An Altimeter Can Kill You
Ignoring even a slight warning
BY J. MAC MCCLELLAN
THE RULES REQUIRE THAT altimeters in airplanes approved for IFR
flight must be checked for accuracy by a qualified technician every
24 months. Even if you don’t fly IFR, all transponders and the altimeter devices that supply Mode C altitude data must be checked for
accuracy on the same 24-month cycle.
The altimeter checking rule has been
around for a very long time and makes
sense. After all, inaccurate altitude information is an obvious hazard for almost
any type of flying.
In the late 1970s my home airport
was Manassas, Virginia, just southwest
of Washington, D.C. I was flying a Piper
Turbo Arrow at the time. The Turbo
Arrow had Continental’s six-cylinder
TSIO-360 engine with a fixed waste gate
turbocharger. The fixed waste gate meant
that you had to move the throttle only
enough to set maximum manifold pressure
because there was no automatic waste gate
to protect the engine from being over-boosted. The system increased pilot
workload, but it was less expensive than
the automatic systems, and Piper wanted
to hold down the cost of its then-new turbocharged model.
The shop where my airplane was based
preferred to satisfy the altimeter testing
requirement by having the altimeter tested
on the bench. The technician would note the
accuracy of the instrument over its certified
operating range and make any adjustments
as necessary. The altimeter would then be
reinstalled in the airplane, and a quick “leak
check” of the static pressure system would
complete the required two-year altimeter
system certification.
The other method to satisfy the altimeter
test requirement is to connect calibrated
equipment to the airplane’s static system
and test the entire system, including the
Mode C transponder output, all at once.
But my shop didn’t do that. Instead,
the mechanics simply removed the altimeter and replaced it with one that had been
tested and certified on the bench several
weeks earlier. I didn’t think anything
about how the rule was satisfied and was
just happy to be able to fly my trip to
Wichita on schedule.
