Narrative:

During preflight at an en route crew change, it was found that a portion of the test function of the wheel well fire detection system was not functional. The warning bell did not test but the warning light did test properly. The bell tested properly with other fire warning system (engine, APU, etc). Maintenance investigated the problem but was unable to repair it. Maintenance control was contacted to consider deferring the test portion of the system. However, in accordance with the MEL, the entire system had to be deferred, rather than only the bell test function. Deferring the entire wheel well fire detection system is permissible provided the brakes are determined cool prior to engine start by either use of the brake temperature indicators (this aircraft did not have that system installed) or by feeling the brakes. The MEL further states that to avoid retracting a wheel overheated due to dragging brakes, one should either: 1) monitor brake temperatures before retraction (not installed), or 2) leave the gear extended for 10 mins. The MEL additionally allows that in the event of engine failure after V1 performance is a prime consideration. In this case the gear should be retracted normally until performance is no longer a factor then extended for the balance of the 10 min period. The brakes were cool prior to engine start and the aircraft did not have a history of brake problems. The taxi distance from parking to the takeoff runway is very short (approximately 200-300 yards). The takeoff performance was not limited by an obstacle, only runway length. After considering the available information, we used the following technique for the takeoff: monitored the wheel well fire light during the takeoff, accomplishing an ICAO type B climb profile, retracting the gear normally, and after well clear of terrain (about 2500 ft MSL) (not required for performance but in the interest of a conservative, safe takeoff) extended the gear for 10 mins. This occurred at about 1 1/2 - 2 mins after takeoff. There were no overheat/fire indications and the flight proceeded without incident.

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Original NASA ASRS Text

Title: A B747-100 WITH WHEEL WELL FIRE WARNING TEST DEFERRED AS INOP PER THE MEL BUT FLC DISAGREES WITH THE SPECIAL PROCS REQUIRED.

Narrative: DURING PREFLT AT AN ENRTE CREW CHANGE, IT WAS FOUND THAT A PORTION OF THE TEST FUNCTION OF THE WHEEL WELL FIRE DETECTION SYS WAS NOT FUNCTIONAL. THE WARNING BELL DID NOT TEST BUT THE WARNING LIGHT DID TEST PROPERLY. THE BELL TESTED PROPERLY WITH OTHER FIRE WARNING SYS (ENG, APU, ETC). MAINT INVESTIGATED THE PROB BUT WAS UNABLE TO REPAIR IT. MAINT CTL WAS CONTACTED TO CONSIDER DEFERRING THE TEST PORTION OF THE SYS. HOWEVER, IN ACCORDANCE WITH THE MEL, THE ENTIRE SYS HAD TO BE DEFERRED, RATHER THAN ONLY THE BELL TEST FUNCTION. DEFERRING THE ENTIRE WHEEL WELL FIRE DETECTION SYS IS PERMISSIBLE PROVIDED THE BRAKES ARE DETERMINED COOL PRIOR TO ENG START BY EITHER USE OF THE BRAKE TEMP INDICATORS (THIS ACFT DID NOT HAVE THAT SYS INSTALLED) OR BY FEELING THE BRAKES. THE MEL FURTHER STATES THAT TO AVOID RETRACTING A WHEEL OVERHEATED DUE TO DRAGGING BRAKES, ONE SHOULD EITHER: 1) MONITOR BRAKE TEMPS BEFORE RETRACTION (NOT INSTALLED), OR 2) LEAVE THE GEAR EXTENDED FOR 10 MINS. THE MEL ADDITIONALLY ALLOWS THAT IN THE EVENT OF ENG FAILURE AFTER V1 PERFORMANCE IS A PRIME CONSIDERATION. IN THIS CASE THE GEAR SHOULD BE RETRACTED NORMALLY UNTIL PERFORMANCE IS NO LONGER A FACTOR THEN EXTENDED FOR THE BAL OF THE 10 MIN PERIOD. THE BRAKES WERE COOL PRIOR TO ENG START AND THE ACFT DID NOT HAVE A HISTORY OF BRAKE PROBS. THE TAXI DISTANCE FROM PARKING TO THE TKOF RWY IS VERY SHORT (APPROX 200-300 YARDS). THE TKOF PERFORMANCE WAS NOT LIMITED BY AN OBSTACLE, ONLY RWY LENGTH. AFTER CONSIDERING THE AVAILABLE INFO, WE USED THE FOLLOWING TECHNIQUE FOR THE TKOF: MONITORED THE WHEEL WELL FIRE LIGHT DURING THE TKOF, ACCOMPLISHING AN ICAO TYPE B CLB PROFILE, RETRACTING THE GEAR NORMALLY, AND AFTER WELL CLR OF TERRAIN (ABOUT 2500 FT MSL) (NOT REQUIRED FOR PERFORMANCE BUT IN THE INTEREST OF A CONSERVATIVE, SAFE TKOF) EXTENDED THE GEAR FOR 10 MINS. THIS OCCURRED AT ABOUT 1 1/2 - 2 MINS AFTER TKOF. THERE WERE NO OVERHEAT/FIRE INDICATIONS AND THE FLT PROCEEDED WITHOUT INCIDENT.

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.