Since an auxiliary power unit (APU) provides electrical power and the pneumatic supply for both air conditioning and main engine start, the impact of a malfunctioning unit at the gate can be severe.
Sometimes, an operator’s only indications of impending problems are a loss in bleed pressure and an increase in exhaust gas temperature. This might occur following lengthy operation in hot environments, where high ambient temperatures require additional fuel to be used by the APU.
This may result in hot section distress. When the unit is stripped down it is not uncommon to see severe degradation in the compressor sections and substantial degradation in the hot section of the engine.
The first step at maintenance induction is to ascertain the level of repair required, and this can often be determined by carrying out an initial borescope inspection. MRO providers may then carry out a preliminary test run of the engine, although this may be avoided if hot section damage or excessive oil leakage is observed on the borescope.
Preventive maintenance can avoid more expensive overhauls, though, as Aaron Stidder, APU services manager for Air New Zealand explains:
“In some circumstances the maintenance of their APU is kept to the minimum and is often overlooked with some operators choosing to operate the engine to the point of automatic shutdown to extract the maximum life from the unit.
“This can often lead to costly repairs and overhaul in order to provide a serviceable unit back to the operator which could have been avoided if a proper maintenance and trend monitoring program had been utilised.”
For a complete guide to the APU aftermarket along with a global directory of APU maintenance providers, see the forthcoming Engine Yearbook 2019.