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 MECO.

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.