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Loss of altitude hold mode of autoplt results in altitude deviation into class a airspace when pilot of VFR small transport has attention diverted by low manifold pressure.
| Attributes | |
| ACN | 634010 |
| Time | |
| Date | 200410 |
| Day | Mon |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | airport : ptv.airport |
| State Reference | CA |
| Altitude | msl single value : 22000 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Operator | general aviation : corporate |
| Make Model Name | Small Transport, Low Wing, 2 Recip Eng |
| Operating Under FAR Part | Part 91 |
| Flight Phase | cruise : level |
| Flight Plan | None |
| Person 1 | |
| Affiliation | company : corporate |
| Function | flight crew : single pilot |
| Qualification | pilot : commercial pilot : multi engine pilot : instrument |
| Experience | flight time last 90 days : 10 flight time total : 3800 flight time type : 750 |
| ASRS Report | 634010 |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : far other anomaly other spatial deviation |
| Independent Detector | aircraft equipment other aircraft equipment : altimeter, oat gauge other flight crewa |
| Resolutory Action | flight crew : overcame equipment problem flight crew : returned to assigned altitude flight crew : regained aircraft control |
| Consequence | other |
| Supplementary | |
| Problem Areas | Aircraft Flight Crew Human Performance |
| Primary Problem | Aircraft |
| Air Traffic Incident | Pilot Deviation |
Narrative:
While cruising VFR at 17500 ft manifold pressure on the right engine dropped slowly about 1.5 inches. While I was troubleshooting the problem (still unknown) the altitude hold inadvertently disengaged. I didn't notice this nor that the aircraft drifted up to 22000 ft until during the manifold pressure troubleshooting. I noticed the OAT (outside air temperature) had dropped about 10 degrees C which led me to the altimeter where I discovered the problem. I immediately began a descent back to 17500 ft. Lessons learned: troubleshooting a problem does not relieve the PIC of their duty to keep the 'scan' going at all times. Sometimes a perfectly good autoplt may fail partially (or even totally in-flight) with no indication other than for the aircraft to drift off. (I was not on flight following nor in contact with ATC.)
Original NASA ASRS Text
Title: LOSS OF ALT HOLD MODE OF AUTOPLT RESULTS IN ALTDEV INTO CLASS A AIRSPACE WHEN PLT OF VFR SMT HAS ATTN DIVERTED BY LOW MANIFOLD PRESSURE.
Narrative: WHILE CRUISING VFR AT 17500 FT MANIFOLD PRESSURE ON THE R ENG DROPPED SLOWLY ABOUT 1.5 INCHES. WHILE I WAS TROUBLESHOOTING THE PROB (STILL UNKNOWN) THE ALT HOLD INADVERTENTLY DISENGAGED. I DIDN'T NOTICE THIS NOR THAT THE ACFT DRIFTED UP TO 22000 FT UNTIL DURING THE MANIFOLD PRESSURE TROUBLESHOOTING. I NOTICED THE OAT (OUTSIDE AIR TEMP) HAD DROPPED ABOUT 10 DEGS C WHICH LED ME TO THE ALTIMETER WHERE I DISCOVERED THE PROB. I IMMEDIATELY BEGAN A DSCNT BACK TO 17500 FT. LESSONS LEARNED: TROUBLESHOOTING A PROB DOES NOT RELIEVE THE PIC OF THEIR DUTY TO KEEP THE 'SCAN' GOING AT ALL TIMES. SOMETIMES A PERFECTLY GOOD AUTOPLT MAY FAIL PARTIALLY (OR EVEN TOTALLY INFLT) WITH NO INDICATION OTHER THAN FOR THE ACFT TO DRIFT OFF. (I WAS NOT ON FLT FOLLOWING NOR IN CONTACT WITH ATC.)
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.