Can an airliner provide ground level cabin pressure, temperature, humidity etc? Apart from the cost angle, which seems to be the biggest factor in airlines, is it technically possible to bring cabin atmosphere (includes pressure, temperature, humidity) etc. at par with ground level, at the cruising altitude of most airliners (say 35000 ft).

I just want to understand what stops airliners from providing this kind of comfort to the passengers. I am assuming there might be issues of hull integrity in order to maintain higher cabin pressure etc.

I'll touch on the humidity a bit.

This is actually a big issue for planes; they tried to make it a bit better on the Dreamliner and were able to bump it up a bit

  Humidity is improved, yet still dry as the desert. Humidity levels on the Dreamliner are 10%-15% — better than 7% on other planes on long trips

source

but it's still a big issue. There are two main reasons they can't really increase it that much.

First off, water is very heavy. Since planes pump in dry exterior air they would have to humidify it which means they need to carry enough water to do so. This weight would greatly affect the useful load of the plane. Now you could try to recycle some of the ambient moisture, but on a large scale that takes power, which takes fuel, which comes back to the weight issue.

The other issue (although not as often thought about) is the longevity of the airframe. The extremely dry conditions that airliners see for most of their time actually helps to prevent corrosion on the airframe as well as rust on any rustable parts. By pumping moisture into the airframe you will run the risk of it causing real damage to the metal components.

The temperature is a simple energy problem. To keep a plane hot you need to heat it faster than the exterior temp is cooling it (or evenly to keep it a constant temp). Insulation can help with this but you are still going to see some cooling. On a rudimentary level you can apply Newton's law of cooling here and keep in mind the exterior temperature is in the -40 (and below) area (according to today's charts).

On to the pressure,

This, of course, is a big one, but, in reality, it's not a big deal. While it may seem a bit irritating, 8000ft atmosphere equivalent is still more than breathable. The FAA does not require continuous O2 until 14,000 ft for unpressurized planes, so an 8000 ft equivalent cabin is more than fine. Now from a comfort standpoint, your ears may pop and you may feel some discomfort, but it's more than safe.

From a purely technological standpoint we can do this, but from an engineering use case standpoint it's better to sacrifice some of the pressure to build a lighter, thinner plane. This saves on fuel and material costs down the line. There is great hope that carbon fiber may bring a change to all this and 8000 ft equivalency will be a thing of the past. Boeing has been pushing this with the 787 having only a 6000ft pressure level which they claim alleviates problems. I have not yet flown one so I don't know first hand how much better it really is. Then again I fly unpressurized stuff often and have become somewhat acclimated to it.

No? You wanna know why?

Strength

Aluminium has lower tolerable stress than steel, while steel can keep its stress tolerance fairly well under repeated use.

Weight

There's a maximum pressure difference of 8 psi for aluminium, with few exceptions. You literally have approximately 4 tons of pressure for 2 square meters of aluminium with around 8 psi of pressure difference. And also people leave aircraft unpainted in the interior to save weight.

Corrosion

As I said about the unpainted interior. Aluminium will fatigue quicker if corroded. Meh, common sense.

Strict safety regulations

8000 ft is the maximum cabin pressure? I think that's too strict. Suggestions.

10000 ft. I mean it's easy to acclimatize to 10000 ft without real hazards or reducing comfort. Average blood oxygen saturation will be a hair less than 95% at 10000 ft. In contrast, it's usually 99% at sea level.

4000 m. Above 90% is OK for patients with COPD. At 4000 meters most people would have blood oxygen saturation around 90%.

5000 m. The standard of acclimatization without too harsh hazards. Might start to see some harsh hazards. So 5000 meters is the maximum cabin pressure most people can tolerate without harsh effects.

So guys, for 5000 meter cabin altitude to work, ban alcohol consumption in planes. At least it'll avoid the headaches.


Why are all engines identical in typical commercial airplanes? I come from the IT world where redundancy is in place to cover several cases, among other:

a statistical failure of a device (so we use more of them expecting that the event is independent)
a rogue usage of a device (hacking into it though a vulnerability)
a specific context where a bug is triggered

In order to account for the last two ones, redundancy is sometimes (or usually - depends on the criticality of the system) done with different devices (form different vendors). This is typically the case with firewalls, for instance.

I was wondering why this isn't the case with aircraft engines?

It would be "easy" with planes having four engines (so two of each kind), somehow less "easy" with two engines (where the differences between their regimes would need to be compensated - the pilot states that they want 80% of power, which, in the specific conditions they are in, means 78% for the left engine and 83% for the right one).

Having two redundant engines which are equally susceptible to be impacted by, say, ashes is probably not interesting, compared to having two different ones which would each have its strong and weak points (but different between the two)

Note 1: "easy" above is used as a placeholder only. I am aware that aircraft software and hardware is complicated, I mainly wanted to highlight the difference between 2 and 4 engines. Having two firewalls is a simple task compared to that.

Note 2: it may be that engine failure is not a big deal, statistically speaking compared to other issues (or in an absolute sense), and therefore the extra maintenance, structural compensations because of the differences etc. are not worth it. Thanks for pointing this out if this is the case

Engines don't fail, on average. And if they do, it's a very low probability that two engines will fail at the same time.

Modern jet engines are extremely reliable, with failure rates on the order of 0.01/1000hrs. And if you do have a failure, you have redundancy, as you have two (or more) engines. A plane can stay aloft and land with a single engine.

It would add a lot of complexity and maintenance costs, with no real benefits.

If the failure is external, such as water in the fuel, lack of fuel, or ash ingestion, two different engines are unlikely to help, because all jet engines works in a very similar fashion. The external problem would simply affect both, maybe at slightly different rates, but ultimately shutting them down anyway.

So it adds cost and complexity, without providing any real improvement. Jet liners simply does not crash because all engines stops due to internal problems.

The one case where it would help is if there's a design fault with the engine, e.g. manifesting in turbine breaking after x hours. But engines are extensively tested before being put into serial production, and this occurs very rarely on production aircraft. In addition, the likelihood of the same problem occurring on two engines in such a sport span of time as a flight, but not at all during testing, is a rather low likelihood.

All commercial airplanes have at least two engine types. This is true even for single engine aircraft.
The first engine type is of course whatever the airplane uses. The second engine is gravity.
As part of passing airworthiness tests, all commercial airplanes must be aerodynamically stable. For an airplane to be aerodynamically stable it must have the property of having a positive static margin. Static margin is one aerodynamic measure of an airplane's stability.
In layman's term, static margin is often called "can glide". Airplane that cannot glide are simply not stable enough to be controlled by a human. Note that airplanes that cannot glide can be controlled by computers. In fact a lot of modern jet fighters are deliberately designed to be unstable (cannot glide) in order to improve areal combat performance.
Because of this, there is nothing to be gained by using different engines on an aircraft while incurring significant disadvantages in terms of engineering, maintenance, drag and fuel management.

Note: for a reference on how far commercial airliners can glide, google "Gimli glider". A typical airliner is almost as efficient as a purpose-designed glider or sailplane.
Also note: I used the word "airplane" instead of "aircraft" because this does not apply to helicopters or (in the near future) multirotors. For helicopters there is a similar reasoning due to autorotation. For multirotors most designs cannot survive total power lost so will just drop like a rock. Some designs are based on helicopter collective pitch so they can also do autorotation


Did Pacific Southwest Airlines ever operate flights under VFR? Does anyone know if Pacific Southwest Airlines (PSA) ever flew certain routes under VFR?

I've read this question and I understand that Part 121 does allow for VFR flight, but my question is specific to PSA. I've heard they routinely flew low altitude VFR flights on shorter routes throughout California.

I was a controller at L.A. Center around that time, and it was extremely common for airlines on the short-haul routes to go VFR-on-top, which is VFR but on an IFR flight plan. There would have been a lot of downsides to flying VFR but every advantage to them to fly VFR on top: with on-top they didn't need clearance into class B (back then the TCA) or get refused or delayed because the controller was "too busy"; didn't have to fly airport patterns (basically guaranteed a straight-in since they would be on an instrument approach); and for flying into the metered airports like LAX or SFO they would have a guaranteed slot, unlike trying to call up as a VFR where they would go to the back of the line and might have to hold for an hour. Plus of course on-top has the benefits of not needing IFR separation, so ATC would almost always give them a direct routing and they could climb all the way up to 16500/17500 instead of being step-climbed under everyone else and capped at maybe 10,000.

So while I don't remember specific routine flights from 25+ years ago, I do remember working a LOT of airlines flying VFR on top on the short routes like LAX to Bakersfield, Burbank to Fresno, etc.

It's important to understand the difference between visual procedures, visual flight rules, and visual meterological conditions.

Airliners always operate under instrument rules because it guarantees them ATC services, and because it's a requirement for proceeding into instrument meterological conditions (IMC).  Also, higher airliner flight level altitudes are reserved for planes on instrument flight plans.

In instrument meterological conditions, only instrument procedures are used.  But in visual meterological conditions (VMC), airplanes operating under instrument flight rules (i.e., those following instrument flight plans) are often required to perform visual procedures like flying visual approaches, or maintaining visual separation from other airplanes.


If the "Fuel Shutoff Valve" is ON, wouldn't fuel be cut off? Ok, so this may be more of an english lanugage question, and I can confirm from context that when the fuel shutoff valve is ON fuel does indeed flow to the engine, but wouldn't that just make more sense to call that valve the fuel valve?

If the component that shuts fuel off is on, operational, in use, functioning, etc., shouln't fuel be shut off, as the name implies?

I realize this could be seen as a quibble, but confusion causes accidents and IMO this is wildly confusing.

EDIT

Looking at the Cessna 152 checklist today I noticed this interesting quirk. On the regular checklist side (pre-takeoff, maneuver, etc.) the checklist reads "Fuel Shuttof Valve - ON", but on the emergency checklist for an engine fire it merely states "Fuel Valve - OFF".

I can see where that can be confusing, but it comes down to perspective.

From a maintenance and engineering perspective, you are actuating the fuel shutoff valve which controls the fuel to the engine.  The actual terminology when referring specifically to the valve would be opened or closed, which would result in the fuel being on or off.

