I am a cargo aircraft loader at Heathrow and while we were offloading cargo, pallets and baggage containers from a Pakistan Airlines B777, the aircraft's Auxiliary Power Unit (APU) failed. It made matters worse when we attached the Ground Power Unit (GPU) ground cables to the aircraft nose because the GPU went into trip mode. This resulted in a complete loss of power and we were unable to open up the front and rear cargo doors. The loading systems' Power Drive Unit (PDU) also failed.

My team leader played with the GPU leads and set them at a certain angle where the trip mode stopped and we offloaded the airplane. What could cause this APU failure, and why would the GPU trip? Is this a safety issue if airlines operate a non-functional APU on their aircraft?

  • $\begingroup$ A functional APU is not a MEL item for many aircraft/operators. $\endgroup$
    – Simon
    Commented Jan 27, 2015 at 20:09
  • $\begingroup$ What is a Mel item sir. $\endgroup$ Commented Jan 27, 2015 at 20:16
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    $\begingroup$ @RAYHANahmed Minimum Equipment List -- it tells what you have to have (or how many) to operate the aircraft and still be complying with the regulations. If the MEL listed the APU as required, you could not operate without it. This would make a good regular question, in my opinion. $\endgroup$
    – Terry
    Commented Jan 27, 2015 at 20:18

1 Answer 1


Dispatching an aircraft with an inoperative APU, regardless of whether it's a passenger aircraft or a freighter, is a normal procedure. While an APU may be started inflight preparatory to it's use after landing, it is not needed inflight, so it's not a safety issue. In my experience (retired in 1999), dispatch of freighters with inoperative APUs is more common than dispatching passenger aircraft that way due to the passenger comfort aspect. Keeping passengers cool while unloading/loading is a priority. Keeping a freighter's crew comfortable is not.

An APU failing while in use is not unusual. GPU failure is also not unusual. If the APU tripped (as opposed to just quit) and the GPU tripped as well, a natural suspect is an electrical problem in the airplane. What is needed, of course, is a competent mechanic to troubleshoot.

APUs can fail for a number of reasons. They're just small turbine engines. In fact, for 747-100 and -200 aircraft, the APUs were often the same as the engines used for SA-226 and -227 Metroliners (with the addition of propellers of course) that I flew prior to flying 747s. Common causes of APU failure include overheating, loss of oil pressure, excessive load, and just plain mechanical breakdown.

The decision as to whether to dispatch a large airplane without an APU would be an economic one primarily. If you're sending the airplane to an airport that is known to have GPU power, it wouldn't make financial sense to delay the flight because of an inoperative APU. If, however, you're dispatching to an airport that doesn't have such, that would be unwise unless such an airport does have a huffer and you want to risk a battery start (never a good idea in my opinion).

  • $\begingroup$ Thank you for your answer. I loaded freight pallets and cargo on many types of aircraft upto A380. One thing many airlines are doing coming in with non operative apu's this included major British airlines on there hub airports. $\endgroup$ Commented Jan 27, 2015 at 20:56
  • $\begingroup$ Huffers are used mainly on PIA Virgin emerates as noted. $\endgroup$ Commented Jan 27, 2015 at 20:58
  • $\begingroup$ Sidenote on APU inop dispatch -- ETOPS twins can't go ETOPS w/ an inop APU (I don't think it applies to three and four holers, though) $\endgroup$ Commented Jan 27, 2015 at 23:34
  • $\begingroup$ Sorry I am only a cargo aircraft loader. I DON'T KNOW $\endgroup$ Commented Jan 28, 2015 at 0:55
  • $\begingroup$ Sorry sir I am only a cargo aircraft loader. $\endgroup$ Commented Jan 28, 2015 at 16:27

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