AviTrader_Monthly_MRO_e-Magazine_2016-03

Cover story: APU maintenance
23
Maintaining APUs
In 1963 the Boeing 727 was the first jetliner to introduce a gas Auxiliary Power Unit (APU). They remain a
critical part of aircraft systems and so is the maintenance and repair process. Keith Mwanalushi reports.
APUs provide secondary power for main
engine starting, cabin air conditioning and
electric power for aircraft while on the
ground and can also provide electric power during
in-flight operation.
The role of APUs is even more crucial in the event
the main power systems fail. The Honeywell 131-
9 APU was used in an emergency landing when
U.S. Airways Flight 1549 splash landed in New
York City’s Hudson River in 2009. Both CFM56
turbofans on the A320 were damaged and electrical
generators went off line.
According to ATC Aerospace depending on the
design, the APU can provide electric power, hydraulic
power, pneumatic power or all three.
Connecting to a hydraulic pump allows the crew
to operate hydraulic equipment such as the flight
controls or flaps. Having this function is also useful
for a backup if there is an engine failure.
The typical APU on a commercial airliner is comprised
of three main sections:
Power Section: This is the gas generator part of the
engine and gives power to the shaft of the APU.
Load Compressor Section: This is a compressor
mounted on the shaft. It provides pneumatic
power to the aircraft. It has two actuated devices,
the inlet guide vanes which regulate airflow
The main criteria that triggers removals will be performance.
to the load compressor and the surge control
valve which allows the surge-free operation of
the turbo machine.
Gearbox Section: This transfers power from the
Photo: Joramco
main shaft to an oil-cooled generator for electrical
power. Inside the gearbox is where power is
transferred to the engine accessories like the fuel
control unit, the cooling fan and the lubrication
module. There is also a starter motor connected
through the gear train. It performs the starting
function of the APU.
The 787 Dreamliner, uses more electric power
than the typical airliner so the APU is only used
to deliver electricity to the aircraft. Not having a
pneumatic system simplifies the design but the
trade-off is the high demand for electricity meaning
the requirement of heavier generators.
Various elements have an impact on the maintenance required.
Photo: AFI KLM
Currently, experts at Lufthansa Technik see three
main developments in the APU maintenance
and repair business: Firstly, APU maintenance
changed to FADEC (Full Authority Digital Engine
Control) drives system with correct maintenance
action to manage the on condition operation.
Secondly, OEM’s have started to change the after
sale market by choosing selected service providers.
And lastly, the APU repair processes will be
released from OEM to the selected service provider.
Cost reduction for repair process versus
damaged part replacement with pool or new
material will be the challenge for the future.
AviTrader MRO - March 2016