When/How may I replace the Turn Coordinator with an Artificial Horizon? I've seen a few panel shots where the traditional turn coordinator has been replaced with a second artificial horizon, as shown below.

The benefits seem obvious - if my artificial horizon failed I'd certainly like to have a fully functional back-up rather than a turn coordinator with no pitch information.

Under what circumstances/conditions are these second horizon gyros acceptable as replacements?
Are there any drawbacks that I should be aware of if I'm considering installing one.

The How It Flies in section on Spiral Dive Recovery mentions:

If you don’t have good outside references, you should not rely on the attitude indicator (artificial horizon). The attitude indicator contains a gyro mounted on ordinary mortal gimbals, which can only accommodate a limited range of pitch and bank angles. A steep spiral can easily cause the gyro to tumble, whereupon it will need several minutes of relatively straight and level flying before it can re-erect itself. Military aircraft have non-tumbling attitude indicators, but you’re not likely to find such things in a rented Skyhawk. Therefore, you should roll the wings level by reference to the rate-of-turn gyro.8 Being a rate gyro (as opposed to a free gyro) it has no gimbals, and therefore can’t possibly suffer from gimbal lock.
8 That is, the turn needle or turn coordinator, whichever you happen to have.

And in section on Spin Recovery it adds:

If you get into a spin in instrument conditions, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled due to gimbal lock. It is better to trust the rate-of-turn, which cannot possibly suffer from gimbal lock, since it has no gimbals. Remember, it is a rate gyro (not a free gyro), so it doesn’t need gimbals.

If you have good enough artificial horizon designed to avoid gimbal-lock, the you don't need turn coordinator. But with ordinary artificial horizon the turn coordinator combined with vertical speed indicator (despite it's problems like rather slow reaction time) are useful backup for recovering from in-flight upsets.


Is lift force less than weight during a descent? In the above image lift is the same during a climb and during a descent. Is lift force not supposed to be less than weight during a descent?

Secondly, does descending at an airspeed much lower than the dive velocity (Vd) of the airplane affect the rate of descent of plane?

The content reproduced in the question is basically exploring what happens if we suddenly bring the wing to the angle-of-attack required to produce some given G-load, while flying at some given airspeed.  The value of lift given in both cases -- 4166 pounds -- is undoubtedly NOT the weight of the microlight aircraft.  It is undoubtedly much higher than the weight of the microlight aircraft.  We are NOT looking at a situation here where net force and net acceleration are zero, so this content really doesn't shed light on the questions being asked.

On the content reproduced in the question, it's not clear why the asker believes that that the red arrow points to a climbing situation and the blue arrow points to a diving situation.  At any rate, the content reproduced in the question has nothing to do with the forces in a steady-state climb or descent.

As to the questions asked--

"Is lift force not supposed to be lesser than weight during a descent?"

Yes, both in a steady-state climb, and a steady-state descent, lift is less than weight.  To understand this better, keep in mind that in the context of winged flight, lift is defined to act perpendicular to the aircraft's flight path through the airmass, not vertically.  Similarly, the drag vector is defined to act parallel to the aircraft's flight path through the airmass, and to a first approximation, the thrust vector can often be considered to also act parallel to the aircraft's flight path through the airmass.  As a result, thrust supports part of the aircraft weight in a steady-state climb, and drag supports part of the aircraft weight in a steady-state descent, and in both cases the wing's lift vector is reduced.  To see the force vector diagrams illustrating these situations, visit these related answers to related questions:

What produces Thrust along the line of flight in a glider?

Does lift equal weight in a climb?

(Note-- to apply the vector diagrams shown in the first link above to the case of a powered glide or powered dive, simply replace the label "drag" with the label "drag minus thrust" –- much as the label "thrust minus drag" appears in the diagrams in the second link above.)

"Is descending at an airspeed much lower than the dive velocity (Vd) of the airplane affect the rate of descent of plane?"

Yes, for a given throttle position or a given thrust force or a given power output, if we change the airspeed, we'll change the rate of descent.  In general the slowest descent rate is obtained flying just a little faster than stall speed.


Is the ground clearance of the B737 engine low compared to similar aircraft? I'm referring to this other Question.
I want to know more about the B737 and the distance between the engine and the tarmac.
There are several Incidents reported, on 737 damaging the tarmac.

Is the distance on a B737 engine really that little comparing to
other Aircrafts (for example A320)?
Is this a design fault? Also have a look at the "Jet-blast pressure study" page 13-14.

Is the distance on a B737 engine really that little comparing to other Aircrafts (for example A320)?

The actual distance depends on the engine installed and the weight and balance of the airplane at the time. But there isn't a huge difference between the two planes. Comparing the figures, the current A320 is about 10 cm higher than the current 737 NG. The A320neo and the 737 MAX will be pretty close, with the MAX potentially getting 5 cm lower.

737 (MAX): 43-58 cm (1.42-1.92 ft)
737 (NG): 46-64 cm (1.5-2.08 ft)
737 (CL): 46-53 cm (1.5-1.75 ft)
737 (100/200): 51-58 cm (1.67-1.92 ft)

A320neo: 46-56 cm (1.84 ft)
A320: 57-85 cm (2.2-2.79 ft)

Those reports also show exhaust velocity profiles at takeoff thrust, but differences in the diagrams make it hard to compare. The IAE engines look to have much lower velocities than the CFM engines on either plane.

  Is this a design fault? Also have a look at the "Jet-blast pressure study" page 13-14.

It depends how you look at it. In the linked report, there are several recommendations for both blast pads and the operator's procedures, but nothing regarding the design of the aircraft. So rather than telling the designers to mount their engines higher above the ground, they are making sure that facilities and procedures are designed to prevent things like this from happening.

As far as being a "fault" unique to the 737, the study mentions:

  ...an accident very similar to that of G-DOCT, which involved an Airbus A320...

So it's not the only airplane that can cause problems. And the newest versions of both types will have similar and smaller distances, they probably decided that there is no facilities-related reason important enough to choose differently.


What is the definition of Lift? Lift is a term that is used very frequently here, but I cannot find any post explaining what it is exactly. I am looking for the exact definition of lift, not an explanation of how it is generated.

It seems a very simple question, but I have not been able to find consistent answers to it. Some very well respected sources even contradict each other:

At https://en.wikipedia.org/wiki/Lift_(force) Lift is defined as:
A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction.

But according to NASA (https://www.grc.nasa.gov/www/k-12/airplane/lift1.html):
  Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air.

If a plane is flying straight and level (at constant altitude) these definitions are the same, but otherwise they are not the same at all. So, which one is correct?

It's the component of the aerodynamic force perpendicular to the direction of the air flow, the aerodynamic force being the reaction of the wing to the relative wind.


What could cause a rear twin-jet's throttle to oscillate on descent? I rode a smaller twin-tail-engined jetliner for a few short hops last month, and I noticed that their airport approaches are much rougher than the widebody airliners I am used to. I believe that it was an MD-80 with United Airlines.

As soon as the pilot began the descent, it seems like the he/she was going wild with the throttle; the engines would appear to idle for 10 seconds, then suddenly and audibly power to full for another 10 seconds, and the cycle would repeat until we were right above the runway. I know that this was not crew specific because I took the commercial flight on 6 different occasions on three different routes.

Question: Why did the pilots increase and decrease the throttle so rapidly and so many times on descent to the runway?

edit: Terrain: flat, Atlanta area; Meteorological conditions: heavy cumulus, twice during the night and the other four during dusk, light to moderate rain under and in the clouds. The clouds had no breaks and often had a strange formation where in some areas they would rise much higher than others. Imagine a series of mountains and valleys with several? miles of distance between cloud peaks and cloud dips.

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


What structural design methods were used for early turbojets? I have had this question for long time. Can anyone please give me some references on early turbojet engine design methodologies?

For example P&W J57 which was designed in early 1950's: How were vibrations, resonance and aero loads calculated and analyzed, most probably without having computers? I'm not too sure whether there is any closed form analytical solutions anyway.

Going back further to the early 40's to the de Havilland Goblin turbojet:
There is an in-depth article from Flight from February 21st, 1946. The first page is here, but you'll need to download the rest of the pages from the navigator on the left. It even comes with this diagram showing the forces on the different parts.

Materials
Reading through the article, the biggest unknown back then was the materials to choose for the hot sections.

Plenty of theoretical information was available on blade form, but little was known about the operating temperatures likely to be experienced. Chief properties required of the selected materials were a high resistance to creep, corrosion and scaling.

They got their hands on the strongest materials, and with the aid of heat-sensitive paint, they scaled back the requirements.

On the first few engines, turbine discs were made of austenitic steel, but later, with the aid of heat-sensitive paint, it was found that ferritic steels with their greater strength, around the working temperature of 750 deg. C. and a lower coefficient of expansion, would prove satisfactory.

Vibrations
For vibrations, what I gather is it's not that big of a deal for turbojets compared to turboprops of that era, but here's the relevant part:

[...] subsequent measurements of vibration for the whole power unit have shown that the vertical oscillation is less than ± 0.001 inch.

Trial-and-error
There is also the test-to-destruction method (not really):

[...] troubles experienced included weaknesses in the welding of combustion chambers, and in the construction of the intake ducts and the tail pipe.

So, there was a fair share of trial-and-error.
Forces
Regarding the forces, and that's a guess here, we find:

Any thrust loading is taken by the front bearing which is located on a stub or pivot shaft between the air intake ducts in front of the impeller.

Post-examination of the bearings may yield information.
For gas-load calculations, back then they've already made very powerful turbocharged engines, so they've had the knowledge. As the article suggests, the chief issue was the materials.


