Narrative:

Approximately one hour into the flight at cruising altitude (FL320); I happened to notice that the 'cabin rate' indicator was descending at 500 FPM. I then looked at the cabin altitude indicator; which indicated 8000 ft. I then noted the differential pressure at 7 psi. The cabin altitude returned slowly to approximately 6000 ft (7.8 inches differential pressure). Referring to abnormal checklist; there were no items to accomplish as the cabin was now normal. No warning lights illuminated at any time. The remainder of the flight was uneventful. A detailed maintenance write up was entered in the logbook. I was to find out later that this airplane had two previous write ups for pressurization problems and that the airplane went on to have three more pressurization problems after mine. I also learned that several of these events involved the activation of the cabin altitude warning horn. This particular aircraft underwent numerous maintenance 'fixes' until the automatic pressurization controller was changed. Ever since the air carrier Y accident and the string of pressurization problems my air carrier experienced; I have been talking to other pilots and thinking about our training; current warning systems and the 'recognition' problems pilots face with a high altitude cabin. Given the development and increasing sophistication of aircraft systems on the B737 and its variants; it is noteworthy that the cabin altitude warning system (an intermittent aural horn) has remained virtually unchanged for nearly 50 years and shares similarities with the B727. Why this system has not been tied into the master caution warning system escapes me. In sharing my thoughts with other pilots; it is clear to me that pilots have had 'recognition' problems with this singular warning during certain events. Our 'traditional' simulator training usually includes a rapid or explosive decompression; followed by a 'master caution air cond' and an amber 'automatic fail' light on the air conditioning panel. Shortly thereafter; the cabin altitude warning horn sounds. This sequence will not occur during a slow loss of cabin pressure caused by improper bleed or pack configuration; air-ground safety sensor failure or certain failures of the automatic pressurization system that do not trigger 'automatic fail' warning lights. Pilots now have to 'interpret' what the horn means. In numerous instances that I have heard of; the pilot response is; 'why is the takeoff warning horn sounding?' interestingly; on the ground; our training response to this horn is 'do not takeoff' or more simply; 'do nothing' which is similar to what some pilot responses are in-flight. Some of these pilots have recently made the transition from other aircraft types that have 'voice' warnings; further complicating recognition of a high cabin altitude. Lastly; in instances where the insidious onset of hypoxia may have been a factor; a singular aural warning may be nearly impossible for a confused hypoxic pilot to properly interpret and respond with the proper procedure (oxygen masks - on/100 percent). I would also like to note that high cabin altitudes as a result of slow pressure changes will not result in a 'changeover' from the automatic to stby pressurization mode until the cabin altitude is approximately 13875 ft (classic models) and 15800 ft (next gen) models. Recommendation: upgrade the warning system to include a new rectangular amber warning light ('cabin altitude') in close proximity to the 'horn silence' button on the overhead panel and the cabin pressurization indicators. The warning light should illuminate at 9000 to 9500 ft cabin altitude (perhaps even lower) and activate the 'master caution air-cond' system lights. These lights should illuminate shortly before activation of the cabin altitude warning horn. Recommended activation altitudes are based on the highest programmed cabin altitudes (8000 ft at FL410) for ng aircraft and give sufficient clear and unambiguous visual warning of an impending condition and create a 'flow' for the pilots in correcting the condition. Positioning of the warning light directs pilots to the indicators. This would prompt the pilots to the appropriate checklist (automatic fail/unscheduled pressurization change) and a switch to stby mode. Should the horn sound; the pilots will have already been thinking of getting their masks on. Additionally; once their masks are on; the light will also prompt a silencing of the horn (due to its proximity position) so that the pilots can 'establish communication' without undue distraction. Emergency action checklist procedures can now be reviewed in a controled manner with pilots on oxygen and communicating clearly. Others may have an even better suggestion than the one I have provided; but this seems to be a reasonable; relatively inexpensive and simple modification.

