To inform flight crews the FMC CDU scratchpad message ENTER IRS POSITION may not display when an incorrect present position is entered for IRS alignment.
The FMC CDU scratchpad message ENTER IRS POSITION indicates the flight crew-entered present position did not pass an IRS comparison check, or the IRS is ready to enter the navigation mode and a present position has not been entered. Operators have reported isolated incidents of IRS alignment without display of the ENTER IRS POSITION message when an incorrect present position has been entered during preflight. As a result, airplanes have inadvertently dispatched with unreliable IRS position information. Engineering analysis indicates the anomaly is the result of a timing problem between the FMCs.
Engaging the autothrottle for takeoff (GPS not available) does not correct the error. If the error is sufficiently large, FMC radio position updating will be locked out in flight. GPS position updating (if available) will function and slowly correct the position error at a rate of 4 NM per minute.
In addition, large initial position errors can cause IRS magnetic variation errors affecting the accuracy of magnetic heading and track. Excessive magnetic track errors can cause AFDS localizer mode failure and subsequent inhibiting of automatic approaches. Complete correction of the FMC/IRS position error requires landing and full realignment in the navigation mode.
The flight crew can detect a large FMC position error by comparing relative positions of the airplane and runway symbols on the HSI. On the POS REF page, individual IRS positions should be compared to the required alignment position (gate coordinates, etc.). If an initial position error is discovered before takeoff, a fast alignment with a correct position entry may not resolve the problem. A full alignment must be accomplished by rotating the IRS Mode Selectors to OFF, then to NAV, and entering the correct present position in the SET IRS POSITION line on the POS INIT page. Also, due to an unrelated anomaly, the use of LAST POS for IRS alignment has not always resulted in correct IRS alignment. If the last positions stored in the FMCs differ, using either for IRS alignment may result in an incorrect alignment position.
This anomaly exists only in PIP FMC and Pegasus FMC. Following are the affected software part numbers:
During preflight, after entering present position on SET IRS POS line, select the POS REF page and verify L, C, and R IRS position coordinates are correct.
If an incorrect initial position is discovered in flight, the situation cannot be corrected without full IRS realignment on the ground.
This anomaly is corrected by Boeing Service Bulletin 757-34-0181 or 757-34-0212.