MD-11 Flight Crew Operations Manual
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Engines -
Description and Operation
Eng.10.9
Compressor Control System
The compressor control system controls performance through the range of engine
operation. A Variable Bleed Valve (VBV) system controls the amount of air into
the high pressure compressor.
At low engine speeds, the low pressure compressor supplies more air to the core
than is necessary for operation. This excess air goes into the fan discharge
airstream and around the high pressure compressor.
The VBV system valves close when the engine speed is sufficient to use the air.
A Variable Stator Vane (VSV) system controls the airflow through the high
pressure compressor.
The VSV system changes the direction of this airflow to the best angle for
compressor performance. This prevents compressor stalls and improves engine
operation during acceleration and deceleration. The ECU controls the operation of
the VBV and the VSV systems.
Engine Vibration Monitoring System (EVMS)
The EVMS consists of the following:
• One Engine Vibration Signal Conditioner (EVSC).
• Accelerometers (1 or 2).
• N1 and N2 tachometer signals wired to the EVSC from each engine (6
total).
The EVSC is powered by 115-volt ac/400-Hz electrical power. All EVMS data is
displayed on the secondary engine display. When engine vibration exceeds a
preset value, the digits will turn amber and will be boxed in amber, and an alert
will be displayed.
Compressor (N1) and turbine (N2) vibration signals for all three engines are
displayed on the secondary engine page of the SD. The secondary engine page is
one of many pages that can appear on the SD. If the secondary engine page is not
on the SD it can be selected with the ENG cue switch on the aft pedestal. The
secondary engine page will appear automatically if there is an engine vibration
alert on the EAD.
The EVSC will perform a self-test on power-up. Missing EVMS data is shown
with an amber X on the CRTs.
An engine may experience normal momentary peaks in vibration levels lasting
between 5 to 30 seconds during certain throttle transients or engine inlet air entry
angles.
The Centralized Fault Display System (CFDS) allows maintenance personnel
access to EVMS maintenance data.
October 02, 2006
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