How to increase your chances of survival in a plane crash? More than 90% of plane crashes in recent years had survivors.

  In America, for example, there were 568 plane crashes between
  1983 - 2000:   Out of the collective 53487 people onboard, 51207 lived
  to tell the tale. That's a survival rate of 96%.

  Some experts believe that almost one-third of deaths in aeroplane
  accidents could have been prevented if people knew what to do and took
  action. (source)

Question:
Beyond bringing on board special items, what are the things a passenger can do to increase his chances of survival in the unlikely event of a plane crash – including its aftermath ?

note: Although plane crashes are very rare, it's nevertheless a question worthy of contemplation both
– theoretically, for argument's sake, as it's an interesting thought all by itself
– and from the point of view that each preventible human death is one too many and unnecessary. For the tens of thousands who experienced a crash as well as their friends and relatives it's certainly not a theoretical question.

« Some of the survival game involves factors passengers largely can't
  control, like the weather, flight crew skills, the design of airline
  seats and the construction, maintenance and age of the plane. But
  passengers themselves can do a lot to improve their chances of
  survival simply by making smart choices and being informed. »⁴

Things you can do:

Preventive Measures

Book the safest seat.
Avoid unsafe airlines.
Fly during daytime (as statistically it slightly increases chances of survival and some kind of incidents are far more common at night).

Preparatory Measures

Bring helpful items on board.
Clothes – what you choose to wear can help or hinder your escape:

Opt for flat, sturdy shoes. Flip-flops and high heels won't help you make it to the emergency exit.²
In case of a fire or glass and metal debris, long-sleeved shirts and heavy cotton trousers will offer more protection than shorts and a T-shirt.²
Form fitting clothes are less prone to catch fire.
Polyester and other synthetic fabrics will melt to your skin.³

Before Take-Off

Pay attention to safety advice. It can save your life. Read the safety card (in the seatback in front of you) as planes do differ.
Memorize the emergency exit locations before take-off. Plan and visualize your escape route. Count the rows (armrests⁴) to your nearest exit (in front of and behind you in case one direction is blocked!). That way, in the event of smoke filling the cabin or if emergency lighting fails and it’s very dark, you will know the number of seat rows and can feel your way to an exit.¹ Many passengers survive the initial impact but are killed by the ensuing fire, smoke and sometimes water. 90 seconds should be used as a time frame for escaping.⁴
Memorize where your nearest cabin crew are, because their help and instructions will make a huge difference.²
Be aware that at 35000ft you have just 30-60 seconds to don the oxygen mask and tug on the line to get the flow going. After that, you lose consciousness.²

In-Flight

Don't drink alcohol.
Seatbelt:

Keep your seatbelt on whenever seated (also prevents turbulence deaths).
Fasten it tightly - every inch of slack increase the potential g-forces you could be subjected to during an impact.²
Wear it across your pelvis, which will handle impact better than your stomach².
Practice opening and closing it:

  After interviewing 1,900 survivors and 155 cabin-crew members,
  Professor Galea made an interesting discovery: Most passengers lose
  valuable time because they struggle to undo their seatbelts. “People
  tend to try and press a button on the seatbelt, because in this
  emergency situation they revert to normal behaviour [learned from] car
  seatbelts [with] a push-button system.”²

During Take-Off and Landing (when ~80% of accidents occur²):

Don't sleep/nap wearing eye mask and earphones.²
Stay alert and aware of your surroundings.²
Keep your shoes on.²

When a Crash is Imminent

Brace for impact:

Put your feet flat on the floor, farther back than your knees ("aft position").²
Consider placing hand luggage under the seat in front of you to act as a cushion (to prevent your feet from hitting the seat, I assume). That way you are less likely to suffer broken ankles that will hinder your escape.²

Escape and Aftermath

After the initial impact it's common for fire to break out. Just a few breaths of toxic smoke can cause you to pass out. In the event of smoke/fire:

Cover your mouth and nose with a wet cloth (e.g. wet the seat back headrest).²
Stay low. (That doesn't necessarily mean crawl on your hands and knees. Although there will be less smoke at floor level, you could be crushed or suffocated under luggage and other passengers.) Instead, keep your head down.²

Don't retrieve your luggage.¹
Don't inflate your life jacket inside the plane. It hinders your escape.²
Move fast but resist the urge to push and shove. It's likely to make matters worse and cause others to lash out.²
Once on the ground get away as fast and as far as you can.²

sources: 1, 2, 3, 4


Why will windshear inop illuminate after landing in an MD-83? Why will windshear inop illuminate on the annunciator panel after landing or before starting taxi in a McDonnell Douglas MD-83?

The WINDSHEAR INOP light tells the pilots that the Predictive Windshear System cannot work:

  System Annunciator Lights

  [...]

  An amber WINDSHEAR INOP annunciator is installed on the annunciator panel.
  This light illuminates any time the windshear systems detects a fault which
  renders the system inoperative.

(MD-80 FCOM Sec. 16 - Instrumentation / Navigation / Communication - Predictive Windshear System)

In order for this system to function, the radar system needs to be active, which is a hazard to the ground crew:

  RDR-4B Radar System Theory

  Like all modern radar systems, the RDR-4B operates by emitting short
  intense pulses of microwave energy which are reflected by objects having
  reflective characteristics within the range of the system. The reflected
  signals are processed to produce visual displays that are representative of the
  size, intensity, bearing and distance of the targets. The RDR-4B incorporates
  an advanced microprocessor design which utilizes the Doppler principle to
  identify areas of moderate and higher turbulence as well as low level
  windshear (microburst) activity. The RDR-4B system is designed to operate
  in the windshear mode automatically any time the aircraft is below 1500’
  AGL, at least one engine is running, and the transponder is not in OFF or
  STBY, REGARDLESS OF RADAR MODE SELECTED. The system also
  operates in the windshear mode below 1500’ AGL using an alternate scan
  technique if the radar is operating in any mode (WX / TURB, WX, MAP). The
  windshear mode is activated at 2300’ AGL. However, no alerts or displays
  are annunciated above 1500’ AGL.

  WARNING:
  Failure to return the transponder to STBY after landing or
  selecting any mode except STBY prior to leaving the ramp area
  allows the radar to operate in the windshear mode creating a
  radiation hazard to personnel on the ground. The RDR-4B
  radiation hazard area extends 13.4 feet from the radar antenna
  in a 120 degree arc left and right of the aircraft centerline.
  This hazard exists even if the radar mode is selected OFF or
  TEST.

(MD-80 FCOM Sec. 16 - Instrumentation / Navigation / Communication - Predictive Windshear System)

After landing, the pilots will switch the transponder to STBY in order to deactivate the radar system before reaching the apron (where ground crews are working). This will disable the Predictive Windshear System and causes the WINDSHEAR INOP light to illuminate.


Is air in the toilet isolated, to prevent seeping into the cabin? This made the news few days ago:

  British Airways flight to Dubai forced to return to Heathrow due to 'smelly poo in toilet'

As can be seen at The Telegraph and The Independent.

Made me wonder: isn't it possible to recycle the air from the cabin and perhaps isolate the toilet? This could lead to a whole new series of chemical attacks on long flights.

If someone can get chemicals on board a flight, they can also get a bomb on. Bombs are easier to make and more effective. Better air-conditioning won't help here, better security will.
It is possible to isolate the toilet... but the problem apparently wasn't just inside the toilet. The toilet isn't pressure sealed, so some smell will escape regardless. It's impossible to completely seal the toilet from the rest of the aircraft, and could be potentially dangerous if it was
The aircraft does introduce fresh air into the cabin, but the compressors to do so only have a fixed capacity. The capacity of these could be increased, but at the price of extra weight, fuel consumption and cost, it's not worthwhile for the 1 time in 100 million that a flight has to turn around for this reason. Even with higher capacities, it's not possible to just constantly flush the cabin air - and remember that all that air needs to be compressed (to maintain pressure) heated, and filtered. All of that costs more money, and adds more complexity, weight etc.


Can I do a VFR climb-out on IFR flight plan? Let’s suppose I wish to fly an IFR flight from KTUS (Tucson International Airport) to KFST (Fort Stockton-Pecos) for example. Given the surrounding terrain there is of course an ODP that reads as follows:

  DEPARTURE PROCEDURE: Rwys 3, 29L/R, climbing right turn direct to TUS
  VORTAC …
  Rwy 21, climbing left turn direct to TUS VORTAC …
  Rwy 11L/R, climb heading 123° to 4000 then climbing left turn direct TUS
  VORTAC …
  ... Climb in TUS VORTAC holding pattern (hold NW, right
  turns, 128° inbound) to 9000 before proceeding on course.

That’s a pretty long departure procedure especially with the temperatures this time of year but it makes perfect sense due to the terrain.

Now since I expect the weather to be CAVOK until the MSA, I would prefer to do a VFR climb-out. In this case, would I present myself as a regular VFR traffic to clearance, ground and tower frequencies and request IFR once at MSA to Center, or would I request an IFR clearance on ground and add the request for VFR climb-out up to the MSA?

KTUS doesn't have a VCOA, but I can think of a couple of alternatives (other people may have more). First, file a composite (VFR to IFR) flight plan. See the AIM section 5-1-7:

  Flight plans which specify VFR operation for one portion of a flight,
  and IFR for another portion, will be accepted by the FSS at the point
  of departure. If VFR flight is conducted for the first portion of the
  flight, pilots should report their departure time to the FSS with whom
  the VFR/IFR flight plan was filed; and, subsequently, close the VFR
  portion and request ATC clearance from the FSS nearest the point at
  which change from VFR to IFR is proposed.

Second, depart VFR and ask for a pop-up IFR clearance when you need it. But this isn't ideal if you know that you'll be in IMC (or in the flight levels) at some point and you want to make sure your clearance isn't delayed.

Third, depart IFR and just ask ATC for whatever handling and routing you want. If weather conditions are good and there isn't much traffic, there's a good chance they'll let you do what you want. Considering the terrain, though, I wouldn't do it unless you're familiar with the area. And of course there's a chance that ATC won't allow it anyway.