From a pilot perspective you are simply turning the fuel selector on and off, which happens to be connected to the fuel shutoff valve.  The pilot doesn't even need to know about the valve, so they placard it from the operational perspective in the cockpit and say whether the fuel is on or off.

Note that these are placarded as a fuel selector and fuel off both of which seem pretty clear to me.

Shutoff has a negative connotation, but the word ON is (highly) associated with the engine running. So if SHUTOFF is ON, is the shutoff valve operational (cutting the fuel off) or do we run with our standard interpretation of the word ON - that something essential (fuel supply) is functioning? I can see a pilot in an emergency, when her or his brain is on the edge of panic, forgetting their training and operating the valve incorrectly. I'd suggest it would be clearer to keep the word Shutoff, but make it one of the options, rather than the name of the whole control.

(source: massey.ac.nz)


Do IFR / VFR weather minimums include ceilings? I'm not really sure how to ask my question, but there are classifications for weather conditions for MVFR, IFR, and LIFR. The FAA also has weather minimums for flying VFR in the different airspaces here.

What is the difference between when we would use one over the other? If I am departing from an airport in class bravo airspace, would I need three statute miles of visibility and remain clear of clouds (second link), or would I also need to adhere to the minimum VFR ceiling of 1000' (first link)?

The FAA does not define "LIFR" or "MVFR"
These are not terms that the FAA makes reference to.  Go look in the FAR/AIM, section 1.2.  You'll find VFR defined, and IFR defined, but not the other two.  Why is that?
Because those are terms used by the National Weather Service, not the FAA
That means that if you want to operate an aircraft, you obey the FAA weather minumums that are provided in your second link.
The information in the first link is about reading weather reports.  As far as the FAA is concerned:

VFR is VFR
MVFR is also VFR
IFR is IFR
LIFR is also IFR

So answer my question
Ok.  If you're departing from an airport, and you will be in class B airspace from the moment your wheels leave the ground, then as long as you have 3 miles visibility and you don't touch a cloud you can fly.
However, it's not likely that you'll be in class B airspace from the moment you leave the ground unless you're departing from the main airport (LAX, ORD, and so on) and if that's the case odds are you're on an instrument flight plan anyway.

Sounds like you wish to depart VFR.

would I need three statute miles of visibility and remain clear of clouds (second link)

Is that Special VFR? Special VFR might be a little less restrictive even.  You can request Special VFR, but it will never be offered (explicitly) to you from ATC.
The Special VFR requirements, to be complete:

14 CFR 91.157 - Special VFR weather minimums. eCFR Authorities (U.S. Code) Rulemaking What Cites Me prev | next § 91.157 Special VFR weather minimums. (a) Except as provided in appendix D, section 3, of this part, special VFR operations may be conducted under the weather minimums and requirements of this section, instead of those contained in § 91.155, below 10,000 feet MSL within the airspace contained by the upward extension of the lateral boundaries of the controlled airspace designated to the surface for an airport.
(b) Special VFR operations may only be conducted -
(1) With an ATC clearance;
(2) Clear of clouds;
(3) Except for helicopters, when flight visibility is at least 1 statute mile; and
(4) Except for helicopters, between sunrise and sunset (or in Alaska, when the sun is 6 degrees or more below the horizon) unless -
(i) The person being granted the ATC clearance meets the applicable requirements for instrument flight under part 61 of this chapter; and
(ii) The aircraft is equipped as required in § 91.205(d).
(c) No person may take off or land an aircraft (other than a helicopter) under special VFR -
(1) Unless ground visibility is at least 1 statute mile; or
(2) If ground visibility is not reported, unless flight visibility is at least 1 statute mile. For the purposes of this paragraph, the term flight visibility includes the visibility from the cockpit of an aircraft in takeoff position if:
(i) The flight is conducted under this part 91; and
(ii) The airport at which the aircraft is located is a satellite airport that does not have weather reporting capabilities.
(d) The determination of visibility by a pilot in accordance with paragraph (c)(2) of this section is not an official weather report or an official ground visibility report.
[Amdt. 91-235, 58 FR 51968, Oct. 5, 1993, as amended by Amdt. 91-247, 60 FR 66874, Dec. 27, 1995; Amdt. 91-262, 65 FR 16116, Mar. 24, 2000]

91.205(d) basically says you have to be equipped for VFR and have a radio. https://www.ecfr.gov/cgi-bin/text-idx?pitd=20170830&node=se14.1.91_1205&rgn=div8
So clear of clouds and 1 mile visibility - sounds more like scud running to me.


Are passengers really not allowed to change seats before take-off due to weight and balance? Just boarded an A320 with many open seats (Spirit), and the flight attendant said we may not change seats until at altitude due to weight and balance.  Is he just trying to prevent pax from moving around while the crew is busy moving around the cabin, or is this a legitimate issue?  I feel it must be the former, in which case I feel they're being a bit deceptive.

The crew gave you the correct reason, but it would had taken a co-ordinated effort from the majority of the passengers to upset the aircraft's balance.

But in smaller aircraft even a single person moving around can shift the center of gravity outside its safe limits. The de Havilland company once produced such an aircraft, the DH86 Express. Wikipedia reports:

  On 2 October 1935 Holyman's VH-URT Loina was also lost in Bass Strait,
  again with no survivors. This time a significant amount of wreckage
  was recovered from the sea and from beaches on Flinders Island.
  Investigation of the wreckage revealed a section of charred carpet on
  a piece of cabin flooring from just ahead of the lavatory door. It was
  thought possible that a small fire from a dropped cigarette had led to
  someone running aft suddenly to stamp it out – the sort of sudden
  change in weight distribution that could set up a fatal loss of
  directional control while the aircraft was on a low-speed landing
  approach.

A dramatic example what a shifting load can do to the controllability of an aircraft was National Airlines Flight 102, a Boeing 747-400 freighter conversion which took off from Bagram on April 29, 2013. The most likely cause was insufficiently secured payload which shifted back when the aircraft pitched up during take-off. A single passenger will not be as heavy as a MRAP, but several of them moving over a significant part of the cabin length in combination with a position of the center of gravity close to the limit could well upset the flight.

CG at the forward limit, passengers moving to the front: The aircraft will need higher speed and much more elevator deflection for lift-off. Especially in combination with an engine failure late in the take-off run, a crash is a distinct possibility.
CG at the rear limit, passengers moving to the back: The static stability and, in manually controlled aircraft, the stick forces are reduced. Flying the aircraft needs more attention, and in extreme cases the aircraft will become longitudinally unstable.

In any case, the pilots need to adjust trim.

It was probably BS and the attendant was just being overly cautious. They probably say that on any flight with a half-full cabin. There is no way to know for sure however without more information about the loading of the aircraft whether it was legitimate or not. If the flight attendant made a general announcement and it was the pursuer who did so, it is much more likely that it was real, because that is who the captain would have given the instruction to. If it was some random attendant just telling you personally not to move, it was probably BS.

That being said, moving around when a plane is taking off is a very bad idea at multiple levels because you could hurt yourself or someone else. You should always remain seated and buckled in when an aircraft is taking off or landing.


What to do if the captain smells like alcohol? Suppose you're the co-pilot on a flight and during pre-flight, talking to the captain, you smell alcohol.

Although you didn't see him drinking and the smell isn't strong, what does regulation stipulate you should do?

Unfortunately, at least in the US, the regulations don't stipulate that you do anything. Airline policy is a different matter, and one which I cannot speak to, as it will be different in every case.

I can provide some background and my own take on this, but it's hard to have one exact answer (what if you're wrong?).

FAA regulations rely on pilots self-regulating. If a pilot drinks within an 8 hour (or, as is often policy for airlines, 12 hour) window prior to the flight, they are required to remove themselves from that flight. If they do not take this action, somebody else must. Often, this person is the First Officer, a junior pilot who has years of flying with pilots who might come to view them as untrustworthy. Perhaps this can help explain why it's such a difficult action to take.

This is a difficult topic, and one which regularly comes up during airline flight training. That said, the courses I took that discussed this scenario essentially landed on a couple of steps (this was years ago, this is probably a bit rusty):

Confront the pilot. Ask them if they've been drinking in the last X hours. If they claim they have not been, or they push back:
Attempt to explain to them the situation they're putting their career (and you) in;
Try to get them to remove themselves from the flight and contact a union representative.

Beyond that, I don't want to make suggestions. Getting them to talk to the union they are a member of is probably a wise idea, as they can offer possible solutions or arrange treatment if the pilot has alcohol abuse problems (which is not a huge stretch, if they're showing up to work reeking of booze). AOPA Flight Training has a blog post that discusses some similar scenarios.

This is clear cut. You are required to wait 8 hours after taking a drink of alcohol before you fly a plane. According to FAA regulations:

  (a) No person may act or attempt to act as a crewmember of a civil
  aircraft—

    (1) Within 8 hours after the consumption of any alcoholic beverage;

    (2) While under the influence of alcohol;

    (3) While using any drug that affects the person's faculties in any
    way contrary to safety; or

    (4) While having an alcohol concentration of 0.04 or greater in a
    blood or breath specimen. Alcohol concentration means grams of alcohol
    per deciliter of blood or grams of alcohol per 210 liters of breath.

  (b) Except in an emergency, no pilot of a civil aircraft may allow a
  person who appears to be intoxicated or who demonstrates by manner or
  physical indications that the individual is under the influence of
  drugs (except a medical patient under proper care) to be carried in
  that aircraft.

  (c) A crewmember shall do the following:

    (1) On request of a law enforcement officer, submit to a test to
    indicate the alcohol concentration in the blood or breath, when—

      (i) The law enforcement officer is authorized under State or local law
      to conduct the test or to have the test conducted; and

      (ii) The law enforcement officer is requesting submission to the test
      to investigate a suspected violation of State or local law governing
      the same or substantially similar conduct prohibited by paragraph
      (a)(1), (a)(2), or (a)(4) of this section.


Can a fighter jet be stolen? What (if any) sort of pilot authentication systems do fighter jets have?

Could anyone with the required skills simply take the aircraft or is a key or authorization code required?

