A PA31T autopilot pitched up uncommanded when the approach mode was selected for an ILS; so the pilo...

Attributes
ACN	1053946
Time
Date	201212
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	PA-31 Navajo/Chieftan/Mojave/T1040
Operating Under FAR Part	Part 135
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain Single Pilot
Qualification	Flight Crew Commercial Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 47 Flight Crew Total 2935 Flight Crew Type 47
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

Narrative:

While on a vector at 2;000 ft MSL to intercept the localizer for the ILS 6; I engaged approach mode on the autopilot. Several seconds after engaging approach mode the airplane initiated an uncommanded GA (go around) mode and pitched up violently. Having never experienced this autopilot behavior before it caught me off guard. I immediately disconnected the autopilot and pitched forward aggressively. I wrestled the aircraft back under control but not before descending approximately 400-500 ft below the assigned altitude. ATC advised me I was below altitude and asked if I was having trouble maintaining altitude. I told ATC that I had a problem but eventually regained control. ATC asked if I was going to be able to shoot another approach (ILS) to which I responded yes. ATC vectored me again to intercept the localizer for the ILS 6. I again engaged the approach mode on the autopilot. The autopilot captured the localizer and GS without issue. This second approach and subsequent landing were without issue.

Original NASA ASRS Text

Title: A PA31T autopilot pitched up uncommanded when the APPROACH mode was selected for an ILS; so the pilot disconnected the autopilot; regained aircraft control but descended below the inbound altitude before finally stabilizing and completing the approach.

Narrative: While on a vector at 2;000 FT MSL to intercept the localizer for the ILS 6; I engaged APPROACH mode on the autopilot. Several seconds after engaging APPROACH mode the airplane initiated an uncommanded GA (go around) mode and pitched up violently. Having never experienced this autopilot behavior before it caught me off guard. I immediately disconnected the autopilot and pitched forward aggressively. I wrestled the aircraft back under control but not before descending approximately 400-500 FT below the assigned altitude. ATC advised me I was below altitude and asked if I was having trouble maintaining altitude. I told ATC that I had a problem but eventually regained control. ATC asked if I was going to be able to shoot another approach (ILS) to which I responded yes. ATC vectored me again to intercept the Localizer for the ILS 6. I again engaged the approach mode on the autopilot. The autopilot captured the localizer and GS without issue. This second approach and subsequent landing were without issue.

Data retrieved from NASA's ASRS site as of July 2013 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.

A PA31T autopilot pitched up uncommanded when the approach mode was selected for an ILS; so the pilot disconnected the autopilot; regained aircraft control but descended below the inbound altitude before finally stabilizing and completing the approach.