C208 pilot on IFR flight plan descends from 6000 ft to 5000 ft when autoplt altitude hold malfunctio...

Attributes
ACN	630778
Time
Date	200409
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sat.airport
State Reference	TX
Altitude	msl single value : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sat.tracon
Operator	general aviation : personal
Make Model Name	Caravan 1 208A
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : multi engine pilot : instrument pilot : commercial pilot : cfi
Experience	flight time last 90 days : 150 flight time total : 3000 flight time type : 550
ASRS Report	630778
Person 2
Affiliation	other
Function	flight crew : single pilot
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude maintenance problem : improper maintenance non adherence : far other anomaly
Independent Detector	aircraft equipment other aircraft equipment : altimeter other flight crewa
Resolutory Action	flight crew : regained aircraft control flight crew : returned to assigned altitude
Consequence	other
Factors
Maintenance	contributing factor : schedule pressure performance deficiency : logbook entry performance deficiency : non compliance with legal requirements performance deficiency : repair
Supplementary
Problem Areas	Flight Crew Human Performance Maintenance Human Performance Aircraft
Primary Problem	Maintenance Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I leveled the aircraft off in cruise flight at 6000 ft MSL. I then engaged the autoplt and observed for 1-3 mins. The autoplt appeared to be in control of the aircraft so I busied myself with some paperwork. When I looked up again the aircraft had descended 1000 ft to 5000 ft MSL. The autoplt had completely failed with respect to altitude hold. ATC did not say a word about my altitude and I quickly climbed back to my original altitude. After a closer examination of the autoplt working, I assumed it was working. Later I spoke with the previous pilot who had flown the aircraft and he told me that maintenance had told him not to squawk the autoplt and they would take care of it as soon as possible. Thankfully the WX was VFR in flat terrain. From now on I'll always have a healthy skepticism towards automation and try to be less complacent. I will also let maintenance know that if something is broken it must be squawked so other pilots can know the condition of the ship.

Original NASA ASRS Text

Title: C208 PLT ON IFR FLT PLAN DSNDS FROM 6000 FT TO 5000 FT WHEN AUTOPLT ALT HOLD MALFUNCTIONS.

Narrative: I LEVELED THE ACFT OFF IN CRUISE FLT AT 6000 FT MSL. I THEN ENGAGED THE AUTOPLT AND OBSERVED FOR 1-3 MINS. THE AUTOPLT APPEARED TO BE IN CTL OF THE ACFT SO I BUSIED MYSELF WITH SOME PAPERWORK. WHEN I LOOKED UP AGAIN THE ACFT HAD DSNDED 1000 FT TO 5000 FT MSL. THE AUTOPLT HAD COMPLETELY FAILED WITH RESPECT TO ALT HOLD. ATC DID NOT SAY A WORD ABOUT MY ALT AND I QUICKLY CLBED BACK TO MY ORIGINAL ALT. AFTER A CLOSER EXAMINATION OF THE AUTOPLT WORKING, I ASSUMED IT WAS WORKING. LATER I SPOKE WITH THE PREVIOUS PLT WHO HAD FLOWN THE ACFT AND HE TOLD ME THAT MAINT HAD TOLD HIM NOT TO SQUAWK THE AUTOPLT AND THEY WOULD TAKE CARE OF IT ASAP. THANKFULLY THE WX WAS VFR IN FLAT TERRAIN. FROM NOW ON I'LL ALWAYS HAVE A HEALTHY SKEPTICISM TOWARDS AUTOMATION AND TRY TO BE LESS COMPLACENT. I WILL ALSO LET MAINT KNOW THAT IF SOMETHING IS BROKEN IT MUST BE SQUAWKED SO OTHER PLTS CAN KNOW THE CONDITION OF THE SHIP.

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.

C208 pilot on IFR flight plan descends from 6000 ft to 5000 ft when autoplt altitude hold malfunctions.