Finally, although this doesn't answer the question you could just fly the ODP. I've never been to Tucson but the airport elevation is 2643' and right now the temperature is 32C and the altimeter setting is 30.10, that means the density altitude is about 5000'. And since you're departing to the east (more or less) it looks like you'll need to get to at least 11000' (IFR altitude) because of terrain, so you have to climb 8000' with limited space to do it and with reduced performance because of the high DA.

I don't know what you fly, but climb performance in those conditions isn't great in many light piston aircraft - at least if they're normally aspirated - and a climb in the hold seems like a good way to get up there safely. Of course, if you know the area well and your aircraft is up to it, then things would look different.


Why does wingwalking use only biplanes? Wingwalking is an aerobatic discipline with shows such as this one.
The definition of wingwalking from Wikipedia gives no other alternative than using biplanes:

  Wing walking is the act of moving along the wings of a biplane during flight [...]

(source: Wikipedia, as of 26 February 2020)

When searching for images on the web, I can only find biplanes, mainly stearman but also other biplanes.

For me, it seems that any aircraft with high wings or parasol wings can do the job, so why do they only use biplanes?

It doesn't only use biplanes, searching "wingwalking monoplane" provides these examples, of some low wing monoplane and high wing monoplane.
Whatever the aircraft, it has to be able to fly slowly.

(source) (not 100% about this one but it looks like one Klemm aircraft)

(source)
(Here one quite rare danish SAI KZ III equipped with a hatch behind the wing, allowing acrobat Palle Johnson to get out.)

(source)
(Jaromir Wagner, Czech wingwalker holding speed record wingwalking at 270km/h here on a Britten-Norman Islander aircraft)


What are the disadvantages of a delta wing biplane design? The idea of a delta wing as I understand it is to reduce stress on the wing and drag on the leading edge. Now using two delta wings on separate planes, on one frame would presumably allow for a reduced wing size, and therefore less drag. I imagine it could also lead to a nice compromise with better control in slow flight than delta wing designs?

However, clearly there are some major issues with this design, as a quick search uncovers no results for this design shape. therefore I pose the question: Why is it that designers have rejected this possibility; what are its major (dis)advantages?

The main benefit of biplanes is that the lift can be distributed across both wings. This provides more lift with less wingspan and places less load on each wing. Less load means the materials can have lower strength. This was very important in the early days of aviation, when engine power, materials, and weight were much more limited.

The major downside of a biplane is aerodynamics. The wings interfere with each other, causing drag. While structural bracing between the wings can reduce weight, it also adds more drag. This means that a single wing will almost always perform better, and modern materials and designs should make that possible.

A biplane configuration does not make much sense for delta wings. The main drawback of a biplane is drag, but drag increases with the square of velocity, and delta wings are best in high-speed flight. While delta wings have lower wave drag, they have higher viscous drag due to the large surface area. A second delta wing will add more area, increasing both wave drag (from the cross section) and viscous drag (from the surface).

Configuration of the aircraft also becomes problematic. Most aircraft with delta wings tend to be fairly flat, allowing lower area and therefore lower drag. The closer the wings are, the more they will interfere. The further apart they are, the more structure is needed, either in a larger fuselage or in structural braces, which increases weight and drag.


Can a view limiting device be used in an aircraft requiring two pilots? Can a pilot use a VLD for IFR currency if the aircraft requires two pilots? Can it be used while carrying passengers? If so, is this ever done?

I'm thinking of part 121 airline pilots under FAA.

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


Is it possible to determine a rotation speed (Vr) using other known V-Speeds? Is it possible to come up with a good estimate of rotation speed using other known V-Speeds?

For example: if I know a plane has a Vs @ 55kt. and a Vy @ 75kt, is there a reasonable method to use to come up with a safe rotation speed?

This is a general question. I'm not looking for an exact rotation speed calculation (unless one actually exists, of course).

From FAR 23.51 that covers normal, utility, acrobatic, and commuter category airplanes:

  For multiengine landplanes, VR,
  must not be less than the greater of
  1.05 VMC; or 1.10 VS1

So 110% of the Vs of a particular configuration works. (Do not use for flight.)

Note from @Pondlife: 23.51 has been superseded with 23.2115 and the quoted definition is no longer there.
Related: What makes V1 one or two knots less than Vr on this performance table?


Is the FAA the only authority with specific medical requirements for transgender pilots? The FAA/CAMI imposes certain testing requirements on transgender pilots, listed in the Guide for Aviation Medical Examiners under Item 41 (G-U System), subheading "Gender Identity Disorder".

Is the FAA the only authority that has these requirements, or do other countries have similar policies?

EDIT:
the link above is broken but I think there might be value leaving the old URL in case people want to try to get there on archive.  The updated reg is at https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/app_process/exam_tech/item48/amd/gd/

As several commenters have noted, medical rules are different in each country. ICAO provides guidelines and recommendations, but it's up to each country to actually implement them. ICAO's personnel licensing requirements are in Annex 1 which is not freely available but their Manual of Civil Aviation Medicine implies that transgender pilots should be considered 'higher risk':

  Applicants with disorders of behaviour (for example regarding habit,
  gender identity, sexuality) should be assessed on the basis of their
  ability to put aside the disorder (or any conflicts related to the
  disorder) in order to attend to the aviation task at hand. These
  persons may have significant conflicts with their environment, leading
  to further difficulties, which may become an impediment for them to
  hold an aviation licence.

Looking at specific countries, the FAA may be an unusual case by explicitly mentioning gender identity issues. I couldn't find any mention of them in the EASA, Canadian or UK medical requirements. But all countries' medical requirements include something general like this (this example is from EASA):

  (g) Personality or behavioural disorder Where there is suspicion or
  established evidence that an applicant has a personality or
  behavioural disorder, the applicant should be referred for psychiatric
  opinion and advice.

That catch-all clause would - I assume - include gender identity disorder and therefore require psychiatric evaluation of the pilot. And in cases where a pilot has had gender reassignment surgery and/or drug treatment, the surgery and drugs regimen would almost certainly trigger additional testing and investigation (so would any other major surgical procedure or long-term drug treatment).

Finally, it's worth noting that whatever the general rules are, every case is different and the only way to get a specific answer for an individual is to consult with an aviation medical examiner for that jurisdiction.

(Disclaimer: I know nothing about medicine except that it's complicated. Consult an aviation medical specialist if you want a real answer.)


Can actual instrument time count as simulated instrument time for the PPL? Just wondering if actual instrument time can be added with simulated instrument time for the requirements of the Private Pilot minimums (3 hour instrument). I have actual instrument time from when my CFI showed me what it was like to fly in clouds. (USA)

Provided your CFI and you were flying on a filed and ATC cleared IFR flight plan at the time of the real instrument time was acquired, then yes, you can use that instrument time toward your PPL Aeronautical Experience requirements.  It does not necessarily need to be done under the hood. 61.109(a)(3) just requires the applicant log 3 hours of dual instruction time flying solely on reference to the instruments, whether simulated or real IMC conditions.

The downside here is that unless you did the dual instrument time with a CFII, you cannot use that time toward an instrument rating or advanced pilot or flight instructor certificates per 61.109.


What are the aerodynamic advantages and disadvantages of the Weltensegler wing? What are the aerodynamic advantages and disadvantages of the Weltensegler Wing?

Would this kind of wing configuration come to help for a glider?
Is there any difference between them and gull wings?
What are the aerodynamic advantages of gull wings?

The inspiration for the Weltensegler wing were birds. The reason for their wing shape is structural, however, not aerodynamic. Your photo shows the 1922 version, called "Baden-Baden Stolz", which was a follow-on to the original and more ambitious version of 1921. The 3-side-view below shows the planform (picture source). However, this airplane never flew, which probably saved the life of its pilot. While pitch and limited roll control was possible with the elevons on the outer wings, no consideration was given to directional control.

The 1921 version was controlled by warping of the outer wings, the washout of which was controlled with pulleys and springs. The idea was that this would help to adjust the wing shape to flight speed. The picture below shows the first and last flight, which is historically significant as the first glider flight in which a considerable gain in height was achieved. However, it ended in a spiral dive which killed its pilot (picture source).

Weltensegler is actually the name of the company which was founded by Friedrich Wenk. Alexander Lippisch was one of his employees.

The configuration cannot be recommended for a glider. It is much better to have a straight inner wing and dihedraled outer wings, as explained in this answer. The anhedral of the outer wings of the Weltensegler wings will produce a downforce on the windward wing, rolling it into the wind which is the opposite of what is needed for coordinated flight. Other configurations using an "M"-shaped wing did so for different reasons and use rudders and much more anhedral on the outer wing panels.


What does the crew of a long range miltary aircraft eat during a mission? The B-52 and other big aircraft often fly long missions.
What does the crew eat or drink?

Is there a list of recommended things to eat, or of specific foodstuff to avoid?

How is it stored onboard?

It's not just the big bombers flying long missions; you'd be surprised how many of the sorties flying over Afghanistan, Iraq, Syria, Libya etc originate from airbases in Northern Europe with refueling stops over the Mediterranean (because closer airbases aren't available or are congested with aircraft from other forces or that have shorter ranges).

The short and simple answer is, aircraft crews eat whatever they can bring with them, that won't make a huge mess in the process. A bigger plane like a B-52 has some room to get up and move, so the pilot may take 15 minutes during ingress or egress, duck out to the entryway and eat a box lunch, then come back and relieve the co-pilot so he can do the same thing. Radar, nav and weapons systems officers typically have a little room behind or between seats to step away and eat, or they'll duck into the entryway as well.

In smaller aircraft, space is more limited, but the pilot's flight suit will still have plenty of pockets to carry food and drink pouches. There are some considerations as to the exact type of food, especially in one- and two-seat jets. A crewman has to prevent "foreign object damage" to the switches and displays in his cockpit, or else his crew chief (and his CO) will be all over him about it. Good choices include pre-wrapped bar-type snacks; granola, protein, energy, some types of candy bars, whatever. Depending on what's available in the mess hall, pilots may be able to assemble simple tortilla wraps with lunch meats and cheese, which are edible with one hand and fewer crumbs than a bread sandwich. Anything that can be put in a drink pouch, like broth or tomato soup, is also a possibility. Bad choices include things like trail mix, peanuts, M&Ms etc, as well as anything too loose to reliably keep together with one hand.

