When crossing a mountain ridge at low height above terrain what consideration is given to turbulence? Some background: I am not a pilot, but a researcher studying air-flow across mountain terrain using Computational Fluid Dynamics (CFD).

During my studies, I have been particularly interested by high-altitude ridges (2 to 3000m a.s.l., say 10000 ft) that are exposed to regional winds that are generally across the ridge. I have been seeing very interesting turbulence patterns above, and in the lee of the ridges (rotors).

Here is the question: what instructions are given to pilots needing to cross such a ridge at low heights above the terrain, in either direction relative to the wind? What does common sense dictate?

I am naturally thinking mostly of helicopter pilots doing mountain work, but any other aircraft are very welcome (fire-fighters, ULMs ...).

From an older FAA Publication on mountain flying:

Mountain Wave
When the wind speed is above about 25 knots and flowing perpendicular to the ridge lines, the air flow can form waves, much like water flowing over rocks in a stream bed. The waves form down wind from the ridge line and will be composed of very strong up and down drafts, plus dangerous rotor action under the crests of the waves. If enough moisture is present, lenticular clouds can form to give a visual indication of the wave action. These clouds are reported in the remarks section of hourly sequence reports as ACSL (altocumulus standing lenticular) or CCSL (cirrocumulus standing lenticular)

Pretty much sums up what you're seeing I guess. As for action:

Ridge and Pass Crossing
On most mountain flights, you will need to cross at least one ridge or pass. Experienced pilots recommend crossing a ridge or pass at the ridge elevation plus at least 1,000 feet. If the winds at mountain top level are above 20 knots, increase that to 2,000 feet. Plan to be at that altitude at least three miles before reaching the ridge and stay at that altitude until at least three miles past it. This clearance zone will give you a reasonable safety zone to avoid the most severe turbulence and down drafts in windy conditions.
If conditions or airplane performance dictate, you may need to fly along the windward side of a ridge to find updrafts for gaining altitude before crossing a ridge. You may also need to circle before reaching the ridge if climbing out of a valley airport. When you actually cross a ridge, you should do so at a 45• angle to the ridge. This allows you to turn away from the ridge quicker if you encounter a severe downdraft or turbulence. Once you have crossed the ridge, turn directly away from it at a go• angle to get away from the most likely area of turbulence quickly. Plan your crossing to give yourself the ability to turn toward lower terrain
quickly if necessary.

As for helicopter mountain flying specifically, I'm afraid you are unlikely to find somebody on here with that experience. You might have better chance on searching for it online and phoning up flight schools that offer such courses (I found a few in the states) for information.


Do pilots need special training to land Lockheed U-2 reconnaissance aircraft? Do pilots need special training to land Lockheed U-2 reconnaissance aircraft as its body is too shallow and is a wide wing aircraft?

All military pilots will receive special training when learning to fly a new aircraft, whether that is in a simulator, a 2 seater version of the same aircraft, or both. An F-16 pilot is not going to fly a F-15 without considerable training. Training is recommended even for light aircraft as well when learning a new model.

If you are asking whether the U2 is harder to land than many other airplanes then yes, the U2 is a very challenging aircraft to land because of it's shape, but also because the pilot can see very little of the runway. In fact the procedure for landing a U2 calls for a vehicle to speed down the runway behind it with another U2 pilot in it to talk the flying pilot down.


Is making faster military fighters even practical? I was curious after reading a book on Cold War plane development about the Soviet ideology of making increasingly powerful jets that could break Mach 3 and so on. With the advent of BVRAAM's such as the Meteor and lasers that are already being scaled down, what are the advantages to increasing the top speed or cruise speed of a fighter jet?

In the 1960, faster fighters were seen as desirable as they could outrun 1960s technology missiles to an extent (see the reports of SR-71s being fired on but never hit by SAMs). Of course, missile manufacturers saw this and developed faster missiles and longer-range radars to negate this advantage. And as it is much easier to make a missile faster than it is to make a large aircraft faster, that's an arms race the airplane manufacturers always lose.

While high speed allows you to outrun missiles, it also reduces maneuverability: at Mach 3, a constant-G turn will be much larger than turning at the same G level at Mach 1.


What is the voltage and amps on the transformer rectifier? On the Airbus A320 ECAM electrical page there are two TR or transformer rectifier and they convert AC or alternating current electrical power into DC or direct current electrical power. But on the ECAM it shows that TR has Volts and Amps but what are they? Are they the new DC current coming out of the TR or the AC current entering the TR? On the subject of Electrics, what even is AC/DC BATBUS and what is there purpose?

What is displayed would be the OUTPUT of the TR.  Those values tell you that (if) the TR is working, and how hard it is working.  If it's supposed to be putting out 12 volts but the actual output is something significantly different, then the TR isn't working correctly.  The amps value tells you how much DC current is being demanded of the TR -- maybe something on the associated bus is consuming too much current (and is thus perhaps the source of smoke that you're trying to eliminate), or another TR in parallel has failed.  The failed TR would show 0 volts & 0 amps, while the operative TR would show normal volts & higher than normal amps.

If you want to know the voltage of the input to the TR, you'd select the generator or bus that is powering the TR to see that.  Voltage is typically a value of a bus, while amps is generally the value of a source, such as a generator, battery, or TR.  The source is putting out however many amps. While you can measure the voltage of a bus coming directly off of an electrical source (so we talk about a generator's or battery's or TR's voltage), you can also measure (and will frequently care more about) the voltage of a bus, independent of what source is powering it right now.  If my Essential DC bus has normal voltage, then the important things on it can be expected to continue operating, regardless of whether the current source for that bus is the #1 TR or the #2 + #3 TR's in parallel or the battery charger or the main battery or the standby battery or whatever else that particular aircraft may have.


Why are fighter pilots seated and not reclined? I have always wondered about this one.

Pulling high G-forces is, I assume, the most physical challenge the pilot of a high performance fighter aircraft needs to be able to sustain without blacking out or worse. Indeed, in most countries in the world would-be fighter pilots cannot qualify for jet-jockey status without first passing the dreaded centrifuge test.

Further, modern jets can in fact pull much higher G-forces than the pilot without damage to the aircraft. Indeed, I read somewhere once that one of the issues with auto-missile-avoidance systems is that the aircraft is limited to performing maneuvers that the pilot can withstand.

As I understand it, the issue with high-G turns is that the blood is drained or driven from the body to the lower (and upper when undergoing high negative-G) extremities of the body resulting in starvation of the brain and consequent blackouts or even embolism in the negative-G case.

I realize pilots wear tight G-suits which have air pumped into them to restrict the amount of blood that can accumulate in their lower extremities, but that doesn't reduce the stress on the heart trying to pump blood up to the head.

It seams rather obvious to me that much of the issue with high-G is caused by the seated position of the pilot. It is an established fact that the more vertical you are the harder it is to withstand G-forces.

Why then are modern jets not designed with the pilot in a more reclined position? For example, the position used by Formula-1 race car drivers:

Don't get me wrong, I understand F-1 drivers do not need to withstand the same kind of sustained vertical forces that a fighter pilot does, however the legs-up reclined position seems to be far more practical for a high-G environment.

I do realize that the whole ejector-seat mechanism would need to be redesigned to eject the pilot differently, and that the pilot's ability to see behind himself would be compromised without technical aids, but surely the increased turn rates and lower profile cockpit would outweigh those issues.

NOTE: I have seen other posts on using the head first "prone" position, including this stack-exchange link but nothing on reclined.

ADDITION: Found this image of a Foka-5 glider pilot position too... note how wonderfully streamlined it is.

Fighter pilots are far more reclined than you might think.

But seat angles are limited by the need for good visibility.  Pilots frequently need to look to down-and-sideways, down-and-forward, and towards their back-quarters.  These are angles that your F-1 Driver couldn't possibly see.

Also, High-G maneuvers are quite rare.  Most modern military planning is about making sure pilots are never even in a position where they need to evade a missile or dog-fight another plane.


Unusual airplane shown in Google Earth At first I thought it was a plane that had its wings removed for some reason.

Then I thought it might be a lifting body, but I don't think so. Any ideas? This was in Jacksonville, FL; I can provide the coordinates if needed.

The United States Marine Corps is facing a dilemma. There are not enough combat ready fighter jets for their pilots around the world. Boeing's answer to this problem is the C+ Program. The program will take 30 retired F/A-18C Hornets and upgrade them to the most state-of-the-art platform, creating the F/A-18C+. (boeing.com, 2016, emphasis added)

Three years after that Boeing article, only three upgrades were completed. Given the slow pace, that's enough time for isolated airframes to be spotted as they're readied for transport and upgrades. The initial volume of 30 was reduced to 25 then 19, enough for one squadron (seapowermagazine.org, 2019). The photo above ticks all the boxes: the missing and covered nose radome, covered canopy, and missing wings.


Is the Progress D-27 really a propfan? On WikiPedia, the Progress D-27 is described as a propfan engine and the airplane it powers, the Antonov An-70, is described as propfan-powered.

This Air&Space magazine article contradicts that; it states that the An-70 is turboprop (not propfan) powered, as is the Airbus A400M.

And indeed the basic configuration of the D-27 and the A400M's
EuroProp TP400 turboprop are very similar:

4.5 m propeller diameter vs 5.2 m
14,000 hp vs 11,000 hp
two-shaft gas generator, third shaft to drive propeller
propeller driven through reduction gearbox
propeller blades mounted before engine
780 km/h max speed vs 780 km/h cruise speed
variabele pitch blades

The only significant difference is that the D-27 has two contra-rotating blade sets while the TP400 uses only one blade set.

Having contra-rotating propellers does not necessarily make a turboprop a propfan though (e.g. the Tu-95 is a turboprop aircraft).

So all things considered, what would make the  D-27 really a propfan but not the TP400?  Is there really a distinction between a propfan and a turboprop?

It depends

It seems that currently the term is more a marketing term than an engineering term.

