HAWK21M From India, joined Jan 2001, 31808 posts, RR: 55 Posted (8 years 11 months 3 weeks 4 days 15 hours ago) and read 3178 times:
Understandably there could be occasions where Pressure Relief valves have functioned if the CPC do not mantain the required Delta P or in case of Rapid Descent.
But have there been occasions of Vaccum relief valves or Negative pressure relief valves functioning.-0.1 to -0.3 psi.
What are the structural Implications.
Avt007 From Canada, joined Jul 2000, 2132 posts, RR: 4
Reply 1, posted (8 years 11 months 3 weeks 4 days 12 hours ago) and read 3146 times:
Any outflow valves I've seen also relieve "negative" pressure. Obviously the pressure vessel is strictly designed to handle internal pressure, not external. And it's equally necessary to allow for the possibility of the cabin pressure being below ambient, althought I don't know of any specific cases. But if an aircraft was at altitude, and for whatever reason was unpressurized, then subsequently the outflow valves were closed and the aircraft descended, then that scenario could arise.
Troubleshooter From Germany, joined Feb 2005, 423 posts, RR: 4
Reply 2, posted (8 years 11 months 3 weeks 4 days 12 hours ago) and read 3141 times:
On our A320´s we have sometimes the message "CAB PRESS LO DIFF PRESS" on the postflight report, which can happen during a (normal) rapid descent. In this case the cabin pressure could not follow its scheduled pressure fast enough for a short time.
TristarSteve From Sweden, joined Nov 2005, 4231 posts, RR: 32
Reply 3, posted (8 years 11 months 3 weeks 4 days 1 hour ago) and read 3083 times:
The B757 has Inwards relief valves in the fwd cargo on the right hand side. I had these open on a normal pax flight last month. The aircraft was descending at 2000fpm when at 5000ft QNH there was a bang under the fwd cabin. The crew felt their ears pop, and noticed that the outflow valve was fully open, and the cabin alt was same as aircraft. They did not know what had happened. There were no EICAS messages, and CPC tested sat.
Aftera while, with much head scratching (also because the crew reported that the aircraft had depressurised) I found that the Cabin Alt Auto Rate knob on the overhead was set to the Min setting.
During the descent the CPC was trying to control the cabin descent to 200fpm. The aircraft was descending at 2000fpm and at 5000ft the aircraft overtook the cabin. This caused the Inwards relief valves to open, and the cabin pressurised back to ambient.
I have seen the overpressure relief valves open many times, due to climbing too fast, but this is the first time I have seen Inwards relief valve open.
So we reset the knob to normal (400fpm) and off they went.
HAWK21M From India, joined Jan 2001, 31808 posts, RR: 55
Reply 4, posted (8 years 11 months 3 weeks 3 days 19 hours ago) and read 3066 times:
Quoting TristarSteve (Reply 3): During the descent the CPC was trying to control the cabin descent to 200fpm. The aircraft was descending at 2000fpm and at 5000ft the aircraft overtook the cabin. This caused the Inwards relief valves to open, and the cabin pressurised back to ambient.
Are you saying Cabin was Depressurising at 2000fpm & Aircraft 200fpm.