I have just watched Mayday episode 11, season 11, "Nowhere to Land" about TACA Flight 110 incident and I was more than surprised to learn that:

  • APU must be turned on,
  • APU needs about 5-10 minutes to start powering anything,
  • APU can be used to power lights in the passenger cabin (???).

This is the exact flow of events (plus voice description) as it is shown in above cited episode:

  • a pilot is shown turning APU's switch,
  • narrator explains that a certain amount of time must pass before APU starts working,
  • lights in passenger section are turned on after this period of time passes.

Contrary to the above, I was always told (based on Air Transat Flight 236 incident, shown in season 1, episode 6, "Flying on Empty" of Mayday, and based on other sources):

  • APU is deployed automatically (gravitational solution) whenever power from engines is lost,
  • APU starts powering equipment nearly immediately,
  • APU is able to power basic instruments only, even pilots cabin remains dark, not to mention passenger sections.

What is the reality?

  • 6
    $\begingroup$ "APU is deployed automatically (gravitational solution)" I think you are talking about a Ram Air Turbine (RAT). $\endgroup$
    – Bianfable
    Commented Jan 14, 2020 at 10:41
  • $\begingroup$ Yes, you'are correct and that makes most of my question not answerable (based on wrong assumptions). But, on the other hand I am more than sure that in Air Transat Flight 236 incident, shown in season 1, episode 6, "Flying on Empty" of Mayday "RAT" is clearly named "APU". Because I have never heard "RAT" name before (until now) and I watched that episode a few years ago. $\endgroup$
    – trejder
    Commented Jan 14, 2020 at 10:44
  • 4
    $\begingroup$ @trejder "Flying on Empty" of Mayday "RAT" is clearly named "APU"" Don't use dramatic TV episodes to determine real-world things. A RAT is never labeled "APU" in any aircraft. $\endgroup$
    – Ron Beyer
    Commented Jan 14, 2020 at 13:44
  • $\begingroup$ So is what is the power output of the APU? How many amps at what corresponding voltages and AC/DC? How much bleed air is available for Airpacs, etc? $\endgroup$
    – mongo
    Commented Jan 15, 2020 at 17:22
  • 2
    $\begingroup$ "5-10 minutes" before the APU can supply power? Not at all. The start cycle is more like 20-30 seconds, after which it can supply electrical power to most of the aircraft systems. $\endgroup$
    – Ralph J
    Commented Mar 6, 2020 at 14:47

1 Answer 1


You are getting the APU mixed up with the RAT:

  • The Auxiliary Power Unit (APU) is an engine powered generator, either a gas turbine or a piston engine. APUs deliver enough power to start engines, power cabin lights, cockpit instruments and radios, and in some cases power hydraulics. Often the APU can only be run at lower altitudes where the air is thicker
  • A Ram Air Turbine (RAT) is a small, automatically deployed propellor run generator which is lowered into the airflow if there is a loss of power. The RAT generates enough power for the most critical systems only, and is designed to give pilots enough electricity to glide until the APU can be started

Often the APU runs from the same fuel supply as the engines, so if the airplane runs out of fuel the APU is unavailable, so the RAT is all they'd have.

  • 2
    $\begingroup$ There are piston "APU's" for ground vehicles, but I've never heard of a piston APU for an aircraft, do you have an example? $\endgroup$
    – Ron Beyer
    Commented Jan 14, 2020 at 13:47
  • $\begingroup$ @RonBeyer: See: What does the PBY Catalina's APU look like? :D $\endgroup$
    – user14897
    Commented Jan 14, 2020 at 14:02
  • $\begingroup$ Nothing recent comes to mind @RonBeyer, some older aircraft like the DC-3 had an optional piston engine APU. Since some are still in service it made sense to include it. $\endgroup$
    – GdD
    Commented Jan 14, 2020 at 14:02
  • 3
    $\begingroup$ The 737-300 APU can provide electrical power up to FL350, so the comment about "at lower altitudes" isn't entirely accurate, at least in this case. Also, the 737 doesn't have a RAT. $\endgroup$
    – Ralph J
    Commented Mar 6, 2020 at 14:45

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