|37000 Feet||Browse and search NASA's
Aviation Safety Reporting System
|Locale Reference||airport : zzzz.airport|
|Altitude||msl single value : 40000|
|Controlling Facilities||artcc : zzzz.artcc|
|Operator||common carrier : air carrier|
|Make Model Name||B747-400|
|Operating Under FAR Part||Part 121|
|Navigation In Use||other|
|Flight Phase||cruise : level|
|Affiliation||company : air carrier|
|Function||flight crew : captain|
oversight : pic
|Qualification||pilot : atp|
|Experience||flight time last 90 days : 150|
flight time total : 14.850
flight time type : 6000
|Anomaly||aircraft equipment problem : less severe|
altitude deviation : excursion from assigned altitude
|Independent Detector||aircraft equipment other aircraft equipment : eicas|
other flight crewa
|Resolutory Action||flight crew : declared emergency|
flight crew : landed in emergency condition
flight crew : diverted to another airport
|Maintenance||contributing factor : manuals|
performance deficiency : fault isolation
Maintenance Human Performance
On our arrival to the aircraft local mechanic briefed us on the aircraft status. He stated that it came in with a log page write-up; EICAS message 'stabilizer trim unschd' in cruise and that this item had been previously written up now 4 times. Additionally; one other recent log page write-up showed an item that the autoplt disconnected twice in cruise. He stated that he was concerned and wanted to troubleshoot the issue using the boeing troubleshooting guidance and it may take some time to resolve this issue. The local mechanic had numerous conversations with air carrier maintenance. The local mechanic stated that air carrier wanted him to change an FCC and do a test. Mechanic explained to us that he changed out an FCC and ran 3 separate tests; isolating each one of the 3 FCC's by pulling a circuit breaker. The test came out good and local mechanic cleared the logbook item as per air carrier maintenance. The takeoff and climb out was normal. First officer did the takeoff and hand flew the aircraft to the top of climb and then requested the right autoplt to command. Approximately 5 mins after the top of climb; an EICAS message 'stabilizer trim unschd' appeared. We followed the QRH; no pitch change was detected; reestablishing system #2; check pitch trim operation and found to be working normally. All operations were normal expect hydraulic stabilizer system #3 was now isolated in cutout as per the QRH. After establishing communications with air carrier; which included dispatch; maintenance; and the chief pilot; I fully explained the issues; the prior log page write-ups and requested their input and directions for us to take. After explaining to air carrier the issues; they said maintenance would contact boeing and then would ACARS us with their recommendations. Shortly after the communication with air carrier; we got another EICAS message 'stabilizer trim unschd.' this time we had a pitch up of 800 FPM. I slowed the aircraft to an in trim speed of 248 KTS; requested and got a clearance to descend to FL300 and divert to ZZZZ. We notified air carrier operations of the latest issues and that we were diverting to ZZZZ. We declared an emergency; but advised ATC that we had the issue under control. We requested priority handling and equipment standing by at ZZZZ as a precautionary measure. 15 mins after leveling at FL300 while hand-flying the aircraft; we got our third EICAS message 'stabilizer trim unschd' but with no pitch change. We confirmed that both stabilizer trim switches were still in cutoff as per the QRH and continued hand-flying the aircraft. Once established on final approach; fully configured for landing and with the radio altimeter alive; we got a strange EICAS message 'darg required.' we continued with a normal landing with the exception of heavy aft control force necessary to maintain the appropriate approach path. Callback conversation with reporter revealed the following information: maintenance discovered; after consulting with the manufacturer; that a position indicating transducer located in the tail of the aircraft was arcing and producing erroneous indications of stabilizer movement. This information is also supplied to flight control computers and may have caused the autopilot disconnects and pitch up.
Original NASA ASRS Text
Title: B747 FLT CREW REPORTS 'STAB TRIM UNSCHD' EICAS MESSAGE IN CRUISE; WHICH HAS BEEN CHRONIC IN THIS ACFT. EMERGENCY DECLARED AND FLIGHT DIVERTS TO NEAREST SUITABLE ARPT FOR NORMAL LANDING WITH STAB TRIM DEACTIVATED.
