Narrative:

The night of 7/thurs/03, I was working on aircraft's #1 engine hmu change. Removed old hmu, found no washers installed. Installed replacement hmu with gasket kit. Torqued locking nuts to 130 in pounds, 6 ea nuts, with inspector there to witness. I then proceeded on with the rest of the hmu installation on #1 engine, in accordance with MM 73-21-10 and torque values inspected by inspector. From here, pushed aircraft out for engine run and performed leak check. No leaks were found. Aircraft was ok for service at this time. Callback conversation with reporter revealed the following information: the reporter stated the hydro-mechanical fuel control unit manufacturer modified the units due to engine performance on B737-800 aircraft. The reporter said the fleet has been equipped with these modified units and as a result, have experienced eight events of flight returns for leaks, fuel imbalances to engine flameout. The reporter stated the technicians are finding the lock nuts on these units are backing off resulting in leaks. The reporter also said the units now operate at a higher pressure. The reporter stated the installation is difficult and special tools are needed to set the correct torque setting of the lock nuts. Callback from acn 590112: the reporter stated the inspection card called for overall inspection and not detailed inspection of the modified hydro-mechanical fuel control unit installation. The reporter said when the unit was found to have loose lock nuts and on removal, it was found to be missing a few washers from under the lock nuts. The reporter stated the FAA safety inspector believed this was the cause of the lock nuts backing off, and scheduled an investigation. The reporter said it was pointed out to the inspector several of nuts were recessed in cavities and cannot be seen. The reporter stated it now appears to be an industry problem of the lock nuts backing off from these higher pressure units. Supplemental information from acn 590112: I witnessed and verified the mechanic in tourqing the six self locking nuts to 130 in pounds. I then proceeded to verify the rest of the installation process and torque values of the hmu per MM 73-21-10. I did not notice any defects, damage or leaks during the testing and leak check function. After 8 flight legs or 25 flight hours, a retorque should be accomplished.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B737-800 RETURNED TO THE FIELD DUE #1 ENG LEAKING FUEL. MAINT FOUND MODIFIED HYDRO MECHANICAL FUEL CTL LOCK NUTS NOT LOCKING.

Narrative: THE NIGHT OF 7/THURS/03, I WAS WORKING ON ACFT'S #1 ENG HMU CHANGE. REMOVED OLD HMU, FOUND NO WASHERS INSTALLED. INSTALLED REPLACEMENT HMU WITH GASKET KIT. TORQUED LOCKING NUTS TO 130 IN POUNDS, 6 EA NUTS, WITH INSPECTOR THERE TO WITNESS. I THEN PROCEEDED ON WITH THE REST OF THE HMU INSTALLATION ON #1 ENG, IAW MM 73-21-10 AND TORQUE VALUES INSPECTED BY INSPECTOR. FROM HERE, PUSHED ACFT OUT FOR ENG RUN AND PERFORMED LEAK CHECK. NO LEAKS WERE FOUND. ACFT WAS OK FOR SVC AT THIS TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE HYDRO-MECHANICAL FUEL CTL UNIT MANUFACTURER MODIFIED THE UNITS DUE TO ENG PERFORMANCE ON B737-800 ACFT. THE RPTR SAID THE FLEET HAS BEEN EQUIPPED WITH THESE MODIFIED UNITS AND AS A RESULT, HAVE EXPERIENCED EIGHT EVENTS OF FLT RETURNS FOR LEAKS, FUEL IMBALANCES TO ENG FLAMEOUT. THE RPTR STATED THE TECHNICIANS ARE FINDING THE LOCK NUTS ON THESE UNITS ARE BACKING OFF RESULTING IN LEAKS. THE RPTR ALSO SAID THE UNITS NOW OPERATE AT A HIGHER PRESSURE. THE RPTR STATED THE INSTALLATION IS DIFFICULT AND SPECIAL TOOLS ARE NEEDED TO SET THE CORRECT TORQUE SETTING OF THE LOCK NUTS. CALLBACK FROM ACN 590112: THE RPTR STATED THE INSPECTION CARD CALLED FOR OVERALL INSPECTION AND NOT DETAILED INSPECTION OF THE MODIFIED HYDRO-MECHANICAL FUEL CTL UNIT INSTALLATION. THE RPTR SAID WHEN THE UNIT WAS FOUND TO HAVE LOOSE LOCK NUTS AND ON REMOVAL, IT WAS FOUND TO BE MISSING A FEW WASHERS FROM UNDER THE LOCK NUTS. THE RPTR STATED THE FAA SAFETY INSPECTOR BELIEVED THIS WAS THE CAUSE OF THE LOCK NUTS BACKING OFF, AND SCHEDULED AN INVESTIGATION. THE RPTR SAID IT WAS POINTED OUT TO THE INSPECTOR SEVERAL OF NUTS WERE RECESSED IN CAVITIES AND CANNOT BE SEEN. THE RPTR STATED IT NOW APPEARS TO BE AN INDUSTRY PROBLEM OF THE LOCK NUTS BACKING OFF FROM THESE HIGHER PRESSURE UNITS. SUPPLEMENTAL INFO FROM ACN 590112: I WITNESSED AND VERIFIED THE MECH IN TOURQING THE SIX SELF LOCKING NUTS TO 130 IN LBS. I THEN PROCEEDED TO VERIFY THE REST OF THE INSTALLATION PROCESS AND TORQUE VALUES OF THE HMU PER MM 73-21-10. I DID NOT NOTICE ANY DEFECTS, DAMAGE OR LEAKS DURING THE TESTING AND LEAK CHECK FUNCTION. AFTER 8 FLT LEGS OR 25 FLT HRS, A RETORQUE SHOULD BE ACCOMPLISHED.

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.