Cabin Pressurization question

[tg743]

The DHC8-100/300 has a limitation regarding cabin differential pressure for take off and landing to be less than 0.5

During normal ops. we only adjust field elevation to equal that of landing airport, and cabin alt should descent at 300fpm.

In our DH1 landing with bleed on and beed rate selector at MIN is approved,which means that we have to reduce bleed rate to MIN as a part of before landing checklist. Normaly at this time cab diff pressure is also 0,5 or less.

Sometimes if we do our before landing check early (at higher alt) or descend steeply without increasing cabin rate of descend accordingly we can end up with too high cabin diff pressure.

To my understanding this is best solved by turning bleed flow to MIN and thus limit amount of pressurized air into cabin which should increase cabin alt and thus decrease cabin diff press. What makes me confused is that sevral commanders have the oppinion that bleed air flow should be MAX until desired diff press is obtained.

Could anyone please clearify

thanks

[TheChitterneFlyer]

Sometimes if we do our before landing check early (at higher alt) or descend steeply without increasing cabin rate of descend accordingly we can end up with too high cabin diff pressure.

I confused by that statement! If you descend at a higher (aircraft) rate of descent and you haven't increased the cabin rate of descent then surely the cabin differential pressure will decrease; thus, you'll depressurise earlier than planned.

[BOAC]

Back to the books, TCF?

[rudderrudderrat]

Hi TCF,

I agree with you. If the cabin altitude (say 3,000) is still above the destination altitude (say sea level), then with a very rapid aircraft descent rate down to sea level, we may overtake the cabin altitude.

When the diff pressure is zero - the inward relief valve will open and the aircraft and cabin will have the same descent rate.

Maybe that's what tg743 was alluding to. By increasing the cabin ROD down to sea level at a comfortable rate (700 ft per min say) then they avoid overtaking the cabin altitude on the descent.

Sorry BOAC, but have TCF and I missed something again?

[TheChitterneFlyer]

Back to the books, TCF?

BOAC I hope that I'm not having a 'senior moment'!

If, for example, that the aircraft is at 20000 feet and the cabin is at, say, 2000 feet, and, that the aircraft is descending at 2000 fpm it will take ten-minutes for the aircraft to reach zero feet (assuming a sea-level runway and 1013). If the cabin RoD is set at 200 fpm both aircraft and aeroplane will arrive at sea-level at the same time i.e. zero diff pressure; just what the good doctor ordered.

If you now increase the aircraft RoD to, say, 4000 fpm, the aircraft will now take only five-minutes to reach sea-level; however, the cabin is still descending at 200 fpm and, during that five-minute period, will have descended by only 1000 feet i.e. the cabin alt will be 1000 feet... or at least that's where it wants to be; however, zero diff pressure would have been achieved much earlier than the landing.

Please correct me if I'm wrong!

[BOAC]

TCF - the last post is correct.

we agree that diff pressure is the diff between inside and outside?

Positive at altitude -agreed?

Now descend, leaving the cabin at cruise altitude for the sake of argument - let's say 7000' and let's propose an airtight cabin with no 'valves' open?

As you pass 7000' what is the 'diff'? Answer gone from positive to zero. So "if you haven't increased the cabin rate of descent then surely the cabin differential pressure will decrease;" is correct.

"thus, you'll depressurise earlier than planned. ." is incorrect.

Now descend further what happens to the diff? It gets more negative. Outside pressure is greater than cabin.

What will happen is that the cabin diff will become more negative until the inward relief valve opens - at whatever setting the designer has chosen - then "you'll repressurise faster than planned."

"thus, you'll depressurise earlier than planned." was the mistake. It was just terminology, I'm sure - you meant, I think, "you will reach zero diff earlier than planned". You will in fact be 'repressurising' the cabin which needs to go from 7000' to sea level.

[TheChitterneFlyer]

What makes me confused is that sevral commanders have the oppinion that bleed air flow should be MAX until desired diff press is obtained.

Indeed, that's correct. This will then provide you with sufficient air mass to maintain a cabin descent rate. What your Commander is trying to establish is a cabin altitude that's similar to the airfield elevation prior to selecting air flow to MIN.

I'm not familiar with the DHC 8, but if the Pressure Controller doesn't have a 'barometric pressure setting capability' i.e. that it's referenced to 1013.2 mb, then unless you do some mental arithmetic to take into consideration the 'actual' QNH then you should set the desired cabin altitude to airfield elevation plus 500 feet. The reason for this quite simple; if the QNH (on the day) is very low and that you have set airfield elevation into the Pressure Controller Cabin Altitude Selector you WILL land with differential pressure remaining.

This might well be the reason why you 'sometimes' land with Diff Pressure remaining! Therefore, 'compare' QNH to 1013 mbs i.e. if the QNH is lower than 1013 (32 feet per millibar); add that difference to the airfield elevation and set it on the Pressure Controller.

If that's too confusing (or you're busy with other things)... don't do it; just set the Pressure Controller to airfield elevation plus 500 feet.

I hope this helps.

TCF

[TheChitterneFlyer]

"thus, you'll depressurise earlier than planned." was the mistake. It was just terminology, I'm sure - you meant, I think, "you will reach zero diff earlier than planned". You will in fact be 'repressurising' the cabin which needs to go from 7000' to sea level.

Yes, I agree; however, as rudderrudderrat mentions, the cabin will now descend at the same rate as the aeroplane. Or to be exact, slightly less, and that you'll have some slight negative diff pressure... near as dammit depressurised; with the exception of course that it's the outside air that's attemting to repressurise the cabin! Semantics dear chap.

I feel sure that if we were within the same room having this very same conversation that we'd all agree.

[rudderrudderrat]

Hi BOAC,

"thus, you'll depressurise earlier than planned." was the mistake. It was just terminology, I'm sure

I'm sure no one else dreamed TCF was implying we'd depressurise to the vacuum pressure of space. I also understood that we were refering to cabin diff pressure, as mentioned in the original question.

[Old Fella]

Cabin Pressurisation

Guys, it is all pretty simple. Normal practice would be to set the CABIN ALTITUDE SELECTOR to destination airfield elevation at TOD, and RATE at desired RATE OF CHANGE. Usually for pax comfort, the RATE OF CHANGE OF CABIN ALTITUDE ON DESCENT, would be a maximum of 300 FPM based on normal rate of descent of the aircraft. So if the cabin altitude at cruise was, say, 3000' the normal rate of descent of the aircraft would need to be no more than would allow at least 10 mins for the cabin to come down from 3000' to Zero. ANY GREATER RATE OF DESCENT OF THE AIRCRAFT WILL RESULT IN THE CABIN ALTITUDE BECOMING THE SAME AS AIRCRAFT ALTITUDE BEFORE THE AIRCRAFT HAS COMPLETED THE DESCENT AND THUS THE AIRCRAFT ALTITUDE AND THE CABIN ALTITUDE WILL DESCEND AT THE SAME RATE FROM THAT POINT. Not exactly rocket science.

[TheChitterneFlyer]

Yeah, I too thought that it wasn't rocket science; however, in typical PPrune fashion there's always someone who wants to complicate the issue.

tg743 must now be well confused?