Relation between lift force and torque produced in propeller used for vertical take-off and landing aircrafts There are many answers for this question but none of them are specific.
I need to find the exact relation between the lift force of a propeller of specific pitch, attack angle , dimensions and material with torque to be produced in a propeller at a specific speed at a certain altitude of a vertical take-off and landing aircraft to hover and fly forward.
Can some experts in the field derive this relation.

The relationship of torque produced by the engine and lift force will be the drag of the rotor.  Increase in lift force by increasing rpm or AOA will require a proportional increase in power output from the engine.  Air density also plays a role.
With a common helicopter, torque is counter balanced by increasing or decreasing torque of the tail rotor.


What should a pilot do in this lost communication scenario? As far as I study, when a pilot faces a lost communication problem, the pilot should consider MEA (Minimum Enroute Altitude, Expected Altitude, Assigned altitude) for altitude and AVEF (Assigned heading, Vector, Expected route, Filed route) for route. I am confuse about what V (Vector) means.

For example, I have filed:

  DVT 1 departure->PHX VOR-> V105 ->DRAKE(PRC VOR)->V257->BISOP->GRAND
  CANYON Airport

I got a clearance like this:

  555AZ cleared to Grand Canyon Airport via DVT one departure PHX vor
  vector to Drake Altitude 10000' Frequency 120.7 Transponder 3435.

However, my communications fail before I arrive at PHX VOR. In this situation, A (assigned) is [DVT one departure->PHX VOR-> DRAKE]. What about V (vector)? Is it DRAKE? Or if not, how can I manage this situation? I think I should go to PHX VOR via DVT one departure. When I arrive PHX VOR, I go to DRAKE (PRC VOR) directly then go to fly as I filed. Is it correct? If not, could you let me know?

It means to expect radar vectors between PHX and DRAKE because ATC expects to have to move you around to accommodate the traffic situation.  If you had a comm failure, once you squawk 7600, ATC will expect you to find your way between PHX and DRAKE by the most logical path.

In this case, because you had filed to fly along V105 you would take that route and that's what the controller would expect.  The controller would quickly figure out what you are doing as you proceed down V105 and would be able to route other traffic to maintain separation to accommodate your track.

The only thing that would create a problem is if you headed off in some direction that made no sense to the controller once at PHX.


Please explain real-world procedures behind this "ATC joke" One of the well-known kind of the "ATC jokes" (funny excerpts from real or invented communication between pilots and air traffic controllers) has recently caught my eye.

I can't seem to understand what actual procedures or rules does it refer (or allude) to:

  Pilot: "Approach, Federated 303's with at 8000' for vectors ILS, full stop.

  Approach: "Unable Federated 303. The ILS is out of service."

  Pilot: "We'll take the VOR then."

  Approach: "Sir, the VOR's in alarm right now. Standby."

  Pilot: "OK, guess it'll have to be the ADF then."

  Approach: "303, unable the ADF right now for traffic saturation."

  Pilot: "OK, approach. State my intentions."

(src)

Could you please enlighten me on what does the last (highlighted) line is intended to mean and/or convey? I understand it's a word play on "state your intentions", but what exactly is it supposed to mean?

As you commented, it's a play on "state [say] your intentions". However, the pilot has said his intentions several times and ATC rejected them each time, so he's turning it around by asking ATC to tell him what intentions won't be rejected.

The "intentions" in this case are the type of Instrument Approach Procedure (IAP) he wants to use. Typically there are several for each runway, for use by aircraft with different levels of equipment; ILS, VOR and ADF (actually NDB) are common options in order of decreasing desirability.

Depending on the weather, there may be two additional IAPs available at any airport which aren't usually charted: visual and contact. There's a possible second-level joke here in that ATC isn't allowed to offer a contact approach, but the pilot is allowed to request it--so if the pilot is asking ATC to tell him his intentions, then in the logic of the joke, ATC could tell him he's requesting that. OTOH, if the weather were such that a visual approach were available, ATC probably would have assigned it to him already without any requests.


Why are some airplane wings not painted? Why is it that some wings are not painted on commercial jets? Couldn't they paint them the same color as the fuselage or would it be a waste of money?

Fuselage present a large vertical surface with greater length than height. Wings are a large horizontal surface that is shorter in the direction of travel. So...

The fuselage paint job is clearly visible on the ground, painting on the wings only if you are above or below the plane. This is probably the main reason. Simple geometry. It would be waste of money.

Sun will heat a large horizontal surface more than a large vertical surface, so making the wings white is more useful than making the fuselage white. While I am not sure that wings would be structurally weaker if heated, the heat would cause thermal expansion, which would indirectly result in increased drag by making maintaining optimal shape more difficult.

Paint has significant effect on drag, more specifically it affects when the boundary layer transitions from laminar to turbulent. In the very long fuselage the difference is probably insignificant and you can paint them as you wish, but for wings the paint should be as smooth as you can make it. While I doubt white paint is smoother than paint of any other color, keeping it simple is probably a good idea.


Is there a common maintenance issue requiring filling a hole in the tail on the A321? I was on a flight recently (AA 705) and a maintenance worker used a forklift to go on top of the plane shortly before the captain announced that we were going to have to wait at the gate for an extra 25 minutes while they filled a hole with putty near the tail of the plane. The aircraft was an A321.

Can anyone tell me what was wrong with the plane? The pilot said it was a common problem and part of routine maintenance. I am wondering if this is a maintenance issue that an A321 expert would immediately recognize as part of routine maintenance or a problem common to these aircraft.

While maintenance records exist at the airline, they're not something that can be accessed by the general public over the internet.  And without those records, it would be guessing to answer what exactly "filled a hole with putty near the tail of the plane" was addressing.

It sounds like the delay might have been to allow the putty to cure, but again, without knowing the exact details, that's as much a guess as anything.

If you know somebody who works in the maintenance department at AA, they might be able to get the info, but again that's not the sort of thing that airlines like being released routinely.

This question may well be closed as being essentially unanswerable, and that's probably the right "answer" in this case.  Without far more specific knowledge, it's not something one can determine from the facts given, even with the World Wide Web available for further research.


How is separation managed when there are only two parallel runways in a very busy airport? In the world's busiest airports, how is separation managed between departures and arrivals when there are only two parallel runways like in London Heathrow. What is the minimum safe separation?

With two runways, usually, one is dedicated to arrivals, and the other is dedicated to departures. That's the simplest way to handle separation -- only in-trail separation is involved.

That's usually the way traffic is managed at Heathrow (LHR) and Dubai (DXB), two of the world's busiest two-parallel-runway airports. At Heathrow, when in a westward flow (70% of the time) the runway functions are swapped about 3pm, to equalize noise impacts.

San Francisco (SFO) has two parallels, 28L and 28R, that are 750 ft apart -- very close. Under visual conditions, both runways are used for VISUAL approaches. Generally, aircraft arrive side-by-side, for wake avoidance.

For about an hour a day. 6am to 7am, when arrivals dominate, LHR allows arrivals on both runways. I don't know what separation rules are used. At SFO, under marginal conditions, both 28s are used for arrivals but aircraft are staggered and capacity is less than for visual operations.


Which UK airport uses different approaches for noise reduction? I remember hearing about a British airport that had two approaches that were used on alternating weeks. They implemented this pilot project (no pun intended) so that different houses would be overflown each week, giving residents the impression that noise from the airport was reduced. What airport was this?

Apparently it's called Runway Alteration. The only hits I got were referring to Heathrow airport, so my guess is this is the only airport that uses this procedure.

Actually, the alternation is in two ways: per part of the day, and per week.
During week 1, from 06:00 to 15:00 runway 27L is used, and from 15:00 until last departure 27R is used.

The week after this, the things are reversed:
From 06:00 to 15:00 runway 27R is used, and from 15:00 until last departure 27L is used.
So the people living under the approach of 27L will have noise in the first part of the day in week 1, and during the second part of the day in week 2.
See the full programme here.

It is influenced by the six factors below (taken from the same source):

  Daytime patterns of runway alternation are different to night-time    patterns: at night there are only a handful of landings and very few take-offs.
  Aircraft land and take off    into the wind, so the wind direction has a big effect on flight patterns.
  Government policy currently favours aircraft taking off    towards the west. Even when the wind blows from the east, aircraft    could
  still fly in from east and take off towards the west. This is because we have a westerly preference.
  A historical restriction means    that aircraft cannot take off towards the east from the northern    runway. This is because of the Cranford Agreement.
  The hour between    06:00 and 07:00 is the busiest time of the day for arrivals so we are   able to use both runways for landings.
  Occasionally we have to suspend    runway alternation for airfield maintenance, in response to bad    weather or when air-traffic control
  want to avoid a build-up of    arriving aircraft.

Actually, alternation also occurs for night flights, but then in 4 one-week periods.


Can a private pilot refuel his small private airplane internationally without passing through immigration or customs? I've been a private pilot for many years, but I have never flown internationally in small private aircraft. I'm about to buy a tiny, high-tech, super-efficient 2-seat airplane that has extended range fuel tanks that make it possible to fly 8000km == 5000 miles without refueling (24+ hours).  The longest flights necessary to cross the south-pacific in hops is 4000km (about 16 hours of flight at 250kph).

Such long range makes it possible for this tiny aircraft to fly anywhere on earth.  Nonetheless, I'd have to be nuts not to stop more often than every 16 hours when that is possible: for a meal, potty stop, walk around and stretch body & legs... and eventually sleep!

This raises the asked question.  Can a pilot, with or without one passenger, stop in other nations JUST to refuel without needing to go through immigration (and get a visa for that nation)... just to land, pull up to the gasoline pumps, fill them up, get back into the airplane, then fly away?

Note that this is NOT a question about flying in a commercial airplane on a conventional scheduled airplane flight. I also presume the stop will not involve going inside any building or leaving the refueling area within the private aircraft portion of the airport. It will involve getting out of the airplane, because refueling of diverse small private airplanes is generally done by hand by pilots, not by the huge trucks that fuel jet airplanes. Typically small airplanes just pull up to a pump much like you'd see at any gasoline station, insert credit card, pump fuel into the wings, then fly away.

