Narrative:

The aircraft had no deferred maintenance discrepancies or current mechanical problems. The crew members were in good physical condition. There was no known traffic on or near the vicinity of the airport. It was my turn to fly. The clearance was copied and read back as follows: 'runway heading as filed, maintain 12000', expect higher 10 mins after departure. Departure control frequency 120.65, squawk 1440.' the checklist was completed, including a double-check of all flight and navigation INS. When handed off to tower, the controller cleared us for takeoff and to turn left on course. The aircraft was light (13 passenger) and we were climbing at 3000+ FPM. During the course of the takeoff, turn or climb the altitude selector on the autoplt mode control panel changed or was accidentally moved to 15000' (cause unknown). We were notified by departure control at about 13200-13300' about the altitude error. I expeditiously stopped the climb by 13500' and was on the way back to 12000' when we were handed to ZDV. Adv instructed us to continue our climb to 23000'. As I said earlier, the cause of the occurrence is unknown. The altitude selector knob could have been inadvertently moved. An electrical transient may have moved it or some mechanical quirk. Also it is common practice at such airports as casper, wy, for ground control to issue your clearance on taxiout. Although it was improbable, it is possible due to the workload during taxi to misset an INS. However, the taxi check does require the 'flight-navigation INS...set/X-checked.'

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Original NASA ASRS Text

Title: FLT CREW OF ACR MLG DEPARTING CPR CLIMBS TO 13500' WHEN ONLY CLEARED TO 12000' DUE ALT ALERTER BEING EITHER MISSET, OR CHANGING FOR REASONS UNEXPLAINED.

Narrative: THE ACFT HAD NO DEFERRED MAINT DISCREPANCIES OR CURRENT MECHANICAL PROBS. THE CREW MEMBERS WERE IN GOOD PHYSICAL CONDITION. THERE WAS NO KNOWN TFC ON OR NEAR THE VICINITY OF THE ARPT. IT WAS MY TURN TO FLY. THE CLRNC WAS COPIED AND READ BACK AS FOLLOWS: 'RWY HDG AS FILED, MAINTAIN 12000', EXPECT HIGHER 10 MINS AFTER DEP. DEP CTL FREQ 120.65, SQUAWK 1440.' THE CHKLIST WAS COMPLETED, INCLUDING A DOUBLE-CHK OF ALL FLT AND NAV INS. WHEN HANDED OFF TO TWR, THE CTLR CLRED US FOR TKOF AND TO TURN LEFT ON COURSE. THE ACFT WAS LIGHT (13 PAX) AND WE WERE CLBING AT 3000+ FPM. DURING THE COURSE OF THE TKOF, TURN OR CLB THE ALT SELECTOR ON THE AUTOPLT MODE CONTROL PANEL CHANGED OR WAS ACCIDENTALLY MOVED TO 15000' (CAUSE UNKNOWN). WE WERE NOTIFIED BY DEP CTL AT ABOUT 13200-13300' ABOUT THE ALT ERROR. I EXPEDITIOUSLY STOPPED THE CLB BY 13500' AND WAS ON THE WAY BACK TO 12000' WHEN WE WERE HANDED TO ZDV. ADV INSTRUCTED US TO CONTINUE OUR CLB TO 23000'. AS I SAID EARLIER, THE CAUSE OF THE OCCURRENCE IS UNKNOWN. THE ALT SELECTOR KNOB COULD HAVE BEEN INADVERTENTLY MOVED. AN ELECTRICAL TRANSIENT MAY HAVE MOVED IT OR SOME MECHANICAL QUIRK. ALSO IT IS COMMON PRACTICE AT SUCH ARPTS AS CASPER, WY, FOR GND CTL TO ISSUE YOUR CLRNC ON TAXIOUT. ALTHOUGH IT WAS IMPROBABLE, IT IS POSSIBLE DUE TO THE WORKLOAD DURING TAXI TO MISSET AN INS. HOWEVER, THE TAXI CHK DOES REQUIRE THE 'FLT-NAV INS...SET/X-CHKED.'

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.