Narrative:

We were operating a boeing 767-300, cruising at FL310, when we decided that FL350 might be a better altitude for us. I entered FL350 into the FMC on the cruise page to determine what our performance could be at that altitude. FL310 was currently displayed in the altitude window on the mode control panel. After determining that our performance would be acceptable, I needed to enter FL310 back into the FMC's cruise page, so it would correspond with our current cruising altitude. I was curious to find out if I slightly adjusted the altitude on the mode control panel a few hundred ft, about 300-400 ft above, then reselected FL310 in seconds would 31000 ft appear in the scratch pad on the FMC's cruise page, which is a time saving feature that is designed into the system. On some airplanes of the same type, a 100 ft change in the mode control panel selected altitude will trigger an altitude display on the FMC's scratch pad. On this particular aircraft, no altitude appeared on the scratch pad. The airplane immediately entered the cruise climb mode, advancing the throttles and increasing pitch. This is the quickest response I have ever seen to a change in programmed altitude. FL310 was displayed in the mode control panel altitude window, we controled the aircraft smoothly or I should say, since the leg was the copilot's, he controled the aircraft smoothly, disconnecting the autoplt and returning the aircraft to FL310. The altitude gained was 280 ft. To prevent a recurrence, I would consider not changing the altitude in this manner in the mode control panel out of curiosity, unless one is certain of the consequences. One curious note. Why didn't the aircraft capture any one number of altitudes in the mode control panel window such as FL311, or FL312, which the system should have sensed when I dialed FL310 back into the mode control panel, of course passing through these altitudes first.

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Original NASA ASRS Text

Title: CAPT CAUSES ACFT TO ASCEND 280 FT ABOVE ASSIGNED ALT.

Narrative: WE WERE OPERATING A BOEING 767-300, CRUISING AT FL310, WHEN WE DECIDED THAT FL350 MIGHT BE A BETTER ALT FOR US. I ENTERED FL350 INTO THE FMC ON THE CRUISE PAGE TO DETERMINE WHAT OUR PERFORMANCE COULD BE AT THAT ALT. FL310 WAS CURRENTLY DISPLAYED IN THE ALT WINDOW ON THE MODE CTL PANEL. AFTER DETERMINING THAT OUR PERFORMANCE WOULD BE ACCEPTABLE, I NEEDED TO ENTER FL310 BACK INTO THE FMC'S CRUISE PAGE, SO IT WOULD CORRESPOND WITH OUR CURRENT CRUISING ALT. I WAS CURIOUS TO FIND OUT IF I SLIGHTLY ADJUSTED THE ALT ON THE MODE CTL PANEL A FEW HUNDRED FT, ABOUT 300-400 FT ABOVE, THEN RESELECTED FL310 IN SECONDS WOULD 31000 FT APPEAR IN THE SCRATCH PAD ON THE FMC'S CRUISE PAGE, WHICH IS A TIME SAVING FEATURE THAT IS DESIGNED INTO THE SYS. ON SOME AIRPLANES OF THE SAME TYPE, A 100 FT CHANGE IN THE MODE CTL PANEL SELECTED ALT WILL TRIGGER AN ALT DISPLAY ON THE FMC'S SCRATCH PAD. ON THIS PARTICULAR ACFT, NO ALT APPEARED ON THE SCRATCH PAD. THE AIRPLANE IMMEDIATELY ENTERED THE CRUISE CLB MODE, ADVANCING THE THROTTLES AND INCREASING PITCH. THIS IS THE QUICKEST RESPONSE I HAVE EVER SEEN TO A CHANGE IN PROGRAMMED ALT. FL310 WAS DISPLAYED IN THE MODE CTL PANEL ALT WINDOW, WE CTLED THE ACFT SMOOTHLY OR I SHOULD SAY, SINCE THE LEG WAS THE COPLT'S, HE CTLED THE ACFT SMOOTHLY, DISCONNECTING THE AUTOPLT AND RETURNING THE ACFT TO FL310. THE ALT GAINED WAS 280 FT. TO PREVENT A RECURRENCE, I WOULD CONSIDER NOT CHANGING THE ALT IN THIS MANNER IN THE MODE CTL PANEL OUT OF CURIOSITY, UNLESS ONE IS CERTAIN OF THE CONSEQUENCES. ONE CURIOUS NOTE. WHY DIDN'T THE ACFT CAPTURE ANY ONE NUMBER OF ALTS IN THE MODE CTL PANEL WINDOW SUCH AS FL311, OR FL312, WHICH THE SYS SHOULD HAVE SENSED WHEN I DIALED FL310 BACK INTO THE MODE CTL PANEL, OF COURSE PASSING THROUGH THESE ALTS FIRST.

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.