When I get the airplane, after a month or two of getting used to it in north america, I plan to flying down to Chile with 1, 2, 3, 4, 5 stops between (depending on these rules), then fly to dozens of islands in the south pacific (and land many places and islands that have airstrips, but no buildings (or at least no legal/government buildings).

In the later cases (south-pacific islands with tiny populations), I suspect nobody bothers or cares, because they assume all air traffic is strictly local. But flying down through (or 20 miles offshore) of Mexico through central America then Columbia, Ecuador, Peru and then finally into northern Chile (where I intend to spend a great deal of time) may be another matter entirely.

While it depends on the country, expect to have mandatory immigration checks on first port of entry in most countries. I know this to be true for the US, Canada, Britain, Russia and the EU (Schengen area). Usually only emergency landings are exempt. But they may lead to investigations so not really an alternative to declare one in each and every country.

Usually it's best to hire a service provider to prepare all the paperwork for your trip. While their main business is to prepare commercial flights (non airliner), most also accept private pilots as clients, a few even welcome them. They will not just plan your journey regarding visas, landing and crossing permits (some countries do require preregistration for each and every flight into their territory), but also make sure to pick only airports with the right fuel available, or making it available. Selecting and booking hotels can be included as well.

Anyone I know, who ever tried to fly further than the next door country, sooner or later took such service as it makes a huge difference. Well, some had to make the experiance to get stuck for a week at some border first :))


Why are cockpit instruments white on black dials? Which is easier to read:  white on a black dial or black on a white dial? It would seem most aviation instruments are white on black.

Right instrument face colors are inverted.

Black and white are perfect complementary colors, meaning:

  When placed next to each other, they create the strongest contrast for those particular two colors.

But for instruments if they were inverted, strong daylight shadows will make the numbers and hands harder to read, while at night with the white typically being electroluminescent or reflective for the instrument's red or white light will cause the glow to wash over the black as illustrated below:

The red is used for easier adaptation to the outside at night. Too much glow in either case goes against that.

As for digital displays, the same principle applies. Try reading an eBook in bed with the background white and the brightness to full -- it's very tiring -- that's why eBook readers have a night option where the colors are inverted.

History

The earliest mention of it I could find was from 1937 in Flight when instrument panels (the six-pack) were being standardized:

  The panel itself (...) is first anodised and then stove-enamelled a dull black -- though it may be said that the matt-grey anodic finish is very much better looking than the black panel and does not reflect light or in any way trouble the eyes of the pilot.

And here's another article from 1939 about solving the lighting issues for the instruments, not just the panel. But that wasn't always the case:

(Flight - 1913)

Image source: wikimedia.org


Where are the fuel tanks located in an Antonov An-225? Does anyone have answer for where the fuel tanks are located on the Antonov An-225?
(photo attached for explanation if anyone could)
Would love to know how many tanks are on the aircraft and how the system works.
*Please let me know if I've colored right sections on the wings indicating the fuel tanks positions.. if not, let me know which ones to modify.
Really appreciate your help!

The 225 is derived from the An-124, which has 10 integral fuel tanks in the wings. Comparing the specs, it looks like fuel capacity is the same as the An-124, so the center section of the wing (the only new part of the wing) doesn't contain fuel.
Based on that, I think the tanks are in sections 2-6 and 13-17. I'm assuming the outer wing sections 1 and 18 are not used as tanks. If those were used as tanks you'd have higher loads on the wing during roll maneuvers.


Is there an FAA regulation to switch off the air-conditioning during take off? The warning above the big white switches in the following picture had me intrigued. Is there some FAA regulation related to the same?

It is not an FAA regulation to have the air conditioning off during takeoff and/or landing, however some manufacturers require it, in which case the pilot is legally obligated to comply with it.

I have flown two aircraft (both jets) which required it to be off:

Some of the Learjets require the AC to be off for both takeoff and landing because it uses a high draw electrical motor to drive the compressor, and when it cycles off and on it causes electrical spikes which can cause problems with the anti-skid system.
Some of the Dassault Falcon jets require the AC to be off for takeoff (actually, they automatically turn it off, but if the automatic system is inoperative the MEL requires turning it off manually) for performance reasons.  Since the AC is these aircraft is using bleed air off of the engine compressor as a source, allowing the air to continue through the engine instead increases thrust.

I know that many airliners have the option to takeoff with it on or off for performance reasons, and provide performance charts or adjustments for both.


What can passengers do if they realise their plane is not on the right path? In the case of flight MH370, I think passengers on board would have long enough time to realise that something is wrong with their flight. I have a habit of carrying a compass with me when I travel abroad. If the plane is not heading in the general direction towards its destination once it reaches cruising altitude, then I would be worried. This is especially so for passengers on board flight MH370 when it appears that it is flying in the opposite direction for a very long time.

What can a passenger do if he realises that the plane is not flying in the right path, i.e. a possible suicide mission or hijack?

Okay, so, going with the idea that you have a compass (and also with the idea that this can be a general question, not necessarily about flight MH370).  If you're heading from Tokyo, Japan to Seattle, Washington, USA (direct), then you could be reasonably certain the craft should be somewhere between headings 45 and 135.  So if you notice the craft is on a heading of 270 (ie., clearly you're going in the opposite direction) and this direction has been sustained for say, an hour, I'd think it would be okay to at least ask the cabin crew what's going on.

As with any time you are raising a concern with the cabin crew, you should be polite.  As has been stated, they are used to passengers being a bit nervous and, as a result, they can easily interpret belligerence as a sign of irrational fear.  And, as you might guess, it's easy to ignore someone if you feel they are being irrational.  So, if you want be taken seriously, be polite.  You're far more likely to get an actual answer, or raise a valid concern even.

So, if you just ask, "why are we headed west instead of east" they may have a very simple answer like, "well, the pilot mentioned we might have to take an odd route to avoid some weather" or "this is the flight to Bombay India, how did you get on it?"  Or, they may have no idea and be concerned themselves, so they go and ask their superior.  Thus, goal achieved, you've alerted the flight crew :).

Either way, so long as you're polite and don't become insistent (or belligerent) that you are correct, I can't see any harm in just asking.  And sometimes asking is what leads to a problem being solved so...  I'd say go for it.


Why does overspeed cause structural damage or failure when limit load factors are not exceeded? The positive and negative limit load factors in this diagram are around 4.5 and -1.8, which means the aircraft experiences structural damage when the aircraft is flown beyond these limit load factors, and beyond the positive and negative ultimate load factors, which are around 6.6 and -2.8 respectively, the aircraft would experience structural failure. These excessive loads typically occur when the aircraft is maneuvered abruptly as in steep dives and steep turns or when the aircraft is flown in turbulent air.

I understand it's excessive loads that cause structural damage or failure when an aircraft is flown beyond the positive or negative limit load factors. But what induces structural damage or failure when the same aircraft is flown within the limit load factors?

My best guess is the parasite drag, but the fact that the normal operating range (green) within the limit load factors jumps to structural failure (red) without going through structural damage (orange) leaves me at a loss, because if the cause is the parasite drag, I don't see the reason structural failure should occur without the structural damage being first experienced.

Edited in response to Simon and kevin's advice

Flying way too fast is an example as shown by the graph. For example descending with full power while maintaining 1 G load factor. Or with a powerful enough engine, exceeding the speed limit in level flight.

The air impacts dynamic pressure on the aircraft, the faster the plane, the higher this pressure—up to the design limitations.

What causes structural failure when the aircraft is merely flown fast?

High speed induces vibrations, flutters, etc., all would likely cause loss of control and/or structural damage/failure.

  Flutter: At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or lead to the destruction of the aircraft.

(Source) Flutter.


What will happen on Airbus A320-200 series when during push back NW STRG DISC in not in the towing position? During pushback what might happen if the NW STRG DISC is not indicated green on ECAM?
Will I have an ECAM warning?
What would be the worst case scenario that might happen?

The name says it all. NW STRG DISC or in full form the Nose wheel steering disconnect. And that is what exactly it does. It physically disconnects the nose wheel steering of the aircraft. In most aircraft, with nose wheel steering disabled, the wheel is allowed to caster. This allows pilots to maneuver the aircraft with the differential braking in case the nose wheel steering fails during taxi. So, yes even an aircraft the size of an A320 can be steered like a small aircraft with a castering nose wheel. So, a tow truck can turn the nose wheel of an A320 just like that.
The problem is that during push back the aircraft engines are started. With the engines started the aircraft hydraulics are powered and pressure is sent for nose wheel steering. The nose wheel, without any inputs always centers it self. During the push back if the nose centers it self, it can damage both the tow truck and the nose wheel of the aircraft. To prevent this the nose wheel steering has to be disconnected for push back. In the A320, there is a lever on the nose wheel which can be put in the towing position. Once it is placed in position, a bypass pin is inserted to ensure that the lever is held in place. Once the lever is positioned an ECAM memo NW STRG DISC is displayed in the cockpit in green. The message turns to amber colour when one of the aircraft engines are started. The whole point of the ECAM message is to increase pilot awareness so that you do not mistakenly leave the lever in tow position before commencing the taxi.

An Airbus A320 with nose wheel bypass pin inserted.


How common and dangerous are fume events? I’ve been hearing about fume events where engine oil containing tricresyl phosphate as an additive can leak onto the HVAC packs and enter cabin air. These organophosphate are incredibly toxic and are used in nerve gases. There are even cases where pilots have died such as here (AVHerald) and here (BBC).

The AVHerald article you linked to pretty much as the answer to your question:

How common are they?Nearly 2,000 events a year in the US alone.

  [According] to the Kansas State University Research there have been 5.3 fume events per 24,000 daily flights in the USA or 1955 fume events a year. However, only 6 fume events per year get reported to the FAA.

