Airliner pressurisation limits

[Jetstream Rider]

Had a small pressurisation issue today - the differential was slightly above the limit in the flying manual - we descended and it came within limits again so carried on, although reported it in the tech log.

It had me wondering what sort of safety factor airliners have? Our limits are 8.6 PSI +/- 0.1 and we were at 8.8 PSI. Now I know we were nowhere near breaking apart, but how far from the ultimate limit were we?

I'm thinking that the limit is set for fatigue life in the main and a few overpressures won't make us pop, there's the relief valves too - but it had me pondering.

Thanks for replies in advance.

[Mad (Flt) Scientist]

§ 25.365 Pressurized compartment loads.

For airplanes with one or more pressurized compartments the following apply:

(a) The airplane structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting.

......

(d) The airplane structure must be designed to be able to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1.33 for airplanes to be approved for operation to 45,000 feet or by a factor of 1.67 for airplanes to be approved for operation above 45,000 feet, omitting other loads.

So your minimum margin is either "flight loads plus nominal pressurisation" or a factor (usually 1.33) times nominal pressurisation.

[Lutefisk989]

So your minimum margin is either "flight loads plus nominal pressurisation" or a factor (usually 1.33) times nominal pressurisation.

Am curious: is it really either of these two conditions, or is it the worst case of the two? Seems like they would have to address both?

[NutLoose]

I’ve always done it this way!

This is an example of what happens when we do not pay attention to detail, and do not follow instructions and checklists! [/SIZE]
A KC-135 Aircraft was being pressurized at ground level. The outflow valves which are used to regulate the pressure of the aircraft were capped off during a 5 year overhaul and never opened back up. The post-investigation revealed: that a civilian depot technician who, "had always done it that way," was using a homemade gauge, and no procedure.[/SIZE]
The technician's gauge didn't even have a max "peg" for the needle and so it was no surprise he missed it when the needle went around the gauge the first time. As the technician continued to pressurize the aircraft, and as the needle was on its second trip around the gauge the aircraft went "boom" - the rear hatch was blown over 70 yards away, behind a blast fence!
An incident like this is never funny and is further regrettable when we consider that this mistake is one that we (the taxpayers) will end up paying for. Fortunately, no one was reported as being injured.[/SIZE]
This was a good "Lessons Learned" for making sure we have trained people, who have the right tools, and who are following detailed procedures. And it should serve as a reminder that just because you’ve always done it that way, it does not make it the “right” way![/SIZE]

The powerpoint presentation with excellent photos of the result is here.........[/SIZE]

http://siri.uvm.edu/ppt/humor/pressure/pressure.ppt.

(Edited by moderator - only to reduce font size, (no need to shout!).

[Bullethead]

G'day JR,

Aircraft pressurisation systems usually have two pressure limits, one that the system should control to automatically and a higher one that the pressure relief valve should blow at.


Exceeding the max diff, with the system controlling the pressure, by a small amount shouldn't harm the aircraft structure. The pressure relief valve is there for when things really go crazy and once blown open will probably stay that way until reset on the ground.

Does your aircraft have a manual mode to control cabin pressure?

Regards,
BH.

[Mad (Flt) Scientist]

Am curious: is it really either of these two conditions, or is it the worst case of the two? Seems like they would have to address both?

Yes, that's what I meant by "either"; careless choice of words.

(Reality is probably slightly better than that, since engineers love margins)

And, regarding the KC-135 story. I understand there's a similar story pertaining to MRA4, albeit on a smaller scale.

[Jetstream Rider]

Thanks all for the replies.

Bullethead - I was flying a B757 - yes we do have a manual mode. Yesterday we were loathe to use it as it has the worlds most sensitive control and you can easily make the rate go from 0 to 1500ft/min in a blink of the eye. After all we were only 0.1 PSI out and inadvertently turning the knob the wrong way could have made an issue into an incident. Our pressure relief valves are doors which blow out and need replacing on the ground - I think they just pop off, but having never seen them in operation I'm unsure. We have two - so there must be a margin there too!

[Bullethead]

Hey JR,

You must've been bored mate! How on earth, or above earth for that matter, did you realise the diff was 0.1psi over the limit?

Did you go to sleep in the seat and wake up looking at the guage?
Regards,

BH.

P.S I drive a B767, it's probably got the same control system. It's got the same limit.

[Jetstream Rider]

The right re-circ fan was u/s, so the pack was operating in high flow. I heard a fairly large fluctuation in the aircon, so I looked at the overhead panel and the needle was further into the yellow than normal. Took a closer look and it was outside our flying manual limits, and still climbing. We descended 2000ft and it came within limits. No drama, wouldn't have worried if it was within limits but thought it should be investigated since our limit is listed as +/- 0.1 and it was 0.1 above that.

I drive both 75 and 76 - the press controls are certainly identical in all the aspects I can see.

[Bullethead]

Fair enough, if you get a large pressurisation bump it'd make you look.

Which do you prefer, the B767 orthe B757?

Regards,
BH.

[Milt]

Overpressures

Way back in the mid 50s three TPs, including Geof Fletcher and Peter Baker, flight tested Comet 2Cs out of Boscombe Down. With some trepidation I sat in one with engines running and a 'doctored' cabin discharge valve until we reached a limiting overpressure of 9 psi. Was not conscious of any effects on myself.

Then there was the occasion that I had a VIP as passenger in a RAAF Mk 20 Canberra. Didn't notice that the VIP had hung his jacket over the cabin discharge valve in the rear bulkhead. Missed scanning the cabin pressure in time before the door seal blew. Jacket was undamaged. Overpressure unknown.

Lesson leaned - the body cannot reliably sense an overpressure that may be enough to burst the cabin. Does your aircraft have any overpressure warning?

[Jetstream Rider]

I can't find an overpressure warning in the QRH, so I think the valves just blow and then you deal with the resulting "cabin pressure" message. It does mean you are reliant on the overpressure valves working though - perhaps that is why there are two?

The 767 has a "whoosh" noise when you retard the thrust levers at TOD as the packs go into high flow - it was a similar noise I heard in the cruise yesterday that made me look.

Bullethead - I love 'em both, but the 767 has the edge I think. Better control response in roll and its just got a bit more space and stability and flexibility. Having said that, I love a light 757, pointing it at the sky and just climbing away. Then again doing improved climb in a 767 at max weight and rotating at 175 knots is good fun too! They are both very capable aircraft but the 767 just feels better. The 75 is a bit more agricultural if that's not too harsh a word to use, and the 767 seems to smell less - horrid toilets and oily aircon in the 75.

[Tightflester]

Mad (Flt) Scientist

Be interested if you could elabourate on the MR4 incident please. Was there a ground over-pressurisation I haven't heard anything about one... yet!

[typhoid]

Mad, I would be interested to find out about the MRA4 issue. To my recollection nothing like that has happened on the ground. We did have a little fun in the air repressurising but that was more of an issue for the ears, than the a/c!

[Mad (Flt) Scientist]

re MRA4. I understand that a ground test of a single system and a mix-up in gauges (or target pressure) resulted in that system being overpressurised (rather than the whole fuselage). Can't recall which system it was - might have been ECS, might have been bleed ducts for WAI (if it has one) - I did hear second/third hand.

[typhoid]

Ah! Now the bells are ringing..