Air Accidents Investigation Branch
Summary:
The pilot started an early descent while attempting to fly an ILS approach to Runway 26 at Exeter Airport. He lost situation awareness with respect to his position relative to the runway and ILS descent point, possibly due to or exacerbated by a malfunction of the glideslope deviation indicator on the course deviation indicator (CDI). His awareness would probably have been rectified by referencing the DME to confirm his position relative to the descent point. The approach controller noticed the early descent three minutes after G-BOTI departed the cleared level and instructed the pilot to climb. The subsequent approach was flown using the localiser and without vertical guidance to a successful landing.
The CDI had been reported as unserviceable prior to scheduled maintenance, which had been completed on the day of the flight, although no fault was found.