Narrative:

On a flight to ontario; california. We were cleared direct to lando intersection on the ziggy 5 arrival. As we were completing the climb to FL330; not quite leveled off yet; ATC gave us 'pilot's discretion to FL240.' the copilot set 24;000 in the altitude window. A few minutes later the copilot said 'what's going on with the altitude?' I then observed that our altitude was passing 34;100 in a slow climb. I immediately descended back to 33;000 and told the copilot to tell ATC we were returning to FL330. The controller responded with 'ok'. I checked our TCAS for traffic and observed no aircraft shown on the screen.there were several factors which came together to contribute to this event. Primarily; the autopilot had apparently not yet captured the 33;000 foot altitude as we initially leveled off from our climb before the copilot twisted in 24;000. The air was smooth. It felt like to me that the aircraft had leveled off like it always does at that point. The light level in the cockpit was in that transition stage as the sun had set; but there was still outside light glaring on the instrument panel. I had just a few minutes earlier switched the panel lighting on; which changes the annunciator lights from bright to dim; causing me not to be able to discern that the 'altitude' light on the autopilot control panel was not illuminated. That light being off means that the altitude hold function is not operating.another factor in this situation is the copilot's lack of experience in this model of aircraft. He is new to our company having recently completed informal training with our chief pilot. This was our 4th or 5th day together; so I was still trying to closely monitor his actions. Unfortunately; he said he was not aware of the requirement to wait until the 'altitude' light illuminates before selecting another altitude. I should have monitored his procedures and also should have noticed the 'altitude' light off followed by the altitude deviation. During this time I was engaged in verifying that the cockpit was set up for the arrival into the ontario area. I should not have assumed that the aircraft was level at 33;000; or that the copilot was monitoring the flight instruments.

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Original NASA ASRS Text

Title: A CE-550 Captain reported receiving a descent clearance while still climbing and immediately entered the newly cleared descent altitude into the altitude window; however the aircraft continued to climb.

Narrative: On a flight to Ontario; California. We were cleared direct to LANDO intersection on the ZIGGY 5 arrival. As we were completing the climb to FL330; not quite leveled off yet; ATC gave us 'pilot's discretion to FL240.' The copilot set 24;000 in the altitude window. A few minutes later the copilot said 'what's going on with the altitude?' I then observed that our altitude was passing 34;100 in a slow climb. I immediately descended back to 33;000 and told the copilot to tell ATC we were returning to FL330. The controller responded with 'OK'. I checked our TCAS for traffic and observed no aircraft shown on the screen.There were several factors which came together to contribute to this event. Primarily; the autopilot had apparently not yet captured the 33;000 foot altitude as we initially leveled off from our climb before the copilot twisted in 24;000. The air was smooth. It felt like to me that the aircraft had leveled off like it always does at that point. The light level in the cockpit was in that transition stage as the sun had set; but there was still outside light glaring on the instrument panel. I had just a few minutes earlier switched the panel lighting on; which changes the annunciator lights from bright to dim; causing me not to be able to discern that the 'ALT' light on the autopilot control panel was not illuminated. That light being off means that the altitude hold function is not operating.Another factor in this situation is the copilot's lack of experience in this model of aircraft. He is new to our company having recently completed informal training with our chief pilot. This was our 4th or 5th day together; so I was still trying to closely monitor his actions. Unfortunately; he said he was not aware of the requirement to wait until the 'ALT' light illuminates before selecting another altitude. I should have monitored his procedures and also should have noticed the 'ALT' light off followed by the altitude deviation. During this time I was engaged in verifying that the cockpit was set up for the arrival into the Ontario area. I should not have assumed that the aircraft was level at 33;000; or that the copilot was monitoring the flight instruments.

Data retrieved from NASA's ASRS site 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.