Air carrier medium large transport altitude deviation overshot. Flight crew inadvertently changed altitude alert setting while arming automatic level off.

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Attributes
ACN	135102
Time
Date	199001
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dtw
State Reference	MI
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other descent other
Route In Use	enroute : on vectors enroute airway : v26
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 140 flight time total : 13000 flight time type : 400
ASRS Report	135102
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 8000 flight time type : 350
ASRS Report	135276
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Just before cetus intersection on V-26 we were cleared to 11,000' from 12,000'. '11,000'' was acknowledged, set and verified per company procedure. Pilot flying (autoflt mode) actuated level change control. Over cetus intersection we were given a radar vector turn to 360 degree. During turn a discussion of correct assigned altitude ensued as the aircraft descended below 11,000' for the 10,000' which was now observed in the altitude window. About this time controller issued 10,000' traffic at 9 1/2 - 10 mi. PNF, confirmed cleared altitude of 11,000' and aircraft was promptly flown back to 11,000'. Traffic was sighted in the distance and no actual conflict ever existed. I have seen the set altitude inadvertently changed by 1000' upon activation of the level change control (pulling) which is also the altitude cranking control (turning). An errant minute twist of control while pulling for activation of level change can change the displayed altitude either 1000' or 100' depending on selected mode. I believe this incident occurred as described above and will insure that any possible future error of this type will not be in excess of 100'.

Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT. FLT CREW INADVERTENTLY CHANGED ALT ALERT SETTING WHILE ARMING AUTO LEVEL OFF.

Narrative: JUST BEFORE CETUS INTXN ON V-26 WE WERE CLRED TO 11,000' FROM 12,000'. '11,000'' WAS ACKNOWLEDGED, SET AND VERIFIED PER COMPANY PROC. PLT FLYING (AUTOFLT MODE) ACTUATED LEVEL CHANGE CONTROL. OVER CETUS INTXN WE WERE GIVEN A RADAR VECTOR TURN TO 360 DEG. DURING TURN A DISCUSSION OF CORRECT ASSIGNED ALT ENSUED AS THE ACFT DESCENDED BELOW 11,000' FOR THE 10,000' WHICH WAS NOW OBSERVED IN THE ALT WINDOW. ABOUT THIS TIME CTLR ISSUED 10,000' TFC AT 9 1/2 - 10 MI. PNF, CONFIRMED CLRED ALT OF 11,000' AND ACFT WAS PROMPTLY FLOWN BACK TO 11,000'. TFC WAS SIGHTED IN THE DISTANCE AND NO ACTUAL CONFLICT EVER EXISTED. I HAVE SEEN THE SET ALT INADVERTENTLY CHANGED BY 1000' UPON ACTIVATION OF THE LEVEL CHANGE CONTROL (PULLING) WHICH IS ALSO THE ALT CRANKING CONTROL (TURNING). AN ERRANT MINUTE TWIST OF CONTROL WHILE PULLING FOR ACTIVATION OF LEVEL CHANGE CAN CHANGE THE DISPLAYED ALT EITHER 1000' OR 100' DEPENDING ON SELECTED MODE. I BELIEVE THIS INCIDENT OCCURRED AS DESCRIBED ABOVE AND WILL INSURE THAT ANY POSSIBLE FUTURE ERROR OF THIS TYPE WILL NOT BE IN EXCESS OF 100'.

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.