Originally Posted by
Derfred
So what did you do next and how did the the pressurisation system respond? I'd be interested to know.
Also, did you ever get any feedback from maintenance as to what caused it?
We did a quick check of the FCOM, which indicated that the AUTO FAIL light should have been on at this stage, but it wasn't.
We then decided to do the "Unscheduled Pressurization Change" NNC, but before we had time to do it we had a rapid depressurization when both Pressure Relief Valves suddenly opened with a loud bang in the tail that all passengers and crew clearly heard. There was a condensation cloud, the cabin became instantly cold and the masks dropped inside of about 30 seconds.
We initiated the Emergency Descent but then recovered pressurization around FL330 when the spring loaded Pressure Relief Valves closed.
This from maintenance:
"Boeing 737NG-FTD-21-10004; Digital Cabin Pressurization fault code 90, Cab Pressure switch activated. Operators have reported pressurisation issues on several airplanes due to improper activation of cabin pressure switch located in outflow valve with no flight deck effect"
Did anyone know that the two Automatic Cabin Pressure Controllers each have an independent Cabin Pressure Switch which is calibrated to automatically close the Outflow Valve when the Cabin Altitude is 14,500 or above and does so without triggering any Master Caution or Light in the Flight deck unless a certain modification is installed ? When FC90 is triggered, the Cabin Pressure Switch sends a "close" order to the Outflow valve which will stay fully closed until pilot action is taken. If action is taken quickly enough......