The bigger concern is, what to do when it comes back out? The B-52 has a rudimentary "head", with what amounts to a camping toilet (a bucket with a plastic bag liner and a toilet seat over the top) called the "honeypot". However, if it's used, the crewman who used it is often tasked by the crew chief with cleaning it, so it's considered a last resort. Crew of smaller aircraft have no such luxury, and unlike NASA astronauts, they wouldn't be caught dead wearing a diaper. Most fighter pilots eat a "low-residue" meal for the two or three meals before a very long mission or ferry flight, to reduce the chance they'll have to BM during the flight (some take an anti-diarrheal like Immodium as a preventive measure). Urination is a little easier to deal with, for men at least; pilots carry packets with an absorbent powder (not unlike what's in diapers) called "piddle packets". The difficulty is typically in... "maneuvering". You have to get the equipment out of your flight suit, when you're strapped down to the point of suffocation in a 6-point harness, then be able to aim into the piddle pack. Finally, you have to store this bag of piss somewhere until you get back to base, and as Rhino Driver is fond of explaining, if that base is a carrier, you have to make sure it's extra-secure, or on capture it'll go zooming forward toward the firewall and explode in a shower of damp absorbent crystals. Even if your squadron-mates don't see you getting out of the plane covered in this stuff, your crew chief will be sure to tell them all about it.


What is the reason aircraft are as long as they are wide? I've noticed most aircraft are as long as they are wide from nose to tail and from wing tip to wingtip, particularly private jets despite recent swept wing technology. You can almost draw a perfect circle with a compass if you could find the centre.  Is it for aesthetic purposes, principles of flight aerodynamics or any other reason.  Please explain

By curious coincidence it is simply what the optimal aspect ratio for speeds around 250–280 knots indicated results in. Aircraft designed for different cruise speeds have different ratios.

The wing span is governed by desired cruise speed. There are two main forms of drag—induced and parasite. Induced drag decreases with wing span and with speed, while parasite drag increases with them. So at lower speed, you need long wings to keep the induced drag at bay, while parasite drag is low simply because of the low speed. And at higher speed, you need shorter wings to avoid too much parasite drag.

Jet aircraft are most efficient at transsonic speeds (Mach number 0.75–0.85) and altitudes around 10-12 km (33,000–40,000 ft) and it so happens that the wing span and cabin length to contain corresponding payload are similar. Turboprop aircraft are most efficient at somewhat lower speeds and altitudes and consequently their wings are a bit longer. And yet slower piston-powered aircraft have even longer wings relatively to fuselage.

(update, as suggested in comments): The extremes are on one side gliders that achieve low drag by having very long slender wings and flying slow and on the other side supersonic aircraft that have short wings with long chord to keep the frontal cross-section low, the very extreme being the very fastest aircraft like X-15 or SR-71.


What button do I push on the instrument panel to activate DME in VOR navigation? For some strange reason I cannot seem to find any information about what button to push on the instrument panel to active the DME.  There is plenty of information about how DME works with interrogation and slant distance, but I cannot seem to locate any information about what button to actually push to activate DME when I am sitting in the cockpit (in a Cessna 172, for example) or where to look on the instrument panel to read the measured distance the DME is telling me.  So if the DME is measuring 8 nautical miles, where on the instrument panel will that "8" be displayed?

Typically, there isn't a "button" as such, you tune the DME to the frequency of the ground station (or slave it to the NAV radio).

The knob on the right is used to tune a frequency (shown before MHZ) when in "frq" mode, which is usually a VOR/DME. The numbers before NM is your distance to/from the station in Nautical Miles.


Would it be beneficial for the Boeing 777x to have a biplane wing design? The Boeing 777x has huge wings to generate lift even to the point of having to have a hinge and being able to bend them.

Would it not be advantageous to have 2 wings like a biplane or one behind another? And what disadvantages would there be?

  A biplane wing of a given span and chord has twice the area of a monoplane the same size and so can fly more slowly, or for a given flight speed can lift more weight. Alternatively, a biplane wing of the same area as a monoplane has lower span and chord, reducing the structural forces and allowing it to be lighter.

This is from Wikipedia so possibly not reliable.

A major disadvantage to that design would be the aft wing had a disrupted airflow over it as it travels through the fwd wing's wake, so lift is greatly diminished.

As to doing a stacked biplane design typical of small sport biplanes in use over the years, this is probably frowned upon for two reasons:

High Drag - more structure in the relative wind always means more drag anemic this was one of the major advantages to the cantilevered wing developed in the 1930s.
Heavier - more wings means more structure both in terms of the second wing itself plus the mounting structure to mate the wing to the fuselage.

A single cantilevered monoplane is by far the most efficient solution save only a flying wing, but these are not commonly used in commercial travel which has been discussed in previous threads.

Now that's not to say a biplane may not be useful for commercial aircraft.  Researchers at MIT proposed a supersonic biplane design which usea the configuration to dramatically reduce its sonic boom.

Whether this idea survives the CGI proposal and becomes actual hardware is another story; this business is full of concept aircraft and precious little flyable hardware.


How should this ODP be flown at KELP? The KELP (El Paso) Obstacle Departure Procedure (ODP) says:

Rwy 4, 8R: climbing right turn heading 120° and ELP R-150 to 9000, then proceed on course.
Rwy 22, 26L: climbing left turn heading 120° and ELP R-150 to 9000, then proceed on course.

Do I fly from the VOR or to the VOR on ELP R-150?
From: would actually mean I use 150 and fly south east
To:   would actually mean I use 330 and fly north west
Do you have a reference for the answer?

In instrument flight, when a Radial is given, it is always the “From” indicated radial on your OBS. In vectoring, turning an aircraft at acute angles is avoided. Flying a Heading of 120° to intercept a Radial of 150°, your only option would be to turn toward a course of 150° degrees (Southeast) until reaching 9000 feet MSL. The field elevation is already almost at 4000 feet MSL. You only have to climb another 5000 feet before you can turn on course. The dead giveaway is the fact that a course of 330° would have you passing in or over the KBIF military Class D. Probably, before you could affect a frequency change


What is an Air Defense Identification Zone (ADIZ)? China recently caused some controversy by announcing an Air Defense Identification Zone (ADIZ) in the East China Sea. There is also an ADIZ surrounding most of North America.

What exactly is an ADIZ and what does it take to fly across one? For example, what would be necessary to fly a private plane directly from Apalachicola to Cedar Key, Florida?

What is an ADIZ?
FAR 99.3 says:

Air defense identification zone (ADIZ) means an area of airspace over land or water in which the ready identification, location, and control of all aircraft (except for Department of Defense and law enforcement aircraft) is required in the interest of national security.

To put it simply, if you're going to fly in the United States ADIZ, the United States government wants to know:

Who you are
Where you are
What you're doing

What do I need to have before I fly through one?
A few things, actually.  They're listed in AIM 5-6-1

You generally need to file a flight plan (but there are exceptions). (FAR 99.11)
You need a two-way radio. (FAR 99.9(a))
You need to have a Mode C transponder, and it has to be on. (FAR 99.13)
If you're going DVFR (rather than IFR), you need to give an estimated time of ADIZ penetration at least 15 minutes before entering it. (FAR 99.15, also reference AIM 5-6-1.4b)

What do I need to do while I'm in one?

Remain no more than 5 minutes off the proposed ETA to your various waypoints
Remain no more than:

10 miles from the centerline of your route, if over land
20 miles from the centerline of your route, if over water


Can I log multi-engine time abroad, when I'm not rated for multi in that country? I hold a multi-engine FAA CPL and want to fly a multi-engine aircraft in another country. The aircraft is not N-registered and I don't have a multi-engine rating in that country. Can I log the time in my FAA logbook? I'd like to log the hours under my FAA license.

More Details:
Country: Colombia (ICAO)
Role: Second in Command

The plane is a multi, I don't have multi license from the registration's country but I have FAA Multi license, so the idea is to fly it and log the hours using my FAA license can I do that ? Even if I don't have the license to fly that plane from the country of the registration ?

FAR 61.3 stipulates what is required to fly N-registered aircraft outside the united States.  There is nothing about flying non N-registered aircraft outside the U.S.  Rightfully so, the FAA does not have jurisdiction in that case.

To answer your question, you will need to reference that country's rule.  If they are an ICAO country, the rules are probably very similar.


What is Engine Pressure Ratio? I kind of know what it is but I don't fully understand it. Wikipedia says,

  the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?

The wiki quote refers to total pressure at intake and outlet. Total pressure is defined as static pressure $p_s$ and dynamic pressure $p_d$ which for incompressible flow = $\frac{1}{2} \cdot \rho \cdot V^2$.

So the Engine Pressure Ratio is
$$\frac {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{exhaust}} {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{freeStream}}$$

The turbofan exhaust gas is a mixture of the jet exhaust and the bypass air exhaust.


Did Flight Engineers progress to be pilots? When 3 man cockpits were common place on commercial aircraft what was the career progression of a Flight Engineer like?

Did FE's normally transition to be pilots? I'm not trying to insinuate here that FE's were in anyway inferior to the pilots and of course some FE's would of had no desire to transition to a pilot capacity at all.

Were there any airlines who operated a policy of starting flight crew out as a FE then training them as pilots?

N.B - Some background information that I'm basing this question on is I've read a number of reports regarding young (early 20 y/o FE's becoming pilots) and in maybe a more extreme case the hijacking of FedEx Flight 705 when the FE who hijacked the aircraft (an ex-Naxy pilot) had been made a FE (this could be part due to the fact he apparently falsified his Navy flying hours).

Before the turn of the century (I can't believe I'm using that phrase, as I retired in 1999) and especially back when all 3 engine and 4 engine airliners had a flight engineer, the usual career path at many airlines was to hire in as an Flight Engineer, or FE (two stripes), progress to an First Officer, or FO (three stripes), and then become a captain (four stripes).

This all began to change, of course, with the advent of large 2-man cockpit aircraft. The change was not without controversy and some pain. For example, at least one initial operator of the 737 ran a 3-man cockpit for a time. The 3rd guy was, as I remember, referred to as the GIB (guy in back). He sat in the jumpseat and read the checklists, which was one of the tasks an FE typically had in 3-man cockpits.

