Narrative:

Cruising at FL200 with requested altitude of FL230, center cleared us to FL210. I selected FL210 while noting the new assigned altitude on the primary flight display. I then pushed the altitude select knob to engage VNAV while I monitored the flight mode annunciator to verify entering climb mode. A few mins later, center told us to maintain FL210 and I noticed we were level at FL220. I don't know how FL220 got in the FMS, but must assume that when I pushed the altitude knob to enter VNAV, the assigned altitude changed 1 digit while I was looking over at the flight mode annunciator. We were at fairly light gross weight and climbing fast when the flight attendant came in to offer coffee. I glanced at the FMA, noticed altitude capture and turned my attention to the flight attendant. As I looked back, we were in managed level flight. I didn't notice we were 1000 ft high. Because of the multifunctional uses of this knob (push, pull, twist) for VNAV modes and level changes, it can create a trap. If each mode/level is verified in sequence it may end up erroneous. The answer with this knob is to reverify all selections affected by this knob after removing your hand from it.

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Original NASA ASRS Text

Title: CAPT OF ACR LGT ACFT MISTAKENLY SET THE WRONG ALT INTO THE ACFT FMC RESULTING IN AN ALT OVERSHOOT.

Narrative: CRUISING AT FL200 WITH REQUESTED ALT OF FL230, CTR CLRED US TO FL210. I SELECTED FL210 WHILE NOTING THE NEW ASSIGNED ALT ON THE PRIMARY FLT DISPLAY. I THEN PUSHED THE ALT SELECT KNOB TO ENGAGE VNAV WHILE I MONITORED THE FLT MODE ANNUNCIATOR TO VERIFY ENTERING CLB MODE. A FEW MINS LATER, CTR TOLD US TO MAINTAIN FL210 AND I NOTICED WE WERE LEVEL AT FL220. I DON'T KNOW HOW FL220 GOT IN THE FMS, BUT MUST ASSUME THAT WHEN I PUSHED THE ALT KNOB TO ENTER VNAV, THE ASSIGNED ALT CHANGED 1 DIGIT WHILE I WAS LOOKING OVER AT THE FLT MODE ANNUNCIATOR. WE WERE AT FAIRLY LIGHT GROSS WT AND CLBING FAST WHEN THE FLT ATTENDANT CAME IN TO OFFER COFFEE. I GLANCED AT THE FMA, NOTICED ALT CAPTURE AND TURNED MY ATTN TO THE FLT ATTENDANT. AS I LOOKED BACK, WE WERE IN MANAGED LEVEL FLT. I DIDN'T NOTICE WE WERE 1000 FT HIGH. BECAUSE OF THE MULTIFUNCTIONAL USES OF THIS KNOB (PUSH, PULL, TWIST) FOR VNAV MODES AND LEVEL CHANGES, IT CAN CREATE A TRAP. IF EACH MODE/LEVEL IS VERIFIED IN SEQUENCE IT MAY END UP ERRONEOUS. THE ANSWER WITH THIS KNOB IS TO REVERIFY ALL SELECTIONS AFFECTED BY THIS KNOB AFTER REMOVING YOUR HAND FROM IT.

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.