WW21 on approach to stp experiences autoplt malfunction, climbs 1500 ft to GS intercept.

Attributes
ACN	602860
Time
Date	200312
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : stp.airport
State Reference	MN
Altitude	msl single value : 2500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : m98.tracon
Operator	general aviation : corporate
Make Model Name	IAI1123 Westwind
Operating Under FAR Part	Part 91
Navigation In Use	ils localizer & glide slope : n/a
Route In Use	approach : instrument precision arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 85 flight time total : 12500 flight time type : 1500
ASRS Report	602860
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	controller : issued advisory controller : provided flight assist flight crew : overcame equipment problem flight crew : returned to original clearance
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Aircraft was on a vector to intercept the ILS. The autoplt was selected on heading and altitude select mode. The approach mode was selected and the indications were localizer and GS arm. Then the localizer and GS capture indications were noted. The autoplt tracked the localizer and the aircraft was configured per company SOP. The aircraft was well outside the GS intercept point and the autoplt assumed it was below the GS and started a gradual climb from assigned altitude to intercept the GS. The altitude deviation was 1500 ft. The crew was monitoring traffic targets on TCASII. There were several aircraft being vectored and requesting practice approachs at stp plus numerous VFR targets in the area. The traffic workload was very high at that time during the approach. Approach control alerted crew to altitude deviation and corrective action was taken to intercept GS manually. Autoplt malfunction was noted and repaired before further flight. That particular situation and company SOP are being evaluated for any corrective action.

Original NASA ASRS Text

Title: WW21 ON APCH TO STP EXPERIENCES AUTOPLT MALFUNCTION, CLBS 1500 FT TO GS INTERCEPT.

Narrative: ACFT WAS ON A VECTOR TO INTERCEPT THE ILS. THE AUTOPLT WAS SELECTED ON HDG AND ALT SELECT MODE. THE APCH MODE WAS SELECTED AND THE INDICATIONS WERE LOC AND GS ARM. THEN THE LOC AND GS CAPTURE INDICATIONS WERE NOTED. THE AUTOPLT TRACKED THE LOC AND THE ACFT WAS CONFIGURED PER COMPANY SOP. THE ACFT WAS WELL OUTSIDE THE GS INTERCEPT POINT AND THE AUTOPLT ASSUMED IT WAS BELOW THE GS AND STARTED A GRADUAL CLB FROM ASSIGNED ALT TO INTERCEPT THE GS. THE ALTDEV WAS 1500 FT. THE CREW WAS MONITORING TFC TARGETS ON TCASII. THERE WERE SEVERAL ACFT BEING VECTORED AND REQUESTING PRACTICE APCHS AT STP PLUS NUMEROUS VFR TARGETS IN THE AREA. THE TFC WORKLOAD WAS VERY HIGH AT THAT TIME DURING THE APCH. APCH CTL ALERTED CREW TO ALTDEV AND CORRECTIVE ACTION WAS TAKEN TO INTERCEPT GS MANUALLY. AUTOPLT MALFUNCTION WAS NOTED AND REPAIRED BEFORE FURTHER FLT. THAT PARTICULAR SIT AND COMPANY SOP ARE BEING EVALUATED FOR ANY CORRECTIVE ACTION.

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.