Military aircraft rarely have any sort of keys or any sort of authentication systems, they rely on physical security measures to prevent people from getting to the airplanes in the first place, like fences, dogs, cameras, guards, and of course guns.

If there were no physical barriers then you have 3 major barriers to being able to steal a fighter jet:

Support: just about every military fighter aircraft has an assigned crew to prepare it for flight and help the pilot(s) get in it. Without this crew you'd have to get the airplane powered up, fueled up, and then you'd have to figure out how to get into it and get the ladder removed. There's a lot of knowledge you'd need to do all this, just knowing how to open the canopy isn't going to be immediately obvious
Starting: starting up a fighter isn't like starting a car, you don't hit a button and everything happens for you, there's fuel tanks and pumps, hydraulic systems, electrical systems, and engine management. You'd have to know where to find all of the switches, dials and levers, and the sequence in which to use them. This isn't something you'd be able to guess, you'd need knowledge of the particular airplane
Flying: high performance jets are challenging to fly, and by flying I mean aviating, navigating, and communicating. Although modern fighters have a lot of computers to help out with this the systems require a great deal of training to use, and if you don't know exactly what you are doing you are most likely going to dig a big hole in the ground

If you know how to start and fly the airplane there's nothing stopping you from doing it, provided you can solve the problems of prepping and getting in the airplane. In fact, it's been done when pilots defect with their airplanes, see the comments below for some great examples.

Once, in 1997, an A-10 pilot 'stole' the aircraft he was flying, (broke off from his flight and his assigned training mission), and disappeared from ground radar. His wrecked aircraft was later found in the side of a mountain in Colorado.  It was assumed that he committed suicide.

See this article for details.

In order to 'steal' a military jet, you would have to be proficient enough in that type of aircraft to safely take off and fly the aircraft. So, the pilots who would be capable of doing this are necessarily limited to a small number of people, most of whom are military trained pilots.


What is the reapplication process for a suspended dispatcher certificate? I had my dispatcher certificate suspended for a year for leaving the testing site before I could provide an adequate urine sample.

I did not realize this was a reportable event. I accept the suspension but am looking for the process to follow to get reinstated after the 1 year period.

I know very little about this, but JO 2150.3C says that if a certificate is suspended for a fixed period of time, it's reinstated at the end of that time if you meet whatever conditions the FAA requires (p. 7-4):

  Indefinite suspensions are used when the Administrator has reason to
  question, but is unable to determine, a certificate holder’s
  qualifications, or when the certificate holder does not comply with
  statutory or regulatory requirements to cooperate with the FAA. For
  indefinite suspensions, the Administrator suspends the certificate
  until certain conditions are met, e.g., until a reexamination or
  reinspection under 49 U.S.C. § 44709(a) is successfully completed. The
  period of time the certificate is not effective is specified in the
  order of suspension, and once the required period has passed, the
  certificate is reinstated.

Your question doesn't have many details - and I understand that you might not want to share them! - but I take that to mean that if your certificate was suspended for one year, it will be reinstated after one year has passed, as long as you've done whatever the FAA wants you to do in order to qualify for reinstatement.

The phrase "until certain conditions are met" implies that there's no standard way to qualify to reinstatement; it probably depends on the reason that the certificate was suspended in the first place. Page 9-15 of the order lists reasons for suspension and they're quite varied, so presumably the "certain conditions" are too.

On the other hand, if a certificate is revoked then it cannot be reinstated (p. 7-4):

  Unlike a suspension, a certificate that has been revoked cannot be
  reinstated. A person whose certificate has been revoked may be issued
  a new certificate provided that the person meets the qualification
  requirements for the new certificate. To be issued an airman
  certificate following revocation, an individual must retake all tests,
  whether written, oral, or practical.

Your local FSDO could probably confirm that, although if your situation is more complicated then an aviation lawyer might be more helpful.

There is no FAR that says your certificate would be suspended for leaving the test site. It is more likely that the suspension was given because your leaving and not returning after being asked to provide the urine sample was interpreted as a refusal to provide the sample. That is grounds for suspension of the certificate. To get it reinstated you need to apply for the certificate following the normal process and take the test(s) again, and don't forget you'll need to provide that urine sample if you are asked for it.


What is the busiest airport? What is the busiest airport in the world, and how is this measured?

Note: Since this answer was first written, updated statistics have been published. The original answer was written against 2015 figures, with the update from 2017 figures.

There is no one simple measure of "busiest" when it comes to airports, there are usually (at least) three.

Busiest by passenger traffic - defined as total passengers emplaned and deplaned, with transit passengers counted once
Busiest by cargo traffic - loaded and unloaded freight and mail measured in metric tonnes
Busiest by traffic movements - total number of landings and takeoffs of all aircraft

The most recent statistics produced by Airports Council International are from 2017.

By passenger traffic the top 3 are

2015

Hartsfield–Jackson Atlanta International Airport,  USA
Beijing Capital International Airport,  China
Dubai International Airport,  UAE

2017

Hartsfield–Jackson Atlanta International Airport,  USA
Beijing Capital International Airport,  China
Dubai International Airport,  UAE

By cargo traffic

2015

Hong Kong International Airport, Hong Kong
Memphis International Airport, USA
Shanghai Pudong International Airport, China

2017

Hong Kong International Airport, Hong Kong
Memphis International Airport, USA
Shanghai Pudong International Airport, China

By traffic movements

2015

Hartsfield-Jackson Atlanta International Airport, USA
O'Hare International Airport, USA
Dallas/Fort Worth International Airport, USA

2017

Hartsfield-Jackson Atlanta International Airport, USA
O'Hare International Airport, USA
Los Angeles International Airport, USA

(Aside: Wow, those Americans like their traffic movements - 8 of the top 10 are USA! | 2017 update: 7/10)

There are a few non-standard claims too. They are interesting, if not winners.

Memphis International is the Home of FedEx, and as such was the highest freight airport for a number of years.
Gatwick Airport is the busiest single-runway airport (There are actually two runways, but only one is ever in use)
Wittman Airport hosts an aviation event which temporarily raises it to one of  the busiest airport in the world for that short period.
The airports of London (Heathrow, Gatwick, City, Luton, Stanstead & Southend) host the most passengers annually for a single city's airports combined.

(Don't get me started on Southend being a "London" airport - it's at least 20 miles from what could sensibly be called London)

References:

ACI: busiest airports in 2015
ACI: busiest airports in 2017
Wikipedia: Busiest airports

This CNN article just reported on 2017's busiest airports.

https://www.cnn.com/travel/article/worlds-busiest-airports-2017/index.html

  Worldwide passenger traffic increased 7.5% to 8.3 billion in 2017, one of the strongest growth rates on record.
  International passenger numbers were up 8.6% globally.
  Worldwide cargo traffic rose 7.7%. The world's largest air cargo hub remains Hong Kong. The US city of Memphis, Tennessee, took the second spot, followed by Shanghai, China.
  Worldwide aircraft movements increased 3% in 2017 to 95.8 million

  The world's busiest airports 2017

  Hartsfield-Jackson Atlanta International Airport (Georgia) -- 104 million passengers
  Beijing Capital International Airport (China) -- 96 million
  Dubai International Airport (United Arab Emirates) -- 88 million
  Tokyo Haneda International Airport (Japan) -- 85 million
  Los Angeles International Airport (California) -- 84.6 million
  Chicago's O'Hare International Airport (Illinois) -- 80 million
  London Heathrow Airport (United Kingdom) -- 78 million
  Hong Kong International Airport (China) -- 73 million
  Shanghai Pudong International Airport (China) -- 70 million
  Aéroport de Paris-Charles de Gaulle (France) -- 69 million
  Amsterdam Airport Schiphol (Netherlands) -- 68.5 million
  Dallas/Fort Worth International Airport (Texas) -- 67 million
  Guangzhou Bai Yun International Airport (China) -- 66 million
  Frankfurt Am Main Airport (Germany) -- 64.5 million
  Atatürk International Airport (Turkey) -- 64 million
  Indira Gandhi International Airport (India) -- 63.5 million
  Soekarno-Hatta International Airport (Indonesia) -- 63 million
  Singapore Changi Airport (Singapore) -- 62.22 million
  Incheon International Airport (South Korea) -- 62.16 million
  Denver International Airport (Colorado) -- 61 million


What is this flange on the engine? Flying home from the Denver ComiCon, on a 737, I saw an angled flange on the engine. Could you describe its purpose?

Photos by CGCampbell licensed CC0

This is called a "strake" or "chine" (Aerodynamic fan cowl strake/chine, Nacelle strake/chine).
They allow the aircraft to generate more lift at lower speeds, which entails such positive consequences as lower stall speeds, lower landing speeds, lower take-off speeds and shorter runways.

Strakes should be used if the nacelles are mounted closely under the wing of the aircraft, see also Why don't all aircraft use/need nacelle chines?. Nacelles are usually mounted like that in order to

maximize the distance between the lower nacelle side and the runway, so that the engine/fan cowl/nacelle doesn't hit the ground during violent landings when the wings bend down, and in order to
minimize the risk for runway debris being sucked into the engine.
Source: Flugzeugtriebwerke, Bräunling Willy J.G., VDI-Buch, Springer Vieweg, 2015

From US Patent 20100176249: Engine Nacelle Of An Aircraft Comprising A Vortex Generator Arrangement:

  With an optimal arrangement and at high angles of attack, such vortex generators, which are known as nacelle strakes or chines, generate a powerful vortex that flows over the wing, where on a slat in front of said wing it delays airflow separation until the aircraft flies at greater angles of attack.

