Ask the Captain: APU failure forces 'overweight' landing

— -- Question: I can see the jet engines under the wing but I have noticed exhaust exiting the tail cone of some aircraft. What is this, as there isn't an obvious rear air intake?

-- submitted by reader wiynot

Answer: The exhaust in the tail of many airplanes is from the auxiliary power unit (APU). It is a small jet engine used for a supplemental electrical and air source. Many transport jets require compressed air to turn the engine starter. This compressed air usually is supplied by the APU. In the event of the failure of a generator, the APU usually can be used to supply electrical power in place of the failed generator. Some airplanes also use the APU compressed air for air conditioning on the ground and in flight.

There are intakes for the APU but they are not as obvious as the intake of the main engines, due to the smaller size.

Q: Hi Captain Cox, I was flying on an MD Super 80 this morning from St. Louis to Los Angeles. The pilot opted to take off with only the APU and one engine electrical generator functioning (he mentioned it during a brief pre-flight delay). Mid-flight, the APU quit working so we were left with only one functioning electrical generator. This prompted a (flawless) emergency overweight landing at Kansas City and we were met by the fire trucks.

This brings up several questions: Would the aircraft have been unflyable if the third electrical generator died? How common are overweight landings? Was there any real danger from the landing?

I fly over 100,000 miles a year and have taken off with many a broken APU, but I've never been in a situation like this. Thanks for your feedback and I appreciate your column.

-- acarron

A: Departing with an inoperative generator is permitted under the FAA approved minimum equipment list (MEL).

The airplane would not have been unflyable, but only battery power would have been available for communication and navigation (some airplanes have a ram air turbine for emergency electrical power but the MD-80 does not). Battery power will last a minimum of 30 minutes.

Overweight landings like the one you encountered are not common but occur occasionally. A maintenance inspection of the airplane is required before another flight.

No, there is no real danger in an overweight landing. It is possible to damage the airplane if a very hard landing is made overweight, but such landings are very rare, making the danger factor very low.

The scenario you describe is unusual, but I faced the same situation in the late 1980s in a 737-200. We did not have to land overweight, but we did have to land with only one generator operating after the APU quit following a departure under the MEL with the right engine generator inoperative. Not dangerous, but inconvenient for all aboard.

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John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.