Incident Overview
A pilot executed a coupled Instrument Landing System (ILS) approach using the autopilot approach mode. The aircraft was in an unstabilized condition at the time of engagement, being high and fast with a rate of descent of approximately 2000 feet per minute.
Approach Details
The approach parameters were outside those required for a stabilized approach. When the autopilot captured the glide slope, the aircraft responded with a series of abrupt pitch movements. The nose pitched down sharply, causing the aircraft to fly through the glide slope. Subsequently, the aircraft pitched up sharply, again flying through the glide slope, this time above it. This sequence repeated with another nose-down pitch before the pilot was able to disengage the autopilot.
Autopilot Behavior
The pitch excursions during the glide slope capture and subsequent oscillations were rapid and extreme. The autopilot's attempt to correct for the initial high and fast condition resulted in overcontrolling, leading to multiple cycles of pitch changes.
Outcome
The pilot successfully landed the aircraft without any further incident. However, the pitch oscillations experienced during the approach imposed loads on the airframe that exceeded the aircraft's designed stress limits.