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Altitude deviation. Aircraft in cruise with autoplt engaged in altitude hold. ATC cleared aircraft to increase speed and descend. PF, first officer, deselected VNAV and changed mode control panel (MCP) to IAS. Shortly thereafter, flight crew noticed the autoplt had reverted to control wheel steering and had commenced a climb. They had gained 2000 ft before noticing the discrepancy.
| Attributes | |
| ACN | 334014 |
| Time | |
| Date | 199604 |
| Day | Fri |
| Local Time Of Day | 0601 To 1200 |
| Place | |
| Locale Reference | atc facility : pdz |
| State Reference | CA |
| Altitude | msl bound lower : 28000 msl bound upper : 30000 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | artcc : zla |
| Operator | common carrier : air carrier |
| Make Model Name | Commercial Fixed Wing |
| Operating Under FAR Part | Part 121 |
| Navigation In Use | Other Other |
| Flight Phase | cruise other |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Qualification | pilot : flight engineer pilot : cfi pilot : atp |
| Experience | flight time last 90 days : 160 flight time total : 8000 flight time type : 1000 |
| ASRS Report | 334014 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : instrument pilot : commercial pilot : atp |
| Experience | flight time last 90 days : 180 flight time total : 5000 flight time type : 200 |
| ASRS Report | 324015 |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance |
| Independent Detector | aircraft equipment other aircraft equipment : unspecified other flight crewa |
| Resolutory Action | flight crew : overcame equipment problem flight crew : returned to intended course or assigned course |
| Consequence | Other |
| Supplementary | |
| Primary Problem | Flight Crew Human Performance |
| Air Traffic Incident | Pilot Deviation |
Narrative:
Inbound to lax from las we were in cruise at FL280. The lax WX was 700 degrees at 2 KTS and the aircraft was approaching the 30-day limit for an autoland check, so we decided to do an autoland. It was the first officer's leg but I would be the PF for the approach, per our airline SOP. As I briefed the approach, ZLA issued first a speed restr, 320 KTS or greater, then a descent clearance, our discretion to FL240. I looked up during the briefing and noticed that we were in a climb through FL300 and control wheel steering was annunciated on the flight management annunciator. We immediately corrected back to our cleared altitude. Before the altitude deviation we were in LNAV and altitude hold with a speed (MCP speed) mode engaged. We don't know at what point the aircraft left altitude hold and went into control wheel steering mode, which resulted in the climb. Later during the descent, the autoplt commanded a jerky, excessive pitch down at GS capture, so we flew a manual approach. It was a short 39 min flight, with a higher than average workload, and we had a very early XA25 report time. This was our second leg. I suspect that strict vigilance of the flight management annunciator when any mode control panel changes are made, even during high workload sits, might detect such a reversion to control wheel steering before it becomes an altitude excursion.
Original NASA ASRS Text
Title: ALTDEV. ACFT IN CRUISE WITH AUTOPLT ENGAGED IN ALT HOLD. ATC CLRED ACFT TO INCREASE SPD AND DSND. PF, FO, DESELECTED VNAV AND CHANGED MODE CTL PANEL (MCP) TO IAS. SHORTLY THEREAFTER, FLC NOTICED THE AUTOPLT HAD REVERTED TO CTL WHEEL STEERING AND HAD COMMENCED A CLB. THEY HAD GAINED 2000 FT BEFORE NOTICING THE DISCREPANCY.
Narrative: INBOUND TO LAX FROM LAS WE WERE IN CRUISE AT FL280. THE LAX WX WAS 700 DEGS AT 2 KTS AND THE ACFT WAS APCHING THE 30-DAY LIMIT FOR AN AUTOLAND CHK, SO WE DECIDED TO DO AN AUTOLAND. IT WAS THE FO'S LEG BUT I WOULD BE THE PF FOR THE APCH, PER OUR AIRLINE SOP. AS I BRIEFED THE APCH, ZLA ISSUED FIRST A SPD RESTR, 320 KTS OR GREATER, THEN A DSCNT CLRNC, OUR DISCRETION TO FL240. I LOOKED UP DURING THE BRIEFING AND NOTICED THAT WE WERE IN A CLB THROUGH FL300 AND CTL WHEEL STEERING WAS ANNUNCIATED ON THE FLT MGMNT ANNUNCIATOR. WE IMMEDIATELY CORRECTED BACK TO OUR CLRED ALT. BEFORE THE ALTDEV WE WERE IN LNAV AND ALT HOLD WITH A SPD (MCP SPD) MODE ENGAGED. WE DON'T KNOW AT WHAT POINT THE ACFT LEFT ALT HOLD AND WENT INTO CTL WHEEL STEERING MODE, WHICH RESULTED IN THE CLB. LATER DURING THE DSCNT, THE AUTOPLT COMMANDED A JERKY, EXCESSIVE PITCH DOWN AT GS CAPTURE, SO WE FLEW A MANUAL APCH. IT WAS A SHORT 39 MIN FLT, WITH A HIGHER THAN AVERAGE WORKLOAD, AND WE HAD A VERY EARLY XA25 RPT TIME. THIS WAS OUR SECOND LEG. I SUSPECT THAT STRICT VIGILANCE OF THE FLT MGMNT ANNUNCIATOR WHEN ANY MODE CTL PANEL CHANGES ARE MADE, EVEN DURING HIGH WORKLOAD SITS, MIGHT DETECT SUCH A REVERSION TO CTL WHEEL STEERING BEFORE IT BECOMES AN ALT EXCURSION.
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.