The Boeing Company Seattle, Washington 98124-2207

737
Number: ELF-11 Date: June 16, 1995 Document Effectivity: 
Subject:  Flight Management Computer Navigation Anomaly 
Reason:  This bulletin provides information contained in Red Bulletin 737-300 87-7R1, dated April 15, 1992, which advised 737 operators of possible FMC navigation errors on the “CERT” 1.0, 1.1, and 1.2 update programs and provided guidance for detecting and correcting errors when they occurred. 

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
There has been one documented case of the FMC computing an erroneous navigation position. An investigation of this anomaly revealed that all FMCs with “CERT” update 1.0, 1.1, and 1.2 programs have the potential for similar occurrences. The anomaly is the result of a specific series of events which are uncommon in the majority of 737-300 operations.
These events start when the FMC uses the VOR-DME position update mode followed by at least 90 minutes of navigation using only the IRS as a reference. In this scenario, the anomaly may be triggered when the FMC experiences some form of position updating. After 90 minutes of IRS NAV ONLY, a navigation position update may cause a software restart. After 140 minutes of IRS NAV ONLY, a position update may produce errors in navigation without a restart. FMC position updating could be any of the following events:
-Radio updating in any mode. -IRS alignment (initial or fast). -Runway position update.
Page 1 of 4
Flight Crew Operations Manual Bulletin No. ELF-11,dated June 16, 1995 (continued)
Although many 737-300 operators fly within a consistent radio update environment (normally DME-DME), they may still be vulnerable. Since the airplane does not radio update while on the ground, it is possible to lane in the VOR-DME update mode (VOR approach) and spend over 90 minutes on the ground without removing power from the FMC. If this period of ground time is sufficient and a position update is performed, the anomaly may be experienced.
Once the position computation is contaminated by the anomaly, errors will continue to grow. Removal of AC poser for a minimum of 15 seconds either in the air or on the ground will reinitialize navigation computations.
Service Bulletin 737-34-1226 or FMC updates 1.3 and later corrected this condition.
Interim Operating Procedures
During taxi for takeoff and in flight, flight crews are encouraged to be aware of the accuracy of navigation performed by the FMC. This may be checked by:
1.
Referring to the POS REF page of the CDU and checking the FMC position against the IRS positions.

2.
Tuning a VOR/DME station and comparing RDMI information with information displayed on the FIX page after entry of the same station identifier.

3.
Comparing the FMC wind and groundspeed information with IRS data.

4.
On EFIS equipped airplanes, checking the small arrowheads at the bottom of the EHSI which display the position of each IRS relative to the FMC position. These will appear when the difference between the FMC position and either IRS position exceeds 12 nm (some airplanes have full time display).

5.
Checking FMC position against visual checkpoints on the route of flight.

In flight, the flight crews should closely monitor the FMC when VOR-DME position updating was used just before the IRS NAV ONLY message appears. If possible, monitor the amount of time that navigation is performed using only the IRS position for reference. If the time approaches 140 minutes or more, frequently check for position differences or other signs of the anomaly after normal radio updating is resumed. The FMC winds should be compared with the IRS winds since the wind information is affected once the anomaly is triggered.
If it is determined that the FMC position is diverging from the desired course, continue flight using normal VOR/ADF navigation procedures. For those airplanes equipped with EFIS, flight crews should realize that the MAP and NAV displays will be in error. LNAV should not be engaged.
If the anomaly is experienced, cycle AC power off for at least 15 seconds to reinitialize the FMC computations (FMCS COMPUTER C/B my be used), before preflighting the computer for the next flight. Completion of a full IRS alignment (10 minutes) will also reset the FMC. Powering down the FMC after each flight is recommended if the airplane is going to be on the ground for more than an hour.
　
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