As it happened, the two jet carriers I worked for hired pilots into the FO seat and used professional FEs. I spent a couple of years on the 727 and then 10 years on the 747.  During that time, every FE save one that I flew with was ex-military, and with few exceptions all were A&Ps. That was desirable in case a maintenance sign off was needed when you were someplace where maintenance was not otherwise available.

Occasionally you would encounter an FE who had been a captain.  The pilots were required to retire at 60 back then, but FEs weren't.  I only knew one such individual, and I always liked flying with him.  The more expertise you have in the cockpit, the better.

In the two 747 carriers I worked for, an FE would sometimes transition to the FO seat, but that was relatively rare, and the costs were typically the individual's to bear.  Interestingly, when I retired in 1999, I had flown with an ex-military FE who years later became the chief pilot of the airline involved.


How to calculate aerodynamic efficiency at minimum power conditions? The drag polar of a propeller driven aircraft is
$C_{D}=0.039+0.071 C_L^2$.
The lift curve slope of the wing ($C_{L_\alpha}$) is 0.09/deg and zero lift angle ($\alpha_{L=0})= -1.5^o$.
Stall angle of the wing is $15^o$.

I am trying to find out the aerodynamic efficiency (L/D). Also if it even can fly with such specifications? If it can, what could be the angle of attack?

At cruise condition, a propeller propulsor can be modeled as: $P=TV$, with constant output power as a function of airspeed. Your objective is to find the cruise condition such that the output power is minimized.

Assuming you know apriori what altitude you want to cruise, then density $\rho$ is determined. At level flight:

$$T=\frac{P}{V}=(C_{D_0}+KC_L^2)\frac{1}{2}\rho V^2S$$

At cruise, weight is equal to lift. At minimum power, we have $\frac{\partial P}{\partial V}=0$:

$$\frac{\partial P}{\partial V}=\frac{3}{2}C_{D_0}\rho V^2S-\frac{KW^2}{\frac{1}{2}\rho SV^2}=0$$

Do some algebra, and we have:

$$3C_{D_0}=KC_L^2=C_{D_i}$$

Therefore, the minimum power condition is not equal to maximum L/D (as an aside, it is equivalent to the condition for maximum rate of climb, which is pretty intuitive).

Now you can find the corresponding $C_L$ and verify whether it's within the valid range of the lift curve. Generally, we want at least 1.3G margin to stall warning or stall buffet. So a 1.4G margin to $C_{L_{max}}$ might not be a bad place to start.


How do you meet pilots looking for passengers? I enjoy being up in the air.  Planes, gliders, helicopters... you name it, I like it.

What is an effective way to meet pilots looking for passengers?

I'm guessing that some pilots would like passengers to have someone with whom to share the experience, or maybe to gather hours for certifications.

I'm not looking to go anywhere in particular.  Although I'm open to flying to any destination, just going up in the air and coming back to the same airport is fine.

I enjoy being aloft in the daytime as well as at night.

I simply enjoy the experience of flying and want to connect with pilots who are looking for some company.

Aviation, for me, is a lifestyle more than anything.

Some background: I'm still a college student, but I'm going into the Air Force, I am on my school's flight team, I am getting a degree in asto engineering (and astro physics).  Almost everyone I know is involved in aviation or space in one way or another.  We fly around the southwest part of the country all the time, but it's normal.  One weekend I'll fly LA, and another a friend will take us to Vegas.

Have you considered going to flight training, yourself?  If you find a good flight school, you wont only learn to fly yourself, but you will meet other people like you.  And once you become friends with them you can all fly each other after you get your licences.

Honestly, I would be pretty uncomfortable flying with someone I don't know next to me.  For example, If you've ever picked up a hitch-hiker you might know how it's a bit awkward and uncomfortable making small talk with a completely random person.  Now imagine what that would be like in an airplane... a plane that most likely has controls in both (front) seats, and could do serious damage if used improperly.


How does ATC protect Air Force One? Air Force One is obviously a big deal.  We close terminals and implement other seemingly crazy safeguards against terrorist attacks while the president is en-route to an airport.

How does ATC protect the president whilst in the air?

I have heard of TFRs for "VIP in the area" reasons — is that for AF1?
I am guessing that the aircraft identification is blocked, but wouldn't they still need to have the transponder on for TCAS?

Specifically, the Wikipedia page on Air Force One has the following quote:

  Air traffic controllers gave Air Force One an ominous warning that a passenger jet was close to Air Force One and was unresponsive to calls. "As we got over Gainesville, Fla., we got the word from Jacksonville Center. They said, 'Air Force One you have traffic behind you and basically above you that is descending into you, we are not in contact with them – they have shut their responder off.' And at that time it kind of led us to believe maybe someone was coming into us in Sarasota, they saw us take off, they just stayed high and are following us at this point. We had no idea what the capabilities of the terrorists were at that point."

Does having the transponder on just paint a huge target on the radar for terrorists?

Whenever Air Force 1 takes off or lands, the FAA establishes a 'Temporary Flight Restriction'.

Essentially this is a big 'upside down wedding cake' of airspace in which Air Traffic Control exercises very tight control of any aircraft operating in the TFR.  Any traffic entering the TFR without prior approval, or any aircraft deviating from ATC instructions will be intercepted by military aircraft.

When President Obama takes a bus trip the FAA establishes a TFR over the bus, that moves so as to stay over the bus as he rolls along!

Wikipedia has a good article on TFRs.


Why was the Electra uniquely vulnerable to whirl-mode flutter? The Lockheed L-188 Electra, notoriously, suffered from early problems with whirl-mode flutter, which caused two Electras to break apart in flight in 1959 and 1960, forcing an expensive redesign of the Electra's engine mounts and killing off Electra sales.

In contrast, the other similar quadturbopropliners of the era (Viscount, Britannia, Il-18, An-10, CL-44, Vanguard) - even those which entered service before the Electra ever flew - had no such problems.

Why?

Flutter is a natural frequency resonance phenomenon.  Something is oscillating, and its oscillation, at some point, starts to match the natural frequency of the structure it's attached to, and a mutual input/feedback cycle starts that stores and returns the energy in such a way that it magnifies the initial input and away you go.

The outer wing was the saxophone... the engine nacelle was the reed.

The other airplanes?  Well, when you get down to it, fortunately they were just out of tune you might say, so they weren't affected.


What principle does the attitude indicator in the G1000 use? The heading indicator in the G1000 uses a magnetometor, but how can the attitude indicator work in the G1000?

Does it work with a tilt sensor, rate sensor or accelerometer?

G1000 is a multi-unit system. The attitude information comes from a specialised unit called AHRS (Attitude and Heading Reference System), typically GRS 77. AHRS uses two solid-state gyros (vertical and directional), plus a magnetometer, to obtain a complete attitude information.

In addition, AHRS does employ accelerometers and rate gyros. Their data is used for stabilisation, filtering, and as an input for autopilot and other systems.


Similar books to this: "konstruktion der luftstrahltriebwerke by Stammer and Jahrig" for Structural and machine element design of jet engine I have been reading recently this book. It is unbelievable and has great content. I would like ask if any of you know any other, similar, more modern, of course focused on structural design with such great figures.
https://www.amazon.de/Konstruktion-der-Luftstrahltriebwerke/dp/B00J6XD28O

Try this " The Jet Engine, by Rolls-Royce 5th Edition


Was a point system implemented to train pilots to land on aircraft carriers? A long while ago a friend of mine told me that early in the history of aircraft carriers, a lot of planes and pilots were lost due to the increased difficulties of landing planes on a moving runway. To address this, he said, a point system was used to train pilots to land more successfully with each action required by the pilot being assigned a number of points (speed, angle of approach, etc).

I have been trying to find information on this for over a year but have been unsuccessful. Have I been misinformed? I'm beginning to think my friend is full of baloney.

Early on in US Naval Aviation, the system began to assign a series of grades for landing performance and set minimum grade requirements to carrier qual and manintain proficiency for naval aviators assigned to carrier based fixed wing squadrons.  The reasoning was to weed out aviators who could not maintain these standards, thus preventing a lot of accidents aboard ship.

A fixed wing carrier pattern and landing has quite a bit in common with that found in terrestrial flying.  However due to the small landing area aboard the ship, the tolerances are considerably tighter and the procedure is more demanding on the aviator.  The Navy found that unless strict standards were adhered to, pilots would begin to lax in their performance and begin to develop bad habits with their flying which could degenerate into dangerous situations.  Therefore all landings aboard ship are now graded by a team of Landing Signal Officers (LSOs).  LSOs are experienced naval aviators who have undergone additional training and experience to become an LSO.  The LSO observes the aviator in the pattern and monitors the pilot’s progress to make sure they’re in the correct position, altitude, airspeed and AoA.  Specific position in the pattern and final approach are graded:

Abeam the platform: the Jet should be abeam the LSO platform on the downwind leg of the pattern, approx 1 mile offset from the port side of the ship, at 600 ft ASL, at a target pattern airspeed, in the landing configuration.
At the 90: the jet should be halfway through the left turn to final approach, perpendicular to the direction the carrier is steaming, about 1 1/2 miles behind the ship, 450 ft AGL, at optimal AoA.
A
Crossing the Wake:  the jet should cross the ship’s wake about a mile behind the ship, 450 ft AGL, at optimal AoA
At the Start: the jet should be lined up in the groove - landing area centerline - and on glideslope - call the ball - about 3/4 mile behind the ship at 450 ft ASL, at optimal AoA.  Major deviations from AoA, lineup and glideslope should be corrected at this point or a go-around should be executed.
In the Middle: in the groove, on glideslope, on AoA.  Any minor deviations should be corrected here. Overshoots and worsening deviations from these targets should require a waveoff.
In close: the jet is 1/4 mile from touchdown, on glideslope, in the groove and very very small corrections being made to maintain these metrics.  Probably the last safe chance for a waveoff, if needed.
At the Ramp:  jet is over the roundout, on glideslope, on groove.  LSO may give the cut signal for the pilot to pull power to idle, if high, but not unsaveable high.
In the Wires: jet touches down (impacts) the deck on the main gear between the #2 and #3 cross deck pendants.  Hook snags the #3 cross deck pendant.  Pilot goes to full power in case of bolter or hook skip and makes ready to go around.
Final Stop: jet comes to a halt in the LA on centerline.  Power to idle.  Disengage the hook from the cross deck pendant.