  When fears arose among airline officials that the word “turboprop” would meet with consumer resistance, the term “prop-fan” was used in a poll of United Airlines passengers. It worked: 50 percent of the respondents said they’d fly on a prop-fan-powered airliner.

From Air&Space Magazine

It depends which particular features you consider an essential defining element for the term "propfan".

Turboprop?

Many features of propfans are also found on some powerplants described as turboprops by their makers.

Contra-rotating blades? - Found on, for example the 1950's NK-12 turboprop.
Supersonic tip speeds? - NK-12 turboprop again.
More than four blades?

The 1945 RB.50 tuboprop had five.
The 1984 PW127 turboprop drove a six-bladed prop.
The 2009 TP400 turboprop drives an eight-bladed propeller.

Curved blades? - TP400 turboprop.

Gearbox?

No Reduction Gearbox - A key distinction of a key forerunner, the GE36, was the lack of a reduction gearbox as used in turboprops.

Other names?

GE called their GE36 an unducted fan (UDF).
Rolls Royce call their design an open rotor engine.
The acronym CROR is also in used; it stands for Contra-Rotating Open Rotor.

D27?

The marketing department of its designers call it a propfan.

Dictionaries

Oxford says

Turbofan: A jet engine in which a turbine-driven fan provides additional thrust.
Turboprop: A jet engine in which a turbine is used to drive a propeller.
Propfan: No results found for “propfan”.

Mirriam Webster - same as Oxford essentially.

Aviation terms.com also defines turboprop etc but not propfan.

Aviation Canada, and other aviation glossaries I looked at, also fail to mention propfan.

Dictionary.com says

  propfan. Aeronautics. a turbojet having a turbine-driven propeller that operates completely outside the jet engine.

But that definition could equally apply to any turboprop.

Wordsense.eu says

  propfan (pl. propfans)
  A modified turbofan engine with the propeller outside the engine duct of an aircraft.

Which is not any better.

Wikipedia tries to define propfan in terms of what it isn't. The nearest it gets to a definition is to say that a propfan has a propellor with many short twisted blades. However the TP400 has many twisted blades - perhaps they are not short enough to qualify?

Conclusion?

There doesn't currently seem to be a clear and short widely-agreed or widely-used set of engineering criteria for labeling a particular engine a propfan rather than an advanced turboprop, an unducted turbofan, an open rotor engine or any other name.

The D27 is a propfan because its maker's marketing department calls it that.


What's the right way to take a dog in a small plane? I would like to take my friend and his dog for a flight. What's the safest way to take the dog? and what's a good way to protect the dog's hearing?

My wife and I have a small dog that frequently travels with us.  We place her in a soft-sided kennel that straps onto the seat belt.  While this doesn't do anything to help in a sudden impact (i.e. crash), it does help to keep her from being bumped around in case of turbulence.  For larger dogs, I know that there are seat belt tethers out there.  My suggestion would be to strap them to a rear seat.  In either case, the big issue is to ensure they don't get in the way while operating the aircraft.

As for the hearing part, a company called Safe and Sound Pets makes a product called Mutt Muffs.  Their website is here.  My wife and I bought a pair, but our dog wouldn't have anything to do with them and they were returned.  Of course, your dog may be better behaved :D


What is nominal n1 and fuel flow behavior during cruise climb? For a commuter airliner (e.g. 737) performing a cruise climb (step climb) at a constant Mach, how should n1 and fuel flow behave?

I know that fuel flow should decrease with altitude (air fuel ratio).
Should fuel flow also decrease with weight (fuel burn off)?

Is n1 kept constant? Does it decrease with weight? Is it unaffected by altitude?

If it depends, then what systems (fadec?) or settings (rated/non-rated mode?) are responsible for setting the behavior.

I know that fuel flow should decrease with altitude (air fuel ratio). Should fuel flow also decrease with weight (fuel burn off)?

Yes, the lighter you are the less N1 you need to maintain the cruise Mach at your cruise altitude. This will result in a lower fuel flow (at fixed environmental conditions, lower N1 always results in lower fuel flow).
The maximum climb N1 is however independent of weight (see below for details), therefore also fuel flow will be independent of weight during climb (but the resulting climb rate is different).

  Is n1 kept constant? Does it decrease with weight? Is it unaffected by altitude?

The cruise N1 depends on both weight and altitude. The following table from the 737 NG QRH (Performance Inflight - Flight With Unreliable Airspeed) shows the target N1 values for various weights and altitudes:

As you can see, N1 goes up with weight and up with altitude. The maximum climb N1 has a more complicated behavior as function of altitude, as the following table from the 737 FCOMv1 (Performance Inflight - General) shows:

The last 3 columns all show climb at fixed Mach 0.78. The N1 usually goes up with altitude, but then slightly down again for the highest altitudes (this is because TAT goes down with altitude up to the tropopause, but stays constant above). The maximum climb N1 is independent of weight, but the actual weight will affect the resulting climb rate.

  If it depends, then what systems (fadec?) or settings (rated/non-rated mode?) are responsible for setting the behavior.

For the Boeing 737, the required fuel flow to reach the target N1 is computed by the EEC (Electronic Engine Control, see also What are the differences between FADEC and EEC systems?). From the 737 NG FCOMv2 (7.20.5 Engine System Description):

  Electronic Engine Control (EEC)

  Each engine has a full authority digital EEC. Each EEC has two independent
  control channels, with automatic channel transfer if the operating channel fails.
  With each engine start or start attempt, the EEC alternates between control
  channels. The EEC uses thrust lever inputs to automatically control forward and
  reverse thrust. N1 is used by the EEC to set thrust in two control modes: normal
  and alternate. Manual selection of the control mode can be made with the EEC
  switches on engine panel.

The target N1 calculations are performed by the Flight Management Computer, but can be overridden by the pilot. From the FCOMv2 (11.32.2 Flight Management):

  Thrust Management

  The autothrottle operates in response to flight crew mode control panel inputs or
  to automatic FMC commands. Reference thrust can be selected on the N1 LIMIT
  page. Automatic FMC autothrottle commands are made while VNAV is engaged.
  The autothrottle system:

And further (11.41.28 FMC Takeoff and Climb):

  N1 Limit Page

  [...]

  Normally, N1 limits are automatically specified. Pilot selection of other limits is allowed.

  Pilot selection of a reduced climb mode does not change the automatic selection
  for other phases of flight.

  Pilot selected mode is automatically replaced by AUTO selection when the
  autopilot next changes vertical mode.

  The active thrust limit is used by the autopilot and is displayed on the thrust mode
  display.


Does EASA (or other regional/national aviation safety authority) have a confidential "close-call" reporting system? Not being a pilot, learned about the ASRS from this site. Have any aviation safety authorities in other countries attempted to emulate the ASRS and to what degree was it successful? After all it was introduced in 1976, which is quite a while ago.

I'm mostly interested in European airspace, although examples from other countries/regions are welcome.

I don't know much about the ASRS, so but I think this is a similar program... It's obviously not Europe, but Australia has the REPCON scheme for confidentially reporting all sorts of safety matters. The ATSB do a bit of investigating, seeking a response from the airline/whatever organisation is involved, and then publish the responses to a couple in CASA's Flight Safety Australia magazine.


Would large, low aspect ratio fan/propeller blades be more efficient? Why not? With reference to this answer
We read in the above that accelerating a larger mass of air by a smaller amount is (theoretically) more efficient. This is an advantage of a larger propeller than is current practice.
However, we also read above that at some point in increasing size, parasitic drag begins to outweigh the benefits of a larger airmass.
So, what if we used propeller/fan blades with larger surface area, but did not maintain the very high aspect ratios we see today? This would increase Reynolds number, decreasing skin friction drag coefficient. In fact, large-chord fan blades are already being rolled out, though more for simplicity than the above reason.

In the case of propellers, there are a number of operating and design considerations which must be taken into account here.
For example, while it is true that imparting a small impulse to a large mass wastes less energy than adding a large impulse to a small mass using a propeller, there are practical limits to how big you can make the propeller diameter before the ground handling of the airplane becomes unmanageably awkward. A huge prop might be more efficient, but you'd need a boarding ladder to get into the cockpit behind that huge prop and the landing gear struts would need to be longer and stiffer and hence heavier. Oh yes- and the prop itself would be significantly heavier.
In addition, to drive a big prop slowly and still develop full power with a piston engine requires a reduction gear of some sort between the 2400RPM of the crankshaft and whatever the turning speed of the slow prop was. The gearbox adds weight and requires an overhaul when its time is run out, and that adds cost- and a potential failure mode that is absent in direct-drive designs.
Finally, gearing down the engine to turn a slow prop magnifies the transmitted torque to the prop and therefore from the prop to the engine and airframe. That reaction torque has to be countered somehow or else opening the throttle will not just turn the prop but also roll the aircraft backwards around the crankshaft axis.
100 years of experience with geared and direct drive piston engines and all sorts of props has yielded the best solution in the form of a direct-drive engine spinning a variable pitch prop with a diameter that puts the blade tips comfortably subsonic at the max power setting of the engine.


How can I work out Sea Level Pressure from QNH? I am a little confused, is the QNH the current pressure at Sea Level? Therefore putting it into the altimeter will give your current altitude on the ground in pressure altitude relative to current sea level?
If not could someone please advise me on how to back calculate sea level pressure given the QNH and ground temperature.

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


What could lead to engine running hot on climb out? I own a turbo Saratoga with an intercooler and gami's. The engine has 100 hrs so it is broken in and I can't get the cylinder head temps below 400 during my climb out and I am operating at full power.

This has required me to run a very low power setting to keep the temp below 400, but my climb rate is terrible.
I am running full rich from take off thru climb.

Does anyone have any ideas on what the problem might be?

I'll provide a bit of a general answer to this so it covers for most cases. The basic question is, "my engine is running hot on climb out, what issues could lead to this"

Cooling Air: Airplane piston engines are air/oil cooled and require proper airflow over the engine to cool properly. A poorly installed cowling or obstructed/damaged air inlets can cause airflow to be incorrect. Some aircraft also have some internal baffling to direct air around the engine bay, these parts can move or get out of place and may be causing improper flow.

