PA-31 Navajo/Chieftan/Mojave/T1040
Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 47; Flight Crew Total 2935; Flight Crew Type 47
Confusion; Distraction; Workload; Time Pressure
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft Equipment Problem Dissipated; Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Aircraft
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.
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.