These positions and metrics are graded and a composite score is released by the LSOs on the platform.  Grades are as follows.

OK Underline:  Superlative pass at the boat conducted with additional complications eg engine failure, barricade landing, etc.  Worth 5 points.
OK “Oh-Kay”: The highest score achievable under standard conditions.  Jet flew pattern and approach with minimal deviations, all promptly corrected for.  Hook snagged the 3-wire.  Good pass worth 4 points.
(OK) “Fair”: A fair pass.  Flying was good but there were a few deviations.  But overall, solid and safe.  Worth 3 points.
OK-Bolter:  a solid and safe pass but hook did not snag a cross deck pendant.  Worth 2.5 points.
No Grade: Too many deviations.  Described by LSOs as “ugly but safe ugly”.  Ironically worth 2 points.
Cut Pass: Unsafe attempt.  Gross deviations outside of the safe margain for flying.  Consider yourself lucky to walk away from this one with only the LSO’s teeth marks in your hinder.  Worth zero points.

A typical review of a pass may read something like.

202 (Jar-Jar): Slightly wide abeam, High at the 90, high and left at the start, slightly high in close, OK.

Squadrons have certain minimum landing GPAs and boarding rates (ratio of traps to total landing passes).  An F-18 squadron might require a GPA of 2.75 with a 60% boarding rate, etc. to remain carrier qualed.

PERSONAL OPINION

I think that grading began as a means of leveraging the competitive nature of the aircrews to motivate better performance.  Again the tolerance between an OK pass and a No Grade are pretty tight compared with landing on terrestrial runways.  Sloppiness can start to creep in unless drilled out.  3-wires start to become 2-wires, then 1-wires, then taxi 1-wires and finally rampstrikes happen and people get killed.  Landing grades are posted in the squadron ready rooms and everyone wants top spot.  If you screw up, everyone’s gonna see it - and it has a serious meaning.


Are rolling takeoffs now more common than powering up while holding on the brakes? I remember back in the 90's that commercial planes would line up on the runway, stop, apply full power and then release the brake to take off.

Now I've been on flight where they've literally rolled from the taxiway straight onto the runway and then powered up without stopping.

Why has that changed? What were the reasons for the older style?

Whether to use a static or a rolling takeoff is usually at the discretion of the captain as long as performance is not an issue and ATC doesn't need you to wait on the runway.

Reasons to do a static takeoff include:

Less runway is required
Better obstacle clearance (mainly because of the earlier liftoff)
Takeoff engine power can be confirmed prior to brake release
ATC may require an aircraft to wait on the runway due to wake turbulence separation requirements
The aircraft manufacturer may not allow a rolling takeoff

Reasons to do a rolling takeoff:

There is sufficient runway available
Obstacles are not an issue
Takeoff engine power can be set relatively quickly and an abort can still be made at a low speed
They take less time to perform, and at busy airports this can mean getting another airplane or two out/in per hour
They offer more passenger comfort because of the smoother acceleration

Not all airplanes provide performance information for rolling take offs. In this case, a commonly used technique is to ensure that takeoff power is set by a certain point on the runway and adding that distance to your calculated takeoff roll for planning purposes.


What are the advantages of twin-fuselage aircraft? During WWII, there were a number of twin-fuselage aircraft (notably bombers but some fighters). They were made by apparently combining two existing single-fuselage aircraft bodies together:

Twin-fuselages seemed to disappear after the 1960s but  recently, Virgin Galactic has come out with a twin-fuselage aircraft, the WhiteKnightTwo.

I understand the Virgin design as the space module (SpaceShipTwo) is designed to fit between the two fuselages. But aside from that special case, are there any advantages to twin-fuselage aircraft?

Was their design and introduction in the 1930s-1950s merely a strange historical accident (and if so, what was the reason d'etre then) or do they currently or potentially have advantages over single-fuselages such that we might see their return?

Depending on the design, one of three advantages made designers choose a twin-boom layout:

No structure directly aft of the fuselage, which helped to integrate the propulsion, or improved access to the cargo area, or improved pilot vision,
Smaller overall surface area for two-engine aircraft, or
Shorter development time for high-performance aircraft.

There are two groups of twin boom designs: Those which were designed that way from the outset, and those which were later derived from single-fuselage designs. You could add a third group, those with two distinct fuselages and an offset pilot position, but an offset cockpit could also be a result of other design considerations. Examples are the Junkers 322 or the Blohm&Voss BV 141, both single-fuselage designs with an offset cockpit.

The first group comprises aircraft like the D.H. 2, the Saab 21 (picture below, source) or the Gotha Go-242. Here the designers wanted to create space for the propeller or a loading ramp. Also, some early jet aircraft used a two-boom layout to keep the tail structure free of the jet blast. The advantage is obvious: No structure at the rear fuselage.

The second group arose due to the performance requirements of modern, all-metal, stressed-skin designs with cantilevered wings. Remember that at the beginning of the 1930s fighters were still biplanes (Hawker Fury, Heinkel He-51), and new designs like the Caudron 460 or the Messerschmitt Bf-108 showed that sleek aerodynamics and high wing loading were the way ahead. However, unlocking the performance potential possible with these designs required much more powerful engines. The engine manufacturers could increase power output only slowly, so a number of designs (Lockheed P-38, Messerschmitt Me-110, Bristol Beaufighter, Northrop P-61) tried to use two engines. Since there were two engines already with a big space requirement for the turbocharger, Lockheed decided to use a two-tailboom design. Here the advantage was a smaller overall surface area to house two engines, their coolers and turbochargers, and the pilot (picture below, source).

During the war, again the requirement for more powerful aircraft arose, and to avoid the delay of a fully new design, existing aircraft were combined, shortening the development time greatly. The first of those was the Heinkel He-111 Z (for Zwilling = Twin), which added a fifth engine to be able to pull the giant Me-321 transport glider. Joining two powerful aircraft was proposed for the Messerschmitt Bf-109 (Bf-109 Z), the Messerschmitt  Me-309 (Me 609), the Dornier Do 335 (Do 635), the Savoia-Marchetti SM.92 and the North American P-51, the only one of those to result in an operational aircraft, the F-82. This resulted in an agile aircraft with high speed and high range, which was very useful for escort or reconnaissance tasks. However, only few of those were used operationally, because soon jet aircraft filled that need in a better way.

The main disadvantages of the twin fuselage designs with their offset pilot position is the high radius of roll inertia and the vertical acceleration suffered by the pilot during rolling maneuvers. Especially for a fighter, a fast roll response is the key to success in aerial combat, and a twin fuselage will impose a penalty not suffered by a two-engined aircraft with a central fuselage. This is compounded by the pilot suffering additional vertical acceleration when rolling, which again will reduce the likely roll agility of the twin-fuselage aircraft.

Might we see more twin-boom or twin-fuselage designs in the future? Probably only for special purposes, like the example you have given yourself. Another might be the Kensgaila VK-7, a Lithuanian airfoil testbed which combines two Blanik gliders with a special, exchangeable center section for testing new airfoils under realistic conditions. This one is not even on Wikipedia's list of twin-fuselage aircraft!

Earlier twin-boom transport aircraft like the Fairchild C-119 or the Nord Noratlas have given way to conventional layouts with a raised tail to accommodate the rear ramp. Twin-boom and twin-fuselage designs are less stiff and more prone to flutter than conventional configurations, so the conventional design prevailed and will do so in future as well.


Induced drag reduction from tandem wing Induced drag is proportional to the square of span loading. The normal method to decrease it would be to increase span. This, however, leads to an exponential weight increase.
So, my question is, if tandem wings are placed with a large longitudinal gap, so as to avoid downwash, would they act as independent wing spans sharing the weight of the aircraft to reduce induced drag? If so, how large would this gap need to be? Would one at the nose and another at the tail be enough?

The downwash is about as high as wide, and angled only by a few degrees (more at slow speed). So the longitudinal distance needs to be many times longer than the wing span to make the wings independent.
And it would still be less efficient than increasing the span:

If you double the span, you will decrease induced drag four times.
If you half the lift, you also decrease the induced drag four times, but then you add the second wing to get your lift back and that also doubles the induced drag, so you have half the induced drag, not quarter.


Why would a diverted airline flight return to its departure airport rather than land at the nearest one? The Vueling flight VY2119 Barcelona-Malaga yesterday (see flightradar) got diverted 50 min into the flight. The nearest airport then was Valencia, but they chose to turn around and fly all the way back to the origin airport, even flying by another airport, Tarragona. Indeed, the whole flight takes regularly 1h40, they could well have flown all the way to Malaga and would  have spent the same amount of time in the air.

Why have they chosen to fly back? Wouldn't an emergency require landing in the nearest airport?

It sounds like it was a non-emergency condition where it wasn't feasible to continue to destination but it wasn't imperative to land ASAP. Something like an anti-icing malfunction where you can't continue into icing conditions (all you need is visible moisture and temps close to freezing; clouds of any kind near or above the freezing level constitutes "technical icing conditions" and you may at minimum have to have working cowl anti-ice to fly through them).

The crew will often consult with airline Operations and Maintenance Control over a desired diversion airport.  Having decided not to proceed to destination, the next best choice is to go back to the departure airport, where passengers that originated at the departure airport can simply go home and return for a later flight.

The other major factor is maintenance related.  Maintenance Control will often ask the capt to divert to airport X because the resources are there, so if the departure airport was a maintenance base, that's another plus.


Does the FAA construe FAR 91.225 to prohibit aircraft w/o electrical systems w/o ADS-B-out from flying above 10,000' MSL unless below 2500' AGL? Does the FAA construe FAR 91.225 as imposing a general prohibition on flight above 10,000' MSL (excluding airspace below 2500' AGL) in aircraft without ADS-B-out, even for aircraft without engine-driven electrical systems?

