User Guide

ENGINE OPERATING TECHNIQUE, PART 3:
“Take care of your engine, and it’ll take care
of you.”
Compared to the “bulletproof” normally-aspirated engine on
the 172, the Mirage’s TSIO-540 is a high-strung thoroughbred--after
all, its displacement is only 50% bigger, but it produces more than
twice as much power under very demanding circumstances.
You’d think the enemy of such an engine is heat, but that’s
only part of the problem. The real culprit is temperature change,
especially if it’s rapid--and a lot more aircraft engines are dam-
aged (cylinder head cracking, etc.) by cooling them off too fast
than by overheating them.
This usually occurs during descents: you’ve reduced power,
so the engine isn’t developing as much heat as it was, while at the
same time your airspeed has increased, so that more cooling air is
moving through the cowling. There are, however, several easy
steps you can take to minimize the ill effects.
One, as mentioned before, is to use aircraft configuration
and add drag for descent, rather than simply pulling off the power
and stuffing the nose down at “Warp speed.” It may look weird to
extend the landing gear at 20,000 feet (admittedly, a severe case),
but no one is watching anyway.
Another, and perhaps the most important, is to reduce
power slowly. A pretty good rule of thumb is “don’t pull off more
than one in. Hg. of manifold pressure per minute,” although in a
pinch--say, if ATC wants you to get down “right now”--you can
pull off two inches, then wait two minutes. Try to adhere to this
rule until you get down to about 55% power or less--and keep an
eye on your CHT gauge, striving to keep it at least above the bot-
tom of the green arc.
Finally--and this is the one that most pilots seem to ignore,
especially when they’re new to high-performance flying--don’t be
in any big hurry to enrich the mixture as you descend, particularly
in turbocharged airplanes. If your fuel injection system is working
right, the mixture you’ve used for any cruise power setting will be
adequately rich for that or any lower power setting. There’s no rea-
son to enrich it, which wastes fuel as well as overcooling the
engine, until you’re down near the ground, where you might need
a richer mixture for a go-around or missed approach...and by that
time, you should be configured for approach and slowed up so
there isn’t as much of that nasty cool air blowing over the cylinders.
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You have three instruments to set the correct fuel mixture in
the Mirage: the fuel-flow indicator, the turbine inlet temperature
indicator (TIT), and, to a lesser extent, the cylinder-head tempera-
ture indicator (CHT).
Takeoff and initial full-power climb are always performed
with the mixture control in its forwardmost full rich position.
Typically, for a cruise climb, the throttle and prop control are set
for the desired power setting and the mixture is pulled back until
the fuel flow indicator shows the correct value as set out in the
pilot’s operating handbook (for example, 35 in.Hg./2500 RPM/32
gallons per hour).
You can use a similar technique for cruise power setting, but
the handbook values are, of necessity, very conservative. Once at
cruise power, you can set the mixture more accurately by referring
to TIT. It will reach its maximum, or “peak,” when the ratio of fuel
and air is exactly optimized. Piper’s operating handbook authoriz-
es operation at peak TIT for all cruise power settings up to a limit of
32 in. Hg/2500 RPM. How much more efficient is this than setting
by fuel flow? Well, although we’ve only reduced power 3 in. Hg.
from the climb setting, fuel flow has dropped to 20 gph--more than
a third! Another reason that these lean fuel flows are authorized for
cruise, rather than for climb, is that now airspeed is higher, so more
air flows through the cowl to cool the engine.
How do you set it? Get the airplane leveled off and trimmed
correctly for cruise, set cruise power, wait for engine temperatures
to stabilize and then slowly start leaning the mixture while watch-
ing the TIT. It will rise to a peak, then begin to drop off again. Note
the peak, and when it starts dropping, enrich the mixture until it’s
back at the peak value.
Flight Instruction
Flight Instruction
Fuel Flow TIT CHT