Piper Navajo PA-31 Service Manual page 479

PIPER NAVAJO SERVICE MANUAL
Overboost
I
Overshoot
An overboost condition means that manifold pressure is exceeding the
limits at which the engine was tested and FAA certified and can be det-
rimental to the life and performance of the engine. Overboost can be
caused by malfunctioning controllers or improperly operating wastegate
in the automatic system or by pilot error in a manual controlled system.
Refer to latest revision of Lycoming Service Bulletin No. 369.
Overshoot is a condition of the automatic controls not having the ability
to respond quickly enough to check the inertia of the turbocharger speed
increase with rapid engine throttle advance. Overshoot differs from over-
boost in that the high manifold pressure lasts only for a few seconds. This
condition can usually be overcome by smooth throttle advance. A good
method for advancing the throttle is as follows. After allowing the engine
oil to warm up to approximately 140
°
F. advance the throttle to 28" to 30"
manifold pressure, hesitate I to 3 seconds and continue advancing to full
throttle slow and easy. This will eliminate any overshoot due to turbo-
charger inertia.
Bootstrapping
Critical Altitude
This is a term used in conjunction with turbo machinery. If you were to
take all the air coming from a turbocharger compressor and duct it di-
rectly back into the turbine of the turbocharger, it would be called a boot-
strap system and if no losses were encountered, it would theoretically run
continuously. It would also be very unstable because if for some reason
the turbo speed would change, the compressor would pump more air to
drive the turbine faster, etc. A turbocharged engine above critical altitude
(wastegate closed) is similar to the example mentioned above, except now
there is an engine placed between the compressor discharge and turbine
inlet. Slight system changes caused the exhaust gas to change slightly.
which causes the turbine speed to change slightly, which causes the com-
pressor air to the engine to change slightly, which in turn again affects
the exhaust gas, etc.
A turbocharged engine's wastegate will be in a partially open position at
sea level. As the aircraft is flown to higher altitude (lower ambient pres-
sures) the wastegate closes gradually to maintain the preselected manifold
pressure. At the point where the wastegate reaches its full closed position.
the preselected manifold pressure will start to drop and this is considered
critical altitude.
I
8-31. REMOVAL OF TURBOCHARGER.
(Refer to the latest revision of Lycoming Service Instructions
No. 1320.)
a. Remove the engine cowling, (refer to Paragraph 8-4.)
1. Disconnect the oil supply and return lines from the center section of the turbo.
CAUTION
Do not spread the v-band couplings to force them over the outside
of the pipe. They must be passed over the end of the pipe. If the
v-band couplings are spread open excessively, their sealing
properties will be destroyed.
Revised: 9/24/81
215 POWER PLANT