For example, does the FAA construe FAR 91.225 to prohibit a glider without ADS-B-out from exceeding 10,000' MSL, unless it is not above 2500' AGL?

If answering "yes", please provide specific examples.

For the purposes of this question, "aircraft without engine-driven electrical systems" means any aircraft that was not originally certificated with an engine-driven electrical system, that has not subsequently been certified with such a system installed, including balloons and gliders.

The motivation for this question is a conversation at a glider club where some pilots stated that their understanding was that gliders without ADS-B-out could not legally exceed 10,000' MSL, unless below 2500' AGL.

This related answer suggests that FAR 91.225(e)(2), which contains the prohibition on flight above 10,000' MSL for aircraft without engine-driven electrical systems, is best interpreted to only apply within the lateral boundaries of Class B or Class C airspace.

The current question is not about best interpretations of FARs, but rather about actual FAA practice.

Also, the current question is not about the significance of the phrase "without electrical systems" rather than "without engine-driven electrical systems" in FAR 91.225.  This letter of interpretation from the FAA Office of the Chief Counsel clarifies that the references to aircraft "without electrical systems" in FAR 91.225 should be understood to instead read aircraft "without engine-driven electrical systems", and that the omission of the phrase "engine-driven" was in fact simply a mistake.

The content in this FAA link indicates that the answer to the question is "no".

  Under the rule, ADS-B Out performance will be required to operate in:

  Class A, B, and C airspace.
  Class E airspace within the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL, excluding the airspace at
  and below 2,500 feet above the surface.
  Class E airspace at and above 3,000 feet MSL over the Gulf of Mexico from the coastline of the United States out to 12 nautical
  miles.
  Around those airports identified in 14 CFR part 91, Appendix D.

  The ADS-B Out rule does not apply in the airspace defined in items 2
  and 4 above for any aircraft not originally certificated with an
  electrical system or that has not subsequently been certified with
  such a system installed, including balloons and gliders. For
  additional requirements for using the exception for item 4, please
  refer to CFR 91.225 section (d) for the requirements.

(italicization added)

(as of Feb 24 2020, the quoted web page had last been updated on August 22, 2019)


Deicing Rod for Propellers Similar to a card hitting the spokes of a bicycle, the card is soft enough to knock the dust off the spokes with no damage. What material would be best to impede the propellers to chip off the ice. Assume the flexible rod is machine held. What heavier duty impeller could be used for this? What about the same idea for a helicopter rotor?
I apologize for the multiple questions but they could not stand alone on their own. If it an issue let me know.

A rod soft enough to not be quickly destroyed by the propeller (or to damage the propeller) would likely be too soft to apply impacts forceful enough to shatter ice along the full length of the blades.
A "baseball card" big enough to handle the full length of the blades, and also soft enough, would likely be blown aft forcefully enough to not touch the prop at all.
Frame challenge:  so-called deicing boots accomplish the same, with no risk of damage and with proven reliability, by electrically heating the blades to melt the ice.


Why is the "Dutch" roll called so? Why is the "Dutch" roll called so? When did this name enter common usage, and what is its origin? Please cite sources if possible!

According to Wikipedia:

  "The origin of the name Dutch roll is uncertain. However, it is likely that this term, describing a lateral asymmetric motion of an airplane, was borrowed from a reference to similar-appearing motion in ice skating. In 1916, aeronautical engineer Jerome C. Hunsaker published the following quote: "Dutch roll – the third element in the [lateral] motion [of an airplane] is a yawing to the right and left, combined with rolling. The motion is oscillatory of period for 7 to 12 seconds, which may or may not be damped. The analogy to 'Dutch Roll' or 'Outer Edge' in ice skating is obvious." In 1916, Dutch Roll was the term used for skating repetitively to right and left (by analogy to the motion described for the aircraft) on the outer edge of one's skates. By 1916, the term had been imported from skating to aeronautical engineering, perhaps by Hunsaker himself. 1916 was only five years after G. H. Bryan did the first mathematics of lateral motion of aircraft in 1911."

Sadly, this question is probably not answerable beyond what wikipedia has on this site (unless there is an English Major looking for a Ph.D. thesis. ;)


Flying on one engine Can a commercial jet with two engines, fly on one. If so, what are the limitations. Maby some are better than others in this situation

Transport Category airplanes (e.g., commercial jets) have performance limitations that are different when operating on one engine versus two engines.

Regulatory requirements for commercial operations require that a flight in a two engine transport category airplane be planned to be successfully flown if one of the two engines fail.

The issues are fairly complicated, but the airplane must be configured and operated considering all existing circumstances (airport/runway/weight, etc.) so that if one of the two engines fails at the most critical time the flight can be continued to a safe landing at the original departure airport, the planned destination, or a suitable alternate airport.

For example, if you are departing Chicago in a B757 and the weather is so bad that although takeoff is allowed, landings are not, and an engine failure occurs (just as the aircraft is lifting off the runway) the capability of the airplane must allow an obstacle free climb, enroute and approach to a pre-planned alternate airport on a single engine.

Another example, if you are halfway between San Francisco and Hawaii in a B767 and an engine failure occurs, the airplane must be able to return to SFO or go on to some place in Hawaii on the remaining engine. (the technical components are a bit more detailed than my simple explanation, but the outcome is the same)

Most trans-ocean flights today are done in two engine transport category airplanes (e.g. B767/777 or similar) and the design criteria (see FAR Part 25) are such that continued operation after a single engine failure, if properly considered in the performance planning for the flight (which needs to happen), ensures a safe continuation to a suitable airport.

Of course, the single engine operational profile will be different (altitude, speed, etc.)


On Air France 447, what would have been the lowest altitude to initiate recovery after the stall developed? What could the lowest altitude of Air France 447 have been to recover the (stalled) flight, where the co-pilot informed the captain that he was pulling up the whole time?

The co-pilot didn't inform the captain until 2000 feet, which obviously was too low.

For the stalled flight to recover, the nose needs to be pointed in the airstream, and then the aircraft pulled up with load factor below the ultimate load. From the accident report:

  The recordings stopped at 2 h 14 min 28. The last recorded values were a vertical speed of -10,912 ft/min, a ground speed of 107 kt, pitch attitude of 16.2 degrees nose-up

Initial situation.

From earlier on in the report we can conclude that these values were typical for the complete stall from 36,000 ft to impact, except for the pitch attitude which was around zero for most of the stall. The flight state parameters were therefore:

Vertical speed was 10,000 feet/min = 51 m/s
Horizontal speed was 100 kts = 51 m/s, which constructs an airspeed of 72 m/s.
The engines were at TO thrust, creating a nose-up moment, compensated by the nose-down moment of the airstream hitting the horizontal stabiliser.
The automatic trim is retained in alternate law, so the stabiliser is trimmed full LE down = 14º.
Elevator is fully up = 30º on top of the trim - it is almost aligned with the stalled airstream.

Nose down manoeuvre.

At 72 m/s the aircraft is at about TO speed, and nose rotation speed should be comparable to TO rotation speed - a bit slower because one side of the elevator is stalled. It takes perhaps 5 - 10 seconds to rotate the nose down 45º (an estimated assumption from my part).
When at 45º nose down, gravity accelerates @ 0.7g and the engines at perhaps 0.25g. At this acceleration airspeed builds up with around 1 g = 9.8 m/s every second. If we take averages for nose down manoeuvre time and acceleration, airspeed builds up with 4.9 m/s for 7.5 seconds = 37 m/s. So when the nose is aligned the airspeed is about 110 m/s, but still accelerating fast.
During the 5 - 10 seconds nose down manoeuvre, the aircraft loses about 1,000 - 2,000 ft in altitude

Speed increase

Before the aircraft can be pulled from the dive, the true airspeed needs to be brought to the manoeuvre speed $V_a$. From the A330 FCOM:

  The load alleviation is only available when :

  The aircraft speed is above 250 knots.
  The FLAPS lever is in the 0 position.
  In normal or alternate law flight mode.

Above 250 knots the load alleviation system is active, in order to keep the maximum load factor at 2.5g. The manoeuvre speed (or cornering speed in military speak) seems to be set at 250 knots = 128 m/s. At 1g acceleration, it takes 2 seconds to increase speed from 110 to 128 m/s, during which the aircraft loses altitude of 500 ft:

$$\Delta h = sin(45) \cdot (V_0 \cdot t + \frac{1}{2} \cdot a \cdot t^2)$$

Pull-up manoeuvre.

Then during the pull-up manoeuvre, the load factor must stay under the limit load = 2.5g. 1g is taken up by gravity, and there is 1.5g available for the pull-up manoeuvre.

$$ m \cdot \Delta n \cdot g = m \cdot \frac {V^2}{R} \Rightarrow R = \frac {V^2}{\Delta n \cdot g}$$

With the values established above, we get R = 10,000 / (1.5 * 9.81) = 1,100 m = 3,300 ft. But that is at constant airspeed of 128 m/s, in reality the airspeed will still pick up at the beginning of the manoeuvre and the radius will be higher - let's say 4,000 ft. The velocity vector of the plane was 45º pointing downwards, so half the radius is used. This is from the moment the AoA is close to zero, which it needs to be in order to start the pull-up, which increases AoA again.

My estimate for the altitude required for a successful pull-up from the stalled situation, is therefore 2,000 + 500 + 2,000 = 4,500 ft if they end up skimming the tops of the waves. If they know exactly what to do, time things perfectly, and manage airspeed perfectly. The aircraft is protected against pulling too many g's, so once the nose is aligned the flight crew can pull the stick fully aft and let the aircraft manage the minimum pull-up radius. If for instance the pull-up is initiated 1 second later than the 2 secs estimated above, the airspeed increases to 138 m/s and the pull-up radius becomes 1,300 m = 4,000 ft, plus the aircraft loses an additional 300 ft during the 1 extra second diving down under full power - this 1 second delay in initiating the pull-up requires another 1,000 ft.

Recovering from a fully developed stall is now trained in Level D simulators. Picture below is from a company that makes the flight model and instructor station extension for any flight simulator to train the manoeuvre. Disclosure statement: I have done business with them in the past.