How dangerous are they?In extreme cases, they can lead to death, but even mild cases diminish flight crew capabilities.


What is this aviation quote? I remember hearing a quote about aviation that was something along the lines of “asking a pilot why he loves flight is like asking an artist (or a painter?).....”. For the life of me, I cannot remember what this quote is; however, once I saw or heard it, I fell in love with it. I wish I had wrote it down but I didn’t. I could have sworn that I saw this quote on the film “One Six Right” but after going through it multiple times, I have yet to see it written on the screen or heard a person on the film mention it. This question has been bugging me for a couple years and I cannot seem to find any help googling it. Thank you for your help!!

This quote is from Barry Schiff, a retired pilot from TWA and aviation consultant (but this is only the beginning of the story, I invite everybody to read the article on Wikipedia):

Photo stolen from AOPA

Your recollection is accurate! This is indeed in One Six Right.

Landing on 16R at Van Nuys, Youtube

He says that at the beginning (3:06) of the documentary. The video is not available online, but the first sentence of this quote is often taken up, e.g. on page 39 of this Midwest Flyer issue.

The quote in its context:

  “Asking someone why they love to fly is almost like asking someone why
  they like Picasso. The beauty is the fact that you can’t really
  describe it, it’s something that you have to behold. Flying takes you
  into another dimension, literally. We see, feel, and experience things
  that people who don’t fly will never know.”

  Barry Schiff, Aviation Safety Expert / Pilot (source)


What does AOE on the sectional chart mean? What does the AOE notation on the sectional stand for? This appears at the bottom of the information for an airport as the final line.

It stands for "Airport Of Entry" (see VFR Terms) In other words there are customs facilities there.

  All civil, private aircraft entering the U.S. must first land at an
  airport of entry before continuing to their destinations, unless other
  arrangements are made with U.S. CBP. Advance notification must be
  provided electronically to CBP by means of the eAPIS. See the APIS
  section for more information.
  (source)

To my knowledge this is also common for international operations outside of the US/FAA jurisdiction although im not sure how its marked on various maps elsewhere.


How much might soft skin reduce aircraft skin friction? According to Peter Kampf's answer here, soft skin can damp a (vibrational?) mode that cause laminar flow to become turbulent. Thus, it could help delay boundary layer transition. My question is, for a modern large transonic jet (737+), how much may this reduce drag?

This has mostly been examined for aquatic flow, e.g. with the soft skin of dolphins. Studies were done with test setups because measuring the boundary layer around live dolphins is exceedingly hard.

One study compared a rigid cylinder with a soft one of the same dimensions, using a polyurethane coating. Below is a diagram from that study with 3 denoting the rigid and 6 the soft cylinder. As you can see, the difference is small but consistent over the range of Reynolds numbers tested. Picture source here.

For dolphins, active regulation of muscle tension might be responsible for a resonant vibration of the skin, but much of the literature is still rather speculative. From the study:

  Based on experimental research on live dolphins, a hypothesis has been formulated (see Chapter 1); this is that by the regulation of the skin muscle tension dolphins promote resonant interaction with a flow. This leads to interaction of fluctuations of the boundary layer and natural frequency of fluctuation of the dolphin skin. As a result, the skin coverings vibrate with resonant frequency practically without energy consumption. Fluctuations of this coating occur at a frequency that controls the frequency of bursting from the viscous sublayer. This is also the reason for the abnormally low resistance, which has been discovered in the authors’ modeling experiments.


Do I need a trim tab system on my twin tail? I plan on building a single engine H-tail aircraft with retractable landing gear, removable wings for transportation, and either split flaps or plain. My only problem is, do I need a trim tab system to handle the workload, or should the air from the prop not interfere with the lift and cause problems with aileron, elevator, and rudder control, increasing the muscle the pilot must use to handle the aircraft?

Yes a trim system would be a most excellent idea. If you can trim the aircraft such that it just flies straight on when you release your hands and feet off of the flying controls, the flight will be more enjoyable than when you need to keep pulling one way for the duration of the flight, like walking while holding up a briefcase in front of you.

Trim adjusts the neutral point of the flying controls: the point where the stick returns to when released.


Why are the B-52's vortex shedding panels angled? Their function is here: What is the purpose of the serrated trailing edge spoilers on the B52?

While watching this video (around the 1:53 mark), I noticed a number of angled devices on the trailing edge of the spoiler.

Why are they angled like that?

The angle of the edge shapes is such, that as the spoilers are stowed, the edge shapes are aligned with the airflow over the upper surface of the wing.

If the shapes were perpendicular to the spoiler trailing edge, they would form an angle of about 35 degrees to the airflow. This would cause turbulence, as the spoilers are not retracted 100% flush with the upper surface of the wing. The turbulence would lessen the lift of the wing and the effectiveness of the flaps (shown extended in the picture), and add drag.


What is the initial direction from Sydney to Santiago on a great circle? When flying the great circle route from Sydney to Santiago, what direction do you head leaving Sydney? Does the path leave Sydney heading SE and approach Santiago strictly from the SW, like is shows in the graphic from this answer?

Yes, the map is accurate. You can see the same thing on this map from gcmap.com:

On a polar orthographic projection, the great circle looks like this:


How many of the airliners that ever existed have been lost in flight? Another way of asking this is: Of all the airliners that have taken off and returned to the earth, what proportion have actually landed?

Or another way: If you were born as an airliner, what is the probability that you would die at work?

I'm specifically interested in:

airliners, being used as airliners (for example, not as emergency troop transports)
hull-losses
aircraft that took off and then crashed or were otherwise lost (i.e. not simply destroyed while on the ground)

We'd have to start with the total number of airliners constructed, which at a guess is a number in the region of 50-60 thousand - but perhaps someone has a reasonably accurate figure already collated from available sources.

Then we'd need to know the total number lost in flight - I guess it's just possible someone has such a figure.

It would be interesting to see how the figure changes for aircraft limited to particular eras.

Related questions

A related question has been pointed out in the comments, What percentage of airplanes are involved in a crash in their lifetime?

That's not explicitly about airliners, though in fact most of the discussion there seems to be based on airliners, and it contains some useful figures, including in one of the answers:

  That's a rate of 1.56 % aircraft with a fatal accident, or 1 / 64.

This is just an extrapolation from data about 2014, but it's a start.

However, it is based on incidents that kill people, rather than flights that damage airframes beyond repair, so it is about something slightly different.

Okay, let's try for a first approximation. Some heavily-produced airliners; for simplicity I'm only counting jets. (ie, I don't want to try and work out how to deal with the DC-3...)

A320 family. 7481 built, 35 hull losses (0.5%)
A330. 1330 built, 11 hull losses (0.8%)
Boeing 727. 1832 built, 118 hull losses (6.4%, mostly out of service)
Boeing 737. 9401 built, 184 hull losses (2.0%)
Boeing 747. 1528 built, 60 hull losses (3.9%)
Boeing 757. 1050 built, 8 hull losses (0.8%)
Boeing 767. 1097 built, 15 hull losses (1.4%)
Boeing 777. 1467 built, 6 hull losses (0.4%)
DC-9. 976 built, 101 hull losses (10.3%, mostly out of service)
MD-80. 1191 built, 35 hull losses (2.9%)
Tu-134. 854 built, 69 hull losses (8.1%, mostly out of service)
Tu-154. 1026 built, 69 hull losses (6.7%, mostly out of service)

This is a partial list, but it includes (I think?) every type with around 1000 or more examples built. It is thus probably representative of the general population of jet airliners. Total 29,322 aircraft, of which 711 (~2.5%) were eventually hull losses.

Now, not all hull losses are flight accidents - there are ground fires, accidents, military action, hurricanes, all sorts of other things - but the majority are. It's probably reasonable to estimate that the overall hull-loss-due-to-flight-accidents-so-far rate is thus a little lower, around 2%, but probably not much below that.

Now, it's worth noting that this isn't quite the same as

  If you were born as an airliner, what is the probability that you would die at work?

...because some of these aircraft will probably be hull losses, it just hasn't happened yet. If Boeing stopped building 747s tomorrow but airlines kept flying them, the hull loss rate would steadily creep up over time until all examples were either crashed or retired. So if you want to work from that perspective, the "eventual accident rate" will be higher. I've flagged the ones which are nearly completely out of service, where these will be approaching a "lifetime" loss rate - they're noticeably higher than the others.


How is the climb/descent performance affected by wind gradient? I am exchanging opinions with other pilots concerning climbing and descending with a wind gradient, and I'm facing strong opposition... In my opinion, a plane trimmed for S&L flight, if it encounters a headwind with a positive gradient (wind getting stronger with altitude) will gain altitude, since–because of the gradient–it will harvest some of the wind energy. In other words, part of the energy of the wind will be converted in potential energy of the airplane.

When descending in a tailwind with a positive gradient, things will take place in reverse: the plane will lose altitude, since a part of the potential energy of the plane will be transferred to the moving air, resulting in an increased KE of the wind...

Is my theory correct?

You are correct.

Suppose the headwind just 10 meters above you is 10 knots stronger then where you are now. Climbing the 10 meters will cost you some kinetic energy which is transformed to potential energy. Suppose you were flying 200 knots airspeed initially, you will end up with 198.1 knots airspeed if the transformation from kinetic to potential energy is 100%.

I derived this from simple kinematic energy equations:

$\frac{1}{2}mV_0^2 + mgH_0 = \frac{1}{2}mV_1^2 + mgH_1$

$V_1 = \sqrt{V_0^2+2g(H_0-H_1)}$

But since your headwind is increased by 10 knots, your true airspeed will be 208.1 so it has increased by 8.1 knots. Free energy from the wind!

