Reporter descends 500 ft past his assigned altitude before correcting back.

Attributes
ACN	297577
Time
Date	199502
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sln
State Reference	KS
Altitude	msl bound lower : 21500 msl bound upper : 22000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zkc artcc : zid
Operator	general aviation : corporate
Make Model Name	Citation I
Operating Under FAR Part	Part 91
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Beechcraft Twin Turboprop Jet Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 125 flight time total : 11000 flight time type : 200
ASRS Report	297577
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : far other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	faa : reviewed incident with flight crew Other
Miss Distance	horizontal : 30000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

During descent from FL350 into sln airport we were cleared to FL200 and advised of a king air 12 O'clock 12 mi climbing to FL190. After a min or so our clearance was changed to FL220 and then the king air was cleared to FL210. We set our altitude selector for the autoplt to level at FL220. We were advised the king air was now at 10 O'clock and 5 mi. We looked and announced to center that we had the king air in sight. As I turned back into the cockpit I noticed our altitude was 21500 ft. I abruptly pulled the nose up and climbed back to FL220 very rapidly. It took maybe 5 seconds to climb back to FL220. As we leveled at FL220 center asked our altitude. We reported that we were level at FL220. (We were when he asked.) we checked our equipment and the altitude selector was indeed set at FL220. However, the autoplt altitude selector was no longer armed. Both pilots had observed it being armed but now it was not. (No explanation.) we were subsequently cleared to lower altitudes. Before being released to sln tower center asked if we previously copied our clearance to FL220 and not FL200 and we answered 'yes.' he then asked, 'and you did not pass below FL220 correct.' we answered that we had not passed below FL220. There was no more conversation about it. We did in fact descend to 21500 ft and return to FL220. Apparently something was bumped in the cockpit to knock the autoplt altitude select off line. It worked properly before and after. We will in the future monitor our altitude and autoplt more closely. The king air at 10 O'clock and 5 mi was not a threat. I could have stayed in the cockpit and let the copilot look for the traffic instead of both of us looking for traffic.

Original NASA ASRS Text

Title: RPTR DSNDS 500 FT PAST HIS ASSIGNED ALT BEFORE CORRECTING BACK.

Narrative: DURING DSCNT FROM FL350 INTO SLN ARPT WE WERE CLRED TO FL200 AND ADVISED OF A KING AIR 12 O'CLOCK 12 MI CLBING TO FL190. AFTER A MIN OR SO OUR CLRNC WAS CHANGED TO FL220 AND THEN THE KING AIR WAS CLRED TO FL210. WE SET OUR ALT SELECTOR FOR THE AUTOPLT TO LEVEL AT FL220. WE WERE ADVISED THE KING AIR WAS NOW AT 10 O'CLOCK AND 5 MI. WE LOOKED AND ANNOUNCED TO CTR THAT WE HAD THE KING AIR IN SIGHT. AS I TURNED BACK INTO THE COCKPIT I NOTICED OUR ALT WAS 21500 FT. I ABRUPTLY PULLED THE NOSE UP AND CLBED BACK TO FL220 VERY RAPIDLY. IT TOOK MAYBE 5 SECONDS TO CLB BACK TO FL220. AS WE LEVELED AT FL220 CTR ASKED OUR ALT. WE RPTED THAT WE WERE LEVEL AT FL220. (WE WERE WHEN HE ASKED.) WE CHKED OUR EQUIP AND THE ALT SELECTOR WAS INDEED SET AT FL220. HOWEVER, THE AUTOPLT ALT SELECTOR WAS NO LONGER ARMED. BOTH PLTS HAD OBSERVED IT BEING ARMED BUT NOW IT WAS NOT. (NO EXPLANATION.) WE WERE SUBSEQUENTLY CLRED TO LOWER ALTS. BEFORE BEING RELEASED TO SLN TWR CTR ASKED IF WE PREVIOUSLY COPIED OUR CLRNC TO FL220 AND NOT FL200 AND WE ANSWERED 'YES.' HE THEN ASKED, 'AND YOU DID NOT PASS BELOW FL220 CORRECT.' WE ANSWERED THAT WE HAD NOT PASSED BELOW FL220. THERE WAS NO MORE CONVERSATION ABOUT IT. WE DID IN FACT DSND TO 21500 FT AND RETURN TO FL220. APPARENTLY SOMETHING WAS BUMPED IN THE COCKPIT TO KNOCK THE AUTOPLT ALT SELECT OFF LINE. IT WORKED PROPERLY BEFORE AND AFTER. WE WILL IN THE FUTURE MONITOR OUR ALT AND AUTOPLT MORE CLOSELY. THE KING AIR AT 10 O'CLOCK AND 5 MI WAS NOT A THREAT. I COULD HAVE STAYED IN THE COCKPIT AND LET THE COPLT LOOK FOR THE TFC INSTEAD OF BOTH OF US LOOKING FOR TFC.

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.