Flight Crew Operations Manual Bulletin No. SHZ-32 R1, dated November 01, 2004 (continued)
Intentionally
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Flight Crew Operations Manual Bulletin
for
The Boeing Company Seattle, Washington 98124-2207
737
Number: SHZ-33 R2 Date: November 01, 2004 Document Effectivity:
Subject: FMC Navigation Anomaly
Reason: This bulletin informs flight crews of an FMC navigation anomaly on airplanes equipped with dual FMCs and FMC Updates U10.3, U10.4, U10.4a or U10.5. The purpose of this reissue is to provide Service Bulletin information.
Information in this Flight Crew Operations Manual (FCOM) bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.
THE FOLLOWING PROCEDURE AND/OR INFORMATION IS EFFECTIVE UPON RECEIPT
Background Information
Boeing has received reports of FMC map shifts on 737-400 airplanes equipped with dual FMCs and FMC Update U10.4a. The root cause of the problem has not been determined, but Boeing and Smiths Industries believe the anomaly may be possible on all 737 airplanes with dual FMCs and FMC Updates U10.3, U10.4, U10.4a or U10.5. The problem can occur on both GPS and non-GPS equipped airplanes.
Each of the reported map shifts was accompanied by a “VERIFY POSITION” alert message and a displayed ACTUAL navigation performance value that was less than the RNP value. In these events, the “VERIFY POSITION” message indicated an actual FMC position error.
The “VERIFY POSITION” message is triggered by differences in FMC and/or sensor positions and RNP. The “UNABLE REQD NAV PERF - RNP” message is generated when ACTUAL navigation performance does not satisfy the RNP criteria. POS SHIFT page 3/3 provides the relative FMC, GPS (if installed), IRS and RADIO positions, as well as RNP and ACTUAL navigation performance.
Flight Crew Operations Manual Bulletin No. SHZ-33 R2, dated November 01, 2004 (continued)
For at least one of the reported events, map shifts occurred on both displays due to the FMC position aligning with the average IRS position. Each FMC’s internal position then reverted to its onside IRS position. When the FMC positions exceeded the RNP criteria, the “VERIFY POSITION” message was triggered.
The ACTUAL navigation performance computation did not reflect the FMC reversion to inertial only operation. Since the computed ACTUAL navigation performance was based on the available, but unused, radio and GPS position data, it was less than the RNP, and the “UNABLE REQD NAV PERF -RNP” message was not displayed.
If the FMC Source Select Switch is positioned to BOTH ON L or BOTH ON R (i.e., equivalent to single FMC operation), the FMC position is updated from available onside sensors with the correct ACTUAL navigation performance. The “VERIFY POSITION” message will then clear if the resulting FMC position is within the RNP. With the anomaly present, the problem will recur when the FMCs are switched back to NORMAL (dual FMC operation) and the “VERIFY POSITION” message will again be displayed.
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