To inform pilots of a VNAV anomaly which may result in a level-off altitude error. Revised to include Service Bulletin information.
Boeing has received operator reports of VNAV over-shooting the intended altitude following a climb. This occurred at intermediate altitudes when the local barometric pressure was significantly lower than standard. Each event occurred when the level-off was initiated shortly after climbing through transition altitude.
The FMC uses a filter to smooth the computed aircraft altitude to compensate for a rapid adjustment to the barometric correction setting. This altitude smoothing function compensates for a barometric correction change at the rate of 1 inch mercury (33.9 hPa) or 1000 feet in two minutes. During climb, when a VNAV level-off is initiated shortly after passing transition altitude and the altimeter is reset from a very low pressure to QNE (29.92 or 1013), VNAV may overshoot the level-off. In descent, an undershoot condition may occur when a VNAV level-off is initiated shortly after passing the transition level and the altimeter has been reset from QNE to a very low pressure QNH setting.
The amount of undershot or overshoot depends on the amount of the barometric correction, when the altimeter is reset, and the elapsed time from altimeter reset to level-off.
If levelling off within 2000 feet after changing altimeter setting from QNE to QNH, or QNH to QNE, do not use VNAV to execute the level-off if QNH is less than 29.70 hg/1006 hPa (low altimeter setting). After the level-off is complete, VNAV may be re-engaged for climb or cruise.
This anomaly is corrected by Boeing Service Bulletin 757-34A0258. This Flight Crew Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified.