PPRuNe Forums - B757 depressurises during descent

We had an incident here last week that may be of interest to you all.
A Pax B757 descending from cruise altitude. Was at 5000ft descending at 2000ft/min when the pilots noticed a pressurisation surge with their ears. Later the cabin crew reported a bang under the fwd cabin at the same time. The pilots looked up at the pressurisation panel and noticed the outflow valve fully open and the cabin altitude at aircraft altitude. There were no EICAS or STATUS messages.
I was called out to the aircraft to investigate. No messages, and no auto event. Bite check carried out on both pressurisation computors and no faults. Outflow valve inspected and exercised and all OK. Then the head scratching started. I had decided to do a ground pressurisation run, when I finally noticed that on the pressurisation panel the Cabin Alt Auto Rate knob was set to the min position. This is normally set to the little triangle which gives 400ft/min rate of change of cabin altitude. At the min posn it gives 200ft/min ROC.
So what had happened was that the pressurisation system was trying to reduce cabin pressure by 200ft/min. To do this the outflow valve had been sent to full open, and as the aircraft desended through 5000ft the aircraft altitude overtook the cabin altitude, and the inwards safety negative pressure relief valves had opened. (On a B757 these are in the fwd freight, just fwd of the cargo door) This was the bang that the cabin crew had heard.
So we reset the ROC knob to normal and off they went.
I asked the pilots if they ever used that knob, and the answer was NO. (So why is it there?)
They will make sure they check it's posn in the future, and the reason I wrote all this is that I hope you all do as well.