Autoplt altitude hold inadvertently tripped off by pilot while entering waypoints in navigation system. Altitude deviation. No indication in text if autoplt altitude hold features an altitude alert device.

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Attributes
ACN	131709
Time
Date	198912
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : psk
State Reference	VA
Altitude	msl bound lower : 8400 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : ztl tracon : cmh
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument pilot : flight engineer pilot : cfi
Experience	flight time last 90 days : 65 flight time total : 1400 flight time type : 45
ASRS Report	131709
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course none taken : detected after the fact other
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation other

Narrative:

During level cruise at 9000' on autoplt, the altitude hold (which had been on during level autoflt for about 1 hour) inadvertently tripped off, resulting in a very gradual descent to about 8400' MSL. During this time I was programming into the RNAV waypoints, which partially diverted my attention from my normal VMC IFR scan. The autoplt control buttons, including altitude hold, are installed at about knee level and within about 6' of the pilots knee. While referring to the approach chart and book and other charts, I suspect the altitude hold button was touched--probably by the nose chart book, and tripped off. The ATC controller called out the altitude deviation to me at 8400' and 9000' was promptly regained. It is suggested that this autoplt control panel have simple, clear plastic, hinged cover guards (the control buttons are lit and labeled), or nearby fence-type protection, to prevent inadvertent activation of the control buttons. An alternative 'fix' would be to install the autoplt control unit in a location not so subject to inadvertent contact.

Original NASA ASRS Text

Title: AUTOPLT ALT HOLD INADVERTENTLY TRIPPED OFF BY PLT WHILE ENTERING WAYPOINTS IN NAVIGATION SYSTEM. ALT DEVIATION. NO INDICATION IN TEXT IF AUTOPLT ALT HOLD FEATURES AN ALT ALERT DEVICE.

Narrative: DURING LEVEL CRUISE AT 9000' ON AUTOPLT, THE ALT HOLD (WHICH HAD BEEN ON DURING LEVEL AUTOFLT FOR ABOUT 1 HR) INADVERTENTLY TRIPPED OFF, RESULTING IN A VERY GRADUAL DSCNT TO ABOUT 8400' MSL. DURING THIS TIME I WAS PROGRAMMING INTO THE RNAV WAYPOINTS, WHICH PARTIALLY DIVERTED MY ATTN FROM MY NORMAL VMC IFR SCAN. THE AUTOPLT CTL BUTTONS, INCLUDING ALT HOLD, ARE INSTALLED AT ABOUT KNEE LEVEL AND WITHIN ABOUT 6' OF THE PLTS KNEE. WHILE REFERRING TO THE APCH CHART AND BOOK AND OTHER CHARTS, I SUSPECT THE ALT HOLD BUTTON WAS TOUCHED--PROBABLY BY THE NOSE CHART BOOK, AND TRIPPED OFF. THE ATC CTLR CALLED OUT THE ALT DEVIATION TO ME AT 8400' AND 9000' WAS PROMPTLY REGAINED. IT IS SUGGESTED THAT THIS AUTOPLT CONTROL PANEL HAVE SIMPLE, CLEAR PLASTIC, HINGED COVER GUARDS (THE CONTROL BUTTONS ARE LIT AND LABELED), OR NEARBY FENCE-TYPE PROTECTION, TO PREVENT INADVERTENT ACTIVATION OF THE CONTROL BUTTONS. AN ALTERNATIVE 'FIX' WOULD BE TO INSTALL THE AUTOPLT CTL UNIT IN A LOCATION NOT SO SUBJECT TO INADVERTENT CONTACT.

Data retrieved from NASA's ASRS site as of August 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.