"Ryan Ferguson" wrote in message
...

Really? Hmm, that's interesting! I've never heard that. What is the
reason


For any piston engine (i.e. normally aspirated or turbocharged), if the
engine is not making maximum power for any reason -- weak cylinder,
induction/exhaust system defects, etc. -- this may become most apparent when
you try to climb to altitude and realize the plane does not have the power
to do what the book says it should do. Loss of power is more obvious in
relation to reduced rate of climb than reduced airspeed. There are planes
out there which simply cannot reach their published service ceiling or
maximum authorized operating ceiling, and the easiest way to verify this is
to go up high. Or to put it another way, if an airplane is capable of
reaching its service ceiling then that is one more datapoint suggesting the
engine and induction system are healthy -- consider this "necessary but not
sufficient" to establish the engine's good health.

For a turbocharged airplane (granted only indirectly relevant to this thread
about a C172), a high-altitude test flight is absolutely essential. Since a
turbocharged engine is worked harder/hotter at altitude (basically the
opposite of a normally aspirated engine), a weak turbo can mask a weak
engine down low and a weak engine can mask a weak turbo down low. The only
way to assess maximum performance from both the engine and the turbo is to
do a "critical altitude test" and assess the altitude at which the turbo
reaches its maximum output. There can also be problems with magneto
firing, anaeroid fuel flow controllers, turbo controllers, and induction
leaks which show up only at altitude.


--
Richard Kaplan, CFII

www.flyimc.com