The idea dates back to 1971 (US Patent 3744745: Liftvanes):

  in operational conditions wherein high angles of attack are encountered, such as in landing or takeoff, the vanes oppose the strong upwash around the nacelle, reducing the flow separation on its upper areas, and providing a strong downwash marked by marginal trailing vortices

From R.S. Shevell. Aerodynamic Bugs: Can CFD spray them away? 1985

  DC-10 wind tunnel tests showed a significant loss in maximum lift
  coefficient in the flap deflected configurations, with landing slat
  extension, compared to predictions. This resulted in a stall speed
  increase of about 5 knots in the approach configuration. The initial
  wing stall occured behind the nacelles and forward of the inboard
  ailerons. The problem was traced by flow visualization techniques to the
  effects of the nacelle wake at high angles of attack and the absence of
  the slat in the vicinity of the nacelle pylons. The solution was
  developed in the NASA Ames Research Center 12 ft. pressurized tunnel and
  turned out to be a pair of strakes mounted forward on each side of the
  nacelles in planes about 45 degrees above the horizontal. The final
  strake shape was optimized in flight tests. The strakes are simply
  large vortex generators. The vortices mix the nacelle boundary layer air
  with the free stream and reduce the momentum loss in the wake. The
  vortices then pass just over the upper surface of the wing, continuing
  this mixing process. The counterrotating vortices also create a downwash
  over the wing region unprotected by the slat, further reducing the
  premature stall. The effect of the strakes is to reduce the required
  takeoff and landing field lengths by about 6%, a very large effect.

I do not believe the teaching in the patent is correct.

Instead of the downwash from the strake suppressing the wing upwash (and remember, the other side of the vortex adds to the wing upwash!)  we believe that the vortex is well organized and can withstand the adverse pressure gradient of the wing better than the 'junk' wake of the nacelle without the strake.

The size and location of the strake determine when the vortex will burst, and hence govern when the inboard wings stalls.  Ideally you would like this to be at the same time as the outboard wing.  Too late and you get unacceptable pitch up.  Too early and you lose low speed performance (higher stall speeds equated to higher speed schedules and longer takeoff field lengths).

A side note, the outboard strake is of little to no value at all, which is why Boeing airplanes only have strakes on the inboard side.


Does a multi-engine ATP with single-engine commercial privileges provide single-engine instrument privileges? Does a multi-engine ATP with single-engine commercial privileges provide single-engine instrument privileges?

I received my ATP certificate recently, and the back of my FAA certificate reads as follows.

AIRLINE TRANSPORT PILOT
  AIRPLANE MULTIENGINE LAND
  ...
COMMERCIAL PRIVILEGES
  AIRPLANE SINGLE ENGINE LAND
...

I left out the specific type ratings and limitations as noted by ....

The ATP supersedes an instrument rating, but the ATP is for the multi-engine category. Shouldn't the certificate reflect an instrument rating in the single-engine category?

The changes to my certificate were completed in IACRA and I have had part of a rating inadvertently removed before using the IACRA system.

Note this is my interpretation of part 61. Double check with the FSDO for a more legally biding answer.

Since the Instrument addon is by category, not class, aka Instrument Airplane, or Instrument Rotorcraft, the ATP Multi-Engine Airplane, counts for Instrument Airplane for the Commercial Single.

ATP certificates do NOT supersede an instrument rating.  Rather an instrument rating is a pre-requisite to obtain an ATPL.  See FAR 61.153 subpart d.1.

An instrument-airplane or instrument-rotorcraft helicopter rating is applicable to all classes or types of aircraft in that category.  I would think the ATPL would have an addendum to it stating INSTRUMENT-AIRPLANE.  I guess the FAA does not list it that way.


What is this white smoke like thing, seen inside the cabin before takeoff? I saw this quite thick white smoke like thing in the cabin in my last flight, before takeoff that is, while it was taxiing and loading passengers:

Video
Please watch in HD to best notice the phenomenon.

It seems to be coming from the AC vents of aircraft. I think it's a quite common phenomenon as the cabin crew didn't even pay any attention to this.
But I saw it for the first time. What is this?

Aircraft Details : VT-IDE (A 3.5 year old A320)

The departure location was Bhubaneshwar and the approximate weather was:
Temperature : 28 °C
Humidity ~ 85%
Clear Sky, good visibility

Direct answer
Air conditioning causes a large drop in cabin temperature, air humidity present in the air condensates into water droplets. It is similar to mist, fog and clouds. If water droplets enter in contact with a solid they form dew and moisture. Accurately, it is an aerosol of water droplets.
Water droplets appear in air when air temperature is lowered below its dew point (see further down). This phenomenon is exactly the same than when a freezer is opened and air over the freezer is in contact with cold air from the freezer tank:

Marcellus ̶̶̶̶̶̶̶̶ freezer. Source.
Quick explanation
In your case, before air conditioning is started the cabin is filled with air from the outside, which happens to be warm and nearly saturated in water vapor. The ambient temperature is close to the dew point. The injection of colder conditioned air in the cabin decreases the temperature around the AC vents (inlet air on the picture below):

Air circulation in large pressurized aircraft, from this answer
The temperature quickly goes below the dew point causing invisible gas (water vapor) condensation into visible liquid water droplets (fog).
Due to the limited instantaneous capacity of the air conditioning system, the temperature is decreased only around the AC vents, not in the whole cabin. When the water droplets mix with the warmer air in the rest of the cabin, they immediately return to the gaseous form (vapor), and doing this they also make this air a bit colder (evaporative cooling).
As pointed out by @JanHudec the humidity level must be kept low during the flight to prevent structure oxidation. Humid air from the cabin returns through the decompression panels to the air conditioning system where it is mixed with dryer air from the packs. After some cycles the cabin air is rather dry and condensation doesn't occur anymore.
Another case of condensation:

Detailed explanation
Why does water vapor condense?
The quantity of water vapor in air is named water saturation ratio (or relative humidity) and is expressed as a percentage: 0% for dry air, 100% for air fully saturated. Saturated air cannot contain more water vapor. If vapor is added then it immediately condenses into visible water droplets.
Saturation occurs at certain combinations of temperature and pressure. For a given pressure, the higher the temperature, the larger the quantity of vapor than can be present without condensation. So condensation can occur when the temperature decreases.
Psychrometrics, the branch of physics studying how liquids and solids boil or condense, revolves around the notion of equilibrium vapor pressure, but for us mere mortals, the notion of dew point is more practical.
Dew point
The quantity of vapor air can contain before being saturated is called its water vapor capacity, it increases exponentially with temperature:

(Source)
Let's define two cases:

Full saturation: If the quantity of vapor is the maximum allowed for the current temperature (100% relative humidity), decreasing the temperature or adding vapor will cause some vapor to condensate into water.

Partial saturation: If air is not fully saturated, there is a (lower) temperature for which it becomes saturated. This temperature is referred to as dew point. So whatever the current temperature, fog can be created by decreasing the temperature until the dew point is reached.

For sake of accuracy: When water vapor is present in air, it is continuously condensed and evaporated. The dew point is the point where condensation occurs at a greater rate than evaporation and water droplets start accumulating.
Whether fog will form depends on the the dew point spread, that is the difference between current temperature and dew point. When the spread is large, the change in humidity or in temperature must be large. In your case, the spread is small, because air is wet and temperature is high.
Bhubaneshwar case
Before air conditioning system is started, cabin air is the same than at the airport: 28°C and 85% humidity. AC is going to inject air at 20°:

Quantity of vapor for 85% RH at 28°C is 23g per kg (85% * 27).
Vapor capacity at 20°: 17g per kg.

When air is injected and temperature is decreased to 20° around the vents, air becomes fully saturated, and the 6g of vapor in excess per kg of air are condensed into fog.

Cabin air relative humidity and AC fog
This air returns to the AC system, and is mixed with air from packs which condition air entering the cabin at the 20% RH targeted for the flight. The 13.6g/kg of vapor in excess (for 20% RH) are progressively extracted by outflow valves, which regulate pressure in the cabin by releasing overboard as much air as packs push in. After some cycles, air contains only 3.4g/kg of vapor (the cycle is simplified, actually we would also account for passengers perspiration and breathing, fog re-vaporization and AC air humidity)
For typical cabin of 150m3 containing about 175 kg of air, 3.5 kg of water are extracted.
Fog in the aircraft is indeed a problem, fog outside the aircraft is another one as visibility is strongly impeded. It requires flying using Instrument Flying Rules (IFR). IFR is allowed only with a specific pilot rating and aviation certification. Knowing the dew point is important to plan a flight, in particular to know whether IFR will be required. Therefore the dew point is measured and published as aeronautical information.
Measuring the dew point
The dew point may be determined with a psychrometer (a system with dry and wet thermometers):

Whirling (how to use it) and digital psychrometers
Pressure, dry and wet temperatures, dew point and relative humidity are linked by the mean of vapor pressure. By knowing three elements, the two other can be determined. So knowing pressure, dry temperature and wet temperature allows to determine the dew point. It exists precalculated charts (Mollier diagrams) to read dew point directly. There are also online calculators.
Here is a vertical profile of the temperature and moisture information (emagram) collected by a radiosonde in Ireland:

Emagram from Valentia Observatory (Ireland). Source meteociel.fr
The vertical scale is the pressure (hPa on the left, equivalent altitude on the right) and temperature is on an oblique scale, with the quantity of water at saturation for this temperature. The dry temperature is shown in read, the wet temperature in blue, and the computed dew point in cyan. When the dew point curve is close to the temperature, moisture is hight (%RH on altitude scale) and cloud/fog can occur (like under 1,600 m for this example).
Dew point and fog/mist information
KLAX ASOS report:

As previously mentioned, when the dew point is close to the ambient temperature and the humidity percentage is high, the visibility is low and fog occurs.
Common examples of condensation

Frost around the freezer pipes: Air with water vapor enter the fridge when the door is open. When air starts to be cooled below the dew point by the pipes, condensation occurs and ice forms around them.

Air exhaled when the weather is cold. Air in our lungs is not saturated due to the high body temperature, but becomes saturated when it starts cooling.

Air saturated by water vapor coming from a pressure cooker.

As pointed out by @RyanMortensen, saturated air on car windows (when the temperature decreases during the night, or when air is saturated without temperature change, because there is more water/rain in the air that can be evaporated naturally -- Increasing car temperature above the dew point is the solution).

Fog and clouds formation: Fog because the ground is colder than the surrounding atmosphere (early in the morning), cloud because air is cooled below the dew point while it climbs (pressure and suddenness play also a role).

Dew :-)

When water vapor changes into ice
When the dew point is below 0°C, water vapor is not condensing into water droplets, it is converted directly into ice crystals (frost).
This effect is particularly dangerous and can lead to icing in the carburetor of a piston engine. This condition can happen in areas of low pressure and low temperature, commonly created by the sudden pressure drop in the throttle venturi, due to venturi effect and the temperature drop due to fuel vaporization.
For a typical piston engine, this effect is centered on a external temperature around 10°C but extends to a large range of temperature when the engine is slowed down (descent):

Induction icing conditions
The remedy to carburetor induction icing is to heat the carburetor and raise the due point to a safe area, especially when descending.
For jet engines, induction icing (not to be confused with flying in atmosphere with ice crystals or supercooled water droplets) can cause ice to form at the engine inlet. Blocks of ice can then break, causing damage to blades, compressor stall and/or combustor flameout. It can also cause EPR probe (PT2) reading errors and subsequent FADEC fatal behavior.

The above explanation is only a technical explanation of how mist forms.
The reason you see it on the A320 in those conditions is this.
Every AC system (home, building, car) uses a plant to generate a cold refrigerant/water, which in turn cools a heat-exchanger. The supply air is passed over this heat exchanger, and the humidity condenses on it and drops off. You never see the mist becasue what is discharged is dry, cold air.
Aircraft AC is different. A plant is too heavy, and AC is rarely required on an aircraft anyway since they spend most of their time in an environment which is -60C. What they do require is pressurisation.
Therefore, the pressurisation and AC are a combined system - and it works on the supply (ambient/outside) air. There is no refrigerant. The air is compressed in the engines or APU, that air is extraordinarily hot. It is cooled (it only needs to be cooled a bit - maybe 50C or so) and then brought back to ambient pressure. When brought back to ambient pressure - even ambient air of 50C is now below freezing. This air is discharged into the cabin. This can't hold the same moisture as outside air, so mist forms. You'll notice that nowhere in the cycle is the moisture able to condense onto a heat exchanger. Some aircraft use dessicant wheels, but these are not as effective as condensing moisture onto a heat exchanger.
After the aircraft takes off, the humidity is lower, the temp is colder and the phenomena is no longer visible.


Why shouldn't the pilot change the aircraft's configuration while avoiding windshear during landing? According to the procedures, when going around, due to windshear, pilots shouldn't change the aircraft's configuration, such as flaps position and landing gear extension. If changing, would the aircraft lose the lift? why?

As far as landing is concerned, it is better to leave the landing gear down because if something goes down and the aircraft touches the ground, it is better to do so with the landing gear rather than the aircraft fuselage. Also, the retraction process may increase drag, not what is desired.

During landing, the retraction of flaps (which may be already at their max) can cause loss of lift (as you noted). Reduced airspeed due to windshear causes loss of lift; retracting the flaps in this condition is not desirable.

There is also the point that during the critical moments when the pilots are trying to pilot the aircraft during wind shear, their attention should be in flying the aircraft and not in fiddling with controls.

From FAA Advisory Circular 00-54 Pilot Windshear Guide:

  CONFIGURATION

  Maintain flap and gear position until terrain clearance is assured. Although a small performance increase is available after landing gear retraction, initial performance degradation may occur when landing gear doors open for retraction. While extending flaps during a recovery after liftoff may result
  in a performance benefit, it is not a recommended technique because:

  1) Accidentally retracting flaps (the usual  direction of movement) has a large adverse impact on performance.

  2) If landing gear retraction had been initiated prior to recognition of the encounter, extending flaps beyond a takeoff flap setting might result in a continuous warning horn which distracts the crew.

Well, the configuration at landing is for low speeds, if the flaps are pulled up, the airplane might stall because of low speed air separation and might take the necessary lift away to get get away from the windshear. And as a comment above says, the flaps also help with stability when manoeuvring the airplane at low speeds. Gears are left down in case of unwanted touchdown (This is windshear, so it might happen).


Forces "felt" by pilot, G-meter, inclinometer--are they the aerodynamic forces generated by the aircraft, or the sum of weight+centrifugal force? Consider a pilot sitting at the CG of an aircraft, with instruments also located at the CG of the aircraft.  (Feel free to consider other cases as a bonus, but the core of the question is aimed at this simple case.)

Consider the apparent force "felt" by the pilot, and measured by a traditional-style panel-mounted G-meter, when the aircraft is upside-down at the top of a loop.  Should this force considered to simply be an expression of the wing's lift vector, plus any other aerodynamic force components that the aircraft is generating?  Does gravity make a contribution to this force?  Does "centrifugal force" make a contribution to this force?  Is the force "felt" by the pilot, and the G-meter, due to the combined effects of gravity and "centrifugal force"?  Can more than one of these things be true at the same time?

Consider the apparent force "felt" by the pilot, and determining the deflection of the inclinometer (slip-skid ball), during a coordinated turn, or during an uncoordinated turn.  Is this force simply an expression of the net aerodynamic force generated by the aircraft?  Does gravity make a contribution to this force?  Does "centrifugal force" make a contribution to this force?  Is the force "felt" by the pilot, and the inclinometer, due to the combined effects of gravity and "centrifugal force"?  Can more than one of these things be true at the same time?

Feel free to substitute "acceleration" for "force" in your reading of the question, if you feel it makes it a better question.

Basically this question is seeking to reconcile the view that a pilot only "feels" the aerodynamic force generated by an aircraft, and the view that gravity and / or centrifugal force also play a role in the forces "felt" in flight.  Can these two views be reconciled?  If so, how?  Or is only one of these two views correct?

A thought problem might that better inform the question -- consider an astronaut in orbit.  Is he or she feeling both centrifugal force and gravity?  Or neither?  Or is this purely a manner of convention, i.e. either answer is accurate?

Can more than one of these things be true at the same time?

Yes. It is only a matter of the point of view, or, more precisely, the reference system.

Consider a trivial case first: The aircraft sits on the ground, engine off. The pilot will feel his weight pressing into the seat even in the absence of aerodynamic forces. What he feels is gravity, and the force is the product of his mass and gravitational acceleration.

Now do the same with the aircraft flying in level flight. The forces acting on the pilot are still the same, so it is again gravity what he feels. Only that now the aircraft is not supported by its landing gear and the ground, but by wing lift (and the ground again).

Next step: The pilot flies a dive, followed by a 2g pull-up, meaning that the g-meter reads 2 gs. Again we look at the moment when the aircraft is level, but now it flies on a curved flight path. This adds a centrifugal load of 1g to the 1g of gravity and the pilot feels twice the weight force that he felt before. The nitpickers will now say this is not weight, because weight is the product of mass and gravitational acceleration, but our pilot couldn't care less at the moment. It hurts in his buttocks, that's what counts.

And neither the g-meter nor the pilot could tell gravity apart from centrifugal force – they all feel the same and only the sum of all is felt as a single sensation.

Now switch reference systems and look at the flow around the wing. Speed and angle of attack combine to produce a lift force of twice the weight the aircraft had in level flight. This is pure aerodynamics, no gravity or centrifugal load involved. Strictly speaking, the aerodynamic force on the wing is even a bit higher in order to compensate for the higher tail downforce resulting from trim to overcome pitch damping. Looking at the full aircraft as one object, its total lift force is now twice as high as in level flight.

This force gets transmitted to the pilot seat and the instrument panel and is felt there as a resistance against the acceleration. Depending on your point of view, either the pilot is accelerated into his seat or the seat pushes against the pilot. Actio equals reactio.

Next switch: The same curved flight path, but now at the moment when the aircraft is inverted. Now centrifugal and gravitational acceleration cancel each other, the g-meter reads zero and the pilot feels weightless. In order to do so, the pilots needs to push the stick forward and reduce wing lift to about zero, or wing lift would pull the aircraft down. At this moment we have no aerodynamic contribution to the vertical forces felt by both the pilot and the g-meter.

Now the pilot pushes the stick even more forward so his flight path becomes straight and level again, only with the aircraft in an inverted position. The g-meter now reads -1 g, the force of the harness pressing on the pilot's shoulders is again his weight times gravitational acceleration, but what allows the harness to support the pilot is the lift of the wing the harness is attached to. Change your reference system, and it is again aerodynamics that the pilot feels. Let him wiggle the stick and the change in aerodynamics is immediately felt on his shoulders.

Throughout this answer we'll use "feel" or "felt" to mean the force or acceleration that is perceived by the pilot and transmitted through the aircraft structure as stresses or strains.  In a "weightless" situation, the net force or acceleration is not zero, but the "felt" force or acceleration is zero.

It is valid to argue that the force or acceleration "felt" by the pilot, and registered by the instruments, in flight is simply the net aerodynamic force or acceleration generated by the aircraft.  To get the actual net force acting on the aircraft, we have to add gravity to the aerodynamic forces.  In other words, the "felt" force or acceleration is the net force or acceleration acting on the aircraft, minus the force or acceleration due to gravity.

Examples--

1) Straight and level upright flight-- net acceleration zero, net force zero, "felt" acceleration 1-G in the upward direction, force "felt" by the pilot is simply equal to his weight, acting in the upward direction.  This force is the aerodynamic lift force generated by the wings, transferred through the aircraft structure to the bottom of the seat and then to the pilot's body.

2) Aircraft inverted at the top of a loop-- again the acceleration and force "felt" by the pilot are simply the result of the net aerodynamic force generated by the aircraft.  If the wings are generating positive lift, the pilot and G-meter will feel positive G's, and if the wings are generating negative lift, the pilot and G-meter will feel negative G's.  If the wings are generating zero lift, the pilot and G-meter will feel zero G's (weightlessness).

3) Steady-state coordinated turn-- the net aerodynamic force generated by the aircraft is simply the wing's lift vector, which contains no lateral (sideways) component in the aircraft's reference frame.  The slip-skid ball is centered.  Meanwhile the net force including gravity is the purely horizontal force vector-- the centripetal force vector-- that drives the turn.  This force vector does have a lateral (sideways) component in the aircraft's reference frame, yet the slip-skid ball is centered.

4) Turn with some sideslip-- the nose of the aircraft is allowed to point toward the outside of the turn.  The airflow impacts the side of the fuselage and generates an aerodynamic sideforce.  Now the net aerodynamic force generated by the aircraft does include a lateral (sideways) component, so the slip-skid ball rides toward the inside of the turn, and the pilot's body tends to lean toward the wall of the cockpit that is on the inside of the turn.

