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the average Lycoming engine
disintegrates is about 3450 RPM,
which doesn't leave you an awful lot
of room when it runs normally (in a
Bell 47, anyway) at 3300! Turbines,
however, are less forgiving than
pistons and give fewer warnings of
trouble because of the closer
tolerances to which they are made.
This is why regular power checks
216 Canadian Professional Pilot Studies
(once a week) are carried out on
them to keep an eye on their health.
The other difference is that damage
to a piston engine caused by
mishandling tends to affect you,
straight away, whereas that in a
turbine tends to affect others down
the line.
In a turbine-engined helicopter,
power used is indicated by the
torquemeter.
Apart from sympathetic handling,
the greatest factor in preserving
engine life is temperature and its rate
of change. Over and under leaning
are detrimental to engine life, and
sudden cooling is as bad as
overheating—chopping the throttle
at height causes the cylinder head to
shrink and crack with the obvious
results—the thermal shock and extra
lead is worth about $100 in terms of
lost engine life. In other words, don’t
let the plane drive the engine, but
rather cut power to the point where
it’s doing a little work. This is
because the reduced power lowers
the pressure that keeps piston rings
against the wall of the cylinder, so oil
leaks past and glazes on the hot
surfaces, degrading any sealing
obtained by compression. The only
way to get rid of the glaze is by
honing, which means a top-end
overhaul. For the same reasons, a
new (or rebuilt) engine should be
run in hard, not less than 65%
power, but preferably 70-75%,
according to Textron Lycoming, so
the rings are forced to seat in
properly. This means not flying
above 8000 feet density altitude for
non-turbocharged engines. Richer
mixtures are important as well. Also,
open the engine compartment after
shutting down on a hot day, as many
external components will have
suddenly lost their cooling. With
some turbine engines (like on the
AStar), you have to keep a track of
the number of times you fluctuate
between a range of power settings
because of the heat stress.
When levelling in the cruise, the
combination of increased IAS and
throttling back cools the engine
rapidly, so close the cowl flaps
beforehand. Don't use the cowl flaps
as airbrakes, either, but to warm the
engine after starting and to cool it
after landing (allow temperatures to
stabilise before shutting down,
especially with turbochargers).
In the cruise, better fuel
consumption may be obtained at
slower speeds and lower power
settings, at the cost of extended
running time, so you might not really
save that much. For example, leaning
to 10° lean of peak Exhaust Gas
Temperature (EGT), without
exceeding the maximum, loses about
5 knots. Typically, EGT probes are
fitted to one cylinder of the engine,
which is not necessarily the one that
reaches peak temperature first, even
though it may end up as the hottest,
so a margin of 25° rich of peak may
still not be enough to stop another
cylinder from getting too close to
peak for comfort, or even lean.
One consideration with using low
power when it's very cold is that the
engine may not warm up properly
and water that forms from
combustion may not evaporate, so
oil won't lubricate properly.
The reason the temperature cools
either side of the peak reading is that
on the one hand (rich), there is too
much fuel and, on the other (lean),
Airframes, Engines & Systems 217
there is too much air (having said
that, the hottest CHT is between 25-
50° rich of peak EGT, because that's
where the peak cylinder pressure
occurs, with a high rate of heat
transfer to the cylinder head, so you
need to lean past it). However,
although being lean of peak works,
there is much more potential for
causing damage to the engine if it is
mismanaged – it needs more
monitoring to be used effectively, as
the temperature at the exhaust will
still be high, which is not good for
the valves, particularly acute with
high performance turbocharged
engines – Australian authorities
found that leaning causes lead
oxybromide deposits to cling to
various parts inside the combustion
chamber, which could become
hotspots and cause detonation (the
　
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