Piper SARATOGA PA-32-301 Maintenance Manual page 594

Overshoot
Bootstrapping
Critical Altitude
PIPER AIRCRAFT
PA-32-301/301T
MAINTENANCE MANUAL
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 overboost in that
the high manifold pressure lasts only for a few seconds. This condition can
usually be overcome by smooth throttle advance.
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 directly back into
the turbine of that turbocharger, it would be called a bootstrap 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 compressor 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 high altitude (lower ambient pressures) 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.
NOTES:
1.
Refer to the latest revision of Lycoming Service Bulletin No. 369 for recommended engine inspec-
tions after any Overspeed or Overboost conditions.
81-00-03
Page 81-07
Revised: September 23, 1981
3B17