Narrative:

I was assigned to assist in troubleshooting a landing gear problem on a company B737-300. The aircraft had already made 2 air turnbacks earlier in the day -- first for aural warning after takeoff, and second because gear lever wouldn't move out of the 'off' position after rotation. My crew determined the cause to be a faulty main gear air sense proximity switch. My co-worker removed the sensor and spliced in a new one, then I completed the installation by mounting the sensor, while he began giving a 'turnover' to the oncoming crew. I helped set the sensor to target gap per the aircraft maintenance manual. We then completed our day by turning over the system chkout to the relief crew. The following day I was informed that the aircraft had made a third air turnback on its first flight attempt following our maintenance. Inspection revealed that I had improperly mounted the new sensor. The face was inadvertently positioned 90 degrees from the target. Somehow I had still been able to set the proper gap -- between the target and the side of the sensor housing! While I was mounting the sensor the previous evening, I was convinced that I was doing it correctly, so did not bother to look closely at the illustration in the maintenance manual. This had been poor judgement -- especially since I had not actually observed the removal of the sensor. I could immediately see my mistake, and that there was an entirely separate flange and set of mounting holes on the sensor -- clearly not 'dummy proof!' of course, this mistake would have been caught prior to returning the aircraft to service, had our relief crew performed the system chkout correctly per the aircraft maintenance manual. Apparently, they were able to retract the gear with the aircraft on jacks, but they inadvertently used the gear lever safety release trigger, which completely bypasses the air/ground sense system that they were supposed to be testing.

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Original NASA ASRS Text

Title: A B737-300 RETURNED TO THE FIELD 3 TIMES DUE TO GEAR GND SENSING PROBS. FIRST INCIDENT WAS AURAL WARNING, SECOND GEAR HANDLE WOULD NOT RELEASE, THIRD, GEAR HANDLE WOULD NOT RELEASE DUE TO SAFETY SENSOR INSTALLED WRONG.

Narrative: I WAS ASSIGNED TO ASSIST IN TROUBLESHOOTING A LNDG GEAR PROB ON A COMPANY B737-300. THE ACFT HAD ALREADY MADE 2 AIR TURNBACKS EARLIER IN THE DAY -- FIRST FOR AURAL WARNING AFTER TKOF, AND SECOND BECAUSE GEAR LEVER WOULDN'T MOVE OUT OF THE 'OFF' POS AFTER ROTATION. MY CREW DETERMINED THE CAUSE TO BE A FAULTY MAIN GEAR AIR SENSE PROX SWITCH. MY CO-WORKER REMOVED THE SENSOR AND SPLICED IN A NEW ONE, THEN I COMPLETED THE INSTALLATION BY MOUNTING THE SENSOR, WHILE HE BEGAN GIVING A 'TURNOVER' TO THE ONCOMING CREW. I HELPED SET THE SENSOR TO TARGET GAP PER THE ACFT MAINT MANUAL. WE THEN COMPLETED OUR DAY BY TURNING OVER THE SYS CHKOUT TO THE RELIEF CREW. THE FOLLOWING DAY I WAS INFORMED THAT THE ACFT HAD MADE A THIRD AIR TURNBACK ON ITS FIRST FLT ATTEMPT FOLLOWING OUR MAINT. INSPECTION REVEALED THAT I HAD IMPROPERLY MOUNTED THE NEW SENSOR. THE FACE WAS INADVERTENTLY POSITIONED 90 DEGS FROM THE TARGET. SOMEHOW I HAD STILL BEEN ABLE TO SET THE PROPER GAP -- BTWN THE TARGET AND THE SIDE OF THE SENSOR HOUSING! WHILE I WAS MOUNTING THE SENSOR THE PREVIOUS EVENING, I WAS CONVINCED THAT I WAS DOING IT CORRECTLY, SO DID NOT BOTHER TO LOOK CLOSELY AT THE ILLUSTRATION IN THE MAINT MANUAL. THIS HAD BEEN POOR JUDGEMENT -- ESPECIALLY SINCE I HAD NOT ACTUALLY OBSERVED THE REMOVAL OF THE SENSOR. I COULD IMMEDIATELY SEE MY MISTAKE, AND THAT THERE WAS AN ENTIRELY SEPARATE FLANGE AND SET OF MOUNTING HOLES ON THE SENSOR -- CLRLY NOT 'DUMMY PROOF!' OF COURSE, THIS MISTAKE WOULD HAVE BEEN CAUGHT PRIOR TO RETURNING THE ACFT TO SVC, HAD OUR RELIEF CREW PERFORMED THE SYS CHKOUT CORRECTLY PER THE ACFT MAINT MANUAL. APPARENTLY, THEY WERE ABLE TO RETRACT THE GEAR WITH THE ACFT ON JACKS, BUT THEY INADVERTENTLY USED THE GEAR LEVER SAFETY RELEASE TRIGGER, WHICH COMPLETELY BYPASSES THE AIR/GND SENSE SYS THAT THEY WERE SUPPOSED TO BE TESTING.

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.