Narrative:

Enroute from las to phx in cruise at FL290, the fire warning light came on, along with the warning bell and the forward cargo fire light. Captain XXX XXX was flying the aircraft. I silenced the alarm and he asked me to pull out the checklist. At about this time, the flight attendant's called to let us know that there was a smoky, acrid smell in the cabin. We could also smell it in the cockpit. I read through the checklist as the captain armed and discharged the fire bottle, then I accomplished the remaining items. The smell disappeared after the recirculation fan was turned off. While I was reading notes, the fire light went out. We were established on the recovery into phx, so we continued and expedited to the field with the aid of phx approach control. I had already declared the emergency and we were given priority to the landing runway and cancellation of the speed restriction. I was given control of the aircraft during an intermediate portion of the recovery while captain XXX coordinated with dispatch. Captain XXX made an expeditious and uneventful recovery on runway 26 and had the fire trucks follow us to our assigned gate. After shutdown, no evidence of fire was found, and additionally we discovered that the fire bottle was not discharged. When I had read the checklist, the captain said he had armed the system. When I looked up to verify this, I saw that the switch was consistent with the 'arm' placard on the panel. I then watched him physically depress the discharge button. I believe the 'arm' switch is not intuitively placarded to facilitate its use. Callback conversation with reporter revealed the following information: reporter stated the cause of the cabin smoke and cargo compartment fire warning is believed to be the recirculation fan. The reporter said the labeling of the arm switch on the overhead control display unit needs correction. The reporter stated the captain wanted to 'arm' the fire bottles, lifted the guarded switch and noted the switch was already in the 'arm' position. The reporter stated the captain then discharged, or thought he discharged, the fire bottle, later proved wrong. Supplemental information from acn 521158: I reached up and removed the guard from the cargo fire switch, looked to see the switch said arm, and pressed the discharge button for the forward compartment. Even though the cargo fire switch pointed to the arm position, the switch is not armed. The switch must be moved away from the word arm, which is different from every other switch I can think of in the overhead panel.

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Original NASA ASRS Text

Title: A B737-500 IN CRUISE AT FL290 DECLARED EMER DUE TO CARGO PIT FIRE WARNING WITH CABIN SMOKE. CAUSED BY A FAILED RECIRCULATION FAN.

Narrative: ENROUTE FROM LAS TO PHX IN CRUISE AT FL290, THE FIRE WARNING LIGHT CAME ON, ALONG WITH THE WARNING BELL AND THE FORWARD CARGO FIRE LIGHT. CAPT XXX XXX WAS FLYING THE ACFT. I SILENCED THE ALARM AND HE ASKED ME TO PULL OUT THE CHKLIST. AT ABOUT THIS TIME, THE FA'S CALLED TO LET US KNOW THAT THERE WAS A SMOKY, ACRID SMELL IN THE CABIN. WE COULD ALSO SMELL IT IN THE COCKPIT. I READ THROUGH THE CHKLIST AS THE CAPT ARMED AND DISCHARGED THE FIRE BOTTLE, THEN I ACCOMPLISHED THE REMAINING ITEMS. THE SMELL DISAPPEARED AFTER THE RECIRCULATION FAN WAS TURNED OFF. WHILE I WAS READING NOTES, THE FIRE LIGHT WENT OUT. WE WERE ESTABLISHED ON THE RECOVERY INTO PHX, SO WE CONTINUED AND EXPEDITED TO THE FIELD WITH THE AID OF PHX APCH CTL. I HAD ALREADY DECLARED THE EMER AND WE WERE GIVEN PRIORITY TO THE LNDG RWY AND CANCELLATION OF THE SPEED RESTRICTION. I WAS GIVEN CTL OF THE ACFT DURING AN INTERMEDIATE PORTION OF THE RECOVERY WHILE CAPT XXX COORDINATED WITH DISPATCH. CAPT XXX MADE AN EXPEDITIOUS AND UNEVENTFUL RECOVERY ON RWY 26 AND HAD THE FIRE TRUCKS FOLLOW US TO OUR ASSIGNED GATE. AFTER SHUTDOWN, NO EVIDENCE OF FIRE WAS FOUND, AND ADDITIONALLY WE DISCOVERED THAT THE FIRE BOTTLE WAS NOT DISCHARGED. WHEN I HAD READ THE CHKLIST, THE CAPT SAID HE HAD ARMED THE SYSTEM. WHEN I LOOKED UP TO VERIFY THIS, I SAW THAT THE SWITCH WAS CONSISTENT WITH THE 'ARM' PLACARD ON THE PANEL. I THEN WATCHED HIM PHYSICALLY DEPRESS THE DISCHARGE BUTTON. I BELIEVE THE 'ARM' SWITCH IS NOT INTUITIVELY PLACARDED TO FACILITATE ITS USE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE CAUSE OF THE CABIN SMOKE AND CARGO COMPARTMENT FIRE WARNING IS BELIEVED TO BE THE RECIRCULATION FAN. THE RPTR SAID THE LABELING OF THE ARM SWITCH ON THE OVERHEAD CTL DISPLAY UNIT NEEDS CORRECTION. THE RPTR STATED THE CAPT WANTED TO 'ARM' THE FIRE BOTTLES, LIFTED THE GUARDED SWITCH AND NOTED THE SWITCH WAS ALREADY IN THE 'ARM' POS. THE RPTR STATED THE CAPT THEN DISCHARGED, OR THOUGHT HE DISCHARGED, THE FIRE BOTTLE, LATER PROVED WRONG. SUPPLEMENTAL INFO FROM ACN 521158: I REACHED UP AND REMOVED THE GUARD FROM THE CARGO FIRE SWITCH, LOOKED TO SEE THE SWITCH SAID ARM, AND PRESSED THE DISCHARGE BUTTON FOR THE FORWARD COMPARTMENT. EVEN THOUGH THE CARGO FIRE SWITCH POINTED TO THE ARM POS, THE SWITCH IS NOT ARMED. THE SWITCH MUST BE MOVED AWAY FROM THE WORD ARM, WHICH IS DIFFERENT FROM EVERY OTHER SWITCH I CAN THINK OF IN THE OVERHEAD PANEL.

Data retrieved from NASA's ASRS site as of July 2007 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.