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
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.