B727 flight crew experiences a possible altitude deviation caused by an altitude alerter/autopilot malfunction. Fatigue was a contributing factor in the incident.
Synopsis
B727 flight crew experiences a possible altitude deviation caused by an altitude alerter/autopilot malfunction. Fatigue was a contributing factor in the incident.
Narrative
The CADC (for some reason) was in DADS 2 which made the altitude alerter inoperative. We asked Center if we could stop the climb at FL180 to troubleshoot the problem. I was concerned that we had missed or not made the correct level off and when Center was queried they indicated that there were no issues and for us to maintain FL180 until we were sure what our problem was. After reselecting DADS 1 and continuing the climb; the alerter performed normally for the remainder of the flight. A causal factor was crew fatigue. This was a difficult trip with 2 'circadian flips' and 2 trip revisions in as many days and we were not well rested. The altitude alerter fail flag is difficult to see at night and there are no aural or visual warnings (other than the light cap for the CADC select) that the altitude alerter had failed.
Second reporter narrative
On initial climbout; we were cleared to 15;000 FT and got a frequency change to Center. We were climbing through 17;000 FT when we questioned ourselves as to whether we had been cleared to FL230. We requested a level off at FL180 and that was approved. We leveled off and figured out that our altitude capture system had failed with no overt indication to the crew; which is why we had passed 15;000 FT. We trouble-shot the system and got it working again. The Controller then cleared us to FL230. We are still unsure whether we had been previously cleared to FL230 and didn't actually bust an altitude or whether we were only cleared as far as 15;000 FT. The Controller said there was no problem and no aircraft were anywhere near the vicinity. Fatigue was a major factor. I flew all night the previous night; had a short day sleep; then we got a last-minute schedule change that slid our departure for this leg 5 hours later. On my original schedule; I would have been off duty and in bed for hours before this incident occurred.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.