Fuel: As JScarry mentions, you need to ensure you have proper fuel flow (which is key to temp control). If you don't have a flow meter installed a shop should be able to check this for you.

Procedure: This sounds like a new plane to you, are you following all book procedures for climb out? Turbo planes often have different op-specs than their NA counterparts. Its not uncommon for there to be full power limitations (e.g. full power 5-min continuous limit then reduce to XX MAP).

Timing: You may have a faulty magneto or your magnetos may be improperly timed causing an advanced ignition situation that would add stress (and heat) to the engine.

Auto-Turbo Settings: I believe the Turbo Toga's had self governing turbos but not all planes do. You should make sure the pop-off mechanism is set correctly and the turbo is operating with in its proper limits.

Set-Up: This is unique to your new engine case but it may be worth bringing in another A&P that is familiar with the Toga Turbo engine and set up to make sure everything is where it should be just in case the engine shop overlooked something.

Incorrect Indication: To add to "Set-Up", it's possible that your CHT is incorrectly calibrated or installed. (this is an edit from another poster, who experienced high oil temps to finally discover that the oil temp thermocouple was in the wrong location, and read higher than actual temps)


Why do the emergency oxygen masks for some 787 premium class seats have an extra 'streamer' to pull them down? Specifically, on Norwegian 787s, you are briefed:

  "In economy seats, reach for your mask. In premium seats, pull the
  streamer for the mask to drop the full distance."

(source)

Now the overheads in premium and economy appear identical:

(Source)

And the premium seats are merely larger versions of economy, they're not beds or fully-reclinable. They're set 2 - 2 - 2 rather than 3 - 3 - 3. I'm guessing there are just two masks in each premium overhead where there would be three in economy, but that doesn't explain the "streamer" (which is just a piece of red cord).

So in premium there's an extra step; of course you end up with exactly the same mask. It puzzles me what the thinking is for the "streamer".

So in premium there's an extra step

For what I see in the picture you posted, not really.

There is always the step "grab the mask and pull firmly" to bring it from the compartment to your mouth (and break the supporting cable, that is now unneeded, in the process).

What I understand from the third picture (first row) inside the image you posted is that they made this step "easier" for the premium class, they attached a string/made the usual one longer, so that you don't have to reach up to the mask pulling yourself up, but you can make the mask come down the full distance by pulling the extra string that will come down near to you. Similar to what is shown in this other card found by mins.

  I can't see how this is not an extra step though. Economy = grab the mask, Premium = pull the string, grab the mask.

I see where you are coming from, but in the economy version you still have to pull.
I agree that having to grab two objects in a sequence rather than only one seems to be counter-intuitive for an emergency situation, but without seeing the mechanism in action we cannot say much more than this.
The leaflets seem to indicate that the "premium" solution is made to be more comfortable to reach, nothing else.


What are the requirement for electrical grounding of aircraft in hangar storage? When an aircraft is being stored in a hangar between flights, are there any requirements to have the aircraft connected to an earth ground?

I believe that there is (although I'm looking for the reference), and if so, why and how should this be accomplished?  Is attaching a cable to the metal structure of the hangar an accepted practice?

In general any aircraft parked in hangar should be electrically grounded. The grounding should be done from basic aircraft structure to a low resistance ground.

Grounding of the aircraft is usually done to,

Protect aircraft and personnel against hazards from lightning discharge.
Provide current return paths
Protect personnel from shock hazard
Prevent accumulation of static charge

During de-fueling or refueling, the aircraft is grounded to the fuel truck.

In general, the aircraft are grounded to a point where the impedance is less than 10,000 ohms referenced to earth for static grounding. For power grounding, it should be less than 10 ohms to power systems neutral.

Aircraft are not grounded to the hangar structure as the external power supply, when connected, is grounded to that.

The best reference, if you ask me, would MIL- HDBK-274(AS)- Electrical Grounding for Aircraft Safety.


Why is a downwind wind speed given in wind checks for aircraft on final? I have a quick question about wind checks. When I spotting at Sydney (YSSY), I often hear tower clearing aircraft to land and giving them spot wind checks at the runway threshold, all pretty standard.

What I don't understand is when that wind check contains a downwind component.  For example:

QFA32, Wind 160/10 Downwind 2, Cleared to land RWY 16R

(That is an example and not a wind scenario I've heard.)

Am I overthinking this? Is the downwind component just given when they are landing with a tailwind?

'Downwind' speed does indeed mean that the aircraft will be landing with a slight tailwind. Each aircraft/airline has a maximum downwind component which is acceptable to land with - generally about 5-10 knots. From my experiences as a private pilot in Australia, I have observed that ATC will tell the flight crew the current downwind component without being requested to do so. I find that they will also advise if the crosswind component is more than about 5-10 knots. It has been a few months since I last heard it but I believe the wording is along the lines of "Qantas 32 heavy, wind 210 at 7, maximum downwind 7 knots, runway 01 cleared to land". This information is also available on the ATIS.


What is the equivalent of Hobbs and Tach in a Jet or Turbine Aircraft? As a piston pilot I am only familiar with tracking hours for my pilot log book and maintenance log book using the Hobbs and Tach times.  Can someone tell me what the equivalent is for a Turbine and for a Jet?

In all of our jets, we have two hobbs meters. One is active from left engine start and the other goes active when the weight of the aircraft leaves the wheels. Neither of these are used for logging our flight time. We do that "chock-to-chock", ie as soon as the chocks are pulled and we start moving to when the chocks are put back in the wheels.

The regs say you can log flight time from when "the aircraft moves under it's own power for the purpose of flight":

  Flight time means:
  (1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or
  (2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.


Why do gusts change the airspeed during landing, but turning (which changes where the wind is coming from) does not? As we know it, aircraft are extremely sensitive to gust during its final landing stage. They can lose airspeed if the wind changes to tail, ad thus loose lift or need to adjust their thrust in order o regain the lost speed. On the other side, the opposite happens when encountering a headwind.

On the contrary, we don't take that into account when we consider a fighter jet. When a jet fly's at 20,000 with headwind of 50 knots, turns 90 degrees to the left and has the wind from the right- is it considered to be a change in wind direction or because the whole system is moving along with the plane and the change is more gradual- the aircraft feels no change in actual lift which is derived from the true speed it feels?

Before I get into the question about the turning fighter jet, a few words about airplanes' being extremely sensitive to gusts during final stages of landing. If this was actually true, I'd be dead by now. Seriously. Airplanes are sensitive to gusts during landing, I'll give you that, but I think it's exaggerating beyond reason to say it is extreme.

Now, back to one of the most persistent myths in aviation:

The myth of the downwind turn

The jet in your question does not make a full turn from headwind to downwind, but it does not matter, the physics are the same as in the myth of the downwind turn.

As you stated the plane is moving along with the airmass. So, for the plane, aerodynamically there is no head-, side-, or downwind. Only us on the ground can feel it, as we are locked into to the coordinate system of the earth with our feet.

The airplane is locked into the coordinate system of the airmass, and as long as the airmass itself does not experience acceleration, the airplane flying in it can turn in whichever way, and it will not notice any change in the wind direction.

The direction of the wind is locked into the coordinate system of the earth, so in your example there is no change in wind direction. Not for the jet aerodynamically speaking, and not in the sense of navigation (relative wind changes, but you use the same formulas for the calculations anyway). What does change is the groundspeed of the aircraft, and this is the reason wind (and gusts especially) causes some concerns for landing planes. Takeoff is also affected, but not quite as much as landing.


Why is Heraklion airport a 'captains only' airport? I was recently watching "EasyJet: Inside The Cockpit" and it said pilots need 4 shoulder stripes to land at Heraklion International Airport, “Nikos Kazantzakis” (IATA: HER, ICAO: LGIR), but it didn't explain why they need the extra experience. Could someone please explain?

Along the approach path there is high terrain that causes turbulence and challenging winds. But it is also easyJet's call to classify any airport as they see fit. On a scale from A to C, they classify Heraklion as B. Other Cat B airports are where the takeoff performance calculations can be tricky as the excerpt below shows:

Category B translates roughly as "slightly out of the ordinary". A good example, according to Captain Martin Dudley, A300 fleet manager for Monarch, is Lanzarote: "There's a little bit of ground on the extended centre line that just comes into play in terms of our performance calculations for take-offs, and that makes it a 'Cat B'. There are a fair number of those."
The UK's biggest airline in terms of passenger numbers, easyJet, classifies Corfu, Dubrovnik, Heraklion, the Isle of Man, Jersey and Rome's second airport, Ciampino, as Category B.
(independent.co.uk)

It came up in the documentary (easy find thanks to searching through the automatic YouTube caption system) when a medical emergency made the crew check the alternate airports on the way, of which Heraklion, was classified by easyJet as captains only. The theme of the series seems to be pilots in training. They couldn't choose it because if the captain became ill, then the first officer would not be allowed to continue the approach.
Another over dramatized sequence in the documentary:
The crew received the latest weather report showing a crosswind that is gusting to 24 knots (if it's been gusting 34 knots then maybe it could have warranted the drama).
The crew consists of a senior first officer, Carl, and the captain is Chris Foster, the company's operations manager. Carl wanted to impress, but [due to company policy] the captain has to make the landing in those crosswind landing conditions, thus robbing Carl the chance to impress and gain experience toward getting his fourth-stripe (this is literally how they present it—a recurring theme in the show as far as I can tell).
(YouTube) Carl and Chris.
Nonetheless, there are tricky approaches out there, like London City, or Kai Tak (RIP). And many others where special training and approval are required. Added safety is not bad, even if it's for tricky windy conditions where experience and additional training do make a difference.