Note that your groundspeed will reduce during such a climb, but you will be able to generate a higher than usual climb rate at constant airspeed in a positive gradient headwind field.

Note that birds make use of this technique, for example the albatross can stay airborne for days with using only minimal energy for keeping in the air.

Albatross in windshear field (youtube)


How should ground crews act to safely extinguish an airliner's brake unit fire? When the brake unit was overheated and had just begun to catch fire due to excessive braking because of long taxi distance upon arrival, the ground personnel (not firefighters) made the first intervention with the fire extinguisher on the parking stand until the firefighters arrive.

I’m wondering, are there any points to be taken into account in terms of ground safety when approaching landing gear during this first intervention or in the case of extinguishing? Is there any guidance procedure for ground crew other than professional firefighters?

What type of extinguisher content should be used to extinguish the fire?

Regarding the related post: How do fire crews handle hot brakes?

I don’t think it’s the same topic. I would like to know if there is a guidance document explaining how the ground crew [not fire crew] should behave in the event of a brake fire upon aircraft arrival.

Approach from upwind. The source of fire is hot brakes so apply maximum cooling, say water-based. But hydraulic fluids under pressure burn, so if brakes are engaged it's best to ensure with flight crew that the brakes are off as soon as the aircraft is stationary. Stay in the vehicle in case tyre ruptures; chances are it will go sideways. If fuel is leaking, use suitable foam.

I once saw a Harrier with a brake fire.  The burning pressurised brake fluid was acting like a flame thrower on the aircraft structure. The aircraft was written off. This happened about 1981.


Can an aircraft or helicopter run into its own wake vortex? I was recently flying my rc helicopter and I would sometimes turn it around and move it forward and sometimes I lost control of the helicopter while doing that a few times and crashed it everywhere (accidently crashed it into my poor cat). Every time I fly this helicopter it randomly starts turning to the left uncontrollably. Also I have had where I kept the thrust at the same level and then I would fly it into a different room and it would start raising in altitude uncontrollably. So can a helicopter or aircraft fly into its own wake and was it the wake of the helicopter that caused these problems with my helicopter?

Helicopters are not easy to fly - whether full-scale manned aircraft or RC models. I would bet that your issues are more related to pilot error than any fault of the machine (other than the fact that it is a helicopter, and inherently unstable).

That said, almost any aircraft can be maneuvered in such a way as to encounter its own wake, and the helicopter is no exception. In fact it's much easier do to this in a helicopter than in a fixed-wing aircraft: A condition known as "settling with power" (or "vortex ring state") can occur when a helicopter descends into its own rotor wash, e.g. by descending directly vertically in calm air. The results of such an encounter can be "unpleasant" - a number of helicopter accidents have been attributed to this sort of encounter.


How are jet engine RPM cycles used to monitor LCF life? Can someone please define what are N1 and N2 throttle cycles? These are used to keep track of the accumulated low cycle fatigue (LCF) life of rotating components. But I cannot find an exact definition of them.

"N1" is the low pressure shaft speed, and "N2" is the high pressure shaft speed. N1 and N2 throttle cycles are when the throttle is moved up-down-up (or vice-a-verse), which causes the N1 and N2 speeds to go up-down-up, and hence "around" like a "cycle".

However, you will not find an "exact" definition, because it varies from engine to engine, even for the same manufacturer. In addition, the exact numbers that define the rpm changes required are usually not publicly available, as they are probably commercially sensitive.

But you can find a general description of "throttle cycles". Low cycle fatigue (LCF), is monitored by counting stress cycles. LCF is what causes a wire to break by bending it back and forth, which is a much easier way to break it, than by pulling each end. Cycling the stress back and forth (up and down) fatigues the wire and it breaks. The same thing happens in gas turbine rotating parts. As they speed up and down (in RPM) the centrifugal stresses increase and decrease. This can cause the rotating parts to fail. So they all have a "hard life", which when reached, the part is thrown away, regardless of condition.

In old engines, that don't have a monitoring system, they don't actually count each time the speed goes up and down. Instead, the engine manufacturer analyses the typical flight, and works out how many cycles occur per hour, on average. This gives an exchange ratio between hours and throttle cycles. See page4.

In modern engines, they actually count the cycles that occur. This means, if they fly the plane slightly differently than intended, the life being consumed is correctly recorded, whereas the exchange ratio would be wrong. (Though the life limit still needs to be verified as correct for the different mission profile).

Typically, 1 cycle is from idle to max speed, and back to idle. 60% to max and back to 60% may be 0.5 cycles, and 75% to max and back to 75% 0.25 cycles, as an example. So, if you take-off, from idle, do a climb at max RPM, throttle back in cruise to 60%, return to max before landing, and then shutdown the engine, you have accumulated 1+0.5+0.25=1.75 cycles in the flight. (Recognising that when you go from 60 to max back to 60, you have meet the criteria for both the 0.5 and 0.25 cycles.)

e.g. Another example of throttle cycles:


Can Ospreys land without running engines? I just have seen a video of a demonstration of an Osprey V-22. While this is an impressive aircraft I wonder whether this might land with failing engines (with the large "propellers" probably hinder proper gliding).

What would a pilot do in the following cases:

one engine failed
both engines failed
tilt mechanism stuck

Welcome to AviationStackExchange and to provide a short answer to your question, yes there are autorotation measures that are similar to those featured on helicopters. The basis for most of my answers pertain the unique drive shaft system, which allows for redundancy between proprotors, something other twin-engine fixed wing aircraft usually lack, so here's a diagram for some reference:

As for the detailed answer, I'll split it into three parts: One engine failure, total engine failure, and a stuck rotation mechanism.

The two RR T406 type engines of the V-22 are connected by a driveshaft, which means that if one engine loses power, the other, provided that it is in good working order can deliver power to both proprotors through said shaft. It lacks the power to actually hover on that one engine though, so it has to be glided down, and protocol presumably dictates a powered glide to somewhere safe.
When both engines fail, autorotation just as on a helicopter is instructed, however, due to the fact that the proprotors are about a third of the diameter of conventional rotors means its low rotational inertia results no autorotation will significantly reduce the impact from a total failure. In other words, it can't really glide like a plane and won't autorotate like a helicopter. It's getting the best and worst of both worlds.
As long as the engines are working, the V-22 can land and take off in both configurations. However, there's an approximately 18-20% (Wikipedia quotes 18, Boeing claims 20) increase in flight efficiency in the turboprop aircraft configuration over the VTOL setting. I can only imagine that number goes higher by 10-15% during takeoff and landing. Planes are simply more efficient this way.

So, yeah.


Why are there some airports not charted as service-available when FBO provides fuel? On FAA's Aeronautical Chart User's Guide - VFR Charts.

It says the tick-mark charted airports provide fuel service. However, at Airport KSIY and 1O5,

There are FBOs at both airports. e.g at Airport 1O5

Is there any reason why these service-available airports not charted as ticked. Do they need to satisfy any FAA requirements before charted?

The chart supplement states that 1O5 is unattended, which fails the attendance requirement. And fuel is self-service for both 1O5 (FBO: Steelman Aviation) and SIY (FBO: Eagles Nest Aviation), which is also a reason for not having the tick marks.
Tick, tock
Your concern was discussed in the Charting Group Meeting 15-01 on April 28-30, 2015 (exactly two years ago):

Many pilots believe that the tick marks indicate the airport has fuel.
Over the past 20 years, many airports have gone to self-service fueling, eliminating the need for an "attendant" to be on duty to provide fuel services.
Several airports have contacted us, requesting the tick marks be placed on their airport symbol because they have fuel.
Recommendations:
Revise the charting criteria to:
Services available – To qualify as an airport with "services available", the minimum requirement that fuel is readily available (self-service or via attendant) 24 hrs/day.
Eliminate the requirement for attendant hours.

Outcome:

Rich Fecht, AJV-5223, reviewed the topic. Rich stated that the IACC Requirement Document has been signed and is ready to implement. Visual Charting is parsing through the data to get a list of airports that meet the fuel availability requirements and will soon start updating the VFR charts. Rich anticipates that the updated airport fueling information to be reflected on all charts within about two years. This change will affect public airports only.
Lynette Jamison, AJR-B11, stated that the proponent of this item should reach out to the state inspectors to let airports know how they can get fuel availability shown at their airport.

That means that both airports should get their tick marks soon enough if they want to.
2018 Update
With the release of new sectional charts on 29 Mar 2018, SIY got the tick marks:

(skyvector.com)

2021 Update
Both now have the tick marks:

Note: the chart supplement does not have to mention temporary shortages according to IACC 8 section 3.1.5.5.4.5:

The following type data shall NOT be published in the SERVICES and REMARKS sections:

g. Temporary shortages of certain types of fuel, starting equipment, or other aircraft services;


Why does a mid-wing aircraft have lower drag? Why does an airplane having a mid-wing have lower drag than high- or low-wing configuration even if they have  wing of same shape and size?
This article (PDF) claims it's true.

