When I converted on to the 757 in 1983 LNAV had not been installed/certified, but we still suffered from numerous uncommanded descents, particularly on LGW-Palma-LGW sectors. Outbound the FMC would decide it was time to start the descent just north of the Pyrenese, inbound it tended to happen around Nantes. Boeing reckoned it was caused by a bug in the FMC program that was triggered by certain DME/VOR combinations. It got so bad at one stage that I used to disengage the autothrottle (which overcontrolled anyway) when in the suspect areas, then selected FL hold when the premature descent was commanded.
Last edited by brakedwell; 24th July 2007 at 18:42.