Yet if we take our airplane and bolt it to a roller coaster and run it through loops, flat turns, etc, the pilot still "feels" apparent forces, and the G-meter and the inclinometer still give indications.  What are causing these apparent forces?  (Assume for simplicity that the roller coaster is in a vacuum.)

We can say that these apparent forces are entirely due to the forces exerted by the rails of the roller coaster upon the wheels of the roller coaster, with gravity playing no role.  This is analogous to saying that the forces "felt" in flight are entirely due to the aerodynamic forces created by the aircraft, with gravity playing no direct role.

Yet we can also say that these apparent forces are entirely due to the combined effects of gravity, and of "centrifugal force".  Obviously, if we don't start a loop in the roller-coaster with enough speed to maintain positive G's all the way around, we'll hang upside-down in the seatbelts as we go over the top.

The key to reconciling these viewpoints is to understand exactly what "centrifugal force" is.  It is the apparent force created by a curvature in a vehicle's trajectory.  It is not a real force.  It is basically just the mirror image of the real forces that are acting to curve a vehicle's trajectory.  For (almost) every centripetal force acting to drive a curvature in a vehicle's trajectory, we can say the occupants "feel" an apparent "centrifugal force" acting in the other direction.  When we do a high-speed turn to the left in a car on flat ground, the tires generate a real force toward the left, and we say we perceive a centrifugal force to the right, and are thrown against the right wall of the inside of the car.  If we want to understand which direction a slip-skid ball would deflect, we could consider the real centripetal force from the tires, or the apparent "centrifugal force" generated by the resulting turn rate, but not both.  If we consider both forces, we end up with zero net force-- they would cancel each other out-- predicting that the slip-skid ball would not deflect at all.  This would be wrong.  We can't mix and match apparent "centrifugal force" with real centripetal force.  We have to use one or the other.

In this context, we're not using "apparent" to mean "the felt component of".  We're just using "apparent" to remind ourselves that "centrifugal force" doesn't actually exist.

We stated that "For (almost) every centripetal force acting to drive a curvature, we can say the occupants 'feel' a "centrifugal force" acting in the other direction".  Why "almost"?  Because we can't "feel" any centrifugal force that is caused by a curvature in a vehicle's trajectory that is due to gravity.  (Consider the forces felt by an astronaut in orbit.)

Why is gravity such a special force?  Forces from wings and forces from wheels must be transmitted through the aircraft structure and to the exterior of the pilot's body.  As a result, stresses and strains are created in the aircraft structure and in the pilot's body.  In contrast, gravity exerts an equal acceleration per unit mass on every molecule of the aircraft structure, and on every molecule of the pilot's body, creating no stresses or strains, i.e. no displacement of one part of the structure or body relative to another part.  Likewise, unless we consider the outer housing of the slip-skid ball or the G-meter to be rigidly fixed in space, gravity has no tendency to displace any element of either of those instruments in relation to any other element.  For example, gravity has no tendency to pull the inclinometer ball to one side within the glass tube, even if the tube is tilted relative to the earth.  (We're assuming that our instrument is much smaller than planetary in scale, so that tidal effects may safely be ignored!)

We've defined "centrifugal force" as an apparent force that is the mirror image of the net centripetal force acting to drive a curve in a vehicle's trajectory.  If the force of gravity is not "felt", then what is the "felt" component of that apparent "centrifugal" force?

Since gravity is already "built in" to the net centripetal force determining a vehicle's trajectory, it follows that an apparent upward force opposite to gravity is already "built in" to the apparent "centrifugal" force that is paired with that centripetal force.  This means that the "felt" centrifugal force is equal to the total centrifugal force plus gravity.  Just as the "felt" centripetal force is equal to the total centripetal force minus gravity.

(Note-- it's really only the component of gravity that acts perpendicular to the flight path that is "built in" to the centripetal force vector.  For simplicity, in the discussion of "centripetal force" in this answer, we'll assume that we're looking at the vehicle's trajectory at a point where the trajectory is exactly horizontal rather than rising or falling, and thus the full weight vector acts perpendicular to the flight path.  Alternatively, we could think in terms of a three-dimensional vector sum rather than a two-dimensional one.)

The key point here is not to argue about whether "centrifugal force" actually exists or not.  Rather the key here is to recognize that we're starting from a situation where the exact trajectory is known, and then we're calculating the centripetal (or centrifugal) force that is involved in creating this trajectory, and then we're subtracting away (or adding) gravity to get the "felt" component of that centripetal (or centrifugal) force.

In essence, "centripetal force" is just another way to say "net force", except that we're discarding components that act tangential rather than perpendicular to the flight path.  In the context of flight, the aerodynamic forces and the force of gravity are both built in to the concept of "centripetal force".  "Centrifugal force" is an apparent force or pseudoforce that is equal and opposite to the real "centripetal force".

In the case of a roller coaster, the "felt" component of the centripetal force would be the force exerted by the tracks on the wheels, and the "felt" component of the centrifugal force would be equal and opposite.  In the case of an aircraft, the "felt" component of the centripetal force would be the aerodynamic force created by the aircraft, and the "felt" component of the centrifugal force would be equal and opposite.  Whether we want to say that the pilot, G-meter, and inclinometer are responding to the "felt" component of the centripetal force or the "felt" component of the centrifugal force is really just a matter of convention, but only the former actually exists.

So all these three things are true:

1) "Felt" force = aerodynamic force = net force minus gravity

And

2) "Felt" force = net centripetal force minus gravity

And

3) "Felt" loading (the sensation of apparent weight due to being accelerated by a force) = ( net "centrifugal" force plus gravity ) / mass

We're taking the convention here that the felt "loading" acts in the opposite direction to the felt "force" or felt "acceleration", just as the apparent "centrifugal" force acts in the opposite direction to the actual centripetal force.

Whether we want to work in terms of "centripetal" or "centrifugal" forces is really just a matter of convention.  Only the "centripetal" forces are actually real.  To get the "felt" component, we subtract gravity in the former case, and we add gravity in the latter case.  Because of the way that gravity can't actually be "felt", yet it is already "built in" to the centripetal and "centrifugal" forces.

Note also that by specifying "centripetal" (or "centrifugal"), we're saying that we're limiting the force and acceleration components of interest to those acting perpendicular (orthogonal) to the flight path, and disregarding those acting parallel (tangent) to the flight path.  The first equation in the list of three above does not have that constraint.

Often one sees diagrams in flight training manuals that try treat "centrifugal force" as just one component out of several forces acting to curve the flight path.  Many diagrams have been published, including by the FAA, that illustrate the deflection of the slip-skid ball in turning flight as being due to a force balance between "centrifugal force", and "gravity", and lift, or between "centrifugal force" and lift.  (Oddly, the real aerodynamic sideforce caused by the fuselage flying sideways through the air is invariably omitted.)  At first this seems intuitively right-- if you are bringing gravity into the picture rather than just purely considering centrifugal force alone, then why not bring lift into the picture too?  But this approach just doesn't work. The effect of the lift vector is already included in the centrifugal force vector.  To explain the forces "felt" in a coordinated or uncoordinated turn, we need to work in terms of the real aerodynamic forces, or we need to work in terms of the balance between "centrifugal force" and gravity, which is essentially the same as working in terms of the balance between centripetal force and gravity.  A diagram combining gravity, centrifugal force, and lift just can't hold water.

If the flight path is fully constrained, then the "centrifugal force plus gravity" (or "centripetal force minus gravity") approach works well.  For example, if the flight path is known to be linear, then centrifugal force is known to be zero, and the slip-skid ball works as a bubble level, telling us which way is down.

But in many situations, the flight path is not constrained to be linear.  Also, unlike the case of the roller coaster, the turn radius is free to vary as the lift force is varied, and the flight path is free to curve up and down in the vertical plane as well.  That is why it is so useful to take the viewpoint the forces "felt" in flight are simply the real aerodynamic forces generated by the aircraft, with gravity and "centrifugal force" playing no direct role.

Consider for example this related ASE answer about why a glider experienced a negative G-load at the top of the loop.  If we imagine the loop to be constrained to follow a pre-defined track with as set radius like a roller coaster, we could imagine that the explanation is simply that we didn't start with enough speed to generate enough "centrifugal force" to counteract gravity and "make it all the way around" without hanging loose in the belts.  But recognizing that the flight path is not constrained to follow a pre-defined track, it is more useful to recognize that when we "felt" a negative G-loading, the wing must have been flying at a negative angle-of-attack and generating negative lift.  Which raises the question, "Why was the wing generating negative lift-- with the stick full aft?  Why didn't the angle-of-attack stay positive, as the pilot was intending to command, even if the resulting trajectory did not result in a perfectly round loop?"  The negative-lift situation is in fact related to the low airspeed at the top of the loop, but for reasons that are much more subtle than the fixed-track roller-coaster analogy would suggest.

The idea that a coordinated turn features a "balance" between centrifugal force and gravity may be technically true, but it has no explanatory power, especially in the context of a flight path that is free to vary in three dimensions as lift and other variables are varied, rather than being constrained to run along a pre-existing track as in the case of a roller coaster.  It doesn't reveal that the key factor in a "coordinated" turn is simply that the fuselage is streamlined to the airflow, so that the aircraft is generating no aerodynamic sideforce and thus has no aerodynamic force component in the lateral (sideways) direction in the aircraft's reference frame.  It can lead one to believe that in a sideslip, an abnormally low turn rate is somehow causing the aircraft to slide sideways through the air, rather than understanding that the aircraft is being forced or allowed to fly sideways through the air which is creating aerodynamic sideforce (as well as drag), which is in causing a reduction in the turn rate.  It can lead one to believe that a pilot should somehow be making adjustments in angle-of-attack to get the "right" lift force for the airspeed and bank angle, or else the aircraft will go sliding off toward the inside of the turn.  (Footnote to be added.)

