B757/767 Cabin Altitude Instrument

[ECAM_Actions]

Hi,

What are the pressure sources for this instrument?

[vapilot2004]

Static pressure & baro comes from L/R Air Data Computers via the ARINC bus.

Each Pressure Controller (located in E/E bay) has integral cabin press sensor.

Cabin altitude warning (10,000 feet) comes from dedicated sensor in E/E.

Cabin differential sensor module in E/E bay uses the alternate static system along with a remote pressure sensor located forward of the module.

Positive press relief valve has own ports. (static remote & local)

[ECAM_Actions]

Interesting... so in a situation where all three static ports are blocked, the cabin altitude instrument would NOT function correctly?

[vapilot2004]

In the case of L, R, and ALT static failure, cabin altitude should still be valid, however difference readout would not and automatic rate scheduling would revert to manual function. While the static system has three independent plumbed sources, there are in fact 6 ports in all.

[Golden Rivet]

sorry mate, you are incorrect. The M19 module which houses the cabin alt gauge, uses an integral port on the back of the module to sense ambient air pressure. None of the three gauges in the module use the ADC's

[vapilot2004]

The overhead cabin press and rate gauges are self-contained as you say GR. The difference gauge receives data from the difference module in the E/E bay which gets ported pressure from the ALT source.

My prior statement is correct:
In the case of L, R, and ALT static failure, cabin altitude should still be valid, however difference readout would not and automatic rate scheduling would revert to manual function.

...however there was clearly ambiguity in my answer. I was focusing on the system while Mr. ECAM's question was regarding the gauge and that ball went sailing right past me. :ok: Thanks mate!

[ECAM_Actions]

Thanks for the replies! Appreciate it.

I was having a discussion with someone about Aeroperú Flight 603 - they crashed into the sea after taking off with the static ports still taped following maintenance work.

They had no idea about their altitude or airspeed, but I had two questions regarding that:

* Why they didn't trust or use the rad alt? GPWS was operational.

* Why they didn't decompress and use the cabin altitude indicator as an altimeter (from your responses it seem this would have worked).

I figured they could have used the IRS GS readout for approximate speed reference, allowing for the fact it accounts for wind.

[grounded27]

Interesting... so in a situation where all three static ports are blocked, the cabin altitude instrument would NOT function correctly?


Would be the least of your worries:ugh::ugh::ugh::ugh::ugh::ugh:

[ECAM_Actions]

In case my comment wasn't clear:

>>>> It appears you could use the cabin altitude as an altimeter.

Their big problem was they didn't know their altitude. The Captain seems to have got on top of the speed by flying pitch/power. They were still flying when they crashed (CFIT).

Understand your systems and use everything available, IMHO.

[grounded27]

Thanks for the replies! Appreciate it.

I was having a discussion with someone about Aeroperú Flight 603 - they crashed into the sea after taking off with the static ports still taped following maintenance work.

They had no idea about their altitude or airspeed, but I had two questions regarding that:

* Why they didn't trust or use the rad alt? GPWS was operational.

* Why they didn't decompress and use the cabin altitude indicator as an altimeter (from your responses it seem this would have worked).

I figured they could have used the IRS GS readout for approximate speed reference, allowing for the fact it accounts for wind.

ECAM Actions.


Uh, NO! Rad alt is good but you loose corrected a/s. No one is prepared to use irs ground speed but suppose it may help. The cabin altimiter will not be accurate as there is a lag in level change to C/P change with the outflow valve full open.

[vapilot2004]

ECAM, I agree, radio altitude would have been a key piece of information in solving the puzzle they were facing, however they had little more than a minute to use the truthful numbers on the ADI before reaching 2,500 AGL.

When dealt conflicting data, we are taught to look for the information that makes sense. Unfortunately for Aeroperu, the crew was not seeing the whole picture and suffered from some "tunnel vision." IRU GS readout could have also been helpful in solving the puzzle, but for control purposes, it is most useful in the middle of the speed envelope. Pitch and power alone would have saved the day.

RE: Cabin altimeters. They could have be trusted but only very coarse altitude and certainly not near the ground. The A/C Press system even when selected off has no 0 sum gain capability for various reasons.