Charts source: http://www.aopa.org.il/userfiles/files/usefull/LGIR.pdf


How to distinguish the induced drag of a lift-producing wing from its pressure drag? Quote John Anderson Fundamentals of Aerodynamics:
"The three-dimensional ﬂow simply alters the pressure distribution on the ﬁnite wingin such a fashion that a net pressure imbalance exists in the direction of V∞ (i.e., drag is created). In this sense, induced drag is a type of “pressure drag."
Wing at 15°AoA  flying in the sky and producing lift same as wing weight...
AXIOM:
Fluid can exert forces at object only in two ways: pressure (normal force) and viscosity(tangential force)
Integration of all tangentional forces at wing ,gives resultant force.Component in backward direction-parallel with freestrem is SKIN FRICTION DRAG
Integration of static pressure over entire wing,gives resultant pressure force.Component of that force in backward direction-parallel with freestrem is PRESSURE DRAG.
What is than INDUCED DRAG, how distinguish induced drag(which is pressure drag) from pressure drag when wing produce lift?

There are multiple ways to decompose lift and drag forces, and they are unfortunately not compatible with each other.
If you know the flow field (for example because you ran a CFD simulation), then to compute lift and drag, you need to integrate:

pressure forces (i.e. local pressure times surface normal, over area)
viscous forces (local viscous stress times shear direction, over area) shear stress is tangential to the local surface, but because not all surfaces are tangential to the flight direction, this influences both lift and drag (though usually a lot more to drag).

That gives you two force vectors, and after you added them up, you can then decompose them into one component which is parallel to the inflow direction (drag), and one which is normal to it (lift). (let's forget spanwise forces for now...).
Looking at drag, you can then of course see which part comes from the pressure forces and which comes from the viscous forces.
Next, induced drag: This is actually a somewhat theoretical definition, and most people today speak about "lift-dependent" drag. This means: How much more drag is the airfoil producing because it produces lift? Assuming an uncambered airfoil, the lowest drag is at AoA=0°, when lift is also zero, so all additional drag we're getting at AoA=15° is lift-dependent.
Assuming a cambered airfoil, the lowest drag is actually not at zero lift, and also not at AoA=0°, so at lowest drag, it's actually producing some lift -- so ... negative lift-dependent drag! Just look at these drag polars:

Does that mean we have negative induced drag? Our definition is already becoming difficult to use. So let's keep the airfoil uncambered for now, meaning that the lowest drag is also at AoA=0, where we produce no lift.
So, under these circumstances, what happens to drag when we increase AoA? Of course, pressure on the upper side of the wing reduces, and it increases on the lower side. This means we're getting a pressure force which is pointing mostly upwards (lift) but also somewhat backwards (drag). But we're accelerating the flow on the upper side, which increases friction there. We're decelerating on the lower side, but that effect is a bit smaller. This means we're getting some additional friction drag. But that's not all! Because of the additional friction on the upper side, the boundary layer grows faster than it would otherwise, changing the streamlines, which in turn changes the pressure distribution, and causes additional pressure drag. This means: If we switched friction off now, we'd actually reduce pressure drag, too!
So, really, we can't point at the change in pressure drag and call it induced drag.
Now, if we make some more simplifying assumptions -- the kind that people used to make all the time when they were still using pencil and paper to design airplanes -- that's when things finally start to add up.
This means we're assuming simple potential flow, and maybe we're adding some estimate of viscous drag based on flight speed and surface area, which is not affected by pressure distribution. In that case, we would have no pressure drag on our symmetric airfoil at AoA=0°, and all the pressure drag we're getting at AoA=15° is purely because the pressure on the airoil is pushing normal to the surface, the upper side is also facing backwards to some extent, because it's at incidence to the flow. Now, all the pressure drag is indeed due to lift, and viscous drag is not affected by lift.
So, until now I was talking about "lift-dependent" drag. But what about "induced" drag? Even the Wikipedia article on induced drag doesn't make a difference between lift-dependent and induced drag, so how big can it be? Fairly large, actually. The most common definition for induced drag is the drag generated because the wing produces trailing vortices. So all the kinetic energy in the wing-tip vortices (but also in the vortex sheet behind the wing wherever the lift is changing in spanwise direction) needs to come from somewhere, and that's called induced drag. At least in simplified physics, that is indeed completely pressure drag -- but it does not necessarily explain all pressure drag. Imagine for example an infinite wing. No change in lift distribution, no trailing vortices, but it must have some pressure drag! Mathematically, this can be solved by assuming that when the wing accelerated or increased AoA, it generated a parallel vortex which it left behind, and keeps feeding via two imaginary wing-tip vortices at infinity. But if you measure a 2D profile in a wind tunnel or simulate one with modern CFD methods, the lift-dependent part of drag is much larger than that, because the theoretical induced drag is pretty small next to all the real effects which happen on top of it.
Now, if you take one more step towards reality, and include cambered airfoils, viscosity, boundary layer displacement, and if you're going fast enough also compression shocks (which produce "wave drag", which another factor influencing viscous and pressure drag...) -- that's when "induced drag" becomes fairly theoretical.
So why on earth does anyone still use it? Precisely because it is simple to compute in simple physics models, where you ignore a lot of real effects. That's when it still does tell you what the lowest achievable lift-dependent drag for your wing shape would be, if all those nasty interactions between pressure field and boundary layer, separations, shock waves and other complications did not exist. This means: Induced drag is a useful construct to explain why lift always produces drag, why long slender wings can be more efficient at producing lift, and how much more efficient. But in a real flow, there's not really a way to extract it separately.
Footnote: Of course there are methods to at least approximately extract the different drag components. The best-known tool to do this is Onera's FFD tool (which only few people outside Onera get to use...). I did not find the original paper quickly, but here is the extension to unsteady flow. You can see the math becomes pretty complicated very quickly. You can also see that they provide a lot of drag components, but a closer look shows that although they include induced drag, and a lot of other components, they're not all adding up to total drag -- that's because there are lots of ways to decompose drag, and most of them don't neatly align.


In light aircraft, non-instrument flight, apply (nearly) full aft elevator before beginning the takeoff roll or at some predetermined speed? Consider a small, light, nose wheel tricycle aircraft, something along the lines of a C152 or similar, sitting at the end of the runway ready for takeoff, with no issues and with all pre-takeoff checks completed.

In general, what is the correct procedure for take-off with regards to the elevator and nose wheel?

One instructor I've flown with taught takeoff as apply nearly full aft (nose up) elevator before beginning the takeoff roll. After rotation occurs, accelerate further to and maintain best climb speed, limited by maximum flaps extended speed (which for the particular type I'm flying are identical). This approach has the obvious advantage of getting the nose wheel off the ground as soon as there is sufficient lift for that, and ensures that one can look straight ahead at the runway, not really having to worry about the instruments at all before rotation, but means you need to be careful to not apply too much aft elevator at the moment the nose wheel lifts off the ground (since you don't yet have sufficient speed to actually fly, and by rotating too quickly you'll end up with a tail strike on the runway).

Another instructor, in the exact same aircraft, advocated neutral elevator until around the maximum speed the nose wheel can take, then slight aft elevator to raise the nose wheel, and more aft elevator for lift-off only when approaching speed of best climb. This has the advantage of keeping you near the ground longer during the takeoff roll, but requires keeping pretty close tabs on the instruments (and possibly adjusting from airspeed to ground speed, because the nose wheel is concerned with ground speed which is likely less than the indicated airspeed because the airplane is facing a headwind component). To me, this seems more complex.

Obviously in both cases, after lift-off, the important number becomes the indicated airspeed (a proxy for which can be engine power and airplane pitch), which should be near the speed of best climb to gain altitude as quickly as possible. That's the easy part.

But while the wheels are still on the runway, why would one prefer one style of takeoff over the other? Are there any particular situations in which either is clearly superior to the other, or is it merely a matter of taste?

It's a matter of the conditions you are faced with.

The first instructor was teaching you soft/rough field technique, where the focus is getting as much weight off the nosewheel as soon as possible to minimize its rolling drag on a soft surface. Besides the drag on the nosewheel, the thrust line acting above the drag axis of the mains on soft ground tends to drive the nose down, so you need to compensate for that as well. The other thing is having the wings producing partial lift from the nose being high also helps reduce the effect of the soft ground.

You might also use this technique if the surface is firm but has a lot gravel, that can get sucked into the prop, and you want the prop as far away as possible while the airplane is moving slowly.  I'll start a run on firm surface with loose gravel with the nose held high but let it come down a bit as speed is gained and there is less tendency for the prop to pull rocks up into it.

Thing is, you need the nosewheel for steering control until the rudder becomes effective, so you give up a lot of potential directional control with tail down technique early in the takeoff, so you only use it to the extent you need to.  Besides directional control, the nose in the air condition is very draggy and can delay acceleration, actually extending the takeoff run.

There are basically 3 procedures that I follow:

Soft/rough field - tail low all though the take off to keep weight off the nose wheel and as much weight off the mains as possible.  You may use flap in this case when you wouldn't ordinarily for the same reason.  You stay in ground effect to accelerate before climbing.
Firm but short field - tail low late in the takeoff and rotating early so the airplane gets to flying speed already at its takeoff pitch attitude. This is to have the airplane lift off at the earliest possible point and may include use of flaps that aren't normally used.  You stay in ground effect to accelerate before climbing.
Normal takeoff - elevator neutral or only slightly nose up, for best steering control and acceleration until rotation at the normal rotation speed.

Students using different instructors always discover quite a bit of variation in techniques as instructors apply their personal preferences.  A perennial problem in the industry.


Are the "centerbody", "fan hub frame", and "intermediate case" all the same thing or separate parts? I'm curious about the frames & cases at the front of an engine. I've seen OEMs call them the centerbody, fan hub frame, and intermediate case. Are these all the same thing or are these each a separate part? It seems that each OEM has their own vocab.

The fan case is the one I can figure out. I'm more talking about the area right behind it before the HPC

On Boeing aircraft, the "centerbody" is part of the exhaust system.  It is the inner is the inner part of the exhaust as shown in the picture below (the outer part being called the primary nozzle).  The centerbody is also referred to informally as the "plug".  Since your question seems to be asking about fan and compressor cases, this doesn't seem to fit.  I'm not sure what Airbus or other airframers call this part, so maybe the word "centerbody" is used differently in different contexts.  But for a Boeing airframe, that's the centerbody.

