jul-aug2012

Flight Safety Australia
Issue 87 July–August 2012
31
Sneaky leaks
The problem of pinhole corrosion
When we think about corrosion in aircraft, most of us probably think of airframe structures.
However, there are plenty of unsettling examples of an insidious corrosion infecting the
network of aircraft aluminium plumbing, as fact-finding investigations for CASA’s ageing
aircraft management plan have discovered.
Corrosion is not just a problem for airframes. It’s also a
cancer for aircraft systems. A small but disturbing number
of the many service difficulty reports sent to CASA concern
pinhole corrosion in the aluminium tubing used in aircraft fuel,
hydraulic, oxygen and instrument systems.
Pinhole corrosion starts when moisture gets inside the
aluminium tubing that is embedded throughout the structure of
almost every aircraft. It collects into a pool at the low point, sits
virtually on one spot and provides the catalyst for corrosion
to start. Aluminium tubing can be found in the powered flight
control system, undercarriage brake system, the instrument
system and the fuel system. Moisture also collects in deactivated
oxygen system lines, but is usually only discovered
when the system is operated.
Little information is available on pinhole corrosion in aircraft.
It is, however, a known issue in household plumbing, where
it is attributed to age, water quality and sometimes, cavitation
induced by sharp bends. But there are sufficient reports
of pinhole corrosion reaching CASA for it to be
something every LAME should be aware of.
Not all pinhole corrosion requires liquid in a
pipe. One event known to CASA described
an AS350 helicopter with gross pinhole
corrosion in the engine compressor bleed
air line required for the windscreen de-icer/
demister.
pinhole
corrosion in the
aluminium tubing
used in aircraft fuel,
hydraulic, oxygen
and instrument
systems.
Such was the pressure loss though the corrosion holes that
the demister system, important for helicopter flight in cold or
humid weather, was inoperative.
One of the most disturbing incidents was recently reported to
CASA by an engineer doing an engine run after a scheduled
inspection. Concerned by the smell of fuel in the cabin, the
engineer immediately shut down the engine and eventually
found the cabin trim fabric and the aircraft’s rear seat cushions
were saturated in avgas.
‘This raised the distinct possibility of flight crew incapacitation
due to the fumes, or even fire or explosion in flight, which, of
course, makes a pinhole in a pipe a major defect,’ CASA senior
maintenance engineer, Roger Alder, notes drily.
‘Over the years, we’ve been receiving defect reports on pinholes
in hydraulic lines and fuel system lines, which can be attributed
to water precipitating out of either the fuel and/or the mineral
hydraulic oil and pooling in the low points of the system.
Airborne moisture typically enters the engine
oil, hydraulic oil, and fuel systems via the
vent systems (just by sitting on the ground
“breathing” due to normal atmospheric
changes) where it will later condense and
form small pools.’
‘Although the numbers involved are small,
there has been a recent increase in this type
of report,’ Alder adds.