|37000 Feet||Browse and search NASA's
Aviation Safety Reporting System
|Local Time Of Day||0001 To 0600|
|Locale Reference||airport : zzz.airport|
|Altitude||msl single value : 34000|
|Controlling Facilities||artcc : zzz1.artcc|
|Operator||common carrier : air carrier|
|Make Model Name||B737-900|
|Operating Under FAR Part||Part 121|
|Flight Phase||cruise : level|
|Affiliation||company : air carrier|
|Function||flight crew : first officer|
|Experience||flight time last 90 days : 85|
flight time type : 85
|Affiliation||company : air carrier|
|Function||flight crew : captain|
oversight : pic
|Anomaly||aircraft equipment problem : critical|
non adherence : company policies
|Independent Detector||aircraft equipment other aircraft equipment : cabin alt warning horn|
|Resolutory Action||controller : issued new clearance|
flight crew : took evasive action
|Problem Areas||Flight Crew Human Performance|
At FL340; we experienced the cabin altitude warning horn. After verifying the cabin altitude we both performed the cabin altitude warning horn memory items; requested a descent and received a descent to 10000 ft with a slight navigation change (direct as apposed to previous clearance). I started the descent; thrust levers idle; speed brake flight detent. The captain ran the qrc/checklists. Once complete; we discussed whether to declare an emergency; and with input from dispatch it was decided not to declare an emergency. In hindsight; this was not the right call; and an emergency should have been declared. If we had declared an emergency our flight would not have changed from what took place; but anytime you read the qrc/perform a memory item an emergency should be declared. The cabin altitude went up to 13700 ft or so (the masks never dropped). Once at 10000 ft we contacted the flight attendants and informed them of our situation; contacted dispatch and let them know of our problems and intentions (this is when the emergency discussion took place). We proceeded at 10000 ft and landed uneventfully. Although the captain made a PA of our situation; I believe a large number of passenger never knew anything had happened. During the descent we omitted the outflow switch manual on the qrc. This may have reduced the maximum cabin altitude; but no other implications. First of all there needs to be another warning system for the cabin altitude warning. It sounds exactly like the takeoff warning horn; and does not in any way bring you to the conclusion to look at your cabin altitude (we need a light or an audible warning telling us of the problem). Second; every time I have had a pressurization problem in the sim it has been a rapid depressurization. This is a very different animal and that may have led to some of our actions. This event (other than donning the masks) seemed to be very straight forward.callback conversation with reporter revealed the following information: the reporter is new to the aircraft. He stated that he recognized the horn in approximately 10 seconds because he had recently been in training. However; all training was done with an explosive decompression and not the slow; insidious depressurization. The reporter again stated that a difference should be made between the takeoff warning horn and the cabin altitude warning. He suggests a different sound or a master caution light positively identifying the cabin altitude.
Original NASA ASRS Text
Title: A B737 AT FL340 RECEIVED A CABIN ALT WARNING. AN EMER WAS NOT DECLARED BUT THE CREW DSNDED TO 10000 FT. THE RPTR COMMENTS THAT AN ADDITIONAL WARNING IS NEEDED FOR THIS MALFUNCTION.
Narrative: AT FL340; WE EXPERIENCED THE CABIN ALT WARNING HORN. AFTER VERIFYING THE CABIN ALT WE BOTH PERFORMED THE CABIN ALT WARNING HORN MEMORY ITEMS; REQUESTED A DSCNT AND RECEIVED A DSCNT TO 10000 FT WITH A SLIGHT NAVIGATION CHANGE (DIRECT AS APPOSED TO PREVIOUS CLRNC). I STARTED THE DSCNT; THRUST LEVERS IDLE; SPD BRAKE FLT DETENT. THE CAPT RAN THE QRC/CHKLISTS. ONCE COMPLETE; WE DISCUSSED WHETHER TO DECLARE AN EMER; AND WITH INPUT FROM DISPATCH IT WAS DECIDED NOT TO DECLARE AN EMER. IN HINDSIGHT; THIS WAS NOT THE RIGHT CALL; AND AN EMER SHOULD HAVE BEEN DECLARED. IF WE HAD DECLARED AN EMER OUR FLT WOULD NOT HAVE CHANGED FROM WHAT TOOK PLACE; BUT ANYTIME YOU READ THE QRC/PERFORM A MEMORY ITEM AN EMER SHOULD BE DECLARED. THE CABIN ALT WENT UP TO 13700 FT OR SO (THE MASKS NEVER DROPPED). ONCE AT 10000 FT WE CONTACTED THE FLT ATTENDANTS AND INFORMED THEM OF OUR SITUATION; CONTACTED DISPATCH AND LET THEM KNOW OF OUR PROBLEMS AND INTENTIONS (THIS IS WHEN THE EMER DISCUSSION TOOK PLACE). WE PROCEEDED AT 10000 FT AND LANDED UNEVENTFULLY. ALTHOUGH THE CAPT MADE A PA OF OUR SITUATION; I BELIEVE A LARGE NUMBER OF PAX NEVER KNEW ANYTHING HAD HAPPENED. DURING THE DSCNT WE OMITTED THE OUTFLOW SWITCH MANUAL ON THE QRC. THIS MAY HAVE REDUCED THE MAX CABIN ALT; BUT NO OTHER IMPLICATIONS. FIRST OF ALL THERE NEEDS TO BE ANOTHER WARNING SYSTEM FOR THE CABIN ALT WARNING. IT SOUNDS EXACTLY LIKE THE TKOF WARNING HORN; AND DOES NOT IN ANY WAY BRING YOU TO THE CONCLUSION TO LOOK AT YOUR CABIN ALT (WE NEED A LIGHT OR AN AUDIBLE WARNING TELLING US OF THE PROB). SECOND; EVERY TIME I HAVE HAD A PRESSURIZATION PROB IN THE SIM IT HAS BEEN A RAPID DEPRESSURIZATION. THIS IS A VERY DIFFERENT ANIMAL AND THAT MAY HAVE LED TO SOME OF OUR ACTIONS. THIS EVENT (OTHER THAN DONNING THE MASKS) SEEMED TO BE VERY STRAIGHT FORWARD.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS NEW TO THE ACFT. HE STATED THAT HE RECOGNIZED THE HORN IN APPROX 10 SECONDS BECAUSE HE HAD RECENTLY BEEN IN TRAINING. HOWEVER; ALL TRAINING WAS DONE WITH AN EXPLOSIVE DECOMPRESSION AND NOT THE SLOW; INSIDIOUS DEPRESSURIZATION. THE RPTR AGAIN STATED THAT A DIFFERENCE SHOULD BE MADE BETWEEN THE TKOF WARNING HORN AND THE CABIN ALT WARNING. HE SUGGESTS A DIFFERENT SOUND OR A MASTER CAUTION LIGHT POSITIVELY IDENTIFYING THE CABIN ALT.
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.