For turbofans which use a "quick engine change" concept, the fan frame actually splits into two pieces.  The "fan hub frame" referred to the inner part.  Hence the word "hub" in the name. I've marked it up in the image below.  This photo also illustrates the concept pretty well.  This shows an engine being assembled.  The outer part of the fan frame (with fan case attached) is on the right, and the fan hub frame (with the rest of the engine core attached) is on the left.  The "fan hub frame" proper is the part just to the right of the yellow tape.

The term "intermediate case" is ambiguous to me.  I could think of two or three different things that that might refer to. On a two spool engine, that probably refers to the booster case, which is usually immediately forward of the fan frame.  But on a three spool engine, it probably refers to the intermediate compressor case, which is usually just aft of the fan frame. Do you have more context?


Why do full service airlines prefer to run a separate low-cost carrier? There are various full service carriers (FSCs) who have a separate low-cost carrier (LCC) subsidiary. For example:

Germanwings by Lufthansa
Air India Express by Air India
JetKonnect by Jet Airways

Why do the FSCs prefer to launch a separate LCC, rather than providing seats at a cheaper rate with reduced comforts within their existing structure?

They still want to get into the low cost segment but want to market themselves differently for better penetration. You can only present so many images of a brand to customers: You can't show both the super-cheap prices with happy youth on them and posh first class at the same time.
They do not want to tarnish a nice brand by mixing it with a low cost carrier and what that is perceived to entail. For instance, Germanwings got a somewhat infamous reputation recently, and Lufthansa is probably happy their name was not more frequently used in media. Low cost is also associated with less good customer service and hidden fees.
Crew are often less well paid, and they are sometimes put under a separate working agreement and company to try to prevent conflicts. Some legacy carriers have very good pay which is difficult to reduce. Management does not want to be bound by these when expanding or adjusting their business.

Each case is pretty specific depending on exactly they are trying to do and achieve. For many American Carriers and Lufthansa, the last one is probably the principle reason.


Do non-towered airports commonly have ILS? Is it common, uncommon or rare for a non-towered airport to have ILS (Instrument Landing System) runways? Can a non-towered airport have ILS?

Nontowered airports absolutely can have ILS approaches, and it's fairly common, though of course not as common as at towered fields.

Two examples in my neck of the woods are KIKK (Kankakee, IL) and KMTO (Mattoon, IL).

Also, at many towered airports, the tower is not 24/7 and closes at night. If such an airport has an ILS, it's still available after the tower closes; ATC services for IFR approaches are provided by a nearby/higher facility like a TRACON or ARTCC, just like a nontowered field. Two examples of this in my neck of the woods are KCMI (Champaign, IL) and KBMI (Bloomington, IL).

I am not, however, aware of any Cat II or III ILS approaches at nontowered fields.

Protecting ILS Critical Areas is usually the responsibility of the tower, but even VFR aircraft are prohibited from entering the ILS Critical Area when the tower is closed:

  (1) Localizer Critical Area. Except for aircraft that land, exit a
  runway, depart, or execute a missed approach, vehicles and aircraft
  are not authorized in or over the critical area when an arriving
  aircraft is inside the outer marker (OM) or the fix used in lieu of
  the OM. AIM 1-1-9(k)


Certification of Aircraft Phone/Tablet Mounts/Holders I would like to know, whether it is required for such items to be certified for application on genaral and/or commercial aircraft:

https://www.rammount.com/part/RAM-B-166-MP1U

https://mgfproducts.com/products/sport-mount-flex-suction?variant=10434660040819

In what countries is it necessary?
Is it compulsory or not for them to be certified?

I would be gratuful for any help!)

For general aviation no certification is required, neither is certification required for at least some commercial operations like flight training, at least for the regulatory bodies I'm familiar with (FAA and EASA). I've seen Ram and Garmin mounts being used on some smaller passenger and cargo operations as well, usually GPS and tablets in older prop airplanes like the DC3, Beechcraft twins and the like.

As for carrier airline operations I am not sure.


Does a vortex/wake turbulence reach ground level nearby? Where I live not many houses are built under the immediate approach/takeoff line of airports. Instead, they use the area for, say, sports fields.

Is it possible, in otherwise no-wind conditions, for a vortex/wake turbulence caused by a landing/taking-off aeroplane to dislodge a cricket bail on a nearby sports field? How close would the sports field have to be to the runway for this to possibly happen?

In the case at hand the (horizontal) distance is about 1.5 km and the plane (737?) was landing (directly overhead) some time before.

According to the FAA vortices from the very largest planes generally do not drop vertically below 1000ft of the flight path. So, large aircraft flying less than 1000ft (300 meters) overhead have the potential of being felt on the ground.

This YouTube video shows a fatal aircraft accident after a medium sized prop airplane flew about 100ft above, and 40secs before the takeoff of a small plane.

The FAA addresses vortex generation and avoidance in AC_90-23G.pdf and the following information encompasses some of that information. In general, pilots are told to be weary of vortexes for 5min, 5mi, and 1000ft below. The following diagram supports this generalized rule...

Flight tests have shown that at higher altitude the vortices from large  aircraft sink at a rate of several hundred feet per minute (upto 300 fpm), slowing their descent and diminishing in strength with time and distance behind the wake-generating aircraft.

6 things determining vortex strength and behavior

1) Aircraft Speed - As the velocity of an aircraft increases, the strength of the vortices is typically reduced. That's because flying faster typically requires a lower angle-of-attack. It's also why takeoff and landing are critical times to pay attention to the flight path of large aircraft around you, because they're going relatively slow and creating large vortices.

2) Aircraft Weight - The heavier the aircraft, the more lift must be produced to sustain flight. Increased lift also increases vortex strength. The A380 below is a great example of this.

3) Angle of Attack - Higher angles of attack result in stronger vortices. You have a stronger difference between low and high pressure around the wing with a high angle of attack.

4) Wing Configuration - Clean configurations result in stronger vortices, because higher angles of attack are typically needed.

5) Proximity to Ground - When your wing is close the ground, wingtip vortices can't get as big, because as they spin around your wingtip, they impact the ground and dissipate. This is a major part of ground effect.

6) Wind - While wind doesn't change the strength of vortices produced, it does impact the way vortices affect you. Wind speed and direction determines how vortices travel and dissipate. For instance, if you have a strong crosswind, the vortices produced by airplanes will float in the direction of the wind.

The aircraft was likely flying near 250-275ft altitude
most approaches are 3deg
1.5km=4920ft
.0524 = tan(3deg)
height(ft) = dist(ft) * tan(deg)
257(ft) = 4920ft(dist) * .0524 (tan(3deg))


Are the VFR weather minimums in 91.155(d) redundant because of 91.155(a)? 14 CFR 91.155(d) specifies that:

  ... no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR, within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport ... unless ground visibility at that airport is at least 3 statute miles ...

But we already know from the table in 91.155 (a) that for Class B, C, D, and E, the minimum flight visibility is 3 sm. So it's not possible for this rule (d) to apply without already triggering the rule (a).

My question is then what's the point of this additional rule? Am I misinterpreting the text?

Short answer: pilots need flight visibility, ATC needs ground visibility.

There are two different definitions of visibility in the regulations you mentioned: flight visibility and ground visibility. They're defined in 14 CFR 1.1:

  Flight visibility means the average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent
  unlighted objects may be seen and identified by day and prominent
  lighted objects may be seen and identified by night.

  Ground visibility means prevailing horizontal visibility near the earth's surface as reported by the United States National Weather
  Service or an accredited observer.

The practical difference is that only pilots can determine flight visibility; ATC and/or ground stations (AWOS/ASOS) determine ground visibility. The regulations aren't redundant because they're talking about different things and also about different needs:

91.155(a) applies to all airspace. At 5000' (or whatever), only flight visibility makes sense to determine if conditions are VFR.
91.155(d) applies to terminal (airport) airspace. That means ATC is also very interested in whether conditions are VFR or not, and they can only work with ground visibility.

Note that the full text of 91.155(d) also covers the case of uncontrolled airports with no weather reporting (emphasis mine):

  (d) Except as provided in §91.157 of this part, no person may take off
  or land an aircraft, or enter the traffic pattern of an airport, under
  VFR, within the lateral boundaries of the surface areas of Class B,
  Class C, Class D, or Class E airspace designated for an airport—

  (1) Unless ground visibility at that airport is at least 3 statute
  miles; or

  (2) If ground visibility is not reported at that airport, unless
  flight visibility during landing or takeoff, or while operating in the
  traffic pattern is at least 3 statute miles.


What plane was this hotel room converted from? The below image is of a converted aircraft now functioning as a hotel of sorts.
I am having trouble identifying the type of the aircraft. It is not on the listing page either. Does anyone know what aircraft this was?


Why is stall speed listed in a POH? In this comment on this question about stall warning systems, it was stated that:

  You can stall at any airspeed (see this question and this one) and in any attitude, only the AoA is important because that's what causes the airflow to separate from the wing.

Why, then, does the Cessna 172's POH (Page ii or 2 of 422) list stall speed, not stall AoA?

I can understand how flying slower than the listed speeds would cause a stall, but the comment above seems to imply that one could stall the plane by flying at 60, 90 or even 120 KCAS but at a high AoA.

Those stall speeds are derived for specific conditions.  In the case of that POH those speeds assume level unaccelerated flight, 2550 lbs gross weight, standard atmosphere and no wind.  They are given as a speed rather than raw AoA because most non-aerobatic small airplanes lack AoA indications in the cockpit, while all have airspeed indicators.  A non-aerobatic airplane also spends most of its time in the part of the flight envelope where the assumptions needed to list a stall speed tend to be valid.

The stipulation of level flight means the relative wind is fixed, thus the AoA depends only on the pitch attitude above the horizon.  For an given weight there is a relation of pitch attitude required to maintain level flight for a given airspeed.  Given these specific stipulations, you can determine the airspeed that results in the critical AoA being exceeded.

