Quote:
Originally Posted by CamasM3e93
If that plug was loose in the fligts leading up to the incident seems quite possible that the plane was losing pressurized air through it and triggering the alarm. As the plug went through pressurization cycles it vibrated and further loosened over time and then the event. This is a hypothesis based on the information that is public.
Subsequent inspections have found loose bolts securing the plug on other aircraft. To say the failed plug was 100% unrelated to the preceding pressurization alarms at this point probably not possible.
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Of course, I can't say with 100% certainty. But I'm still unconvinced that they're directly related-- they may be separate lines of incidence.
What I meant in my previous posts that the plug was NOT giving a Door Open light (and is in fact, incapable of giving a light), but that the light the NTSB references was the auto-press controller indicating a failure in the pressurization system. Those two systems are not directly interrelated in that the door open/closed light system has no direct interaction with the auto-press controller-- i.e. the systems are not hooked up together.
There are two cabin sensor ports that input into the pressurization controller, but those are different than the door open/closed lights.
There are a couple of things that make me go "hmmmm", though.
First is that you already have an outflow valve that's opening and closing to meter cabin pressurization. If there was a leak around the plug, the outflow valve would simply adjust its movement since it's already compensating for air exhausted through toilet and galley vents, miscellaneous fixed vents, and by seal leakage. In other words, if the plug was leaking like a door seal (which is usually noisy and irritating) it would probably be compensated for by the outflow valve metering the cabin altitude.
Also, the auto-press light (technically, Auto-Fail) illuminates for loss of DC power, a controller fault, an outflow valve control fault, excessive differential pressure (> 8.75 PSI), excessive rate of cabin pressure change (±2000 sea level feet/minute), high cabin altitude (above 15,800 feet), or if the controller is not responding properly.
Any of these conditions will push the system to automatically swap to the secondary controller (which is identical to the first.) Additionally, the auto-press light will stay on with a Master Caution- which is a big hint for the crew to crack open the checklists and take corrective action.
The checklist is pretty directive as to what to do (think: If/Then chase chart)-- if the plug door was affecting cabin altitude enough to force a controller swap to the Alternate mode (i.e. excessive differential pressure or rate of cabin pressure change or high cabin altitude), that would show up on the cabin altitude gauge or the cabin rate gauge and would direct specific actions via the checklist.
Those actions would be to go into Manual Mode on the pressurization controller in an attempt to control cabin altitude yourself (you essentially force the outflow valve full closed to control cabin altitude). To date, I haven't seen anything that indicates the crew had anything other than an auto-press controller fail in the primary mode (i.e. they ran it in Alternate)-- if they were driven by the checklist to operate in Manual mode, the cabin pressure controller would have been replaced at a minimum, based on troubleshooting/fault codes.
Although the two things may end up being related, so far it looks to me that there's a very real probability that they were two separate things-- there was an issue with the cabin pressurization controller and the plug gave up the ghost due to faulty installation. That being said, I reserve the right to revise my thought processes based on new information coming to light.
R.