HANDS ON
FIREWALL FORWARD
richer mixture to also increase detonation margin. If you follow
the manufacturer’s guidelines for CHT and mixture settings, deto-
nation will not be a problem.
Most turbo engines have a turbine inlet temperature (TIT)
limit as well as the typical EGT readings. TIT is designed to protect the turbine wheel from damage. So, no matter how rich or
lean of peak the engine is operating, the turbine can only withstand so much temperature before it may be damaged. When
leaning a turbo you need to be sure that you don’t exceed TIT
before reaching the desired EGT rich or lean of peak.
At lower altitudes where the air is dense the turbo does not
need to work at full blast to produce the desired manifold pressure, so some of the exhaust gas must be diverted from the turbo
to keep it from overboosting the engine. A valve called a waste
gate opens to send some exhaust gas around the turbo so it won’t
produce too much pressure from the compressor.
In most turbo engines the waste gate functions automatically.
Engine oil pressure is used to move the waste gate to maintain the
manifold pressure the pilot has selected with the throttle. As the
airplane climbs and air density drops, the waste gate moves
toward closed and sends more and more of the exhaust gas
through the turbo. At some air density the waste gate is fully
closed and the turbo is producing its maximum boost for the
engine. This air density is called the critical
altitude for the engine and is the density alti-
tude at which the engine can produce
maximum rated power. Of course, if the air
aloft is colder or warmer than standard, the
critical altitude will also move up or down
because it’s density altitude, not true alti-
tude, that matters.
J. Mac McClellan, EAA 747337, has been a pilot for more
than 40 years, holds an ATP certificate, and owns a
Beechcraft Baron.