If any of the criteria listed above are not true, then those published stall speeds are not meaningful to you.  You can stall at any attitude and at any airspeed.

To see the true relation between airspeed and stalling you need to consult a Vg diagram.

The curved lines starting at 0 MPH and 0 load factor represent the stall speed for a given load factor.  The stall speed listed in PoH you quote is specifically the point noted "Normal stall speed" on this chart (this chart isn't for the same airplane, so the numbers will differ).


How to use Rudder during takeoff in single engine propeller aircraft? e.g. Cessna 172 Background: I am a complete beginner on Flight Simulator X: Steam edition, using a simulator setup with rudder pedals with realism set to the max (to simulate more realistic flying).

I understand that rudder use is essential to align the aircraft to the runway centreline during takeoff due to the left turning tendencies. However, I cannot seem to consistently align the aircraft to the runway centreline during takeoff. Sometimes, I just run parallel to the centreline.

Questions:

Is this common in real life flying? Are actual pilots consistently able to align the nose wheel to the runway centreline during takeoff in such single engine propeller plane or am I being too idealistic?
How is the operation of rudders in an actual aircraft like? Slow smooth gentle depression e.g. to the right pedal each time the aircraft swerve to the left or are the left and right legs constantly moving in quick succession to keep the aircraft aligned to the runway centreline?
Generally, how much of a push is required on the pedals during takeoff? It seems to be very very slight 1cm touches to my rudder pedal on the simulator. Is that realistic?

Thanks so much everyone! All answers are appreciated!

Is this common in real life flying? Are actual pilots consistently
  able to align the nose wheel to the runway centreline during takeoff
  in such single engine propeller plane or am I being too idealistic?

In my experience flying primarily light singles its not terribly difficult to stay aligned on the centerline during takeoff. Even my first solo was nicely centerline. However the more powerful the aircraft the more the tendency and the more control/care may be required.

  How is the operation of rudders in an actual aircraft like? Slow
  smooth gentle depression e.g. to the right pedal each time the
  aircraft swerve to the left or are the left and right legs constantly
  moving in quick succession to keep the aircraft aligned to the runway
  centreline?

All control inputs should be smooth and with care. I generally ease onto the right pedal as I apply throttle and ease off/back on to stay center line.

  Generally, how much of a push is required on the pedals during
  takeoff? It seems to be very very slight 1cm touches to my rudder
  pedal on the simulator. Is that realistic?

This is what most home sims are worst at. Real light aircraft rudder pedals take a fair bit of force to work an have far more than a 1cm throw. I would add a curve to your response so that the initial throw of the pedals is less sensitive.


From which stages is the bleed air from a GE90? I'm doing a university project on the GE90-115b engine and I'm having a hard time understanding this EASA type-certificate data sheet. In particular I can't figure out the air bleed extraction section (page 10 section 10). Is the stage number counted from the fan stage or from the LPC 1st stage?

Are there some specific reason for the choice of extraction air from "middle stages" instead of extracting it partially from low pressure stages and partially from high pressure ones?

The base engine (up -94B rating) has 10 stages HPC, the growth engine (up to -115B) has 9 (reference https://www.geaviation.com/commercial/engines/ge90-engine).

In both cases, the type certification is counting bleed starting from the first stage of the HPC (not the LPC).

The reference to "10th stage" bleed in the type certification is misleading.  There is no 10th stage bleed in either engine.  Rather it is "CDP" bleed.  "CDP" stands for "compressor discharge pressure", i.e. right after the last stage of the compressor.

So in the base engine, the last bleed is extracted immediately after the 10th stage, and on the growth engine it's extracted right after the 9th stage.

Below is a diagram showing the bleed air flow that may (hopefully) clarify.  The CDP bleed is in orange.

(image from this and this answer, original source)


Is it okay to ask the Call Sign of the plane to Pilots / Flight crew during boarding? I sometimes open "Live ATC" app on my iPhone and listen to the "status" of my airplane when I'm taking off until my plane reaches the runway (I know the phone have to be turned off but its exciting to listen to ATC communicating with your pilot). In order to know the status, I have to know the call sign of the plane. If I'm taking off from a small airport, I kinda guess the call sign based on the conversation between the ATC and the pilots and the position of my plane (for example if the ATC says United 220 hold short of Juliet and if my plane is on Juliet, I know my plane is United 220). There have been times when the pilot announces the call sign of the plane before pushback.
My question is, Is it okay or weird for me to ask the call sign of the plane to the pilots / flight crew during boarding.

Mostly you wouldn't need to ask. Most commercial flights use the airline's standard call sign and the flight number printed on your ticket. Sometimes the call sign can be confusing - British Airways use 'Speedbird'; PAN AM used to use 'Clipper'. There's a list of common callsigns on http://www.pilotcareercenter.com/Search-Air-Carrier-By-Name-Region-Aircraft-Type-Radio-Callsign#callsign

The flight number should be enough - just listening for calls referring to 'two-two-zero' will almost certainly identify United 220. Code-sharing flights, however, will generally use the flight number of the operating airline. Your ticket might say 'United 220', but if the flight is operated by, say,  American Airlines so the call sign might be 'American 406'.

All that said, if you're using your phone when you have been explicitly asked to turn it off the airline is within its rights to confiscate your phone or remove you from the flight.


Why the moment arms in x-direction are considered as -x? Thanks for anyone reading this question. I am able to understand the signs of the forces, but I could not understand why a negative sign is added to the moment arm in x-direction (please check the last line in the picture).
Thank you!

In your drawing, force is defined positive when upwards. By convention, pitching moment is defined positive in aircraft nose up direction. Origin of the moment is the wing leading edge, so a positive force yields a nose down moment which is negative.

Horizontal forces result in a nose up moment when above the origin, and nose down when below the origin.

That is assuming:

Positive is up, for force and position.
Positive is aft, for both force and position.
Positive is nose up, for moments.

That is the sign convention for which the equation in the book holds. The book should state the sign convention it is using, perhaps you can post that as well.


Is the A-10 Warthog being discontinued? Popular forum feedback on the A-10 Warthog is that it is held in high esteem however I also read that it is no longer being manufactured. Is this true and why would a highly successful combat aircraft be discontinued?

A-10 entered service in 1976 and is the only production-built aircraft that has served in the USAF that was designed solely for close air support(CAS). With huge losses of A1 (226) in Vietnam war called for development for a aircraft capable for providing air support to ground troops and conduct counterinsurgency. If we look at the history of development of CAS it had direct competition from Army helicopter ( e.g AH-56 Cheyenne) for the above mentioned roles.

The CAS study provided two important recommendations for the Air Force: “(1) the Air Force should take steps to highlight in official USAF doctrine, tactics and procedures publications the methods for accomplishing those missions for which the armed helicopters were provided and which the Air Force considered part of the close air support function; and (2) to fulfill the requirements for the 1970 plus time period, the Air Force should take immediate and positive steps to obtain a specialized close air support aircraft, simpler and cheaper than the A-7, and with equal or better characteristics than the A-1.Based on Air Force and contractor studies, the estimated unit flyaway cost for the A-X was \$1 - \$1.2M (FY70)depending on purchase quantities. Research and Development costs were estimated at 240Million \$. Also for the projected unit cost (FY70)for the turboprop version was \$0.837M (600 aircraft buy) - \$0.937M (400 aircraft buy). The projected unit cost for the turbofan version was \$0.989M (600 aircraft buy) - \$1.092M (400 aircraft buy). Hence with the budget that airforce had they put up a order of 715 aircraft.

Production of the 715 A-10 aircraft finished in 1984. With A10 seeing most of action and noteworthy performance in Afghanistan and gulf wars by the turn of 1990s the aircraft was outdated in terms of avionics and more modern helicopters capable of performing same role (e.g Boeing AH-64 Apache). In 1992 air force purposed F-35 multi role fighter to replace A-10. To put numbers into perspective the U.S. Air Force stated that retirement of A-10 would save \$3.7 billion from 2015 to 2019.

With delay in F-35, air force retrofitted all A-10 and OA-10 aircraft in the fleet to the A-10C standard with a new flight computer, new glass cockpit displays and controls, two new 5.5-inch (140 mm) color displays with moving map function, and an integrated digital stores management system.

References:

https://web.archive.org/web/20150910033808/http://www.dtic.mil/dtic/tr/fulltext/u2/a530838.pdf

http://www.gao.gov/htext/d07415.html

http://www.airforcemag.com/MagazineArchive/Pages/2008/June%202008/0608fade.aspx

http://www.defenseindustrydaily.com/a-highertech-hog-the-a10c-pe-program-03187/

http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104490/a-10-thunderbolt-ii.aspx


Does TRACON airspace override Class B airspace in terms of control authority when controlling approach or departure procedures? This is a simple schematic description of Class Airspace.

My understanding is that Class B has strict rules, specific lower and upper altitude limits and requires clearance to enter. However, I have also come across the Standard Operating Procedures Manual for the N90 TRACON (ORDER 7110.1D), which designate the sectors of local airspace differently. Those also define lower and upper altitude limits. Clearly, the N90 TRACON overlaps with the Class B airspace around the JFK, LGA, TEB and EWR airports.

My confusion has to do primarily with this overlap. I was thinking that Class B airspace has to do strictly with terminal procedures (approaches/departures), that is, all take-off and landing procedures take place within this airspace.

However, I have seen SID and STAR procedures running over fixes located close to the airports, which define much higher altitudes than the Class B upper limits (around JFK and the above airports, the upper limit is 7000ft for Class B airspace, while lower limits vary).

Take, for example, the LENDY SIX ARRIVAL, of JFK airport. I read over the JENNO fix information: "Expect FL230".

