes Gogo's cellular towers in the US and Canada, as opposed to satellite internet.

(Left; Right) Click to view.

Related external links:

Inside a Gogo ATG4 installation
How Does In-Flight Wi-Fi Really Work?

Thanks to @mins for their help on the answer.


What requirements are there to fly into KDCA?

The "special notices" are a section of the A/FD, but those notices paint a very incomplete picture. Both the FAA and the TSA have requirements regarding DCA. The very basics are that you must get authorization from both the FAA and TSA 24 hours in advance, you must have an armed security officer, and you must depart from a gateway airport.
The details are, well...
FAA Requirements

Regulations require an arrival/departure slot to be reserved via the e-CVRS system during most hours, just like other airports designated as high-density airports.
Flights to DCA must originate from within a 1250 mile perimeter (with limited exceptions).

TSA requirements
The TSA requirements are referred to as the DCA Access Standard Security Program (DASSP). An overview is given on this tsa.gov page (scroll down to "DCA Access Standard Security Program").
Details can be found in the Interim Final Rule... because calling the ruling both interim and final won't confuse anyone. Here is a summarization of that document:
Section A. Aircraft Operator Requirements:

Security Coordinators The aircraft operator (company) must have a designated security coordinator who is the point of contact for the TSA and is responsible for implementing the DASSP requirements. This person must undergo a TSA security threat assessment, and a fingerprint-based criminal history records check (CHRC) by the FBI.

DCA Access Standard Security Program (DASSP) TSA will verify the operator's status with the FAA before sending the DASSP document which contains "sensitive security information" which must be implemented, and the operator must notify TSA when all requirements have been implemented.

Flightcrew Members Each flight crew member (does not include flight attendants) must undergo a fingerprint-based CHRC and an FAA record check. Almost any prior restricted airspace violation is an immediate disqualifier.

Flight Approvals Each flight into or out of DCA requires a tentative slot reservation from the FAA (see FAA reqirement #1 above) before applying for TSA authorization. The TSA application must be submitted at least 24 hours prior to aircraft departure. If TSA approves the application, they will transmit this to the FAA which will issue the final slot reservation, and TSA will notify the operator. The TSA only approves up to 48 applications for a given date. The application must include (once per round-trip):

The name, address, and citizen status of each passenger and crewmember (includes flight attendants).
The registration number of the aircraft.
The flight plan.
and... "any other information required by TSA."

Operating Requirements Each flight must:

be operated under instrument flight rules.
not make any intermediate stops between the designated gateway airport and DCA.
have the aircraft, crew, passengers, and property inspected by TSA prior to departure.
have the cockpit door closed and locked for the duration of the flight (if equipped).
have at least one armed security officer who meets the armed security officer requirements.

Costs TSA charges \$15 for each crew member and passenger to conduct the security threat assessment. Additionally, they charge for the screening services at the airport which are "currently estimated at \$296 per round trip into and out of DCA."

Protection of Sensitive Security Infonnation The specific security details described in the DASSP must only be shared on a need-to-know basis.

Other Security Procedures This paragraph basically says TSA can make you do whatever they want... as if that wasn't obvious by this point in the list.

Compliance TSA can perform any inspection or test it wants to verify the operators compliance with DASSP.

Section B. Fixed Base Operator Requirements
That's right, there are two section B's, one right after the other... in case you weren't already convinced the TSA has strong attention to detail.
This section just says there are FBO requirements for gateway airports. Here is a list of Gateway Airports dated March 21, 2013. The list is always subject to change and seems to have more to do with whether there is an FBO at the airport who wants to put up the DASSP more than anything else.
Section C. Armed Security Officer Requirements
Armed security officers must be an active or retired law enforcement officer, or an individual approved by the TSA. This requires a similar background check as the security coordinator and law enforcement training which TSA finds acceptable. For more information, see the Armed Security Officer (ASO) Program.
Flying under Part 91?
At first glance it doesn't seem like there is any rule explicitly prohibiting Part 91 operations in 49 CFR Part 1544, and §1544.1 Applicability of this part. says:

(a) This part prescribes aviation security rules governing the following:
(1) The operations of aircraft operators holding operating certificates under 14 CFR part 119 for scheduled passenger operations, public charter passenger operations, private charter passenger operations; the operations of aircraft operators holding operating certificates under 14 CFR part 119 operating aircraft with a maximum certificated takeoff weight of 12,500 pounds or more; and other aircraft operators adopting and obtaining approval of an aircraft operator security program.

The general definition of an aircraft operator is not limited to an operator with a Part 119 operating certificate. However, the possibility of using part 91 seems to be ruled out by §1544.101 Adoption and implementation. where it enumerates the scenarios TSA is willing to approve, and there are no Part 91 operations in the list.
There are probably a few reasons why Part 91 is not allowed. Most notably:

TSA has specific aircraft operator requirements which an individual operator is unlikely to be able to comply with, and
There are only 48 slots available per day, and therefore preference is given to commercial operators - the explicitly given explanation for why non-airline flights are permitted at all is to permit "operators, their customers, and affected local businesses to recover from the adverse economic impacts brought on by the events of September 11, 2001."

