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Air carrier medium large transport altitude deviation excursion from clearance altitude. Reporter says FMA changed flight mode and altitude select by itself.
| Attributes | |
| ACN | 92507 |
| Time | |
| Date | 198808 |
| Day | Fri |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | atc facility : gsh |
| State Reference | IN |
| Altitude | msl bound lower : 34500 msl bound upper : 35000 |
| Environment | |
| Flight Conditions | Mixed |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | artcc : zob |
| Operator | common carrier : air carrier |
| Make Model Name | Medium Large Transport, Low Wing, 2 Turbojet Eng |
| Navigation In Use | Other |
| Flight Phase | descent other |
| Route In Use | enroute : on vectors |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Qualification | pilot : atp |
| Experience | flight time last 90 days : 190 flight time total : 8000 flight time type : 250 |
| ASRS Report | 92507 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : commercial pilot : instrument |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude |
| Independent Detector | other flight crewa |
| Resolutory Action | flight crew : returned to intended course or assigned course |
| Consequence | Other |
| Supplementary | |
| Primary Problem | Aircraft |
| Air Traffic Incident | other |
Narrative:
The first officer was flying the aircraft. We had been issued several vectors and turns by ATC for flow control into chicago O'hare. I was on the P/a explaining the en route delay to the passenger when I noticed the FMA had changed from 'perf cruise' to 'perf descent,' and the altitude select had changed from 35000 to 33000'. I asked the first officer if we had been cleared to FL330. He said no. The aircraft altitude was 34600' when I noticed the problem. The descent was stopped at 34500'. I don't know why the autoplt entered a descent mode. An altitude warning didn't occur because the altitude select had changed also. I suspect a power surge in the electrical system may have caused the problem. I have experienced this problem in the past with the medium large transport flight guidance system when a hydraulic pump is turned from low to high.
Original NASA ASRS Text
Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT. REPORTER SAYS FMA CHANGED FLT MODE AND ALT SELECT BY ITSELF.
Narrative: THE F/O WAS FLYING THE ACFT. WE HAD BEEN ISSUED SEVERAL VECTORS AND TURNS BY ATC FOR FLOW CTL INTO CHICAGO O'HARE. I WAS ON THE P/A EXPLAINING THE ENRTE DELAY TO THE PAX WHEN I NOTICED THE FMA HAD CHANGED FROM 'PERF CRUISE' TO 'PERF DSCNT,' AND THE ALT SELECT HAD CHANGED FROM 35000 TO 33000'. I ASKED THE F/O IF WE HAD BEEN CLRED TO FL330. HE SAID NO. THE ACFT ALT WAS 34600' WHEN I NOTICED THE PROB. THE DSCNT WAS STOPPED AT 34500'. I DON'T KNOW WHY THE AUTOPLT ENTERED A DSCNT MODE. AN ALT WARNING DIDN'T OCCUR BECAUSE THE ALT SELECT HAD CHANGED ALSO. I SUSPECT A PWR SURGE IN THE ELECTRICAL SYS MAY HAVE CAUSED THE PROB. I HAVE EXPERIENCED THIS PROB IN THE PAST WITH THE MLG FLT GUIDANCE SYS WHEN A HYD PUMP IS TURNED FROM LOW TO HIGH.
Data retrieved from NASA's ASRS site as of August 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.