It's because interference drag (the vertical flow field forced to share space with the horizontal flow field) is minimized when the wing meets the fuselage at a 90 degree angle or more.
With a low wing airplane with dihedral, even if the sides of the fuselage are vertical, there is still less than a 90 degree angle due to the dihedral angle.  To mitigate the effect of an acute angle at the junction, traditionally a fillet fairing is installed to provide a curving transition between vertical and horizontal, reducing the intensity of the interference closest to the fuselage and minimizing flow separation.
Ideally however, it's better to have as close to a 90 degree intersection, as possible, or better yet greater than 90 degrees, and a mid wing makes this easy to do.  And if the fuselage cross section is circular, like the Grumman Wildcat, even better because the intersection is effectively a bit more than 90 degrees because the sides are curving away from the joint.  The mid wing has a number of packaging problems however (like the Wildcat's crazy landing gear, spar going right through the key cargo space on transport aircraft, etc).
The Corsair fighter killed two birds with one stone in going with the gull wing configuration; it lowered the wing at the landing gear attachment to allow a much shorter strut, and it maintained a 90 joint at the fuselage even though it was attached near the bottom.  The Corsair's gull wing achieved the benefit of a mid wing design aerodynamically, and a low wing design in its landing gear configuration.  Look at a Corsair and there is no fairing at the wing root you normally see on low wing fighters.  No fairing was necessary.
The Thorp T-18 homebuilt addressed the problem to avoid an acute angle at the wing to fuse junction, with a simple low wing, by starting the dihedral half way out on the panel, so that the mid section of the wing was flat and came out of the slab sided fuselage at 90 degrees (most T-18s use a very small fillet fairing at the back to reduce turbulence from flow separation aft of the thickest part of the wing, although some builders go with fancier ones). Thorp got the added benefit of a panel that could be built in two 4ft panels with the dihedral joint between them, allowing the wing to be skinned from a single trailing-edge-to-trailing-edge wrap of a 4 ft sheet of aluminum for each panel. Very easy to build.


Why does the CF6/CFM56 Turbofan Fan Frame contain 12 circular holes between the primary and secondary flow? This question is related to the answer I found on this particular thread, about the Frames in a Turbofan.

In a turbofan what holds the spinning axis?

I went on this thread after some research on Google, since recently I started working in the industry. I'm working on this particular part of the frame, and I would like to understand more about it. Also I am currently working on understanding more about jet engines in general and how they work, I find this absolutly fascinating since I found a lot of information and answers to my questions - and now for the curiosity :)

My question is about the Fan Frame here (GE CF6/CFM56 Turbofan on pics)

I would like to know the use of the 12 Circular Holes on the inside of the frame, between the 2 flows (Low-pressure compressor way, Right?), you can see here what i'm talking about on those pictures (Red arrows).

(I don't think this is to make the part lighter, because the outside edges of thoses holes, are re-inforced a lot with a big extra thickness.)

Thank you.

Short answer

These holes are vents used to discharge air from the primary (core/hot) flow into the secondary (bypass/cold) flow. They are normally closed by valves (variable bleed valves, VBV). The valves open to maintain the compressor pressure/flow within a safe area.

VBV in operation: Youtube

VBV on a CFM56-5 (also visible the VSV). Source

VBV are a mean to prevent a compressor stall in turbo-engines. A  stall appears in situation like this:

For some reason, e.g. when thrust is reduced, pressure at the compressor exit exceeds by a large amount its inlet pressure.
When the pressure ratio is beyond the compressor capability, air at the exit starts opposing compressor airflow, a reverse pressure wave is created, which propagates to the inlet.
Flow over one or more compressor blades is disturbed and the airfoil stall angle is reached creating an aerodynamic stall.
The stall is extended to the next blades, i.e. a rotating stall develops.
The pressure ratio is getting unstable.
Thrust is suddenly reduced, attempts to increase thrust accelerate the stall.

The VBV are used to adjust the compressor pressure ratio, by discharging some air between the LPC and the HPC. This decreases both the pressure ratio of the LPC and HPC and helps preventing compressor stalls.

Holes location

The fan frame assembly shown here without its outer shroud:

Fan frame assembly, without its outer shroud, of a CFM56-7B. Source

comprises two rings of 12 stationary vanes which are crossed by:

The primary (core/hot) flow for the inner vanes
The secondary (bypass/cold) flow for the outer vanes.

The 12 holes/vents in the mid-box structure of the fan frame assembly are closed by 12 variable bleed valves (VBV):

VBV in a CFM56-5A. Source

The valves are protected by a grille:

Source

Location of these elements in the engine:

Location of the fan frame in a CFM56-5. Source

Details:

VBV on a CFM56-5. Source: CFM56-5 Training Manual

The master valve is controlled directly by a motor, the other valves are linked to the master VSV by a flexible shaft.

5: Fuel gear motor powered by HMU, controls position of the master VBV
1: Master VBV
2: Position feedback
4: Position sensor
7: Flexible shaft controlling the other valves

VBV use

As regard to the cold flow, the inner duct is located between the low pressure compressor (aka booster) and the high pressure compressor. The pressure delivered by the compressor section is optimized for maximum engine power.

When the engine speed is low or the engine is decelerated, the LP compressor pressure can increase and clear the LP compressor stall margin. In such a case, the VBV are opened to release some air into the secondary flow and restore the margin. Conversely, when thrust is increased, the risk of stall disappears, the valve is closed to restore efficiency. The following diagram shows how the action on the valve contributes to keep the compressor flow in the designed working area.

Stall prevention using VBV. Source

More on VBV and stall prevention:

What is the difference between variable and transient bleed valves?
How does the FADEC prevent compressor stall during rapidly increasing thrust demand?

VBV for debris rejection

On military aircraft, unsuccessful attempts have been made to use VBV to capture and reject debris and water. The principle is to create a bend in the airflow with an opening at the start of the bent section. Heavier elements are captured by the hole. CFMI has reintroduced this technique in the LEAP engine.


Why must the long cross-country flight for a commercial rating be solo? Assuming I am interpreting the below FAR correctly, one must either be solo, or have an instructor present to complete the long cross country requirement. More specifically, it seems to me that you could meet the requirements with a passenger present if an instructor is on board.

§61.129   Aeronautical experience.
(4) Ten hours of solo flight time in a single engine airplane or 10 hours of flight time performing the duties of pilot in command in a single engine airplane with an authorized instructor on board (either of which may be credited towards the flight time requirement under paragraph (a)(2) of this section), on the areas of operation listed under §61.127(b)(1) that include—
(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point.

I'm trying to understand what the purpose of the solo requirement is, and how it could be alternatively achieved without being solo, but only if an instructor is onboard.
It's clearly not for safety of the passenger, since, as a private pilot, its completely legal for me to make the very same flight with a passenger; I just can't use it to meet the commercial requirement quoted above. Why not? And how could adding an instructor to the plane make any difference?

I got this answer to my question from a CFI...

The idea behind the solo requirement is to assure the pilot can accomplish the trip unassisted. Other passengers are permitted if an instructor is present because the instructor will assure that the passengers do not assist with the flight.


Why is it 'aileron right and elevator up' with a front-right wind in a tailwheel airplane? Question: How should the flight controls be held while taxiing a tailwheel airplane into a right quartering headwind?

Answer: When taxiing a tailwheel airplane into a right quartering headwind, use up aileron on the right hand wing and up elevator.

My answer: put the control to the right should be enough, why bother the elevator?

The wind could pick up the tail in tailwheel aircraft which is why they recommend elevator up to prevent that scenario.  If the wind is really bad the tail could possibly get higher than the wings and flip the airplane over.  This will not be an issue for tricycle gear aircraft so elevator doesn't matter in this case.  This will be tested on the FAA knowledge exam so you will want to remember this.


Why is magnesium not used more often as a structural metal in aircraft? Magnesium is lighter than aluminum for the same strength.  If the answer is corrosion, how much work has been done to improve the corrosion resistance of magnesium?  How much lighter would a magnesium structure be than an equivalent aluminum structure?

Magnesium has initially been held back by technical difficulties in the removal of chloride inclusions. These had been solved only after a lot of progress had been made in aluminum alloys. Also, magnesium parts should only be used in a dry environment - when the metal is exposed to water, rapid corrosion must be expected.

The first application of magnesium in aerostructures was in the 1930s (mostly under the trade name Elektron), with a small boom in the 1950s. Lockheed even built one F-80C fully from magnesium. Magnesium was widely used in the former Soviet Union, but eventually phased out in newer designs due to corrosion problems. Flammability, by the way, was never a real concern with magnesium.

With improved protection techniques, magnesium might make a limited come-back. It is already used in castings and extruded parts. Per kilogram, it is twice as expensive than aluminum, but very easy to machine. However, at higher temperature it will lose strength even faster than aluminum.

Parallel to that, aluminum sheet can be made lighter and stiffer by adding up to 2.5% of lithium. These Al-Li alloys have found their way into the newest aircraft designs and will raise the bar for magnesium applications.

Magnesium and water really don't mix well: Trying to extinguish a magnesium fire with water is a memorable experience, as adding water will intensify the reaction. Use iron chips or sand instead to cool down the burning metal and to keep oxygen away. Historically, though, magnesium fires on aircraft have been extremely rare.


Why does a turbofan shut off when rapidly retarding the thrust levers? In a turbofan engine, when the thrust lever is at maximum and is then suddenly pulled back to idle, the engine will shut off and lose power.

How can this be the case as the engine should be at idle when the thrust lever is brought to idle? But in this case, the engine shuts off because of the sudden movement. What is the specific reason for this?

The rotational inertia of the fan will continue to push air through the engine after the fuel supply has been reduced. Reduce the fuel supply too quickly and the result is like holding a match in a gale: the wind blows the flame out.

To prevent this, modern engines use digital controls (FADEC) to ensure that the engine is kept within its operating parameters regardless of what the pilot might do with the throttle.


Do helicopter rotors have anti-ice systems like propellers and wings? Icing disrupts the aerodynamic qualities of wings and props, is there a similar danger with helicopter rotors?  Are there de-icing systems to prevent this?

Yes, helicopter blades are no different from wings and propellers when it comes to ice, and there exist rotor de-ice and anti-ice systems for helicopters that require the ability to fly in known icing conditions, typically military types. Some examples include the Sikorsky S-92 and MH-60, and the Kamov Ka-50.

Rotor anti-icing systems are usually electric devices like heated mats, which are mechanically simpler that typical fixed-wing solutions like pneumatic boots and bleed-air-heated leading edges.

Not only the blades require anti-ice systems, however: elements like the engine intakes and air data sensors also need to be kept free of ice just like in fixed-wing aircraft.


