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Altitude bust. Reporter cites aircraft equipment design as not being friendly to pilot in event of a malfunctioning FMC system in that the autoplt disconnects entirely instead of at least maintaining heading and level off capability.
| Attributes | |
| ACN | 152892 |
| Time | |
| Date | 199002 |
| Day | Tue |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | airport : jfk |
| State Reference | NY |
| Altitude | msl bound lower : 32500 msl bound upper : 33400 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Operator | common carrier : air carrier |
| Make Model Name | Widebody Transport |
| Navigation In Use | Other Other |
| Flight Phase | climbout : intermediate altitude |
| Route In Use | enroute airway : any |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Qualification | pilot : atp |
| Experience | flight time last 90 days : 220 flight time total : 25000 flight time type : 1000 |
| ASRS Report | 152892 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : atp pilot : commercial pilot : instrument |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance |
| Independent Detector | aircraft equipment other aircraft equipment : unspecified other flight crewa |
| Resolutory Action | flight crew : returned to intended course or assigned course flight crew : regained aircraft control |
| Consequence | Other |
| Supplementary | |
| Primary Problem | Aircraft |
| Air Traffic Incident | other other |
Narrative:
At approximately 32500' both FMC's failed. West/O FMC autoplts trip off. Aircraft overshot clearance altitude of 33000' by 400' before aircraft brought under control. Under normal conditions this widebody transport climbs at 1500 to 2000 FPM at these altitudes with loss of autoplt reaction time for pilots is seconds or less. In this case 900' was required to control aircraft and level off at 33000'. Design of having the autoplt trip off when there is a route discontinuity is weak and unsafe. When an FMC fails it's a signal that there is a 'route discontinuity' and hence autoplt trips off. Under such conditions, autoplt should revert back to basic mode ie, altitude hold and heading hold, not off!
Original NASA ASRS Text
Title: ALT BUST. REPORTER CITES ACFT EQUIPMENT DESIGN AS NOT BEING FRIENDLY TO PLT IN EVENT OF A MALFUNCTIONING FMC SYSTEM IN THAT THE AUTOPLT DISCONNECTS ENTIRELY INSTEAD OF AT LEAST MAINTAINING HDG AND LEVEL OFF CAPABILITY.
Narrative: AT APPROX 32500' BOTH FMC'S FAILED. W/O FMC AUTOPLTS TRIP OFF. ACFT OVERSHOT CLRNC ALT OF 33000' BY 400' BEFORE ACFT BROUGHT UNDER CTL. UNDER NORMAL CONDITIONS THIS WDB CLBS AT 1500 TO 2000 FPM AT THESE ALTS WITH LOSS OF AUTOPLT REACTION TIME FOR PLTS IS SECS OR LESS. IN THIS CASE 900' WAS REQUIRED TO CTL ACFT AND LEVEL OFF AT 33000'. DESIGN OF HAVING THE AUTOPLT TRIP OFF WHEN THERE IS A RTE DISCONTINUITY IS WEAK AND UNSAFE. WHEN AN FMC FAILS IT'S A SIGNAL THAT THERE IS A 'RTE DISCONTINUITY' AND HENCE AUTOPLT TRIPS OFF. UNDER SUCH CONDITIONS, AUTOPLT SHOULD REVERT BACK TO BASIC MODE IE, ALT HOLD AND HDG HOLD, NOT OFF!
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.