Narrative:

Flight aircraft arrived at airport, gate zz, on dec/xx/96. The flight crew verbally informed me that it appeared the left pack turbo fan continued to run even with the left pack switch placed in the off position. I informed my lead. My lead and I began to troubleshoot the problem. We noticed that the pack turbo fan was running continuously with left pack switch in the off position and APU bleed air at 40 psi which is normal for a pack off condition. All the warning lights were normal. Because the pack showed no abnormal condition other than a continuously running pack turbo fan. We consulted the MEL for deferral of the pack turbo fan. This required the removal of the cannon plug to the shutoff valve and stowing it. Making sure the valve indicator showed closed and manually pushing the solenoid pin to be sure it was closed. We additionally turned the APU bleed on and made sure the valve didn't run. We left it in this condition for 5 mins and the 3 mechanics present didn't observe any abnormalities such as pack noise or drop in APU pressure indicating that the pack was running or warning lights. We then advised the crew that they could run the right pack on the ground but the left pack was restr per the MEL to flight only. Having complied with the MEL procedures for inoping the turbo fan shutoff valve we did the paperwork and made the airplane ready for dispatch. It took about another half hour to dispatch the airplane because baggage. After dispatching the aircraft I was informed that the airplane had filled with smoke and we needed to go out and tow it back in. Once the aircraft was back at the gate I asked the flight crew what happened. They informed me that everything was ok until they set up the airplane to start the engines. When they turned off the right pack with the APU bleed on, they said that the left pack overheat light came on and then the cabin filled with smoke. We opened the left pack bay door and noticed that the heat exchangers were warm and the ducting to the air cycle machine was warm. We then realized that something further was in error. We put the inoped left pack turbo fan shutoff valve back in its normal condition, then we went up to the cockpit and selected the APU bleed to 'on' and pack switch to 'off.' we went downstairs and discovered by touch that the air cycle machine was running at a very low level. At this point we decided to inoperative the whole left pack per the MEL. When we did this per MEL procedures, it was discovered that the pack still was running. We consulted the maintenance manual and found a service tip. Item a suggested that we change the pack flow control valve. We did that per maintenance manual procedures. This also did not correct the problem. Item B said to remove the engine pressure regulator source air and see if the pack shuts down. It did. This indicated that the engine pressure regulating valve had a leaking diaphragm and required changing.

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Original NASA ASRS Text

Title: A B737-200 RETURNED TO THE GATE AFTER PUSHBACK AND STARTING #1 ENG DUE TO SMOKE IN THE CABIN. TURBO FAN PREVIOUSLY DEFERRED. FOUND ENG PRESSURE REGULATOR LEAKING CAUSING PACK SHUTOFF VALVE TO REMAIN OPEN.

Narrative: FLT ACFT ARRIVED AT ARPT, GATE ZZ, ON DEC/XX/96. THE FLC VERBALLY INFORMED ME THAT IT APPEARED THE L PACK TURBO FAN CONTINUED TO RUN EVEN WITH THE L PACK SWITCH PLACED IN THE OFF POS. I INFORMED MY LEAD. MY LEAD AND I BEGAN TO TROUBLESHOOT THE PROB. WE NOTICED THAT THE PACK TURBO FAN WAS RUNNING CONTINUOUSLY WITH L PACK SWITCH IN THE OFF POS AND APU BLEED AIR AT 40 PSI WHICH IS NORMAL FOR A PACK OFF CONDITION. ALL THE WARNING LIGHTS WERE NORMAL. BECAUSE THE PACK SHOWED NO ABNORMAL CONDITION OTHER THAN A CONTINUOUSLY RUNNING PACK TURBO FAN. WE CONSULTED THE MEL FOR DEFERRAL OF THE PACK TURBO FAN. THIS REQUIRED THE REMOVAL OF THE CANNON PLUG TO THE SHUTOFF VALVE AND STOWING IT. MAKING SURE THE VALVE INDICATOR SHOWED CLOSED AND MANUALLY PUSHING THE SOLENOID PIN TO BE SURE IT WAS CLOSED. WE ADDITIONALLY TURNED THE APU BLEED ON AND MADE SURE THE VALVE DIDN'T RUN. WE LEFT IT IN THIS CONDITION FOR 5 MINS AND THE 3 MECHS PRESENT DIDN'T OBSERVE ANY ABNORMALITIES SUCH AS PACK NOISE OR DROP IN APU PRESSURE INDICATING THAT THE PACK WAS RUNNING OR WARNING LIGHTS. WE THEN ADVISED THE CREW THAT THEY COULD RUN THE R PACK ON THE GND BUT THE L PACK WAS RESTR PER THE MEL TO FLT ONLY. HAVING COMPLIED WITH THE MEL PROCS FOR INOPING THE TURBO FAN SHUTOFF VALVE WE DID THE PAPERWORK AND MADE THE AIRPLANE READY FOR DISPATCH. IT TOOK ABOUT ANOTHER HALF HR TO DISPATCH THE AIRPLANE BECAUSE BAGGAGE. AFTER DISPATCHING THE ACFT I WAS INFORMED THAT THE AIRPLANE HAD FILLED WITH SMOKE AND WE NEEDED TO GO OUT AND TOW IT BACK IN. ONCE THE ACFT WAS BACK AT THE GATE I ASKED THE FLC WHAT HAPPENED. THEY INFORMED ME THAT EVERYTHING WAS OK UNTIL THEY SET UP THE AIRPLANE TO START THE ENGS. WHEN THEY TURNED OFF THE R PACK WITH THE APU BLEED ON, THEY SAID THAT THE L PACK OVERHEAT LIGHT CAME ON AND THEN THE CABIN FILLED WITH SMOKE. WE OPENED THE L PACK BAY DOOR AND NOTICED THAT THE HEAT EXCHANGERS WERE WARM AND THE DUCTING TO THE AIR CYCLE MACHINE WAS WARM. WE THEN REALIZED THAT SOMETHING FURTHER WAS IN ERROR. WE PUT THE INOPED L PACK TURBO FAN SHUTOFF VALVE BACK IN ITS NORMAL CONDITION, THEN WE WENT UP TO THE COCKPIT AND SELECTED THE APU BLEED TO 'ON' AND PACK SWITCH TO 'OFF.' WE WENT DOWNSTAIRS AND DISCOVERED BY TOUCH THAT THE AIR CYCLE MACHINE WAS RUNNING AT A VERY LOW LEVEL. AT THIS POINT WE DECIDED TO INOP THE WHOLE L PACK PER THE MEL. WHEN WE DID THIS PER MEL PROCS, IT WAS DISCOVERED THAT THE PACK STILL WAS RUNNING. WE CONSULTED THE MAINT MANUAL AND FOUND A SVC TIP. ITEM A SUGGESTED THAT WE CHANGE THE PACK FLOW CTL VALVE. WE DID THAT PER MAINT MANUAL PROCS. THIS ALSO DID NOT CORRECT THE PROB. ITEM B SAID TO REMOVE THE ENG PRESSURE REGULATOR SOURCE AIR AND SEE IF THE PACK SHUTS DOWN. IT DID. THIS INDICATED THAT THE ENG PRESSURE REGULATING VALVE HAD A LEAKING DIAPHRAGM AND REQUIRED CHANGING.

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.