Slight over pressurization before landing

[737Andi]

Hi,

I have read that passenger jets (e.g. A320 or 737) are landing with a slight over-pressurization, which needs to be removed before opening the doors.
Can anyone please explain the reason in simple words?

I think I do understand why planes are taking off with a slight over-pressurization.
During takeoff the cabin looses air very rapidly via the outlet valve and the engines are not delivering enough bleed air as a fast replacement so that the pressure drop in the cabin is quite fast.
When starting a little over-pressurized the passengers will feel more comfortable.

But as mentioned above I can not explain the reason for the over-pressurization for the landing :(

Thanks!

Best regards Andi

[eckhard]

Hi Andi,
Both the take-off and landing case are to do with passenger comfort. Some aircraft pressurise a bit after engine start, others do it during the take-off run. I don’t think there’s a problem with the engines’ supply of bleed air, nor with the rate of cabin pressure loss as you describe. It just means that the pressure controller has smoother control, as it’s starting from a known, small differential pressure. After landing, the cabin is slowly depressurised. This avoids pressure “bumps” that could be caused by the landing gear squat switches, for example. Some aircraft try to reach a zero differential pressure by about 200ft before landing, which has a similar beneficial effect on comfort.

[737Andi]

It just means that the pressure controller has smoother control, as it’s starting from a known, small differential pressure.

Ok...is there a technical reason why the pressure controller is working smoother in over pressurized condition? To me that makes unfortunately no sense :(

[Meikleour]

To be fully depressurised the outflow valves would have to be fully open. If the cabin is slightly pressurised then the valves are starting from a known partially closed position and from there they modulate. Does that make sense to you?

[eckhard]

Hi again Andi,
Thanks for your question.

I mean that the initial pressurisation is started from a static, constant environmental pressure, as the aircraft is on the ground. The outflow valves can close at a slow rate and the differential can increase slowly, to a small value. If the aircraft was airborne and climbing when this happened, the external pressure would be reducing and the rate of differential increase (cabin rate of climb) would be more difficult to control.

Here are some extracts from the B787 FCOM:

“For takeoff, the system supplies a small positive pressurization prior to rotation to cause a smooth cabin altitude transition to the cabin altitude climb schedule.“

“During descent, cabin altitude decreases to slightly below the FMC planned landing altitude. This ensures that the airplane lands pressurized. Landing altitude barometric pressure correction comes from the captain’s altimeter setting.
At touchdown, both outflow valves open to depressurize the cabin.”

I guess I didn’t explain it very well.

[Sidestick_n_Rudder]

IIRC the slight over-pressurization has to do with change of airflow around the fuselage during rotation, which may result in an uncomfortable pressure bump for pax. To counter this, the cabin is pumped to a slightly higher pressure.

[hoss183]

If the valves were fully open (take off or landing) even a say 200ft change in a few seconds would cause discomfort, hence having a pressure slightly above ambient allows the system to control it and be stable & smooth.

[capngrog]

Way back on November 20, 2000 (20/11/2000), a fatal accident occurred when the automatic depressurization system failed after an Airbus A300 emergency landing at Miami International Airport (KMIA). In short summary, outflow valves were blocked by improperly installed insulation mats which had shifted in flight. The blocked outflow valves caused cascading problems, including improper pressurization and multiple fire warning alarms. Prior to the ensuing emergency landing, the flight crew failed to follow depressurization checklists which ultimately resulted in the cabin crew attempting to open cabin doors of a pressurized aircraft. As the aircraft was stopped on a taxiway, numerous fire alarms caused the captain to issue an evacuation order; however, the cabin crew were initially unable to open the doors for evacuation. A male flight attendant was eventually able to crack open the L1 door which caused an explosive decompression of the cabin and ejection of the male flight attendant onto the concrete 20 feet below.

For details on this somewhat unusual accident, here's a link to the NTSB Report:

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20001212X22314&AKey=1&RType=Final&IType=FA

Regards,
Grog

[eckhard]

Thanks Capngrog, for that interesting report.
And of course there’s always the dreadful Saudia TriStar accident at Riyadh, where the aircraft remained pressurised, engines running, for quite a while after the emergency landing. I think the doors were impossible to open, due to the diff pressure. No squat switches on that one, it seems.

[orion1210]

Pre pressurisation ensures the pressurisation system is already in full control of the cabin pressure prior to commencing the climb ultimately for comfort. As others have said, this is because the outflow valves have already modulated to a position, where there is less outflow than inflow, and therefore can act accordingly in an instant to any changes to the inflow or altitude.

All the aircraft I’m rated on must land unpressurised which I believe is to enable rapid exit in an emergency because the emergency exits are plug type doors. In reality however there is always a slight positive cabin pressure due to a slight resistance to the outflow of cabin conditioned bleed air but this almost instantly depletes when the conditioned air ceases.

[megan]

IIRC the slight over-pressurization has to do with change of airflow around the fuselage during rotationThat I don't know enough to comment, but with open valves the speed of the aircraft will reduce cabin pressure below the outside static, to be seen in unpressurised aircraft with the static source using cabin pressure as when static line behind instrument panel becomes disconnected. Acceleration will show a climb on the VSI (cabin pressure reducing) and de-acceleration a descent (pressure increasing). Had it occur on one occasion.

[320busboy]

If the valves were fully open (take off or landing) even a say 200ft change in a few seconds would cause discomfort, hence having a pressure slightly above ambient allows the system to control it and be stable & smooth.

Packs off takeoff....Packs on at thrust reduction. Ever noticed the “bump” ? I haven’t. Not saying it doesn’t. Just saying I haven’t noticed it. Maybe those with better senses than me can notice it.

From an engineering perspective, a coke can which is pressurised is much stronger than one that isn’t. This makes a bit of sense to me. Also better to know about a pressurisation problem at thrust set than climb.

just some additional thoughts.