Narrative:

Cleared to FL190 during climb. Approaching FL190, ATC called TA. While looking outside for traffic, autoplt did not hold altitude at FL190. When I noted the altimeter again, I saw it climbing above FL190. I immediately stopped climb and returned to FL190 after FL195. I believe that glare from sunlight gives a false indication on altitude hold annunciator. Altitude hold function can appear to be on when it is not. Future prevention actions: be on lookout for sunlight glare causing false annunciator indications, and ensure proper internal illumination of the altitude hold annunciator. Also, I will be especially careful as to altitude control when distracted by ATC and looking outside of cockpit for traffic. Supplemental information from acn 336120: ATC called us about the altitude after the altitude was back at about FL190. We never saw any traffic nearby and do not know of any conflict.

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Original NASA ASRS Text

Title: WHILE LOOKING FOR TFC DURING THE CLB, A LR35 FLC OVERSHOT THEIR ASSIGNED ALT. THE SUN'S GLARE CAUSED THE ALT HOLD TO APPEAR ILLUMINATED AND SELECTED WHEN IT WAS NOT, AND AS A RESULT, THE AUTOPLT DID NOT CAPTURE THE ALT.

Narrative: CLRED TO FL190 DURING CLB. APCHING FL190, ATC CALLED TA. WHILE LOOKING OUTSIDE FOR TFC, AUTOPLT DID NOT HOLD ALT AT FL190. WHEN I NOTED THE ALTIMETER AGAIN, I SAW IT CLBING ABOVE FL190. I IMMEDIATELY STOPPED CLB AND RETURNED TO FL190 AFTER FL195. I BELIEVE THAT GLARE FROM SUNLIGHT GIVES A FALSE INDICATION ON ALT HOLD ANNUNCIATOR. ALT HOLD FUNCTION CAN APPEAR TO BE ON WHEN IT IS NOT. FUTURE PREVENTION ACTIONS: BE ON LOOKOUT FOR SUNLIGHT GLARE CAUSING FALSE ANNUNCIATOR INDICATIONS, AND ENSURE PROPER INTERNAL ILLUMINATION OF THE ALT HOLD ANNUNCIATOR. ALSO, I WILL BE ESPECIALLY CAREFUL AS TO ALT CTL WHEN DISTRACTED BY ATC AND LOOKING OUTSIDE OF COCKPIT FOR TFC. SUPPLEMENTAL INFO FROM ACN 336120: ATC CALLED US ABOUT THE ALT AFTER THE ALT WAS BACK AT ABOUT FL190. WE NEVER SAW ANY TFC NEARBY AND DO NOT KNOW OF ANY CONFLICT.

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.