ATPL ADY question!

[Howard Ja Blome]

Hi All

Just did ATPL AGK today, had a question on Pressurisation Systems.

Question was in a typical pressurisation system in a modern airliner, which modes are used?

a. Negative Pressure and Positive pressure
b. Negative Pressure and Differential
c. Isobaric and Differential
d. Isobaric and Negative Pressure

I answered C. Isobaric and Differential, Got this wrong. Can anyone tell me what the correct answer should be, because none of study texts I have say so.

Thanks in advance

[the wizard of auz]

b). negative pressure and differential.

[The Bullwinkle]

I agree with the wizard of auz

I don't quite understand where isobaric pressure would fit into the equation, which leaves a. or b.

The PRESSURE DIFFERENTIAL must not exceed the maximum specified for the aircraft. For example, on the B737, the maximum allowable difference between ambient pressure and cabin pressure is 9.1psi. If the cabin pressure is increased to the point where the cabin pressure exceeds the atmospheric pressure by more than 9.1 psi, the airframe could sustain damage

The pressurisation system has a NEGATIVE RELIEF valve so that the pressure inside the cabin is never lower than the outside pressure (the airframe is only designed to expand, and would be crushed if the atmospheric pressure was higher than the cabin pressure)

Therefore, I think b. would have been the most appropriate answer.

[HEALY]

HJB...did you pass though?

[Howard Ja Blome]

Thanks guys for your help.

I realise that the negative pressure relief valve works so that the hull doesn't get crushed during rapid descent or even a normal descent where the cabin altitude cannot descent quick enough.

My understanding from ATPL text books is that in normal operating modes were

Ground Mode

Proportional Rate Mode

Isobaric Envelope mode

Differential Mode

Over Pressure Mode

So from above, one would naturally pick C. But as I said got it wrong.

Bull Winkle

Isobaric (quote from an ATPL text)

"In this range of altitudes the cabin altitude is maintained until the maximum differential is reached"

Differential (quote)

"When an altitude is reached where the max differential pressure is reached, the differential pressure is maintained thereafter"

I believe that the max differential pressure is reached and maintained, the cabin altitude rises at the same rate as the aircraft until it reaches 10,000 feet cabin altitude at which time the a warning comes on.

Just for your note, I have run this question past two indepedent ATPL theory providers who have both agreed with my answers. But then again, I wonder what CASAs answer is?

Thanks again to all

[ROB-x38]

For what it's worth I would have answered C also if it was a question re: how the system operates on climb.

Maintain cabin altitude (isobaric) until max differential is met, then maintain the differential. Oh well. Classic CASA

There may have been some wording in there which totally changed the direction of the question. They seem to do that a bit Can you remember the exact wording?

Answer B could be an answer to a question along the lines of what safety mechanisms are built into a typical pressurisation system?

Words to live by when it comes to CASA exams are RTFQ! (sometimes a few times)

Cheers

[savyslug]

the question posted does actually say what modes are used.
is negative pressure relief a mode or a safety device???
one again wording of the question. i thik the way you have worded that question answer c is correct.