Other fun quotes:

TSA may, at its discretion, require any flight into or out of DCA under this subpart to comply with additional security measures.

and...

In addition, if TSA or the FAMS requires that the aircraft have one or more Federal Air Marshals (FAMs) on board, the aircraft operator must allow the FAM(s) onboard, at no cost to the Federal Government.

and...

TSA may, at its discretion, cancel any or all flight approvals at any time without prior notice to the aircraft operator. For example, if the threat level in the Washington, D.C., area or in the vicinity of any of the gateway airports is set at ORANGE or RED, TSA is likely to cancel any and all flight approvals into and out of DCA.

So, happy flying everyone!


Since when had fighter jet designer stopped caring about Whitcomb's Area Rule?

Of course designers still care about the area rule, but it has become less important. The original area rule is just for Mach 1; at higher speeds you need to look at the cross section distribution along the Mach cones - but which ones? They change with angle of attack and Mach number, and there is no single point anymore where performance needs to be optimized. Rather, the aircraft has to perform well from subsonic to mild supersonic speeds, and the higher speeds for which the older designs from the Fifties and Sixties were optimized turned out to be irrelevant in practical use.

Next, which configuration needs to be optimized? The F-106 carried exactly one type of AA missile. Today, the typical aircraft will carry everything from dumb bombs to laser illumination pods, with fuel tanks and electronic warfare pods thrown in between. With that kind of variety in accessories who can tell which configuration exactly it is which needs to obey the area rule?


What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?

According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off.

Exhaust velocity isn't generally quoted, perhaps because it only bears a loose relation to the performance characteristics of the engine. I suppose if you wanted you could find the area of the exhaust and use the density of air to find the velocity.

Looking specifically at the GE90, we see that it has take-off SFC (specific fuel consumption) of 7.9 mg/N-s. Using the formula $I_{sp} = 1/(g_o·\text{SFC})$ given in paragraph 4 of the Wikipedia article Specific Impulse we get a specific impulse of 12,285 s. If we use the cruise SFC of 15.6 mg/N-s, we get a specific impulse of 6,536 s. Is this an effect of the lower density of air, or simply of the lower fuel required in the cruise? I don't know.


Can a commercial pilot fly passengers for hire without an air carrier certificate?

If you are referring to the USA with the regulating body being the FAA, this link will give you your answer. All text below came from this site:
http://www.cfidarren.com/cpinfo.htm

Here is the answer I think you are looking for:

  If your customer approaches you, with his own airplane (either he rented it himself, or he owns it).  In this case you may fly for hire, because you are not offering your services as a package or "holding out" as a commercial operator -- air taxi or charter.

Answer details:

  Commercial Pilot Certificate

  A certificated Commercial Pilot is paid for services as a pilot, provided you meet the qualifications which apply to the type of operations you are doing.  Some of this flying is  controlled by Part 91, but most commercial flying is controlled by Part 119, 121, 125, 129, and 135.

  Do I need a new medical certificate?   To exercise your commercial pilot privileges, you must get a new medical (at least 2nd class) every year.  If your 2nd class medical expires (and becomes a 3rd class), you cannot exercise the privileges of a commercial pilot.

  What can I do with a commercial certificate?    The following are things commercial pilots can do under part 91 -

  Local scenic flights within 25sm of departure airport.
  Ferry Flights
  Crop Dusting
  Banner Towing
  Aerial Photography
  Fire fighting
  Go on to earn your flight instructor certificate

  Can I fly people around for money?  You can certainly be paid for being a pilot with a Commercial Pilot Certificate.  You may work for an Air Carrier certificated under Parts 119, 135 or 121.  You may not be the Air Carrier conducting the business without getting an Air Carrier Certificate.  When you offer a package (airplane, price, pilot), you are no longer operating under Part 91 and would need an Air Carrier Certificate. An example would be "Can you fly me to Atlanta?"   If you provide the aircraft, you need to have an Air Taxi Commercial Operator certificate under Part 135.  There are different rules regarding maintenance, airworthiness requirements, crew member qualifications, and expanded VFR and IFR equipment lists that make Part 135 very different from Part 91.

  So the answer is NO, you cannot fly people around for money.  A Commercial Pilot Certificate simply allows a pilot to be PIC engaged in a commercial operation (airline) that has an Air Carrier Certificate.  A pilot certificate does not entitle the holder to become a commercial air-taxi operator.

  If your customer approaches you, with his own airplane (either he rented it himself, or he owns it).  In this case you may fly for hire, because you are not offering your services as a package or "holding out" as a commercial operator -- air taxi or charter.


How to get registration from ICAO 24 bit code?

Apparently (according to these folks) some countries use an algorithm to generate their ICAO (Mode S) 24-bit IDs. For those countries, you can simply reverse the algorithm and get a registration given the Mode S ID.

Others countries allow you to download their registration database, complete with the Mode S identifier (you can get the US registration database here) - it's a simple matter to load this into your own software and map the ICAO/Mode S ID code to a registration number.

As the folks at Gatwick Aviation Society have noted, some countries are tight-lipped about how their codes are assigned (and who they're assigned to), so these lookups are probably not complete.