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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 speed 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).
One point with low power settings
when it's very cold is that the engine
may not warm up properly and water
forming from the combustion
process may not evaporate, so oil
won't lubricate properly.
Although many flight manuals state
that as soon an engine is running
without stuttering it's safe to use it to
its fullest extent, try warming up for
a few minutes before applying any
load, at least until you get a positive
indication on the oil temperature
(and pressure) gauges. This ensures a
film of oil over all parts.
Even better, warm it before you start
it, because the insides contract at
different rates – in really cold
weather the engine block may have
the grip of death on the pistons and
strain them when the starter is
Engines & Systems 57
turned. Equally important is not
letting it idle when cold, as you need
it to be fast enough to create a splash
of oil inside (1,000 RPM is fine).
After flight, many engines have a
rundown period which must be strictly
observed if you want to keep it for
any length of time. As engines get
smaller relative to power output,
they have to work harder. Also, in
turbines, there are no heavy areas to
act as heat sinks, like the fins on a
piston engine, which results in
localised hotspots which may
deform, but are safe if cooled
properly, with the help of circulating
oil inside the engine (75% of the air
taken into a turbine is for cooling
purposes). If you shut down too
quickly, the oil no longer circulates,
which means that it may carbonise
on the still-hot surfaces, and build
up enough to prevent the relevant
parts from turning. This coking up
could sieze the engine within 50
hours or less.
If the starter light remains on after
you release the starter button on a
piston engine, you should shut it
down, as it indicates that the starter
is still engaged with the engine and is
being driven by it.
Lubrication
Friction can be quite handy, but not
inside an engine. Without some way
of making the various surfaces rub
smoothly against each other, they
would get hot, and suffer from
scoring damage.
Oil actually does many things,
including cooling, cushioning, flushing,
lubrication and sealing.
There are two main methods of
lubrication, wet sump and dry sump.
The first is very simple, with the
engine oil in a sump under the
engine, in which the crankshaft and
other moving parts rotate, splashing
it all around (splash and mist).
Dry sump uses a tank outside the
engine, and oil is force fed around
under pressure where it is needed
　
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