What is this supposed to mean: "Expect FL230"? Will an approaching aircraft enter the procedure at a Flight Level of 23000ft or be at that flight level whenever approaching the airport at that fix point? How does Class B relate to these operations? I thought that approach operations take place within Class B but I am confused because the altitudes I see in SID/STAR procedures or the lower/upper altitude limits in the various airspace sectors of Standard Operating Procedure (SOP) plans* are quite different. It appears as if flights that approach for arrival, for example, are controlled before entering Class B (i.e. at higher altitudes) when inside a TRACON, e.g. N90.

*(I cannot link to an SOP Manual because they are intended for internal distribution, so I cannot find something official online, however, the information is pretty much like that document).

Class B is a legal restriction for pilots. ATC has to give them clearance to enter it. It's there to protect the traffic around a busy airport from aircraft that are not being controlled by ATC.

The TRACON airspace is just for controllers. Pilots aren't expected to know sector boundaries. They don't have to worry about that at all. Each controller will tell the pilot the frequency to contact the next controller whose airspace they are about to enter. Few TRACONs make their sector boundaries public and they change often.

A pilot flying under VFR does not necessarily even need to talk to controllers outside of A, B, C or D airspace, regardless of what ATC sector he might be in.

As long as you have the correct type of transponder, it is entirely legal to fly VFR through the layer of air above the class B below 18,000 feet without contacting atc. (This is class E airspace). Whatever controller whose sector you're flying through will keep any IFR traffic away from you, but they don't need to give you vectors or anything. You have to stay away from clouds. It's entirely the pilot's responsibility to see and avoid things.*

SIDs and STARs are only used by IFR traffic that is being controlled by ATC, but not necessarily by TRACON the whole time. Some STARs start hundreds of miles from the airport. When they begin the arrival they will be controlled by en-route controllers, who will tell them when they need to switch to TRACON and what frequency to use.

* I'm leaving out flight following for simplicity.


What is the purpose of the ball and string attached to the tip of the vertical fin on test aircraft? This question applies to aircraft being tested, presumably within a certification program.

I've seen what looks like either a ball or "shuttlecock" at the end of a tether attached to the top/rear extremity of an aircraft's vertical fin. An example can be seen toward the end of this video.

What is its purpose? Is is part of or does it contain any instrumentation, or is it there to provide direct visual indication of something - such as trailing airflow patterns?

That is a static pressure line. The "shuttlecock" device on the end pulls the line out behind the aircraft, where it can get a more accurate pressure reading, away from the air flowing around the aircraft that affects the regular pitot-static system. It will also be more accurate during flight conditions that normal aircraft rarely experience, but need to be accurately analyzed during testing.

Cropped from this image

Static pressure has an effect on the performance of the aircraft, so during testing that is one value they want to measure accurately during flights. Once testing is complete there is no longer a need for that information, and the regular pitot-static system takes the measurements needed.

The line can be seen in this photo, and in this video, you can see the reel inside the rear of the aircraft that is used to pull in the line when it is not in use.


What are the dimensions of the A320's wingtip fences? Does anyone know the size (relative to the wing) or have a schematic draw of these wingtip fences?

Dimensional information can be found on Airbus's Airport and Maintenance Planning Documents for the entire product range.

You will have to do a little digging and calculation to find the sizes of the wingtip devices but they are there.

Specifically, on the A320, subject 2-3-0 lists the height of both of the bottom and top of the wingtip fences relative to the ground, and 2-2-0 provides dimensions from the top.


Has a "fight" ever happened in the cockpit? Somehow, a variation of How do long-haul pilots cope with boredom? .

Has it ever happened that the pilots started some small talk about politics, or football, or favourite rock band, or that one cheated with the other's SO, and the discussion degenerated in some strong arguments, if not a fight? Is there anything in place to prevent that (like, procedures on what you're supposed to talk about during free moments)?

I've never seen anything about pilots fighting during the flight but on Air India 611 they fought before the flight then proceeded to fly anyway. There are differing accounts of whether it was physical or just verbal. The captain claims he was punched for calling his older copilot "uncle."

On Express Jet 4453 apparently they got into such a heated argument that when they landed, the flight attendant made the passengers disembark. They continued on in a different plane with a different flight crew.

There are reports of a fight between pilots on a China United Airlines flight but finding out what actually happened will be difficult.

On JetBlue flight 191 the captain started ranting and raving to the point where flight attendants and passengers had to remove him and physically restrain him for the duration of the flight. There's video of the incident. He was charged with interfering with a flight crew but was found not guilty by reason of insanity.

And on Delta 2598 the pilots did an emergency landing when two flight attendants got into a brawl.


What FAA sources of weather information can pilots use to mitigate the risk of flying in icing conditions? Pilots flying under the FAA jurisdiction have many official sources of weather information available to them. Specifically as it relates to icing conditions, what official sources of weather information can pilots refer to in order to avoid or mitigate the risk of flying in icing conditions?

Here's a list of official FAA weather information sources that pilots can analyze to avoid or mitigate icing risks:

AIRMET Zulu

An AIRMET Zulu will be issued for areas where icing conditions are forecast to be light or moderate.

SIGMET

A SIGMET will be issued for icing conditions forecast to be severe.

PIREPs

PIREPs are the best source of icing information. Unlike weather information sources which are forecasts, PIREPs are reports from pilots flying in the actual conditions and should be considered the most accurate source of icing information.

Here's an example of a PIREP where a pilot of a TBM-700 was descending into Atlanta. He reported a cloud layer topping at 7900 feet down to 500 feet AGL and picking up light rime ice between 7900 and 5000 feet.

UA /OV ATL /TM 1200 /FLUNKN /TP TBM7 /SK OVC015-TOP079 /IC LGT RIME 050-079 /RM DURD

Significant Weather Prognostic Charts

The top two panes of a prog chart help pilots visualize the freezing level. A zig zag line shows when freezing level is at the surface and height contours for the highest freezing level are drawn at 4000 foot intervals with dashed lines. This information gives pilots a forecast of temperatures which when combined with known precipitation can predict ice and help to avoid it.

Area Forecast (FA)

An FA is an additional source of information about where cloud tops and bottoms are. If you know that along with the freezing level you can get a better idea of where icing conditions are most likely.


What are the major aircraft systems (LRUs) using the ARINC429 Protocol? I want to know the different aircraft systems (LRUs) using ARINC429 protocol.
Why is it used instead of the ARINC 629 ?
How is it different from the different ARINC protocols like 629 and AFDX(664)?

On modern aircraft all equipments that needs to communicate to each other in binary format may use ARINC 429 protocol. For instance all fly by wires computers on Airbus 320, 330, 340 use arinc 429 protocol.

Nowadays other protocols do exist too such as arinc 629 protocol or fiber optic protocols etc.

As a summary all aircraft systems that need to transmit and receive binary data may use 429 protocol, but other digital protocols are possible.

However a standardized LRU that doesn’t use ARINC 429 protocol is the FDR ( flight data recorder), because the data that it receives is already sequentially ordered, the label is not transmitted with the data value

On the following website starting page 43 you find all the possible equipment that may use A429

http://read.pudn.com/downloads111/ebook/462196/429P1-17_Errata1.pdf

Also in the same document starting page 13 you have the list of the parameters that may be transmitted on A429

Hope this added information is helpful


If the cabin temperature controller does not function, does the pilot have to make an emergency landing? The cabin temperature controller in commercial airlines, controls the cabin temperature. But if for any reason, it does not function, does the pilot have to make an emergency landing, or can he cruise to a lower altitude for normal temperatures?

I'll use the B757 as an example, on it you would not divert due to temperature control issues. In fact the Master Minimum Equipment List items 21-61-1 and 21-61-2 give permission to dispatch the aircraft with inoperative zone temperature control.

The Quick Reference Handbook (PDF) for the 757 gives the following instructions for failure of zone temperature control.

I also think most airlines and crew think passengers would rather get to where they are going than being comfortable.

As for the emergency landing, aircraft can and do divert all the time without declaring an emergency (Mayday) or urgency (Pan-Pan). So worse case the crew would choose to divert to a suitable airport.


Constructing flight recorder data By just going off the public information about a flight and what one knows about an aircraft, is it possible to construct a differ version of a flight recorder's data well enough to fool investigators?

Theoretically? Yes.
Practically? No.

In order to construct an alternative Flight Data Recorder narrative good enough to fool investigators you would have to acquire a flight data recorder to feed the data to (this is the easy part). You would then need to feed the appropriate parameters to the flight data recorder to fill its memory (modern recorders can hold over 10 hours of data - this could encompass several flights that would have to be believably reconstructed - speeds and altitudes matching radar track data for example).
A flight simulator could probably be rigged up to supply the data to the recorder, so the primary difficulty here would be matching your forgery to other data sources like RADAR tracks, radio calls, the CVR (unless you intend to falsify that as well), etc...

Once filled with your falsified/reconstructed data the recorder would need to be injected into the chain of custody, which is perhaps more difficult. replacing one which has been recovered would require fooling the investigating agency, which has already seen, photographed, and cataloged the device they recovered (and would likely notice any subtle differences), or by finding the real recorder before investigators and replacing it with your forgery.

While it is not impossible to do all of the above it is highly unlikely that anyone would succeed in such a scheme.


Why aren't the passenger seats designed as jump seats (foldable) in commercial airlines? We know that the crew seats in an airplane are "jump seats (foldable)", which made me wonder why it isn't the same case with passenger seats.

Some advantages which I see if passenger seats are converted to the "jump seat" design:

Better access and thus easier cleaning of the aircraft
Easy entry exit by passengers
Already available tech (as in the crew jump seats)

You're in luck. Airbus has filed a patent for that. (Not a joke.)

  Airbus said the design was a response to recent demands from low-cost airlines to cram more seating into existing plane interiors – often to the detriment of passenger comfort.

Just when I thought the below can't get any worse.

(Original, sardine edit by @TomMcW)

There is also this concept:

  Although the patent has been filed, whether or not the seats will become popular depends on whether or not airlines want to pick up the design.


What other materials apart from conventional nickel and steel superalloys can be used as a first stage turbine blade material? Steel and nickel chromium alloys have been prominent and most used as turbine blade material although there are other materials such as tantalum carbides, tungsten and many more which I have no knowledge of. But these materials are not being used or even planned on being used in the near future why is that?
Do the cons of these materials beat the pros of the ni-cr superalloys?

