B757/767 cabin alt

[jaytee54]

On the overhead panel of the B757 and B767 there is a group of 3 round dials in the pressurisation panel.
The left one is cabin diff pressure
The middle one is cabin altitude
The right one is cabin rate of climb/descent.

Does anyone know if these are self contained? Are they worked from an internal input of cabin pressure?

OK the differential pressure might need an input from the ADC system, but surely the cabin alt would use a very simple barometric altimeter sensing cabin pressure in situ?
I am guessing it would read altitude based on 1013/2992 standard pressure?

[Piper19]

In the 757:
There is a pressure port on the back of that panel with the 3 indicators (it's officially called the press and ind warning module). The pressure port provides the cab alt and cab alt rate info. The diff pressure comes from a differential pressure sensor below the cockpit floor (the elec equipment bay). That sensor gets static pressure from the alternate static ports (the ones that also provide your standby instruments) and also senses cabin pressure. A diaphragm in the sensor then differentiates the two inputs to indicate your differential pressure.

The two ADC systems come into play for the auto operation of the pressurization, not for the 3 indicators you mention. (1 ADC inputs into each control computer, and the computer itself has a pressure port on it's face for cab pressure).

Then there is also a cab alt warning switch below the cockpit floor, that provides your EICAS warnings+ aural+ the dedicated lights next to the 3 indicators + master warning lights.

[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?

[jaytee54]

piper19
Thank you for that reply. Pretty much what I suspected.

Recently I watched an Accident Investigation programme on TV about the Aero Peru 603 accident in Oct '96.
It struck me they could maybe have depressurised the cabin and used the cabin alt indicator for a gross error check on their altitude (they thought they were at something like 9000 ft when they hit the sea). All static ports were sealed with tape.

[Sailvi767]

Or they could have looked at the radar altimeter.

[jaytee54]

Radio alt reads only below 2500 feet (radio), and by that time they were in so much trouble, so confused, that it didn't seem to help.

The SSR readout on ATC radar read 9000 feet and they believed it, despite its source being ADC derived like everything else.
So, as I thought, the only altitude source not connected to the static vents was the cabin alt, and by depressurising the cabin it would have read aircraft alt.

Once established on a stable approach the rad alts would have been very useful.

[vapilot2004]

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.

[172_driver]

Thanks vapilot!

[vapilot2004]

No worries 172!