Originally Posted by
Cloud Cutter
Thanks for that info. Does the system on the 787 not use envelope modulation for mode 4? My understanding is that this is standard on all new Honeywell EGPWS, so you should get ‘TOO LOW, GEAR’ at 500 ft above the runway, rather than 500 ft RA.
I'm not sure - Honeywell says that envelope modulation is used at certain locations to either expand the Mode 4 envelope for improved alerting, or to desensitise it to prevent nuisance alerts. However, the only Boeing reference I can find says that envelope modulation lowers the radio altitude limits to prevent nuisance alerts.