I watched a couple of cockpit videos on the 737NG landing, and I noticed GPWS in some of the videos does not announce 500ft, 400ft, 300ft, etc. Does this mean the pilots have routed these sounds into the headphones, or they have been disabled on the particular operator’s aircraft?
No, the pilots cannot disable audio warnings or switch them over to headphones only. They are always send to the speakers and all headphones. The pilots cannot even adjust the volume for these warnings. This is to make sure that these warnings can always be heard regardless of any intentional or accidental misconfiguration of the audio control panel.
Audio warnings for altitude alert, ground proximity warning, collision avoidance, and windshear are also heard through the speakers and headsets at preset volumes. They cannot be controlled or turned off by the crew.
(Boeing 737 NG FCOMv2 5.20.1 - Communications - System Description)
The typical altitude callouts from the GPWS are:
Radio Altitude Callouts
[Option - Typical]
The GPWS provides the following altitude callouts during approach:
- 2,500 feet – TWENTY FIVE HUNDRED
- 1,000 feet – ONE THOUSAND
- 500 feet – FIVE HUNDRED
- 100 feet – ONE HUNDRED
- 50 feet – FIFTY
- 40 feet – FORTY
- 30 feet – THIRTY
- 20 feet – TWENTY
- 10 feet – TEN.
Note: Callouts at 1000 feet and 500 feet are based on barometric altitude above the landing field elevation; callouts at 2,500 feet, and below 500 feet are based on radio altitude.
The GPWS provides height callouts based on the altitude set by the Captain’s Minimums selector.
Callouts are based on radio altitude when the MINS selector is set to RADIO. Callouts are based on barometric altitude when the MINS selector is set to BARO:
- DH/MDA plus 100 feet – PLUS HUNDRED
- at DH/MDA – MINIMUMS
(Boeing 737 NG FCOMv2 15.20.17 - Warning Systems - System Description)
All of these callouts are airline options and can be disabled by individual operators. There are also callouts at other altitudes, which are not listed here. See also Does the Boeing 737 NG have "5" radar altitude callout?.