<< >> NASA ASRS Report 598804, October 2003

A 500 ft altitude excursion by C441 ATP pilot when the aircraft's autoplt altitude hold disconnects from that function on frequency with ZSE, wa.

Attributes
ACN	598804
Time
Date	200310
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : olm.vortac
State Reference	WA
Altitude	msl bound lower : 7500 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	general aviation : corporate
Make Model Name	Cessna Aircraft Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 90 flight time total : 10000 flight time type : 600
ASRS Report	598804
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	atc equipment other atc equipment : mode c other controllera
Resolutory Action	controller : issued advisory flight crew : became reoriented flight crew : returned to assigned altitude
Supplementary
Problem Areas	Environmental Factor Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Aircraft cleared to descend and maintain 8000 ft. 8000 ft was entered in altitude alerter. Armed light illuminated. Nearing 8000 ft, autoplt captured normally. Capture light illuminated. Preparations started for landing. Call was received from center inquiring about altitude. Responded level 8000 ft. (I had assumed I was still at 8000 ft.) responded 'I'll change to the #2 transponder' (because occasionally it would send an erroneous altitude.) center said I was not at 8000 ft and at this time I was shocked to see 7500 ft and in a shallow descent on my altimeter. Center said 'climb back to 8000 ft,' which I did. They then handed me off to approach and nothing else was mentioned. I believe what happened was that, in preparing for landing, I bumped the autoplt trim wheel that knocked the altitude hold feature off. While reaching for charts, checklists, etc, it can be very easy, in retrospect, to hit the trim wheel and not know it. There is no audible horn until you are 1000 ft off either way.

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Original NASA ASRS Text

Title: A 500 FT ALT EXCURSION BY C441 ATP PLT WHEN THE ACFT'S AUTOPLT ALT HOLD DISCONNECTS FROM THAT FUNCTION ON FREQ WITH ZSE, WA.

Narrative: ACFT CLRED TO DSND AND MAINTAIN 8000 FT. 8000 FT WAS ENTERED IN ALT ALERTER. ARMED LIGHT ILLUMINATED. NEARING 8000 FT, AUTOPLT CAPTURED NORMALLY. CAPTURE LIGHT ILLUMINATED. PREPARATIONS STARTED FOR LNDG. CALL WAS RECEIVED FROM CTR INQUIRING ABOUT ALT. RESPONDED LEVEL 8000 FT. (I HAD ASSUMED I WAS STILL AT 8000 FT.) RESPONDED 'I'LL CHANGE TO THE #2 XPONDER' (BECAUSE OCCASIONALLY IT WOULD SEND AN ERRONEOUS ALT.) CTR SAID I WAS NOT AT 8000 FT AND AT THIS TIME I WAS SHOCKED TO SEE 7500 FT AND IN A SHALLOW DSCNT ON MY ALTIMETER. CTR SAID 'CLB BACK TO 8000 FT,' WHICH I DID. THEY THEN HANDED ME OFF TO APCH AND NOTHING ELSE WAS MENTIONED. I BELIEVE WHAT HAPPENED WAS THAT, IN PREPARING FOR LNDG, I BUMPED THE AUTOPLT TRIM WHEEL THAT KNOCKED THE ALT HOLD FEATURE OFF. WHILE REACHING FOR CHARTS, CHKLISTS, ETC, IT CAN BE VERY EASY, IN RETROSPECT, TO HIT THE TRIM WHEEL AND NOT KNOW IT. THERE IS NO AUDIBLE HORN UNTIL YOU ARE 1000 FT OFF EITHER WAY.

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.