Date: 201601
Location: ZZZ.ARTCC, US
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 129
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Design; Failed; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion; Human-Machine Interface; Time Pressure; Training / Qualification; Workload
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I just read a few articles regarding the fact that the B737Max was fitted with a maneuvering characteristics augmentation system (MCAS) that the pilots were unaware of; and that no mention was made of this feature in the B737's FCOM (Flight Crew Operations Manual). I had a similar experience on a Boeing 737-800 three years ago; that I would like to share with you. On January XX; 2016; I was [Captain of] a Boeing 737-800. While [flying] at FL370; the aircraft suddenly depressurized without giving us any Master Caution; aural or visual warning. What I later learned was that the outflow valve suddenly fully closed; causing the cabin altitude to reach its 9.1 PSI structural limit; at which time both overpressure valves suddenly opened; causing a rapid depressurization. We had to perform an emergency descent. The [airline] engineering department determined; with the support of a Boeing provided document (FLEET TEAM DIGEST: 737NG-FTD-21-10004); that the aircraft depressurized because of the faulty activation of a safety pressure switch. The issue I am raising here; if that the safety pressure switch which activated at the wrong time; causing the depressurization; was not; and as far as I know; is still not mentioned in the 737's FCOM. I first learned of the existence of this safety feature after the incident; while reading the document Boeing had provided to [airline] Maintenance; and which was labelled 'Confidential'; or 'Secret' or something to that effect. This is why when I read the articles about the 737 Max; I realized that maybe there was now at Boeing; a culture of installing additional safety devices that pilots need not know about. Without going into too much technical detail; the 737NG' pressurization system has two identical automatic cabin pressure controllers (CPC); one of which controls the cabin; the other which is a backup. They interchange at each aircraft cycle. The three position selector switch which controls the pressurization are labelled NORM; ALTN; and Manual. The aircraft is normally flown on NORM. Should a fault occur; the CPC can sometimes automatically switch from the NORM to the ALTN; at which time a master caution rings and an AUTOFAIL light illuminates. If the switchover does not occur automatically; the pilot; after a Master Caution and an Autofail light; can manually switch to the second CPC by selecting ALTN. Should that fail to recover the pressurization; the MANUAL position allows direct control of the outflow valve position. The aircraft is normally flown on NORM since the CPC associated with these switch positions NORM and ALTN swap positions at each cycle. If one of the CPC system fails; it is supposed to automatically switch to the other CPC; at which time a Master Caution warning light comes on with a chime; and an AUTO FAIL light comes on. If there is no auto switching; the crew can manually go to ALTN position; and should that fail; the crew can go to MANUAL control. All this is clearly explained in the FCOM and there are non-normal checklist and procedure that cover them. Boeing decided to add an additional safety feature; which for reasons that baffle me; is not listed in the FCOM; and that I knew nothing about until my incident. It kicks in should an aircraft depressurize and the pilots take no action; as occurred in [an accident in 2004]. In that accident; the pilots had inadvertently taken off with the Pressure Selector on Manual and an outflow valve set at an intermediate position. The pilots had failed to recognize that they were pressurizing slowly and when the visual and aural signal did come; they misinterpreted them and did nothing; until they lost consciousness. The added safety feature Boeing added; was; I imagine; installed to thwart any such reoccurrence. Each of the CPC is associated in the outflow valve with a cabin pressure switch calibrated at 14;500 feet of cabin pressure. Should the continuous cabin altitude warning go off at 10;500 feet; and the masks deploy at 14;000feet and still no action taken by the pilots; this pressure switch; calibrated at 14;500 feet of cabin pressure; overrides the commands of the CPC to the outflow valve and sends a close command; regardless of the Rotary Switch position. The idea behind this is that should we again have a situation where neither pilot took any action after depressurizing; this device would automatically close the outflow valve and restore cabin pressure; and hopefully bring the two unconscious pilots back to life. The idea is good. On my flight; the cabin pressure switch; which was calibrated to send a close signal to the outflow valve at 14;500 feet cabin altitude; faulted and activated at a much lower cabin altitude with no cockpit warning; save the cabin pressure differential gauge in the overhead panel which crept up to 9.1 PSI. When it reached the limit; the overpressure valves opened; as they were meant to do in such a case; and the cabin depressurized quickly. Boeing indicated to [company] that several 737NG operators had experienced the same problem before mine. The object of this report is not to talk about the depressurization or of the failure which caused it; but to highlight the fact that Boeing decided to install this additional safety feature without making any mention of it whatsoever in the FCOM. So obviously; pilots do no train for this occurrence; since they ignore its very existence; I only learned about it after my incident. The second thing I would like to highlight; is that in the aircraft in which I had the incident; the faulty activation of the cabin pressure switch did not set off a master caution or an AUTOFAIL light; although the Boeing document did say that there was a fix that would trigger a warning in case of faulty activation; and that was Nord Micro Bulletin CSB 21933-21-003. But it seems that the installation of that fix is optional. Our Flight Ops did not even know this device existed; so how could we decide to install an upgrade for something we knew nothing about.




Synopsis: 737 Captain reported concern that the aircraft manufacture is creating a culture of installing addition safety devices that pilots need not know about.