In short, the "centrifugal force plus gravity" approach is often not the most useful way to look at the problem when the flight path is free to vary rather than constrained to be some particular shape like a straight line, or a perfectly circular loop of pre-defined radius, or a round horizontal circle with a pre-defined turn radius and bank angle or airspeed.  Unless we are back-engineering the "felt" forces (i.e. the aerodynamic forces) from a saved track log that was recorded with tremendous precision, the "centrifugal force plus gravity" approach doesn't have much application.  To know what the "centrifugal" force is, we have to know what the trajectory is, and to know what the trajectory is, we usually have to know the forces acting on the aircraft, including the aerodynamic forces, in which case we already know the answer to question of what will be the result of "centrifugal force plus gravity".

The "centrifugal force plus gravity" approach is generally better suited to understanding the forces felt in race cars travelling along tracks of fixed bank angles and fixed turn radii, or the angle taken by a rope of fixed length tied to bucket full of water swung round in a circle, than to the forces "felt" in an airplane whose path is free to vary in three dimensions.  But when applied properly, it is not an invalid way to look at the mechanics of flight.


What is more environmentally friendly? An A320 or a car? Presuming the A320 is at least 80% full (of passengers) and comparing it to a typical road car with 2 people inside, which one is more environmentally friendly, per passenger mile?

I have attempted to do some rudimentary calculations, but then was reading that because aircraft release their emissions at altitude, this offsets (?) some of the environmental impact.

I'm going to simplify and assume that jets and cars burn the same fuel, and output the same exhaust, CO2, NOx and all. I'm going to compare only short-haul flights against cars.

According to Wikipedia, an A-320-NEO does 1.95L/100km per seat. Assuming flying at 80% capacity, that gives us 2.4L/100km per seat. According to
The Car Guide, a 2019 Honda Civic does 6.5L/100km. Assuming an average car carries 2 people, that gets 3.2L/100km. It would seem like flying is slightly better, using this simplistic measure.

Keep in mind that the longer you fly, the better the mileage, since the takeoff and climb are the fuel guzzling segments.

The answer is that it is dramatically more environmentally friendly in terms of CO2-equivalent greenhouse gas release to drive any given distance than to fly it. The average American drives three times the distance from San Diego to London, yet the relative contribution of that year of driving versus a single flight from America to London is less than 150% more (just eyeballing the difference in the plot in the link below).

And as others have pointed out, aircraft burn through fuel at a precipitous rate during take-off and landing, so for shorter distances this disparity is likely to be even greater as the total ratio of cruise altitude efficiency becomes lower.

The dramatic impact that aircraft contrails have on this matter really can't be overstated.

Comparison of air versus road travel: https://phys.org/news/2017-07-effective-individual-tackle-climate-discussed.html

Average distance driving: https://www.fhwa.dot.gov/ohim/onh00/bar8.htm


Is it allowed and possible to hold at the intersection of two VOR radials using a single VOR receiver? First of all, is it permitted? I am practicing VOR tracking on my simulator and the missed approach procedure (KLGB VOR 30) takes you out to an intersection off two VOR radials. Since the plate doesn't specify "DME required" I figured yes.

Assuming yes, are there any tricks/protips to doing this successfully?

As I understand it you are asking two questions:

is it permitted to hold at an intersection of two VOR radials with nothing more than a single VOR receiver? and
what are some tips and tricks for doing it?

Answer to question one:
As others have mentioned, it was permitted at one time (and I believe it is still permitted). I'm old enough to have received training on how to do it at a time where it was much more common to fly in GA aircraft with no DME or GPS and sometimes only a single VOR receiver. I spent over an hour researching this question for you in the AIM, Airplane Flying Handbook, Instrument Flying Handbook, and Instrument Procedures Handbook, and online generally, and was not able to find anything saying it was permitted or prohibited. I don't see any problem with it other than it adds a bit of workload to the pilot, but it can be done safely and relatively easily. If I find an definitive answer I'll update my question.
Answer to question two:
Here are some tips and tricks for flying an intersection hold with nothing but a single VOR receiver. This explanation assumes you know how to hold, including the FAA recommended hold entries.
There are two radials, hence the intersection. I'm going to refer to them here as the main radial and the cross radial. (It is possible to approach the fix on the cross radial then switch to the main radial for the hold after executing the hold entry, however, for purposes of this explanation, I'm going to assume that you are approaching the fix on the main radial (i.e. the radial on which the inbound holding course is located).
In the figure below the aircraft is on the main radial and is approaching the cross radial from the West. However, this aircraft could also be approaching from the opposite side of the fix from the East.

So, while you are on the main radial flying towards the fix, you want to spend most of your time monitoring the main radial because it is your primary course navigation, however, you also want to initially tune the cross radial well before reaching it in order to verify, and take a mental note of which side the CDI is deflected. This can be confusing to some pilots because the CDI will be either on the right or the left depending on where the station of the cross radial is located in relation the main radial. (When you fly a G1000 or HSI, the CDI rotates such that it is easy to tell when you are approaching the cross radial or have already passed it, but this is not the case in stand-alone old-school VORs). Whether the CDI of the cross radial is on the left or right also depends on if the pilot tuned in the radial or the reciprocal. For simplicity in this example, lets assume you tune in the cross radial well before reaching the intersection and the CDI is fully deflected to the left.
You then re-tune the main radial (hopefully your NAV radio has a flip-flop button), and center the CDI. You want to be as precise as possible and note the wind correction angle required and bug that heading! It's important to be precise, because you will be flying the bugged heading when you switch to the cross radial.
As you fly towards the holding fix on the main radial you will periodically switch back and forth to the cross radial until you detect the CDI is alive. At this point, you want to watch the CDI to get an idea of how fast it's moving.  I like to take a note of the deflection of the CDI, lets say it is 8 degrees to the left (still haven't reached it), then take a note of the clock to determine how long it takes to fly 1 degree. Let's say that it takes 30 seconds to go from 8 degrees to 7 degrees, if so, then you know you have approximately 3.5 minutes before reaching the holding fix (7 degrees x 30 sec/deg = 210 sec or 3.5 min). You can then switch back to the main radial and continue navigating on it, until about 1 minute before you reach the intersection, at which point you would tune in the cross radial and fly your bugged heading until the cross radial is centered. At which point you would peform the appropriate hold entry.
At the moment you reach the holding fix, you will be flying the bugged heading and the cross radial will be tuned into your VOR. If you are on the main radial on the inbound heading, then it will be a direct entry and you just turn (right turn in the above figure) and fly outbound. On the outbound leg, tune the VOR to the main radial to the inbound course so that you can intercept it when you turn inbound. If you are approaching the holding fix from the opposite direction (from the East in the above figure) such that you are flying the outbound heading of the hold, then you will need to do a teardrop or parallel entry. In this case, I like to do the parallel entry because it requires less math and turning. Since you are already flying the outbound heading and are parallel to the inbound course you don't have to do anything when you reach the holding fix from this direction other than start the timer or take a note of the time and fly out for 1 minute. During this one minute of flying outbound on the parallel entry, you will tune in the main radial to the inbound course, so that when you make your greater than 180 degree turn you can intercept the inbound course.
Whether you do the direct entry or parallel entry, once you are established on the inbound course and tuned into the main radial, you will need to bug in that heading, and promptly tune the VOR to the cross radial, because you've only got a minute or so on the inbound leg to intercept the inbound course, determine the WCA, bug it, then tune the cross radial to identify reaching the holding fix.
Then repeat until you're ready to leave the hold.
This was a long explanation, but it's really not as bad as it sounds. When you are practicing this on the sim, you can try flying 2 minute legs, which gives you more time to intercept inbound, then tune the cross radial. As you get more efficient, you can shorten the legs until you get to 1 min. BTW, in actual IFR, you can ask ATC for a non-standard leg length. I imagine any airplane in this day an age with no DME or GPS that only has a single VOR is probably a very very slow airplane, and ATC can probably accomodate 2 or 3+ minute legs, just ask.

Yes, you can hold at an intersection with only the radial from one VOR  identifying the intersection as long as you have either DME or GPS. And yes, you can hold at an intersection without DME or GPS if you have the intersection identified by the radials of two VORs. You can even hold with nothing but a GPS alone.

If you want to hold at a non-published hold position of your choosing, you could do so at a specific distance and direction from any identifiable point along a specific bearing from your hold fix. You can substitute the distance with the bearing from a second identifiable point. This includes NDBs, airports, Visual Reference Points, GPS waypoints, etc.

I always find it easiest to track inbound to the fix using either a radial, bearing, or GPS direct-to to the fix. When using VOR and DME, it is easiest to intercept the identifying radial way prior to reaching the appropriate distance. If you are using two VORs to identify the hold fix, intercept one of the identifying radials using one of your OBSs first. Use your other OBS to identify your second radial. Which OBS you choose depends on the radial or bearing of the fix along which you will be holding. That should be your primary OBS.

One of the holds given to me by the DPE on my instrument checkride was an unpublished hold at a self-declared, unofficial waypoint/fix. Since, I was on an IFR flight plan, I had to request the hold from ATC with the following format.

  Skyhawkxxxxx, requesting a hold xx direction on a ### bearing, ## nautical miles and XX direction from intersection xxxxx, along the ### radial of xxxVOR, 1 minute legs, standard turn, at #### feet.

Admittedly, this was a bit of an over the top scenario given by the DPE. He was a legacy carrier captain who believed in making sure he tested beyond the capabilities of the magenta-line generation. It was more of a test of how I could think through, draw out , and communicate the hold while maintaining directional control under the hood rather than how to fly it. The bearing from the fix put me right in line with the IAF of my first IAP that had a hold-in-leau of procedure turn anyway.


How are Max Brake Energy Speed & Weight affected by other factors? I'm a student pilot and I'm trying to understand how different factors affect VMBE and Maximum Brake Energy Limited Take-off Mass. I made some researches but I couldn't find anywhere a clear answer.

What is the effect of Density, Flaps, Mass, Runway Slope, Runway Contamination ?

I know that to solve these kind of problems we use the kinetic energy formula: $\small Eb = \frac 1 2 MV^2$ but I don't know how to use it to explain the effect of every factor mentioned above.

(I need your help because I don't want to just learn things by heart and not be able to explain that in an interview or whatever).

