Engine Fault Detection and Identification
The OMS engine FDI function detects and identifies off-nominal
performance of the OMS engine, such as off-failures during OMS
thrusting periods, on-failures after or before a thrusting period
and high or low engine chamber pressures.
Redundancy management software performs OMS engine FDI. It is
assumed that the flight crew arms only the OMS engine to be used;
the OMS engine not armed cannot be used for thrusting. FDI will
be initialized at SRB ignition and terminated after the OMS-1
thrusting period or, in the case of an RTLS abort, at the transition
from RTLS entry to the RTLS landing sequence program. The FDI
also will be initiated before each OMS burn and will be terminated
after the OMS thrusting period is complete.
The OMS engine FDI uses both a velocity comparison and a chamber
pressure comparison method to determine a failed-on or failed-off
engine. The velocity comparison is used only after MECO since
the OMS thrust is small compared to main propulsion thrust before
The measured velocity increment is compared to a predetermined
one-engine and two-engine acceleration threshold value by the
redundancy management software to determine the number of engines
actually firing. This information, along with the assumption that
an armed engine is to be used, allows the software to determine
if the engine has low thrust or has shut down prematurely.
The chamber pressure comparison test compares a predetermined
threshold chamber pressure level to the measured chamber pressure
to determine a failed engine (on, off or low thrust).
The engine-on command and the chamber pressure are used before
MECO to determine a failed engine. The velocity indication and
the chamber pressure indication are used after MECO to determine
a failed engine. If the engine fails the chamber pressure test
but passes the velocity test after MECO, the engine will be considered
failed. Such a failure would illuminate the red right OMS or left
OMS caution and warning light on panel F7 and the master alarm
and produce a fault message. In addition, if an engine fails the
chamber pressure and velocity tests, a down arrow is displayed
on the maneuver CRT next to the failed engine.
When the flight crew disarms a failed engine by turning the arm/press
switch on panel C3 to off , a signal is sent to the OMS thrusting
sequence to shut down the engine and to signal guidance to reconfigure.
Guidance reconfigures and downmodes from two OMS engines, to one
OMS engine, to four plus X RCS jets.