Narrative:

Several engineering orders were released (7/2000) to address overheating bun warmers specifically on boeing 767-400 aircraft. As a precautionary measure, because the 767-200, 300 and 777-200ER fleet utilizes similar warmers, eo's were issued for inspection and possible modification of them as well. The warmer mfg (monogram) determined that some overheat protection circuits could fail in the closed position and in such a way as to cause a runaway thermal condition. This caused charring and burning of the warmers insulation. There were 2 possible configurations of the overheat protection circuit. One, an earlier design used a single thermostat that protected all three electrical phases. The other design utilized an over temperature protection thermostat for each phase (3) with a resistor backup, this design was the one in question. The eos instructions required an inspection to determine the over temperature configuration, and modification procedures if the warmer had the 3 separate protectors. After modification several tests were to be performed. One of these was to test the newly installed thermostats. This test was not being performed. The manufacturer provided the initial training for performing this modification. According to the personnel that attended this training overheat protection test was never addressed. The manufacturer provided testing fixtures that did not include a 'control thermostat bypass circuit', which was necessary to simulate a failure of the control thermostats and allow testing of the overheat thermostats. That led to some confusion. The eo test instructs to 'close switch on control thermostat bypass circuit. Observe that heater turns 'on' as indicated by ammeter on test fixture. Record maximum temperature (not to exceed 260 degrees F). The question of how to perform the overheat protection test came up during a review associated with the reassignment of warmer repairs. Discussions with technicians that previously performed these modifications indicated that the overheat protection test had not been performed. An internal self audit finding was filed with companies quality assurance. That resulted in an investigation by company team. Initial indications are that all affected bun warmers be deactivated, and removed for required testing. Additional discussions at the shop level indicate that supervision will need to perform more timely and frequent reviews, and more closely monitor training provided by outside agencies. It has been my experience that companies compliance standards tend to be more stringent than many of these outside agencies who in turn may not provide the necessary reinforcements for adhering to test or repair procedures. Callback conversation with reporter revealed the following information: the reporter stated two emergency diversions due to smoke in the cabin have been traced to galley bun warmers that overheated charring the interior insulation. The reporter said the cause was found to be the overheat sensors that were added to each electrical phase to prevent thermal runaway, but in fact were failing closed. The reporter stated the training given to the shop technicians by the manufacturer did not address any test of the sensors in the training, nor did the manufacturers maintenance manual have any required procedure for sensor testing. The carrier issued an engineering order to test the sensors on each unit coming into the shop. The reporter claims that the manufacturer never updates its maintenance manuals.

Google
 

Original NASA ASRS Text

Title: A SHOP LEAD TECHNICIAN REPORTS TWO EMER DIVERSION INCIDENTS ON B 767-200 AND B 777-200ER WITH SMOKE IN CABIN REPORTS TRACED TO GALLEY BUN WARMERS.