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Original NASA ASRS Text

Title: A B737-400 FLT CREW EXPERIENCED A SLOW UNSCHEDULED CABIN PRESSURIZATION CHANGE WITHOUT A WARNING HORN. AN EMER WAS NOT DECLARED.

Narrative: APPROX ONE HOUR INTO THE FLT AT CRUISING ALT (FL320); I HAPPENED TO NOTICE THAT THE 'CABIN RATE' INDICATOR WAS DSNDING AT 500 FPM. I THEN LOOKED AT THE CABIN ALT INDICATOR; WHICH INDICATED 8000 FT. I THEN NOTED THE DIFFERENTIAL PRESSURE AT 7 PSI. THE CABIN ALT RETURNED SLOWLY TO APPROX 6000 FT (7.8 INCHES DIFFERENTIAL PRESSURE). REFERRING TO ABNORMAL CHKLIST; THERE WERE NO ITEMS TO ACCOMPLISH AS THE CABIN WAS NOW NORMAL. NO WARNING LIGHTS ILLUMINATED AT ANY TIME. THE REMAINDER OF THE FLT WAS UNEVENTFUL. A DETAILED MAINT WRITE UP WAS ENTERED IN THE LOGBOOK. I WAS TO FIND OUT LATER THAT THIS AIRPLANE HAD TWO PREVIOUS WRITE UPS FOR PRESSURIZATION PROBLEMS AND THAT THE AIRPLANE WENT ON TO HAVE THREE MORE PRESSURIZATION PROBLEMS AFTER MINE. I ALSO LEARNED THAT SEVERAL OF THESE EVENTS INVOLVED THE ACTIVATION OF THE CABIN ALT WARNING HORN. THIS PARTICULAR ACFT UNDERWENT NUMEROUS MAINT 'FIXES' UNTIL THE AUTO PRESSURIZATION CTLR WAS CHANGED. EVER SINCE THE ACR Y ACCIDENT AND THE STRING OF PRESSURIZATION PROBLEMS MY ACR EXPERIENCED; I HAVE BEEN TALKING TO OTHER PLTS AND THINKING ABOUT OUR TRAINING; CURRENT WARNING SYSTEMS AND THE 'RECOGNITION' PROBLEMS PLTS FACE WITH A HIGH ALT CABIN. GIVEN THE DEVELOPMENT AND INCREASING SOPHISTICATION OF ACFT SYSTEMS ON THE B737 AND ITS VARIANTS; IT IS NOTEWORTHY THAT THE CABIN ALT WARNING SYSTEM (AN INTERMITTENT AURAL HORN) HAS REMAINED VIRTUALLY UNCHANGED FOR NEARLY 50 YEARS AND SHARES SIMILARITIES WITH THE B727. WHY THIS SYSTEM HAS NOT BEEN TIED INTO THE MASTER CAUTION WARNING SYSTEM ESCAPES ME. IN SHARING MY THOUGHTS WITH OTHER PLTS; IT IS CLR TO ME THAT PLTS HAVE HAD 'RECOGNITION' PROBLEMS WITH THIS SINGULAR WARNING DURING CERTAIN EVENTS. OUR 'TRADITIONAL' SIMULATOR TRAINING USUALLY INCLUDES A RAPID OR EXPLOSIVE DECOMPRESSION; FOLLOWED BY A 'MASTER CAUTION AIR COND' AND AN AMBER 'AUTO FAIL' LIGHT ON THE AIR CONDITIONING PANEL. SHORTLY THEREAFTER; THE CABIN ALT WARNING HORN SOUNDS. THIS SEQUENCE WILL NOT OCCUR DURING A SLOW LOSS OF CABIN PRESSURE CAUSED BY IMPROPER BLEED OR PACK CONFIGURATION; AIR-GND SAFETY SENSOR FAILURE OR CERTAIN FAILURES OF THE AUTO PRESSURIZATION SYSTEM THAT DO NOT TRIGGER 'AUTO FAIL' WARNING LIGHTS. PLTS NOW HAVE TO 