To inform flight crews of incorrect FMC holding pattern size when flying without LNAV engaged.
The FMC calculates holding pattern size based on the target holding airspeed, current winds and altitude when crossing the holding fix, and FAA/ICAO holding protected airspace limits. The FMC updates holding pattern size, as required, each time the airplane crosses the holding fix. However, current FMC software prevents pattern size update if LNAV was not engaged on the previous crossing of the holding fix. If a holding pattern has been modified after crossing the holding fix without LNAV engaged, the FMC will not update the holding pattern size when the fix is crossed again. The airplane must cross the holding fix twice with LNAV engaged for FMC holding pattern update to occur. This does not affect holding pattern size for initial holding pattern entry.
The anomaly can occur when descending in holding or modifying a holding pattern. In one reported event, the flight crew created a holding pattern in the FMC at an initial approach fix and entered the holding pattern using HDG SEL. Upon receiving ATC clearance, the flight crew began a descent and engaged LNAV. Before crossing the holding fix twice with LNAV engaged, the flight crew descended to an altitude where the original holding pattern size no longer met protected airspace criteria. A GPWS warning was generated from nearby terrain. This terrain alert would have been avoided had a holding pattern update occurred.
The anomaly does not exist with PEGASUS FMC, P/N S242T102-455.
When possible, enter and fly FMC holding patterns with LNAV engaged. Holding pattern size will be updated, as required, each time the airplane crosses the holding fix and the airplane will remain within FAA/ICAO hold protected airspace. Cross check lateral and vertical navigation for proper operation.
If 1) climbing or descending in, or 2) modifying, a holding pattern without LNAV engaged, fly the holding pattern in HDG SEL and use time/distance techniques. Under these conditions, use the FMC holding pattern for reference only since pattern size is not updated when the holding fix is crossed.
This anomaly is corrected by Boeing Service Bulletin 757-34-0192 or 757-34-0210 or 757-34-0211 or 757-34-0212. Refer to individual Service Bulletin for applicability.
This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operators fleet have been modified.