Narrative: SEVERAL ENGINEERING ORDERS WERE RELEASED (7/2000) TO ADDRESS OVERHEATING BUN WARMERS SPECIFICALLY ON BOEING 767-400 ACFT. AS A PRECAUTIONARY MEASURE, BECAUSE THE 767-200, 300 AND 777-200ER FLEET UTILIZES SIMILAR WARMERS, EO'S WERE ISSUED FOR INSPECTION AND POSSIBLE MODIFICATION OF THEM AS WELL. THE WARMER MFG (MONOGRAM) DETERMINED THAT SOME OVERHEAT PROTECTION CIRCUITS COULD FAIL IN THE CLOSED POSITION AND IN SUCH A WAY AS TO CAUSE A RUNAWAY THERMAL CONDITION. THIS CAUSED CHARRING AND BURNING OF THE WARMERS INSULATION. THERE WERE 2 POSSIBLE CONFIGURATIONS OF THE OVERHEAT PROTECTION CIRCUIT. ONE, AN EARLIER DESIGN USED A SINGLE THERMOSTAT THAT PROTECTED ALL THREE ELECTRICAL PHASES. THE OTHER DESIGN UTILIZED AN OVER TEMP PROTECTION THERMOSTAT FOR EACH PHASE (3) WITH A RESISTOR BACKUP, THIS DESIGN WAS THE ONE IN QUESTION. THE EOS INSTRUCTIONS REQUIRED AN INSPECTION TO DETERMINE THE OVER TEMP CONFIGURATION, AND MODIFICATION PROCEDURES IF THE WARMER HAD THE 3 SEPARATE PROTECTORS. AFTER MODIFICATION SEVERAL TESTS WERE TO BE PERFORMED. ONE OF THESE WAS TO TEST THE NEWLY INSTALLED THERMOSTATS. THIS TEST WAS NOT BEING PERFORMED. THE MANUFACTURER PROVIDED THE INITIAL TRAINING FOR PERFORMING THIS MODIFICATION. ACCORDING TO THE PERSONNEL THAT ATTENDED THIS TRAINING OVERHEAT PROTECTION TEST WAS NEVER ADDRESSED. THE MANUFACTURER PROVIDED TESTING FIXTURES THAT DID NOT INCLUDE A 'CONTROL THERMOSTAT BYPASS CIRCUIT', WHICH WAS NECESSARY TO SIMULATE A FAILURE OF THE CONTROL THERMOSTATS AND ALLOW TESTING OF THE OVERHEAT THERMOSTATS. THAT LED TO SOME CONFUSION. THE EO TEST INSTRUCTS TO 'CLOSE SWITCH ON CONTROL THERMOSTAT BYPASS CIRCUIT. OBSERVE THAT HEATER TURNS 'ON' AS INDICATED BY AMMETER ON TEST FIXTURE. RECORD MAXIMUM TEMPERATURE (NOT TO EXCEED 260 DEGS F). THE QUESTION OF HOW TO PERFORM THE OVERHEAT PROTECTION TEST CAME UP DURING A REVIEW ASSOCIATED WITH THE REASSIGNMENT OF WARMER REPAIRS. DISCUSSIONS WITH TECHNICIANS THAT PREVIOUSLY PERFORMED THESE MODIFICATIONS INDICATED THAT THE OVERHEAT PROTECTION TEST HAD NOT BEEN PERFORMED. AN INTERNAL SELF AUDIT FINDING WAS FILED WITH COMPANIES QUALITY ASSURANCE. THAT RESULTED IN AN INVESTIGATION BY COMPANY TEAM. INITIAL INDICATIONS ARE THAT ALL AFFECTED BUN WARMERS BE DEACTIVATED, AND REMOVED FOR REQUIRED TESTING. ADDITIONAL DISCUSSIONS AT THE SHOP LEVEL INDICATE THAT SUPERVISION WILL NEED TO PERFORM MORE TIMELY AND FREQUENT REVIEWS, AND MORE CLOSELY MONITOR TRAINING PROVIDED BY OUTSIDE AGENCIES. IT HAS BEEN MY EXPERIENCE THAT COMPANIES COMPLIANCE STANDARDS TEND TO BE MORE STRINGENT THAN MANY OF THESE OUTSIDE AGENCIES WHO IN TURN MAY NOT PROVIDE THE NECESSARY REINFORCEMENTS FOR ADHERING TO TEST OR REPAIR PROCEDURES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED TWO EMER DIVERSIONS DUE TO SMOKE IN THE CABIN HAVE BEEN TRACED TO GALLEY BUN WARMERS THAT OVERHEATED CHARRING THE INTERIOR INSULATION. THE RPTR SAID THE CAUSE WAS FOUND TO BE THE OVERHEAT SENSORS THAT WERE ADDED TO EACH ELECTRICAL PHASE TO PREVENT THERMAL RUNAWAY, BUT IN FACT WERE FAILING CLOSED. THE RPTR STATED THE TRAINING GIVEN TO THE SHOP TECHNICIANS BY THE MANUFACTURER DID NOT ADDRESS ANY TEST OF THE SENSORS IN THE TRAINING, NOR DID THE MANUFACTURERS MAINT MANUAL HAVE ANY REQUIRED PROC FOR SENSOR TESTING. THE CARRIER ISSUED AN ENGINEERING ORDER TO TEST THE SENSORS ON EACH UNIT COMING INTO THE SHOP. THE RPTR CLAIMS THAT THE MANUFACTURER NEVER UPDATES ITS MAINT MANUALS.

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.