Can you receive traffic advisories without a specific destination or intention? I’m wondering if it is possible to call up the local approach facility and ask them for flight following even if you are just doing sightseeing or something. Do you need to tell them your intended route, altitude, and destination? How would you ask them for flight following without a specific destination?

You call up ATC, tell them that you are out sight-seeing and would like flight following and traffic advisories.  On a workload permitting basis ATC would provide the service.
Here is an excerpt from the FAA Air Traffic Control Handbook:

Section 6. Basic Radar Service to VFR Aircraft- Terminal
7-6-1. APPLICATION
a. Basic radar services for VFR aircraft must include:

Safety alerts.
Traffic advisories.
(...)

This is a very common request and depending on the location there should be very little difficulty in receiving this service.


For a teardrop shape, which orientation has the least drag? Here's the shape:

(Well, that's a cross section of the shape. The actual object is a 3D shape obtained by rotating that outline around the y-axis.)

Let's pretend it's a solid, rigid object flying through the air. It can fly point-first or curve-first. Which way produces the least drag?

Assume it's normal air at room temperature. The angle of attack will be constant. It will be subsonic speeds.

BTW, is there a huge difference in drag? If not then it doesn't matter much.

The design looks similar to the wheel fairings found on many Cessna-type aircraft. They lead curve-first.

If it was going tip-first, you could have a nice laminar airflow initially, but once it reaches the curve, the airflow will separate from the surface and produce a big wake with associated drag. By leading with the curve you can have a laminar flow right to the tip, with only a little bit of drag.


In the traffic pattern, what is the appropriate distance from the runway for the downwind leg? Specifically for a piston general aviation aircraft, is there a recommended distance from the runway to fly the downwind leg? Does this change based on the speed of the aircraft?

Most instructors say that (in a low wing plane) you should see the runway on the tip of the wing or between the tip and 1/4 in the wing. AOPA has a nice over view of non towered airports here. This FAA brief states it should be flown 1/2 to 1 mile out (page 7-4 of FAA Airplane Flying Handbook, FAA-H-8083-3B).

  This leg should be
  approximately 1/2 to 1 mile out from the landing runway,
  and at the specified traffic pattern altitude.


Where is helicopter rotor attached on Allison Model 250 engine? Which section of the Allison Model 250 engine is attached to helicopter rotor?

It's not. The engine is connected to a gearbox that transfers power to the rotor head. I don't know of any helicopter where the engine is directly connected to the rotor head. If it were, the helicopter would not be able to autorotate after an engine failure, unless the failure allowed the engine to continue to rotate.


Why are autorotations to a full touchdown not practised any more? What is the history behind a decision to discontinue helicopter autorotations all the way to ground contact? In the 1960s, our Army flight program directed full ground contact for any and all autorotations. I will never accept that 'simulations' are a substitute for actual practice. We were taught every possible aspect of autorotating a helicopter possible, including a vertical autorotation.

Too many broken off tail booms.  The Army can afford to replace damaged machines; flight schools that can barely afford hull insurance on recip helicopters in the first place, if at all, not so much.  (Funny story: I once asked an aviation insurance broker why the hull premiums on recips were so high, like over 5% of hull value back in the 80s (before Robinson's program).  He said, "Cuz they all crash!!!!" and laughed.  He said mainly it was because they are used for the most high risk activities: ab-initio training and Ag.)

Anyway, the FAA decided some years ago to remove the full auto from all but the instructor rating because, in a nut shell, it was calculated that the number of chopped off tail booms from training accidents attributable to full autos is way more than the number of chopped off tail booms from real full autos, where the hard landing could be blamed on removing the training requirement. Effectively, choosing one risk profile over another because the statistics favored it.

Not too different from ending the old practice of live engine cuts during multi engine training.  Except for a simulator, a multi pilot never experiences a real engine cut on takeoff until it really happens.  Back when live cuts were done, there were way more failure-to-maintain-control crashes in training than crashes from real engine failures.


Why is wireless avionics intra-communications (WAIC) not used for air-air or air-ground communication? Since ITU approved the use of WAIC systems, aircraft designs have evolved and benefited greatly from fuel consumption savings, reduced maintenance (by removing/replacing costly and heavy cable wiring with wireless systems) and improved environmental footprint. However, these wonderful lightweight WAIC systems are restricted to provide bounded inflight services and does not provide air-to-ground or air-to-air communications. What is the reason for this?

WAIC is designed to be extremely low power.

Its low power is not be enough to reliably reach a receiver on the ground, satellite, or other aircraft.  Increasing the power would create all sorts of other problems, including problems with spectrum allocation, and data collisions.

There is simply no demand for a more complex system to do these other tasks.

The basic engineering principle is KISS


How does Jeppesen create its US charts? The Jeppesen and FAA IFR plates represent the same information in different ways. The FAA publishes its charts for free, but the underlying data that goes into the charts (e.g., exact coordinates of fixes, frequencies, etc) doesn't appear to be published in a structured format. The PDF files that the FAA publishes appear to be created using Bentley MicroStation, but do not seem to embed any useful metadata about what the various symbols mean.

Based on that preliminary research, I am wondering how Jeppesen creates their various charts, such as IFR approach plates? Do they simply take the FAA charts and manually create their own derived charts (which strikes me as tedious and error-prone)? Or is there some hidden repository of structured FAA data available somewhere that they use to create them?

FAA/NACO charts aren't masters, either

The master form of a FAA instrument approach procedure isn't the FAA/NACO approach plate for that procedure -- that plate is a derived document, just like the Jeppesen plate for that approach is.

The master, instead, is kept by the FAA in textual form as a set of Form 8260s. These provide a precise, formulaic description of any FAA approach procedure -- they're simply not in a form that's at all usable in the cockpit.

An example set of 8260s for the (rather special) KASE LOC/DME 15 approach can be found here.  Note that this contains exact records of fixes, governing obstacles, holds, and other procedure parts, as well as documentation about standards waivers and notes of changes.


Can a runway have a displaced threshold and a closed section in the same area? There is tricky question we weren't able to find a clear answer to.

Following roughness tests we decided to close part of the runway to all aircraft movements (1400 ft out of a total 3000 ft runway). The taxiway is located at the center of the runway (photo 1).

Actually the airport is certified under Canadian regulation (CARS 302) and this is what is indicated in this case :

Closed marking is placed at each end of the portion thereof, declared closed.
Runway threshold is temporarily displaced from the runway end, arrows are provided on the portion of the runway before the threshold.

It's not clear what the best practice is if we have the two situations at the same time; a displaced threshold, and a closed section in the same area.

For the moment, we decided to maintain the two type of marking at the same time; closed marking are installed on the side of the runway (photo 3).

This is a temporary measure. Phase 1 of corrective works was done. Roughness test still not acceptable, we have to come up with a phase 2 corrective works before this winter.

Has anyone seen this before and what do you do in this case? I'm also interested to see what do you do under FAA regulation.

Thanks

This photo show the marking this winter without the arrows. To be able to paint the arrows for displaced threshold marking we installed the closed marking on the side of the runway (see below).

Picture with displaced threshold and closed marking

In any case, threshold and runway numbers are highly desirable in a new position. Possibly it is the reason why pilots were confused with your initial variant.

At least by FAA regulations, if you don't want to allow takeoff from this area, it should not be a displaced threshold. I believe that you have a relocated threshold (part of runway closed for takeoff and landing). It can still be available for taxi though, but you don't have a taxiway from that end of a runway. It should look like this:

FAA reference is here, in "Partially closed runway" paragraph.

I saw partially closed runway in Europe as well, and it looked like that (actually they marked that part as a blast pad, and I saw that several times):

Unfortunately did not find anything similar for Canada (used Canadian AIM), but "relocated threshold" is mentioned in some accident investigation in Canada: report. And I still don't understand why the runway should not be marked just as "closed", without threshold displacement markings. Like here:

Arrows may confuse pilots, because it usually means that that part of a runway can be used for takeoff, or landing in opposite direction.


How to calculate the best climb rate and glide rate? Considering the lift equation

$$L = C_L \frac{1}{2}\rho v^2 s$$

Given $P$ is 1,000 ft, $\rho$ is 0.002308, and $V$ is the cruise speed of a Piper Archer or 126 knots, we get

$$L = (0.4) (0.002308) (\frac{1}{2}) (126)^2 (160)$$

or 1,172.5 Newtons.

What are the pencil-and-paper formulae for calculating the best-climb and best-glide rates?

Did you ever consider to look at the units in your calculation?

You can't just throw together some numbers and expect a correct result. Start by converting all input to the much more sane SI units and see how it goes from there. If you plug in the units instead of the numbers on the right side of the equation, you can check for correctness by comparing to the intended unit on the left side.

Hint: 1 Newton = 1 kg·m/s².


How Realistic (or not) was the crash scene in Flight? In the movie Flight, Denzel Washington's plane loses one engine and some other controls. To keep the plane gliding, he inverts the plane until it reaches a very low altitude. At the last second he flips it back right side up so that the belly of the aircraft takes the grunt of the crash landing.

Obviously this is all hollywood and takes a lot of creative license, but I'm a bit curious as to how realistic it was. Would you actually be able to fly a large jet upside down like that?

It's apparently loosely based on Alaska Airlines Flight 261 and like in the actual incident, I don't think he deliberately inverts the plane. In the actual event, the horizontal stabilizer gets jammed in an extreme position thus forcing the plane to invert. Now if it were possible to fly a commercial airplane that big inverted, Alaska Airlines too would've landed safely eventually (which didn't happen, despite the pilots trying to fly it upside down according to the CVR transcripts).


Has the FAA banned FPV for model aviation? On the FAA website the FAA says that you should not fly your model aircraft out of line of sight. Does this mean you cannot use FPV goggles?

The FAA has interpreted the rules to mean that the operator must have a line-of-sight view of the model aircraft at all times, and that FPV does not qualify.