Narrative: ON OUR ARR TO THE ACFT LCL MECH BRIEFED US ON THE ACFT STATUS. HE STATED THAT IT CAME IN WITH A LOG PAGE WRITE-UP; EICAS MESSAGE 'STABILIZER TRIM UNSCHD' IN CRUISE AND THAT THIS ITEM HAD BEEN PREVIOUSLY WRITTEN UP NOW 4 TIMES. ADDITIONALLY; ONE OTHER RECENT LOG PAGE WRITE-UP SHOWED AN ITEM THAT THE AUTOPLT DISCONNECTED TWICE IN CRUISE. HE STATED THAT HE WAS CONCERNED AND WANTED TO TROUBLESHOOT THE ISSUE USING THE BOEING TROUBLESHOOTING GUIDANCE AND IT MAY TAKE SOME TIME TO RESOLVE THIS ISSUE. THE LCL MECH HAD NUMEROUS CONVERSATIONS WITH ACR MAINT. THE LCL MECH STATED THAT ACR WANTED HIM TO CHANGE AN FCC AND DO A TEST. MECH EXPLAINED TO US THAT HE CHANGED OUT AN FCC AND RAN 3 SEPARATE TESTS; ISOLATING EACH ONE OF THE 3 FCC'S BY PULLING A CIRCUIT BREAKER. THE TEST CAME OUT GOOD AND LCL MECH CLRED THE LOGBOOK ITEM AS PER ACR MAINT. THE TKOF AND CLBOUT WAS NORMAL. FO DID THE TKOF AND HAND FLEW THE ACFT TO THE TOP OF CLB AND THEN REQUESTED THE R AUTOPLT TO COMMAND. APPROX 5 MINS AFTER THE TOP OF CLB; AN EICAS MESSAGE 'STABILIZER TRIM UNSCHD' APPEARED. WE FOLLOWED THE QRH; NO PITCH CHANGE WAS DETECTED; REESTABLISHING SYS #2; CHK PITCH TRIM OP AND FOUND TO BE WORKING NORMALLY. ALL OPS WERE NORMAL EXPECT HYD STABILIZER SYS #3 WAS NOW ISOLATED IN CUTOUT AS PER THE QRH. AFTER ESTABLISHING COMS WITH ACR; WHICH INCLUDED DISPATCH; MAINT; AND THE CHIEF PLT; I FULLY EXPLAINED THE ISSUES; THE PRIOR LOG PAGE WRITE-UPS AND REQUESTED THEIR INPUT AND DIRECTIONS FOR US TO TAKE. AFTER EXPLAINING TO ACR THE ISSUES; THEY SAID MAINT WOULD CONTACT BOEING AND THEN WOULD ACARS US WITH THEIR RECOMMENDATIONS. SHORTLY AFTER THE COM WITH ACR; WE GOT ANOTHER EICAS MESSAGE 'STABILIZER TRIM UNSCHD.' THIS TIME WE HAD A PITCH UP OF 800 FPM. I SLOWED THE ACFT TO AN IN TRIM SPD OF 248 KTS; REQUESTED AND GOT A CLRNC TO DESCEND TO FL300 AND DIVERT TO ZZZZ. WE NOTIFIED ACR OPS OF THE LATEST ISSUES AND THAT WE WERE DIVERTING TO ZZZZ. WE DECLARED AN EMER; BUT ADVISED ATC THAT WE HAD THE ISSUE UNDER CTL. WE REQUESTED PRIORITY HANDLING AND EQUIP STANDING BY AT ZZZZ AS A PRECAUTIONARY MEASURE. 15 MINS AFTER LEVELING AT FL300 WHILE HAND-FLYING THE ACFT; WE GOT OUR THIRD EICAS MESSAGE 'STABILIZER TRIM UNSCHD' BUT WITH NO PITCH CHANGE. WE CONFIRMED THAT BOTH STABILIZER TRIM SWITCHES WERE STILL IN CUTOFF AS PER THE QRH AND CONTINUED HAND-FLYING THE ACFT. ONCE ESTABLISHED ON FINAL APCH; FULLY CONFIGURED FOR LNDG AND WITH THE RADIO ALTIMETER ALIVE; WE GOT A STRANGE EICAS MESSAGE 'DARG REQUIRED.' WE CONTINUED WITH A NORMAL LNDG WITH THE EXCEPTION OF HVY AFT CTL FORCE NECESSARY TO MAINTAIN THE APPROPRIATE APCH PATH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINTENANCE DISCOVERED; AFTER CONSULTING WITH THE MANUFACTURER; THAT A POSITION INDICATING TRANSDUCER LOCATED IN THE TAIL OF THE ACFT WAS ARCING AND PRODUCING ERRONEOUS INDICATIONS OF STAB MOVEMENT. THIS INFORMATION IS ALSO SUPPLIED TO FLIGHT CONTROL COMPUTERS AND MAY HAVE CAUSED THE AUTOPILOT DISCONNECTS AND PITCH UP.
Data retrieved from NASA's ASRS site as of January 2009 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.