Narrative:

While cruising IMC in light turbulence at assigned altitude 10;000 ft; I was called by center inquiring about my altitude and was told my mode C indicated 10;500 ft. I had been cruising on autopilot with altitude hold; but apparently accidently bumped the altitude hold button on the control wheel while reading the chart on my ipad; disengaging alt hld. The aircraft climbed 500 ft before I and center noticed it. This aircraft has an s-tec 50 autopilot with two small momentary push buttons on the left horn of the pilot's control wheel. One is 'a/P off' and the other is 'alt hld on/off'; and they are identical except for their color. The buttons are unguarded; protrude well above the control horn itself; and are very sensitive to light pressure. There is no audible or visual warning sound when either the a/P or the alt hld function is disengaged other than a small annunciator light on the a/P control head (on the instrument panel) extinguishing; and in this aircraft that control head is partially obscured by the control horn and a portable GPS mounted on the control horn. This is not the first time in the last 3 years I have done this; but it is the first time I've deviated more than 200 ft from my assigned altitude. It was my fault; obviously; and the situation was compounded by the IMC conditions; turbulence; and absence of any significant warning that the altitude hold disengaged. In the past 3 years that I've been flying with these avionics I've become more attuned to this phenomenon; but this time it got away from me.

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Original NASA ASRS Text

Title: PA24 pilot reports inadvertent disengagement of the autopilot altitude hold function. The aircraft drifts 500 above assigned altitude before ATC queries the reporter and the deviation is corrected.

Narrative: While cruising IMC in light turbulence at assigned altitude 10;000 FT; I was called by Center inquiring about my altitude and was told my mode C indicated 10;500 FT. I had been cruising on autopilot with altitude hold; but apparently accidently bumped the altitude hold button on the control wheel while reading the chart on my iPad; disengaging alt hld. The aircraft climbed 500 FT before I and Center noticed it. This aircraft has an S-Tec 50 autopilot with two small momentary push buttons on the left horn of the pilot's control wheel. One is 'A/P Off' and the other is 'Alt Hld On/Off'; and they are identical except for their color. The buttons are unguarded; protrude well above the control horn itself; and are very sensitive to light pressure. There is no audible or visual warning sound when either the A/P or the Alt Hld function is disengaged other than a small annunciator light on the A/P control head (on the instrument panel) extinguishing; and in this aircraft that control head is partially obscured by the control horn and a portable GPS mounted on the control horn. This is not the first time in the last 3 years I have done this; but it is the first time I've deviated more than 200 FT from my assigned altitude. It was my fault; obviously; and the situation was compounded by the IMC conditions; turbulence; and absence of any significant warning that the altitude hold disengaged. In the past 3 years that I've been flying with these avionics I've become more attuned to this phenomenon; but this time it got away from me.

Data retrieved from NASA's ASRS site as of July 2013 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.