  Based on the plain language of the statute, the FAA interprets this
  requirement to mean that: (1) the aircraft must be visible at all
  times to the operator; (2) that the operator must use his or her own
  natural vision (which includes vision corrected by standard eyeglasses
  or contact lenses) to observe the aircraft; and (3) people other than
  the operator may not be used in lieu of the operator for maintaining
  visual line of sight.

But also note that this does not apply if:

  ...the aircraft is operated in accordance with a community-based set of
  safety guidelines and within the programming of a nationwide
  community-based organization;

The above is from the linked NPRM released in June 2014. The latest NPRM from February 2015 has the same requirements.

So members of an organization like AMA are exempt from that rule. The AMA requires a spotter to be used when flying FPV.


How does wind speed affect helicopter rotor speed? So on the left it shows wind flowing with the direction of the rotor, yet it represents that it would reduce the rotor speed. I'm wondering how it does this, wouldn't the wind minimally help increase rotor speed by pushing it? Even if the effect was minimal to the point that it was neutral, why would it slow the rotor down?

If anyone could explain that would be great! Thanks.

From your image, the advancing blades (right side) would indeed be faster (relative to the air).
The rotor RPM is fixed all around, but in forward flight—say at 10 knots forward flight like your example—the advancing rotor will have an additional free 10 knots of airspeed (check image below). The other side will be rotating away from that airstream, losing 10 knots.
Red arrows indicate direction of motion, dotted arrows indicate airstream direction, same direction add, different subtract.

The faster side will produce more lift compared to the slower side. This is called dissymmetry of lift.
Solution?
The blades are allowed to flap (change angle) independently from each other, to be able to counter this phenomenon.

If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize lift across the rotor disk. Articulated rotor systems, usually with three or more blades, incorporate a horizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as they rotate. A semi-rigid rotor system (two blades) utilizes a teetering hinge, which allows the blades to flap as a unit. When one blade flaps up, the other blade flaps down.
— danubewings.com

Video in French explaining the flapping (good video even if you don't speak French, it's well illustrated).
Animation for flapping.


How do ground effect vehicles perform turns? Considering that Wing-In-Ground-Effect (WIGE) vehicles fly with a rather low ground clearance, there is not much room for banking. How do WIGE vehicles turn? Do they turn just very slow with a low bank angle? Or do they turn uncoordinated with wings level?

I can imagine that banking has an opposite effect: the raised wing has less ground effect so more drag so it creates some countering yaw moment. Is this correct?

It turns like an airplane by tilting the lift vector. Watch carefully in this video, especially about 50 seconds in, and you can see the craft is turning by banking.

It just can't bank very hard unless it climbs a little higher. This is one of the problems of GE aircraft; they aren't very maneuverable because they can only make gentle turns at the normal cruising level close to the surface.

The alternative to banking, if you don't have enough room to roll, is skidding, where you are redirecting the thrust vector and also generating some lateral force from the sideways presentation of the craft's body. Basically like a hovercraft. A lot less efficient though because of the drag rise from the skid. Some of those really big GE machines like the Ekranoplan would have used skidding (kind of obvious from the huge rudder that it's designed to skid) and maybe a little bit of bank.

It's probably a good thing that this concept has not been all that successful. If the world was full of these types of machines crisscrossing every which way over oceans and lakes, all at the same level and going pretty fast, you probably have a lot more mid-airs than you get with airplanes, or boat collisions for that matter.


Why is the B737 altitude display unneccessarily ambiguous? I was playing FSX again lately and was flying the B737-400. So I hope it is simulated correctly and this question isn't dead on arrival :-)

It always bugged me, that the altitude display on the MFD is very ambiguous when you are immediately below a "round" value:

On first glance everyone (ok..me) would read the altitude to be 9900 ft. Only if you know what to look for and spot that the numbers carry on downwards you can deduce that you are actually at 10000. The reason is that the second significant digit does not roll over smoothly but ticks over appruptly when the next round value is surpassed.

Granted, 100 ft may not be a huge difference, but it seems very unneccessary to have it like that. If the first two significant digits behaved more smoothly without this round-down like behavior I would recon that it would be way less ambiguous.

Why is the altitude display designed that way?

Edit: Here is what it looks like in motion:

Because thats how old school "steam" altimeters work/look and glass cockpits were designed to work similarly such to reduce unfamiliarity when transitioning. Prior to the screen age a drum altimeter would have read out and increased in much the same way the glass cockpit does now.

(source)

You will also notice the 10's and 1's place's are joined and rounded off by 20's. This has to do with the common arrangement of tick marks on any old altimeter as well as the fact that in a fast climb the one's digit would be unreadable.

On any note, its not ambiguous to a pilot, its actually quite accurate if you know what you are looking at.


What are the differences between wings made for 2 engines and wings for 4 engines? I have always wondered the differences between a wing made to hold 2 engines and a wing made to hold 4 engines, could someone please explain?

The differences are surprisingly small, and the implications of two versus four engines are not very great as far as design of the wing is concerned.

Weight implications

The wing is an immensely strong structure - after all, it's able to bear the weight of the entire aircraft, including of course the engines.

Engines are heavy, but only a small fraction of the weight of the whole aircraft.

A similar aircraft with four engines rather than two will have smaller engines, so the total weight difference will not be large.

Space implications

You need more space to fit two engines on each side, but not a vast amount. For example, the British Aerospace 146 has four remarkably small Lycoming ALF 502 engines. What's more, four small engines can be easier to mount, because they need less ground clearance.

Same wings, more engines

In practice, it's not very difficult to add engines to a wing (for aircraft companies, that is; I don't think I'd be able to do it without seeing someone do it first at least once).

For example, the 747 can optionally carry a fifth engine - obviously, this engine is just being carried, not used, but it shows that simply placing the engine there doesn't present insurmountable aerodynamic or other engineering difficulties.

And then we have the Airbus A330/A340, designed and built in parallel, which are essentially the same aircraft - same fuselage and same wing - one with two engines and one with four. (Actually the -500 and -600 A340 variants do have slightly longer wings, but that's because they are heavier, and nothing to do with the additional engines.)

That's not to say that there are no implications in having additional engines. In this case, wing flutter caused by the outboard engines of the A340 required some stiffening.

And of course, additional engines will require additional fuel and control lines. However, these aren't part of the structure of the wing itself, and are not in themselves likely to have implications for it.


What main landing gear actuator does the Boeing 777-300ER have? I can't remember the name of this system was, but all I do know is that is a big actuator that only exists on the Boeing 777-300ER variant. It will pressurize the actuator according to the thrust input the cockpit. It will automatically tilt-up to get a better AOA.

The Boeing 777-300ER is the only variant of the 777, which has a Semi-Levered Gear. From the 777 FCOM (v2 14.20.3 Landing Gear - System Description):

Semi-Levered Gear
[777-300ER]
The semi-levered gear consists of an additional hydraulic actuator that connects the forward end of each main gear truck to the shock strut. During takeoff, the actuator locks to restrict rotation of the main gear truck and allow takeoff rotation about the aft wheel axle, thereby improving airplane performance capability. During landing, the actuator is unlocked to permit rotation of the main gear truck and provide additional damping.

The 777-300 and 777-300ER are both equally long at $ 73.86 \, \mathrm{m} $, which is about $ 10 \, \mathrm{m} $ longer than the -200 variants. Due to the higher risk of tailstrikes, both variants are equipped with a Tail Skid:

Tail Skid
[777-300 and 777-300ER]
The airplane is equipped with a tail skid system. The tail skid extends for takeoff and landing and retracts during flight. It helps protect the pressurized part of the airplane from contact with the runway. The tail skid uses the main lading gear actuation system.
The EICAS advisory message TAIL SKID is displayed when the tail skid is not in the correct position.

The -300ER however has a MTOW (maximum takeoff weight) of $ 352 \, \mathrm{t} $ compared to only $ 299 \, \mathrm{t} $ for the -300, which increases the risk of a tail strike during takeoff rotation.
This Boeing article explains it as follows:

The Boeing 777-300ER's new semi-levered landing gear system has performed flawlessly during the flight-test program. The unique gear, which is manufactured by Goodrich Corp., allows the airplane to rotate early by shifting the center of rotation from the main axle to aft axle of the three-axle landing gear truck. As the airplane rotates, the nose is allowed to rise higher earlier.

You can see the difference here:

(image source: airliners.net and youtube.com)


Is there a way to **decelerate** significantly without an "eyeballs-out" sensation? I'm not sure how to describe this, but the problem came about as I was computing some acceleration profiles. Let's say you want to get up to Mach 3 (939 m/s) quickly. If you accelerate at 3 g's, this will take 31.923 seconds (not bad), and in level flight would cover a ground path of 14.991 km (kind of a lot, but not really comparable to typical flight lengths).

Only problem is, when it comes time to land, you have to slow down. If you want to do that quickly too, well, 3 g's in the "eyeballs-out" direction is a little hard on the eyeballs. Doing 1 g would take 3 times longer but also cover a ground path more than three times longer because of that pesky time squared in the equation for uniform acceleration ($\Delta x = \frac12 at^2$, if initial x and inital v are zero).

So you could gradually slow down over an even longer distance, or...is there some other, quicker way that doesn't involve much eyeballs out? Could you maybe fly a circle over the airport and decelerate that way, so that you feel the g-force mostly pointing down into your seat? Is there some other way you could "cut into another plane"?

The goal is to decelerate quickly so that the average flight speed (over the whole journey) is very close to Mach 3.

If you need to have an average speed of Mach 3, then cruise at a speed higher than Mach 3 and land at Mach 0.2. And decelerate at a comfortable rate.

Or fly in a cryogenic wind tunnel to get the speed of sound down so much that Mach 3 is actual a very low speed.