Originally Posted by
172_driver
Are the indicators (Cabin Alt and Rate of Climb/Desc) independent of electrical supply a la Piper19, i.e. the needles move mechanically by expanding bellows? How about diff pressure indicator?
The cabin ALT and pressure/rate indicators on the 757 are electrical. They are powered by the AC Standby Bus.
All 3 indicators are independent of the ADC system and pressure controllers as is the cabin altitude warning system. Both pressure controllers have their own cabin altitude sensors but use ADC data for scheduling.