B727 flight crew overshoots altitude due to malfunctioning autoplt.

Attributes
ACN	438038
Time
Date	199905
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : mol.vor
State Reference	VA
Altitude	msl bound lower : 35000 msl bound upper : 35400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : enroute altitude change
Route In Use	approach : traffic pattern enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : commercial pilot : flight engineer pilot : instrument pilot : multi engine pilot : private
Experience	flight time last 90 days : 150 flight time total : 15000 flight time type : 4500
ASRS Report	438038
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	aircraft equipment : altitude alert other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to original clearance
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Approaching assigned cruise altitude, flight crew was momentarily distraction by frequency change to new controller. PF noticed high rate of climb approaching assigned altitude, and took appropriate measure to avoid altitude deviation, disconnecting autoplt. Normal cues were presented, enforcing the belief that the autoplt had been disconnected, but the aircraft continued through the assigned altitude despite efforts to return it manually to the assigned altitude. It was quickly discovered that although all cues indicated the autoplt had been disconnected, it was still engaged and resisted the efforts of the PF to correct back to the assigned altitude. Multiple disconnect commands finally succeeded in disconnecting the autoplt and the aircraft was flown back to the assigned altitude. Maximum altitude deviation at apex of climb was +400 ft. Primary cause of deviation was failure of the autoplt to disconnect properly. A secondary factor was distraction during a critical change of phase of flight.

Original NASA ASRS Text

Title: B727 FLC OVERSHOOTS ALT DUE TO MALFUNCTIONING AUTOPLT.

Narrative: APCHING ASSIGNED CRUISE ALT, FLC WAS MOMENTARILY DISTR BY FREQ CHANGE TO NEW CTLR. PF NOTICED HIGH RATE OF CLB APCHING ASSIGNED ALT, AND TOOK APPROPRIATE MEASURE TO AVOID ALTDEV, DISCONNECTING AUTOPLT. NORMAL CUES WERE PRESENTED, ENFORCING THE BELIEF THAT THE AUTOPLT HAD BEEN DISCONNECTED, BUT THE ACFT CONTINUED THROUGH THE ASSIGNED ALT DESPITE EFFORTS TO RETURN IT MANUALLY TO THE ASSIGNED ALT. IT WAS QUICKLY DISCOVERED THAT ALTHOUGH ALL CUES INDICATED THE AUTOPLT HAD BEEN DISCONNECTED, IT WAS STILL ENGAGED AND RESISTED THE EFFORTS OF THE PF TO CORRECT BACK TO THE ASSIGNED ALT. MULTIPLE DISCONNECT COMMANDS FINALLY SUCCEEDED IN DISCONNECTING THE AUTOPLT AND THE ACFT WAS FLOWN BACK TO THE ASSIGNED ALT. MAX ALTDEV AT APEX OF CLB WAS +400 FT. PRIMARY CAUSE OF DEV WAS FAILURE OF THE AUTOPLT TO DISCONNECT PROPERLY. A SECONDARY FACTOR WAS DISTR DURING A CRITICAL CHANGE OF PHASE OF FLT.

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.