Turbine blades must withstand high tensile stresses at their blade roots and be resistant to cyclic stresses from fluctuating fluid dynamic loads- all of this at very high temperatures, and over great numbers of thermal cycles. This argues in favor of metallic blades and against ceramics in this application, because they perform better against fracture in tension and fatigue crack propagation.


What is this "flap-like" thing on the back of the F-15? I saw an image of an F-15 landing. What is that flap-like thing in the top of the plane in this picture? I couldn't find it on any other pictures or drawings of this aircraft.

It is a speed brake (air brake). Here is a closeup of it.

Image from aero-cafe.com

It is usually used reducing speed landing (approach). It is also used for other functions- to slow down quickly at high speeds and to prevent too much speed buildup in a dive.

This image shows it being used during approach.

"F-15A 21st TFW landing with extended speed brake 1984" by SSgt. K. Vail, USAF - U.S. DefenseImagery photo VIRIN: DF-ST-85-11671. Licensed under Public Domain via Wikimedia Commons.

Similar airbrakes are also found in other aircraft, notably the Su 27 and its derivatives.

Image from aerospaceweb.org


What crosstrack should I have during a holding pattern? I want to know if the ATC is expecting a maximum crosstrack depending of the plane's speed.

I've learn to be at 2.2nm for my holding (with 175kias) but is it just a distance to help me or a rules ?

I think that my protecting side is higher if my speed is higher too but can I use the whole protexting side  ?

Thx

If you follow the holding guidelines you will never leave the protected area. This includes performing an appropriate entry, applying correct wind correction, and flying the correct leg length.

ATC expects you to slow to holding speed no later than 3 mins before arriving at the holding fix for planning purposes, and generally doesn't expect you to use the entire protected area. However, the entire area is protected, so you could use it if you wish.


Does fly-by-wire permit smaller tail fins (area and thickness) for jetliners? To prevent an XY problem, here's the context:

A striking feature of the 777 is how small and thin its tail fin is compared to the 747.

When I asked about this many, many years ago, someone told me it's because of the 777's fly-by-wire, which makes better use of small fins. But I started to question that recently.

Above are overlaid scaled drawings (747 and 777). Right away the engine placement seemed to provide the answer: one engine inoperative (OEI) controllability.

Using each plane's lateral engine placement and max thrust (282 kN and 513 kN for the 747-400 and 777, respectively), and with a worst case scenario OEI for the 747 (which would be an outboard engine), the 747 ends up with ~27% more yawing torque. Which would explain the tail fin areas of ~77 and 53 $m^2$.

But (big but), I recalled a famous A340-600 OEI takeoff video (a quad just like the 747 and with equal thrust of ~280 kN), and decided to compare it with the 777-300.

And here the tail fin areas are ~53 (777) and 48 (A346) $m^2$:

So, back to square one.

Of course there's the upper-deck (hump) of the 747; if that is one of the main reasons for the bigger tail, then it still does not explain how the A340 (-500 and -600) manages its OEI by having a smaller tail and more outboard engines compared to the 777.

Is it really fly-by-wire that permitted the smaller fins (area and thickness) of the A345/6 and 777?

One possibility that came to mind is perhaps the use of asymmetric spoilers when an extreme yaw is commanded – which can be seen in the linked A340 video at one moment (before/after the ailerons showed big deflection without the use of spoilers). Of course that presents its own problem: the accompanying roll (and with the video's low-quality and lack of telemetry, it is not really evidence).

Tail fin areas via https://booksite.elsevier.com/9780340741528/appendices/default.htm
Scaled drawings via Boeing and Airbus (for the 777 I used the -300, not the -300ER, in case you're wondering about the wingspan).

After much google-fu, for the 777 it's an astounding yes (that someone was right after all).
From a 1996 ICAS paper (see PDF pages 2, 4, and 5) about the 777's innovations, the use of relaxed stability and FBW laws indeed helped reduce the tail area and its weight:

The paper mentions a wheel/rudder crosstie. The wheel here being the yoke. Before the 777 Boeing sized the tail based on controlling an in-flight engine-out using only the yoke. This digital crosstie allowed the smaller fin:

Vertical Stabilizer. Conventional sizing of the vertical stabilizer is based primarily on meeting requirements associated with static directional stability. Sideslip characteristics and the pilot’s ability to control the effects of in-air engine failure using wheel only have sized the vertical stabilizers on previous Boeing airplanes. With the wheel/rudder crosstie, the same desired sideslip and engine-out control characteristics can be achieved with the smaller vertical stabilizer. The rudder input due to the crosstie compensates for the reduced area.

The even smaller fin of the quad-A345/6 remains open.
It turns out my guess had merit. The A340 does use the roll surfaces to supplement the yaw:

The flexibility of fly-by-wire was used to optimize the VMCG speeds. In fact, the rudder efficiency was increased on the ground by fully and asymmetrically deploying the inner and outer ailerons on the side of the pedal action as a function of the rudder travel: the inner aileron is commanded downwards and the outer aileron complemented by one spoiler is commanded upwards.$^1$

And watching the engine-out test video again, this becomes clearly visible:

Note: either the author mixed inner with outer, or Airbus updated it since or flipped them for the -5/600, either way, it doesn't change the answer.

$^1$: C. FAVRE (1994) Fly-by-wire for commercial aircraft: the Airbus experience, International Journal of Control, 59:1, 139-157, https://doi.org/10.1080/00207179408923072 p. 153


Why are the top speeds for jet engines higher than for propellers? I realize turbines are more efficient than piston engines, but if that were true, then why don't turbo-props reach jet speeds?

The thrust of a propeller is proportional to the inverse of airspeed, while the thrust of a pure turbojet is roughly constant over airspeed in the subsonic region. This means that two airplanes with the same static thrust, one propeller-powered and the other jet-powered, will reach very different top speeds.

And, no, piston engines are more efficient.

A piston engine produces a constant torque $\tau$, independent of speed. This torque drives the propeller which produces work $W$ per unit of time on the air passing through it. This work per unit of time is power $P$ and proportional to the product of torque and propeller speed $\omega$, which is again constant over airspeed $v$. The power to propel the aircraft is the product of thrust and airspeed and equals engine power times propeller efficiency $\eta_{Prop}$. When airspeed goes up, thrust must go down proportionally for power to stay constant.
$$P = \tau\cdot\omega = T\cdot v\cdot\eta_{Prop}$$
$$\Rightarrow T \varpropto \frac{1}{v}$$

Turbojets, on the other hand, profit from flight speed because the intake pre-compresses the air when it slows down ahead of and in the intake. This pre-compression lifts the pressure level of the whole engine, so it sees an increased mass flow with increasing speed, producing higher thrust. This effect by itself would increase thrust in proportion with the square of flight speed, but the same effect which reduces the thrust of a propeller acts on a turbojet as well. This effect, however, is less pronounced because the jet accelerates less air to a higher speed, and both cancel each other, roughly.

Turbofans are more similar to propeller engines, so here the thrust goes down over airspeed, and more so for higher bypass ratios. Turboprops are even closer to piston-powered propellers, so their thrust drops even more with increasing speed. This means that the speed at which drag equals thrust drops when you move from turbojets to turbofans, and further to turboprops and is lowest for piston-powered propeller aircraft.


What is meant by a "congested area of a city/town/settlement" in FAR 103? FAR 103.15 states:

  No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.

I'm wondering what is meant by "congested area".  It sounds like you can fly over a city, town or settlement so long as it's not congested but...how is that defined?

I could not find a definite definition of congested area.

As mentioned here:

  [...] neither the FAA nor the NTSB has ever provided [...] a precise
  definition of [...] a "congested area." Rather, a "congested area" is
  determined on a case-by-case basis. According to the Board, "the
  determination must take into consideration all circumstances, not only
  the size of an area and the number of homes or structures, but, for
  example, whether the buildings are occupied or people are otherwise
  present, such as on roads."

The same is echoed here:

  [For the definition of congested area,] FAA and the NTSB have opted
  for taking a "case-by-case" approach in determining how to apply
  certain terms.

AOPA states that:

  Congested areas. "Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of
  1,000 feet above the highest obstacle within a horizontal radius of
  2,000 feet of the aircraft." The FAA does not define congested area in
  the FARs or in the Aeronautical Information Manual. Interpretations in
  low-flight enforcement cases are not consistent for purposes of
  drafting a precise definition. Such a determination is usually decided
  on a case-by-case basis, and in the cases that we've seen, congested
  has been interpreted rather broadly. For example, a highway with
  moderate traffic was found to be "congested," as was a seaside area
  where 200 to 300 persons were sitting on the beach or bathing in the
  water.

The same can be seen here, and here, and here.


What is this aircraft and its submarine? I got this picture from an anonymous source and I'm curious to learn more about its mission and I want to know if it's a genuine picture. It happens to be a submarine aircraft carrier.

It's a joke. It was first posted – at least as far back as 2001 – on skyhawk.org with the caption:

  The Royal Australian Navy Skyhawk, being an all-purpose aircraft, was the subject of a 1970s experimental sea trial of an A-4 on board a Royal Australian Navy submarine. There was no objection from the aviators to operating the Skyhawk from the somewhat limited deck space of the submarine. A problem with the Skyhawk's turbine cavitating while underway submerged was overcome. The diesel catapult worked just fine. However, the experiment was abandoned when the submarine's XO objected to the jet blast burning the paintwork off the sail.

More here:

http://skyhawk.org/article/humor-page-two (the current live version)
http://warbirdsnews.com/warbirds-news/fun-facts/1965-mig-flies-bridge-a-4-submarine-fake-real.html
https://www.airliners.net/forum/viewtopic.php?t=1007945

Anyway, the plane is a McDonnell Douglas A-4G Skyhawk, the sub is reportedly an Oberon-class, and the helicopter is a Westland Wessex.