Was there more than one incident or accident where an Airbus FBW aircraft reverted to mechanical backup? There was an accident Smartlynx A320 training flights MYX-9001 at Tallinn on Feb 28th 2018, where ELAC pitch control failed due to fault in the stabilizer actuator, and then both SECs failed due to the command and monitor boards responding differently to a short bounce, so only mechanical backup was left for pitch control (fortunately the crew managed to land the aircraft and had just some broken panels).
It is the only instance I heard of where the mechanical backup had to be used. Was there any other incident or accident of an Airbus FBW aircraft (A320 or later model) where only mechanical backup control was available?

Sadly yes, there was at least one other flight, XL Airways Flight 888 from 2008 on which the use of the mechanical backup trim wheel might have saved the plane.


How would exhaust heat recovery into the combustion chamber affect the efficiency of a jet engine? So, a jet engine basically sucks in air, heats it up and spits it out, right?
The hotter the air gets, the faster the exhaust velocity and thus the more efficient (Higher specific impulse).

Now my question is how this all gets affected if you use a heat recovery system, either a heat pump or a simple heat exchanger. Let's ignore the annoying practicalities of metals melting and all that stuff and just look at the theoretical side.

If you recover heat from the exhaust and put it back into the combustion chamber how does this affect the efficiency of the engine? If we were to apply the heat into the combustion chamber using a heat pump, pumping heat from the exhaust into the chamber could we get even higher efficiencies? While "Yes/No" answers would suffice I would be much more interested in a thermodynamic explanation and possibly even some estimations of the theoretical efficiency which could be reached using the Carnot cycle for pumping the heat.

Onto the practical, has anything like this been experimented with? To my knowledge we don't have heat pumps capable of operating at 2000 degrees Celsius and pumping heat quickly enough to compare to any type of engine, but maybe someone has experimented with simple heat exchangers like how rocket engines use regenerative cooling (for different purposes though, but still similar to this).

Extracting heat from the exhaust will cool and compress it, slowing it down and reducing thrust. Recirculating the heat back into or downstream of combustion will raise the temperature at that point and on downstream to the extraction point. The inevitable thermodynamic losses mean that the returned heat does not quite compensate for the heat drawn out and overall efficiency will fall a little.

What does improve efficiency is to draw through additional air mass in order to increase the exhaust mass flow, even at the expense of lower overall velocity and temperature. This is what the bypass turbofan does.

A more radical modification is to swap the combustion chamber for a heat exchanger and heat the gas via hot fluid from an external source; the hotter the fluid the better. I have seen this proposed for the hydrogen turbopump in some versions at least of the SABRE air-breathing rocket engine. It was also proposed for nuclear-powered bombers in the 1950s, though I cannot recall if those included turbojets as well as ramjets.


What is the aerodynamic reason military parachutes are square? Until these, I had only ever seen round military parachutes. The slots also differ between paratroop

and cargo.

The top pic is the new T-11 canopy adopted by the US Army.  Has a significantly lower descent rate and less opening shock compared to the old T-10.


What's the function of the plate behind the wheels of the Pilatus Porter? Anyone knows what's the plate behind the wheel of the Pilatus Porter for?

Looks like a sort of gravel kit.
Real gravel kits are used by jet powered aircraft to land on rough airfields.
In this case though, the engine is in front, so it is likely that it is only there to reduce the amount of dirt raised when landing/taking-off so to reduce the chances of damaging the aircraft.
From the official PC-6 Porter brochure via pc-6.com (PDF), they call this option a mudguard:

MUDGUARD

And according to a service bulletin (pilatus-aircraft.com; PDF) there were prone to fatigue:

Operators have reported failures of the mudguard supports and stone guard mounting yokes. The failures are caused by fatigue damage.


What powers an aircraft prior to the APU being switched on? I know that APUs are power plants for non-propulsion related purposes and that they are turned on before takeoff.

What powers an airplane prior to the APU being turned on?

Other than the APU, there are multiple ways to provide electrical power to an aircraft:

Battery: The battery is typically the first thing you would turn on and it usually provides DC power to emergency systems only (at least on an airliner, smaller aircraft are fully powered by the battery). Running only on battery power will however deplete the battery quickly. The battery can also be used to start the APU.
Ground Power: Most (maybe all) airliners can accept a connection to a Ground Power Unit (GPU), which will supply AC electrical power to run all aircraft systems on the ground. These can be either mobile units (typically powered by Diesel):

(source)

or direct cable connections to the airport power grid:

(taken from this question)
Engine: This would be the method of last resort, but you can run an engine to provide electrical power via its generator. Note however that starting a jet engine would typically require bleed air from the APU or an Air Start Unit (ASU). Some turboprops can run an engine with a stopped propeller to provide power (Hotel mode). The engines also provide all electrical power during the flight, which is also used to re-charge the battery.

Running an APU on the apron is typically discouraged or restricted1 by the airport because of the noise and pollution. It does however provide one advantage over ground power: the APU also supplies bleed air to the air conditioning systems. Without it, air conditioning is only possible via a Pre-Conditioned Air (PCA) hose. See What is this tube connected to a 757 for? for details.

The APU is usually switched off after starting the main engines. It can however be used in the air on some aircraft. See In what conditions is the APU used in midair? for details.

1 Some airports have restrictions on when an aircraft is allowed to run the APU, e.g. Amsterdam:

  The use of aircraft APU is forbidden in these stands in the period between 2 minutes after blocks for the arrivals and 5 minutes before off blocks for departure.

The APU will still be used to start the main engines during push-back.


If a Boeing 707 hits turbulence that registers as +5 G on the FDR, would the 787 experience lower G forces due to the flexibility of their wings? A 707 being a much older plane with relatively stiff wings compared to the 787. What percentage would the g forces be reduced on a 787 given its wing flex?

And yes, I mean if the 707 and 787 were flying through the exact same turbulence at the same time.

The 787 will not experience lower G-forces, it will experience them later. If the turbulence is of short duration, the 787 might not register much while a 707 is shaken thoroughly. If the updraft persists, both aircraft will eventually feel the same forces.

When an aircraft enters turbulence, the wings experience more lift and start moving upwards. If the wings are heavy (full tanks) and floppy, it takes quite a while before the fuselage experiences any forces. After the wings reached their upper bending limit, the fuselage starts moving upwards together with the wings. In an aircraft with lightweight, stiff wings, the fuselage follows any wing movement immediately.


Does a flight training company need to make a copy of my driver's license? I want to get a Sport Pilot license, but the company that will train me wants to make a copy of my driver's license. I don't mind showing it, but is a copy required?

According to 49 CFR 1552.3 a flight school is required to keep a copy of the documents used to provide proof of citizenship for five years. This is usually a passport or birth certificate. A driver's licenses is not proof of citizenship in the US.

However, a valid photo ID (such as a driver's license) is required in addition to your pilot's license in order to exercise the privileges of the pilot's certificate because the FAA issued pilot's licenses do not have a photo for identification. So a business that is renting an aircraft to you has a legitimate interest in retaining evidence that they are handing over their valuable asset to a licensed pilot, and that you are who you say you are.

They may not be required to keep a copy of your driver's license, but they are also not required to rent you an aircraft or provide flight instruction.


What is the purpose of the fin sweep on small piston airplanes? Why some small piston airplanes, like Cessna 152/172, Tecnam P2008 and many more, have the vertical stabilizer with a sweepback? I don't think that those airplanes may encounter compressibility effects, so what is the reason?

Two benefits to fin sweep on a type like a 152 or 172:
Marketing, and aerodyanamics.
The airplane looks "cooler", "going fast when it's tied down" with the sharply swept fin; this makes it easier to sell to casual pilots and those who'll rent the plane to casual pilots.
However, there's also another advantage.  The sweep moves the tail area to the rear without adding dead weight of structure far out in the tail.  Thus rudder authority and yaw stability are gained with little increase in overall weight, compared to the previous generations of the same models in the Cessna line, for instance (look at a 180 from 1960 for comparison to a modern 182).


Does a jet engine keep running when it can no longer be controlled from the cockpit? On the Qantas Flight 32 incident, the Airbus A380's number two engine suffered from an uncontained engine rotor failure and partially damaged the controls of number one engine as well.

According to Wikipedia, engines one and four entered a 'degraded mode' after the explosion, and number one could not be shut off after landing.

So it seems that in case an engine loses all communication signals from the aircraft's control systems in the cockpit, it could somehow remain operational.  To me that definitely sounds a lot safer compared to shutting down and risking the plane lose all thrust.

So do jet engines, by design, keep producing thrust in case they lose contact with the control systems?

General rule here: the engine will remain at its last power setting should the FADECs lose comms with the flight deck controls (in your A380 case) or the thrust lever cables come unhooked (in the case of an older aircraft with mechanical cable-and-pulley controls).

However, the fire handle will kill the engine even if the main controls fail -- it operates the firewall fuel and hydraulic shutoff valves.  In QF32, this didn't happen because the cable for the #1 engine fire handle itself was damaged by shrapnel entering the wing.


What is a GDS (in the context of the airline industry)? I have been looking at this article and throughout the article they mention GDS's but do not explain the terminology. Does somebody know what they are talking about -

For e.g. -

  Three GDSs emerge from the previous CRSs and sold off by airlines;
  pricing transparency grows - centerforaviation.com

CRS I am assuming/presumng means Computer Reservation System.

GDS stands for Global Distribution System:

  A Global Distribution System (GDS) is a network operated by a company
  that enables automated transactions between travel service providers
  (mainly airlines, hotels and car rental companies) and travel
  agencies. Travel agencies traditionally relied on GDS for services,
  products & rates in order to provision travel-related services to the
  end consumers. A GDS can link services, rates and bookings
  consolidating products and services across all three travel sectors:
  i.e., airline reservations, hotel reservations, car rentals.

These are the databases that maintain travel reservations and allow travel agents to see available inventory and book tickets. As noted in the Wikipedia article, they may also host airlines' computer reservations systems and passenger service systems. For example, Amadeus operates a system used by hundreds of airlines, not to mention thousands of hotels, rental car companies, tour operators, and other travel businesses to maintain reservations and track inventory, and a global distribution system used by tens of thousands of travel agencies, ranging from storefronts to online booking websites, that allows agents to book travel with these providers.


