What happened
On 22 October 2022, a Piper PA-28-151, registration G-BOTI, was conducting a private instrument approach to Runway 26 at Exeter Airport. While flying in instrument meteorological conditions, the pilot began a descent from 2,600 ft approximately 5 nm before the intended descent point.
Believing the aircraft was on the correct glidepath, the pilot continued the descent, which averaged 600 ft/min. The aircraft eventually reached an altitude of approximately 1,150 ft, which was only 400 ft above the undulating terrain. The approach controller noticed the deviation and instructed the pilot to climb. Following a go-around, the pilot identified that the course deviation indicator (CDI) was not functioning correctly. A subsequent approach using localizer guidance and GPS waypoints allowed for a successful landing.
The investigation
The AAIB examined the aircraft's navigation equipment, specifically the Garmin GNS430. Prior to the flight, the CDI had been reported unserviceable, but maintenance performed at Biggin Hill had cleared the defect. The investigation reviewed radar data, radio transmissions, and GNSS track logs.
Investigators found that the pilot's awareness of his position relative to the runway was compromised. The investigation also looked into air traffic control procedures, noting that the controller's clearance did not use the specific phraseology required by unit procedures for intercepts outside of 10 nm, which would have prompted a distance check via DME.
Findings
- The pilot initiated an early descent due to degraded situation awareness regarding his position relative to the runway and the descent point.
- The pilot relied on a static glideslope indicator that appeared centered, which reinforced his incorrect belief that he was on the correct path.
- A potential intermittent malfunction of the glideslope deviation indicator may have contributed to the pilot's error.
- The pilot failed to use the DME to independently verify his distance from the threshold, which would have revealed he was too far out to descend safely.
- The air traffic controller did not use the prescribed phraseology, which would have made the descent conditional on a DME check.