The factors you mention will affect how much energy the aircraft has that's going to be transferred into the brakes, and how much will be lost otherwise.  The more energy you have, the slower you can go before the brakes are overwhelmed -- thus, reducing your VMBE.

With more flaps, energy will be dissipated in aerodynamic drag, although probably not all that much (though going from the least possible to the greatest possible flaps may make some difference -- or it may be sufficiently slight that the charts are published with a worst-case scenario & if you have greater flaps then your situation will be slightly better than what's charted).  With runway gradient, you'll dissipate some energy rolling uphill, while you'll have more energy (potential energy becomes kinetic energy) rolling downhill.

The really significant factor is aircraft weight, as your formula makes clear... an aircraft stopping at 100,000# will have 2/3 of the energy of an aircraft stopping at 150,000#, all else being equal.  So, in those two cases VMBE would be much higher for the 100k case... if Limit = 0.5 * wt * V-squared, you can have a higher V-squared to reach the same limit with a lower weight.

Contamination probably wouldn't matter much: while you would be transferring energy more slowly into the brakes stopping on a slick runway than on a dry one, the total energy absorbed into the brakes would be the same.  So unless the rate that the energy is absorbed changes the maximum energy that the brakes can absorb (seems like it would be unlikely, although in some cases... who knows), contamination wouldn't affect VMBE.  (Unless the charts take stopping credit for the deceleration due to "plowing through" contamination -- which would actually help out to some extent.

The most likely effect would be to make the stopping distance more limiting in the contaminated case than it would be otherwise -- so you might be limited by the ability to stop before the end of the runway in a contaminated case, where you'd be limited by brake energy in a dry runway case.

The effect of weight, flaps, and slope on the actual VMBE would be that things which give the aircraft more energy (more weight, less flaps, downhill slope) would reduce the VMBE.  Think of it in an extreme case... if you had the brakes of a bicycle, a one-pound vehicle could go insanely fast before you could put enough energy into the brakes to cause a problem, but if your vehicle were, say, the Space Shuttle, even at slight speed, the brakes would be overwhelmed by the KE of that much mass.  Consider likewise, a 45 degree downslope vs a 45 degree upslope, or the drag caused by a huge drag chute vs the drag of a pencil flying through the air.

Maybe it's clearer to say that VMBE is the maximum speed to reject the take-off within the energy capacity of the brakes alone (Max brake energy speed). Brakes convert kinetic energy into heat. Once the limit is reached, the brakes fade. So above VMBE the brakes will not work.

VMBE is most limited by:

Heavy Mass
Downhill Slopes
High Temperatures (Low Density)
High Pressure Altitudes (Low Density)

On the other hand, if the Allowed T/O Mass is Brake Energy Limited, we need to decrease the Mass (weight) to be able to stop without fading the brakes.

However, runway up-slope will actually help the aeroplane to stop, therefore the brakes will not have to absorb so much energy and the allowed mass will be higher. With a runway down-slope it is the opposite A down-slope will degrade the stopping capability of the aeroplane and therefore the brakes will have to absorb higher energy => brake energy limiting take-off mass will be decreased.

I think in fact that The Brake Energy Limited T/O Mass is mostly dependent on the Decision Speed (V1).

"A maximum V1 speed, limited by brake energy (VMBE), exists for each TOW, this is why it seems to grow more limiting as V1 increases. To achieve a higher V1 speed, it is necessary to reduce TOW"

Correct me if I'm wrong.


Who decides how fast commercial aircraft fly: the pilots or ATC? Once a commercial aircraft is in the cruise phase, who determines the speed of the aircraft?
Do pilots have the authority to fly at up to the maximum permissible speed of that plane, or does ATC decide on that depending on the approximate time it will take to reach the destination? Is it stated in the flight plan that the pilot cannot go beyond some specific speed even if the aircraft is capable of it?

ATC can assign speeds en-route for separation or in the TMA area for sequencing aircraft onto the approach. Speeds en-route in upper sectors are assigned using Mach numbers:

R: "DLH123, maintain speed Mach .78"

In the TMA area, aircraft are sequenced for an approach and reduced to similar speeds to maintain lateral separation. These restrictions can be paired with conditions, until when the speed needs to be maintained:

R: "BER456, reduce speed 210 knots"
R: "BER456, cleared ILS approach runway 24,
    maintain 170 knots indicated until outer marker / 5 DME"

Certain approach or arrival charts list maximum speeds for certain turns, marked with MAX 210 KIAS, indicating that this turn should not be flown with more than 210 KIAS to make the radius.

Speed restrictions imposed by ATC need to be maintained until cancelled by the ATC unit:

R: "DLH123, no speed restrictions / resume normal speed"

The pilot however is responsible for ensuring a safe flight and can report that he is unable to follow the instructions due to performance limitations.

A: "DLH123, unable to comply, indicated airspeed will be 150 knots"

There is also legal considerations when it comes to aircraft speeds, such as general rules for traffic below certain altitudes or in some jurisdictions, only for certain airspace classes (related: What is the speed limit in European Airspace).

The FAA CFR 91.117 says:

  §91.117   Aircraft speed.

  (a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

The German LuftVO however rules that a speed limit of 250 KIAS below FL100 only applies to VFR and IFR aircraft in airspaces class D,E,F,G and only to VFR aircraft in airspace class C. IFR aircraft in airspace class C do not have a speed limitation.

Regarding the general authority over a flight, see the related question:
Who has the higher authority, the pilot in command or ATC?

ATC is providing collision separation services only. It is the responsability of the pilot to operate the airplane and to keep it under a safe flight envelope. So, speed:pilot, Heading-altitude:ATC :)


In a piston aircraft engine, how much heat must be removed by the oil cooler? It is pretty straightforward to compute the heat and power generated by the combustion process, given the displacement, RPM and fuel mass flow rate. Similarly, the power exhausted can be computed with knowledge of EGT and conservation of mass.  The difference is dissipated as heat, from air cooling of cylinders and oil cooler.  I'm looking for a rough ratio between oil cooler and cylinder heat dissipation.

According to Fundamentals of Powerplants for Aircraft, quoted in Mike Busch on Engines (chapter 2), "Other thermal losses" account for 12.2% of fuel energy:

Conduction to air: 7.2%
Conduction to oil: 1.6%
Radiation and misc: 3.4%

The other two areas of loss are exhaust (51.6%) and mechanical (36.2%).

I don't know how "cylinder heat dissipation" would fit into this classification (some combination of air conduction and friction losses?), but looking only at conduction losses gives a ratio of air to oil of 4.5:1.

Typically in an air cooled, reciprocating engine, for the total amount of chemical energy released during combustion, about 40-45% exits the tailpipe as heat in the exhaust gases, 25-30% of that energy is transferred to the propeller through the crankshaft as mechanical energy, 15-20% or so exits the engine from cool air flowing through the cowling over the engine block, and the final 5-10% is removed through the oil cooler.

SOURCE: THE AIRCRAFT ENGINE AND ITS OPERATION - PWA 109702, pp 19 - Pratt & Whitney Aircraft Corporation, 1949.

I knew I had this in my library somewhere.

If we take a look at an airplane eg an SR-22 on takeoff, the IO-540 engine consumes about 25 gal/hr of 100LL and produces 310 BHP, or is outputting 8.3E8 Joules of mechanical energy per hour. With a chemical energy density of 40MJ/kg for 100LL, this means that about 2.6E9 Joules of chemical energy are supplied to the engine every hour, thus giving us around a 31% efficiency in this example, and that’s about what we would expect.


Why don't commercial aircraft adopt a slightly more seaplane-like design to allow safer ditching in case of emergency? For example :

Why don't they have a more hydrodynamically shaped fuselage underside ?
Specifically for the aircraft that have their engines mounted under the wings, isn't this too dangerous for ditching ? As far as I understand when an engine touches water during ditching the whole aircraft would/could break apart ?
Or, couldn't they have some kind of (retractable) floats ?

I'm far from being an aviation expert myself, but I imagine some things like the above (or possibly others) could be done, in order to allow safer ditchings for commercial aircraft. Would all these be too costly for airlines ?

Because ditching is extremely, extremely rare, so the costs of redesigning aircraft along with the extra drag and weight (increased fuel burn) it would no doubt add to the airframe far outweighs the potential benefits.

That argument might sound weird to someone, but think about it this way: would it seem reasonable to redesign every single car on the planet so that they can float, just because every year a few people around the world accidentally drive their car into a lake?

A quick search on The Aviation Herald suggests that, since 2005, a total of four flights worldwide have been forced to land on water. To give you an idea of the scale, there are about 100,000 commercial flights worldwide daily. So that's something like 4 flights out of half a billion, which is practically nothing. Also, in those 4 accidents, more than half of the people onboard actually survived. Actually, the increased greenhouse gas emissions caused by adding extra weight, such as floats, to aircraft worldwide would probably kill many more people than such safety equipment would ever save.

It's not that the aviation industry isn't investing heavily to improve safety. It's just that making ditching safer is an area where there is almost nothing to be gained. So the money can be much better spent elsewhere.

Most of the special features of a seaplane are about taking off from the water, because of the need to minimize drag.  Almost none of this is useful for an airliner.

When you're ditching, you're probably going to ditch on the ocean, and unless you go all-out on the seaplane design, salt damage alone is going to destroy the airplane.  Design of airliners for ditching is based around the assumption that the plane won't be re-usable:

The fuselage is reasonably hydrodynamic as a side effect of streamlining.  It lacks the "step" of a seaplane, but you only need that to reduce hydrodynamic drag during takeoff.
The engine pylons are designed to be a weak point.  In the event of a water landing, the engines will break off without significant damage to the wings.
Floats are only useful to keep the airplane steady during taxiing, loading, and other surface operations -- on a seaplane, the floats are the first thing to leave the water during takeoff, and the last thing to touch down during landing.  An airliner is designed to float with the wings right at the waterline, to provide the same stabilization effect.

Any airliner designed for over-water operations is required to be able to ditch safely in conditions where a seaplane would be able to land, and remain floating long enough for the passengers to evacuate.  Adopting more "seaplane-like" features wouldn't improve on this, and would reduce performance during normal operations.