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 ...
- 4,332
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 ...
- 61.3k
44
votes
Accepted
In what conditions is the APU used in midair?
This answer applies to the AVRO RJ (BAe 146), because this is the aircraft I've flown. Other aircraft may or may not have similar features/issues/restrictions.
Reasons for APU use in mid-air
For ...
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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 ...
- 12.8k
33
votes
What is this open panel at the rear of this airplane?
It is the Auxiliary Power Unit (APU) inlet door.
Image from Boeing Aero Magazine
- 100k
33
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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 ...
- 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 ...
- 12k
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 ...
- 100k
24
votes
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When do commercial flights turn off the APU?
This varies due to operational need and airline standard operating procedures.
On my aircraft (an Embraer E-Jet) we turn on the APU roughly ten minutes prior to pushback and shut it down shortly ...
- 3,113
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 ...
- 54.3k
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 ...
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 ...
- 126k
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 ...
- 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 ...
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15
votes
Accepted
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, ...
- 8,541
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).
...
- 126k
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 ...
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12
votes
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Can airliners operate without an operating APU?
Yes, you can operate without an APU. If the APU is inoperative you will need to write it up and coordinate with maintenance and dispatch to operate under the stipulations of the MEL. This may ...
- 35.2k
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 ...
- 72.8k
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 ...
- 61.3k
11
votes
In what conditions is the APU used in midair?
Auxiliary Power Units (APUs) are small gas turbine engines used primarily during aircraft ground operation to provide electric power, air conditioning etc and to start the main engines.
In flight, ...
- 100k
11
votes
Accepted
What is this device protruding from the rear of this 787?
As you have already imagined, it's the APU fuel line shroud drain, as visible here:
(Source)
Detail:
The fin gives an aerodynamic profile to the drain. The reason this drain exists seems to be:
...
- 71k
11
votes
Accepted
Where is the APU in the case of an S-duct (like the L-1011)?
(Source via aircraft.wikia.com)
APU is number 134 on the cutaway, which you can find near root of left stabilator leading edge. It is ahead of the engine, below the intake duct, on the left side.
By ...
- 56.2k
9
votes
Can airliners operate without an operating APU?
Yes it is.
Without the APU the airconditioning and electronics also won't work when the engines are shut down, so a ground power unit is connected to provide the electricity and enough compressed air ...
- 27.4k
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 ...
- 39.2k
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