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HAWK21M
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Relief Valves Practical Use

Fri Oct 20, 2006 11:12 pm

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.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
avt007
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 2:41 am

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
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 2:55 am

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.
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Tristarsteve
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 1:19 pm

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.
 
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HAWK21M
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 7:52 pm

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.

Quoting TristarSteve (Reply 3):
So we reset the knob to normal (400fpm) and off they went.

Any Structural checks carried out thereafter.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Tristarsteve
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 7:59 pm

Quoting HAWK21M (Reply 4):
Are you saying Cabin was Depressurising at 2000fpm & Aircraft 200fpm

No the aircraft was descending at 2000fpm, and the cabin altitude was descending at 200fpm, instead of 400fpm which is normal.

Quoting HAWK21M (Reply 4):
Any Structural checks carried out thereafter

No, the relief valves operated as advertised to save the fuselage from negative pressure.
 
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HAWK21M
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RE: Relief Valves Practical Use

Sat Oct 21, 2006 8:05 pm

Quoting TristarSteve (Reply 5):
No, the relief valves operated as advertised to save the fuselage from negative pressure

True.But Exposure to Negative pressure of -0.1 to -0.3psi can cause damage to the Fuselage structure as the Aircraft is built to hold Positive pressure.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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