'INTERPRET' WHAT THE HORN MEANS. IN NUMEROUS INSTANCES THAT I HAVE HEARD OF; THE PLT RESPONSE IS; 'WHY IS THE TKOF WARNING HORN SOUNDING?' INTERESTINGLY; ON THE GND; OUR TRAINING RESPONSE TO THIS HORN IS 'DO NOT TKOF' OR MORE SIMPLY; 'DO NOTHING' WHICH IS SIMILAR TO WHAT SOME PLT RESPONSES ARE INFLT. SOME OF THESE PLTS HAVE RECENTLY MADE THE TRANSITION FROM OTHER ACFT TYPES THAT HAVE 'VOICE' WARNINGS; FURTHER COMPLICATING RECOGNITION OF A HIGH CABIN ALT. LASTLY; IN INSTANCES WHERE THE INSIDIOUS ONSET OF HYPOXIA MAY HAVE BEEN A FACTOR; A SINGULAR AURAL WARNING MAY BE NEARLY IMPOSSIBLE FOR A CONFUSED HYPOXIC PLT TO PROPERLY INTERPRET AND RESPOND WITH THE PROPER PROC (OXYGEN MASKS - ON/100 PERCENT). I WOULD ALSO LIKE TO NOTE THAT HIGH CABIN ALTS AS A RESULT OF SLOW PRESSURE CHANGES WILL NOT RESULT IN A 'CHANGEOVER' FROM THE AUTO TO STBY PRESSURIZATION MODE UNTIL THE CABIN ALT IS APPROX 13875 FT (CLASSIC MODELS) AND 15800 FT (NEXT GEN) MODELS. RECOMMENDATION: UPGRADE THE WARNING SYSTEM TO INCLUDE A NEW RECTANGULAR AMBER WARNING LIGHT ('CABIN ALT') IN CLOSE PROX TO THE 'HORN SILENCE' BUTTON ON THE OVERHEAD PANEL AND THE CABIN PRESSURIZATION INDICATORS. THE WARNING LIGHT SHOULD ILLUMINATE AT 9000 TO 9500 FT CABIN ALT (PERHAPS EVEN LOWER) AND ACTIVATE THE 'MASTER CAUTION AIR-COND' SYSTEM LIGHTS. THESE LIGHTS SHOULD ILLUMINATE SHORTLY BEFORE ACTIVATION OF THE CABIN ALT WARNING HORN. RECOMMENDED ACTIVATION ALTS ARE BASED ON THE HIGHEST PROGRAMMED CABIN ALTS (8000 FT AT FL410) FOR NG ACFT AND GIVE SUFFICIENT CLEAR AND UNAMBIGUOUS VISUAL WARNING OF AN IMPENDING CONDITION AND CREATE A 'FLOW' FOR THE PLTS IN CORRECTING THE CONDITION. POSITIONING OF THE WARNING LIGHT DIRECTS PLTS TO THE INDICATORS. THIS WOULD PROMPT THE PLTS TO THE APPROPRIATE CHKLIST (AUTO FAIL/UNSCHEDULED PRESSURIZATION CHANGE) AND A SWITCH TO STBY MODE. SHOULD THE HORN SOUND; THE PLTS WILL HAVE ALREADY BEEN THINKING OF GETTING THEIR MASKS ON. ADDITIONALLY; ONCE THEIR MASKS ARE ON; THE LIGHT WILL ALSO PROMPT A SILENCING OF THE HORN (DUE TO ITS PROX POSITION) SO THAT THE PLTS CAN 'ESTABLISH COM' WITHOUT UNDUE DISTRACTION. EMER ACTION CHKLIST PROCS CAN NOW BE REVIEWED IN A CTLED MANNER WITH PLTS ON OXYGEN AND COMMUNICATING CLEARLY. OTHERS MAY HAVE AN EVEN BETTER SUGGESTION THAN THE ONE I HAVE PROVIDED; BUT THIS SEEMS TO BE A REASONABLE; RELATIVELY INEXPENSIVE AND SIMPLE MODIFICATION.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.