What is the role of baffles in Fuel Tanks? I think there are many baffles inside the shape of Fuel tank. What kind of advantage they do?  Do they help in managing the oscillations in-flight?

They help in keeping the fuel from sloshing around during maneuvering. Some sloshing is impossible to avoid, but a reduction is extremely helpful in avoiding unwanted dynamics.

Consider a sudden pitch-up into a climb, or a sudden acceleration when the afterburners are lit: Now the fuel will rush backwards, shifting the center of gravity backwards. If this happens suddenly and the fuel starts oscillating, the aircraft would fly a wavy flightpath without pilot input. Now add the input of the pilot who tries to keep the flightpath steady, which is delayed by maybe 0.2 seconds, and add the latency of the control system, and you might have ideal conditions for pilot induced oscillations.

Some combat aircraft don't use baffles, but a coarse, sponge-like foam which takes up maybe 2% of the tank volume but dampens fuel oscillations very effectively.


Is it possible to re-engine aircraft with different engine options? Most airliners have multiple engine options and so I was wondering if it would be possible for operators to fit a different engine type to their existing aircraft; let's take for instance the Boeing 747-400 fitted with RB211-524Gs. Could these be interchanged with PW4000s or GE CF6s or would the differing wing loadings require structural changes to be made?

It's doable, but not easy. Unless the aircraft was designed with that in mind, even then it does pose challenges. An example would be the Boeing 787:

The two different engine models compatible with the 787 use a standard electrical interface to allow an aircraft to be fitted with either Rolls-Royce Trent 1000 or General Electric GEnx engines. This interchangeability aims to save time and cost when changing engine types; while previous aircraft could exchange engines for those of a different manufacturer, the high cost and time required made it rare.

The benefit of interchangeability makes leasing aircraft easier for the lessor, as their fleet would be compatible with all the airlines and their existing facilities.

[Once] a plane has one type of jet engine under its wings, it’ll always have that type of engine. It’s just too time-consuming and expensive to swap out a Rolls-Royce in favor of a new General Electric.
This complicated the sale and leasing of jets. If Airline A wants to lease a plane from Airline B, but the two didn’t use the same kinds of engines, that usually would sink the deal (Source).

Related and further reading:

How much would it cost for a large airline to switch engine manufacturer for their planes?
Why bother having different engine options on one aircraft?
Boeing still hasn’t solved engine swap ‘challenges’, 2007


Can anyone identify this aircraft in the AMARG area of Davis Monthan? Anyone has an idea what this thing in AMARG area of Davis Monthan could be?

Googlemaps link: https://www.google.com/maps/@32.1512545,-110.843407,25m/data=!3m1!1e3?hl=en

Problem solved, a close friend of mine pointed to these two very interesting articles on the topic:

http://foxtrotalpha.jalopnik.com/a-new-angle-on-this-shadowy-stealth-f-a-18-hornet-conce-1727910676

http://aviationweek.com/awin/super-hornet-stealth-test-rig-seen


How is the effect of wind on arrival time handled? If a commercial plane encounters strong tailwinds, would the pilots set the autopilot for a fixed arrival time (if that is possible), or would it be ok to arrive way ahead of schedule?

I imagine this could disturb airport operations, and that there are fuel economy considerations: if I was the airline, I would try to minimize fuel consumption.

But if I were ATC, random arrival times could cause chaos. Passengers in turn would probably be happy to arrive as fast as possible. It seems like there are conflicting interests.

It will depend on constraints at the arrival airport. If there are no constraints, they will choose a speed that minimizes fuel burn, and arrive early if that's how it works out.

It's possible that the arrival airport may have constraints on arrival volume. As you pointed out, for busier areas, if aircraft arrive earlier than expected, it could cause things to get backed up. In that case ATC may have an aircraft adjust their speed or enter a hold in order to delay their arrival until capacity allows. Slowing down early is more efficient than arriving too soon and having to fly a holding pattern.

In areas without radar coverage, such as oceanic crossings, aircraft may be required to hold a certain airspeed in order to maintain separation from aircraft ahead and behind. In this case a pilot may not have much choice until they exit the oceanic airspace, and any adjustments would have to be made after that point.

ATC will do their best to ensure the safe and expeditious flow of traffic, but safety has to come first. While a pilot can choose to not follow ATC instructions in the case of an emergency, and in the interest of safety pilots have some amount of discretion in declaring emergencies as they see necessary, doing so just to reduce cost or for convenience would not be acceptable and may attract some scrutiny.

It's also possible that there would be constraints on the ground at the arrival airport. If the aircraft is scheduled to arrive at a certain gate at a certain time, and it arrives early, there may be another aircraft still occupying the gate, or support staff may not be ready to receive the aircraft. This would require the pilots to coordinate with the airline. If alternate arrangements can't be made, the airline might actually prefer the aircraft to be not arrive early. They would probably prefer to sit on the ground, with the engines off if possible, as that burns much less fuel than doing the waiting in the air.


Where to get spatial data for ATC airspace? I am looking for free or open spatial vector data (geoJson, .shp, .kml, …) which maps the responsibilities of air traffic control in South East Asia, worldwide would be very nice too. SkyVector.com seems to have an excellent data source but its not open. any ideas? Cheers!

Data about ATC sectors in easy to use format, such as shapefiles, is generally not publically available. Take it from someone who has spent hundreds of hours manually importing such data from PDF text files to various GIS applications. You can find a few individual data sources here and there via Google, but they are limited geographically and more often than not, outdated.

What you need to do is locate the aeronautical information publication (AIP) of your country of interest, and navigate to the En-route section. Airspace data is usually contained in section ENR 2.1. What you will find is a more or less well formatted PDF file containing textual airspace definitions (WGS-84 based coordinates). You will need to manually transfer this text data into which ever format you need, using your preferred text editor or spreadsheet application.


Are all the engines on a multi-engine plane the same? In general, are there any differences between the engines on a multi-engine aircraft? Are engines designed to be specifically for the left or right wing, or inboard or outboard for a 4-engine aircraft?

What about the tail engine on a tri-jet like the DC-10? Is it the same as the two wing-mounted engines?

Generally yes, but not always. If you include the APU as an engine, the answer would need to be different, but I understand your question concerns just the engines used for propulsion.

For jets, the direction of rotation doesn't make much of a difference, so the same engines can be used on all stations. However, with propellers the swirl does influence flying characteristics, especially at low speed, so special left- and right turning versions of turboprop and piston engines are available.

Maintenance and logistics become much simpler if only one type of engine is used. In the past, not so much emphasis was placed on this, so some airplanes used different types of engines. Examples were:

Convair B-36B, which combined piston and jet engines in the same airframe
Junkers G-38, which used a Junkers L88 inboard and a Junkers L8 outboard. Later, the outboard engines were also changed to the L88. This can be seen from the number of propeller blades: If the outer engines drive a two-bladed propeller, it is the earlier version with the L8.
Hawker-Siddeley Trident 3, which was almost a four-engined jet, because it had a small, tail-mounted RB-162 engine in addition to the three tail-mounted RB-163 Spey to provide thrust on takeoff.
Rutan Voyager, which used a bigger front and a smaller rear engine.


What materials are used for the gas bags in hydrogen airships? I heard that to construct a hydrogen blimp or airship, the gas bag must be coated with aluminium, which got me confused: doesn't it add significantly to the weight? Is aluminium really used, and is there a specific reason to use it?

I'm also interested in knowing what is the best lightweight material in general for building an airship/blimp bags.

Have you ever heard of goldbeater's skin? This was the preferred material before synthetic materials became available.

What is it? Wikipedia says:

  Goldbeater's skin—the outer membrane of a calf's intestine—is a
  parchment traditionally used in the process of making gold leaf by
  beating, reducing gold into mere 1 μm-thick leaves. […] To manufacture goldbeater's skin, the gut of oxen (or other cattle) is soaked in a dilute solution of potassium hydroxide, washed, stretched, beaten flat and thin, and treated chemically to prevent putrefaction.

From the 1880's on it became the preferred material for gas bags, displacing rubberized cloth or silk which would become brittle quickly. The first man-carrying hydrogen ballon used already silk which had been rubberized by varnishing it with rubber that had been dissolved in turpentine. Again Wikipedia:

  Large quantities of goldbeater's skin were used to make the gas bags
  of early balloons created by the Royal Engineers at Chatham, Kent
  starting in 1881–82 culminating in 1883 with "The Heron", of 10,000 cu
  ft capacity. The method of preparing and making gas-tight joins in the
  skins was known only to a family from Alsatia called Weinling who were
  employed by the RE for many years. The British had a monopoly on the
  technique until around 1912 when the Germans adopted the material for
  the internal gas bags of the "Zeppelin" rigid airships, exhausting the
  available supply: about 200,000 sheets were used for a typical World
  War I Zeppelin, while the USS Shenandoah needed 750,000 sheets. The
  sheets were joined together and folded into impermeable layers.

There is a more detailed description in the Wikipedia article on the ZR-1 which reveals that the goldbeater's skin was reinforced with cotton cloth:

  The gas cells were made of goldbeater's skins, one of the most
  gas-impervious materials known at the time. Named for their use in
  beating and separating gold leaf, goldbeater's skins were made from
  the outer membrane of the large intestines of cattle. The membranes
  were washed and scraped to remove fat and dirt, and then placed in a
  solution of water and glycerine in preparation for application to the
  rubberized cotton fabric providing the strength of the gas cells.
  The membranes were wrung out by hand to remove the water-glycerine
  storage solution and then rubber-cemented to the cotton fabric and
  finally given a light coat of varnish.

Today the preferred material is polyethylene therephtalate, a polyester, which is also used for blister packages or soda bottles. By itself, it would have far too high a rate of permeation, therefore it is sealed with a metallic layer of a thickness of only a few atoms. Such metallized films are also used for thermal blankets, in spacesuits, on satellites and for toy helium balloons. To increase its tensile strength, the film is stretched in two perpendicular directions such that the molecule chains become elongated and the film becomes thinner. The result is called biaxially oriented PET (BoPET) and metallized by vapor deposition of a thin layer of aluminium.