PA-24 Comanche
Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 30; Flight Crew Total 3000; Flight Crew Type 300
Distraction; Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors; Weather; Aircraft
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.
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.