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75 votes
Accepted

Why is the 787 tail cone made out of titanium?

Whenever you see titanium being used on aircraft, look for heat sources. The metallic alternative aluminum is usually lighter and much cheaper. In most commercial aircraft the APU (Auxiliary Power ...
Gypaets's user avatar
  • 4,385
73 votes
Accepted

Why isn't the APU a standard diesel generator?

Weight. Piston APUs for trucks are designed for frugal and quiet operation. This one produces 5.2 kW electrical power and weighs 375 lbs. The APU for the A320 and B737 is a noisy screaming unit that ...
Koyovis's user avatar
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43 votes
Accepted

What is the loud noise of a helicopter when the rotors are not yet moving?

The sound you're hearing is the APU (Auxiliary Power Unit) running. Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to ...
HiddenWindshield's user avatar
33 votes
Accepted

Does the Antonov An-225 have an Auxiliary Power Unit (APU)?

No. It doesn't have an APU — it has two of them. They are located close to the main landing gears. From An-225 Mriya is the world’s largest aircraft (English version): Auxiliary power plant ...
PerlDuck's user avatar
  • 3,186
31 votes
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Was the miracle on the Hudson saved exclusively by the APU?

Yes, the solution to start APU was important. The ditching procedure directs the use of maximum available slats and flaps for the final approach and touchdown (source, chapter 10.3). This is not ...
h22's user avatar
  • 12.1k
28 votes
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What role did the RAT (Ram Air Turbine) play on US Airways flight 1549?

The full NTSB report can be found here and is the official source of information on the incident. According to that report Sullunberger did immediately start the APU which did deliver power and they ...
Dave's user avatar
  • 101k
23 votes

What powers an aircraft prior to the APU being switched on?

Other than the APU, there are multiple ways to provide electrical power to an aircraft: Battery: The battery is typically the first thing you would turn on and it usually provides DC power to ...
Bianfable's user avatar
  • 56.5k
21 votes

What is the loud noise of a helicopter when the rotors are not yet moving?

You may be expecting the rotor to spin up in lock-sync with the engine. That's not quite what happens on a turbine helicopter. Turbine helicopter engines (turboshaft engines) have a turbojet ...
Harper - Reinstate Monica's user avatar
17 votes
Accepted

What powers the air-conditioning on commercial planes?

What powers the airconditioning on most airliners is APU or main engine compressor bleed, which is being supplied at fairly high pressure, say, 80 psi, and temperature, around 3-400F, to the air ...
John K's user avatar
  • 132k
16 votes

What is an "all-electric APU"?

Usually an APU provides electricity and bleed-air, just like the aircraft engines do. An all-electric APU only provides electricity. This can be done in the Boeing 787 because of the bleedless ...
MadMarky's user avatar
  • 669
15 votes

What is the fuel source for the APU on a B737?

It comes from the left side of the fuel manifold, which means it is most often fed from the #1 main tank, typically by turning on either or both boost pumps in that tank. However, it can be fed from ...
Ralph J's user avatar
  • 51.8k
15 votes
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What powers an aircraft prior to the APU being switched on?

Onboard batteries for DC and a ground power supply for AC. The ground supply can come either from an airport vehicle or from the stand itself. Since the standard AC in aviation is 115V and 400Hz, ...
AEhere supports Monica's user avatar
14 votes

Is the APU classified as a component of Airframe or Powerplant?

I was a tech writer in another life. Under ATA100/2100, the Air Transport Association system of categorization of systems for maintenance manuals, the APU is Chapter 49 (Airborne Auxiliary Power). ...
John K's user avatar
  • 132k
13 votes
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What does the PBY Catalina's APU look like?

The link you provided is for a Canadian PBY-5A which were called "Canso" in Canada. I flew these aircraft in water bombing operations during the early 1980's. In your link it describes the 1955 ...
Mike Sowsun's user avatar
  • 37.8k
12 votes

Why isn't the APU a standard diesel generator?

Three words: They’re much heavier. Gas turbines have a very high power to weight ratio As opposed to reciprocating diesel engines. So for the needs of an airplane which requires large amounts of ...
Romeo_4808N's user avatar
  • 74.1k
11 votes

Was the miracle on the Hudson saved exclusively by the APU?

The engines provide electrical and hydraulic power to the systems that control the flight path of the aircraft: elevator, aileron etc. If all power is lost, a Ram Air Turbine (RAT) is deployed which ...
Koyovis's user avatar
  • 61.8k
9 votes

What is the APU actually used for?

Using 747-100/200 aircraft at the two 747 carriers I flew for in the 1990s as an example, the specific uses (not in any particular order) were: pneumatic power to start the engines pneumatic power to ...
Terry's user avatar
  • 39.2k
9 votes
Accepted

How does an ETOPS APU start in-flight?

Some aircraft may have a special "ETOPS APU" that can be purchased, but many aircraft simply have THE APU that they come with. It is maintained to the standards required in the approved ETOPS program, ...
Ralph J's user avatar
  • 51.8k
9 votes
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Why is powering on the APU so low on the A320 emergency checklist?

The assumption was that a multiple engine failure happens at altitude, for instance in cruise while flying through volcanic ashes. This happened to British Airways Flight 9 and to KLM flight 876, with ...
Koyovis's user avatar
  • 61.8k
9 votes

Why isn't the APU a standard diesel generator?

A primary reason, aside from weight considerations, is that a gas turbine APU has a built in air compressor - the compressor stage of the turbine. As such, it can supply air under pressure, or bleed ...
tj1000's user avatar
  • 8,787
9 votes

Why isn't the APU a standard diesel generator?

You need to look at the overall picture. Yes, a diesel is cheaper, but it will weigh more and be less reliable. Also, the operating time of an APU is rather a small fraction of total airframe ...
Peter Kämpf's user avatar
9 votes
Accepted

How are the services kept running on a plane without an APU?

The aircraft is connected to a ground-based supply. The ground-based air conditioning is connected using a large yellow pipe, and electricity is connected using a set of cables. In the photo below ...
kevin's user avatar
  • 39.8k
8 votes

Where is the APU in the case of an S-duct (like the L-1011)?

The APU on the B727 was placed inside the main gear wheel well. image source It drew air from inside the wheel well, so it was only for use on the ground. The exhaust was on the top of the right ...
TomMcW's user avatar
  • 28.6k
8 votes

Can the APU provide emergency thrust?

The Antonov An-26B cargo aircraft (produced in the Soviet Union) has a Tumansky RU19A-300 combined APU and jet engine mounted on the wing behind the right main engine. This unit provides additional ...
pots's user avatar
  • 81
8 votes
Accepted

Why do the jet turbines spin while the engines are turned off?

After shutting down they still have momentum. Or like you said, because of Windmilling - freely turning when the wind impacts the compressor blades. The APU can, but more often than not doesn't, ...
PIXP's user avatar
  • 306

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