What happened
On 19 December 2011, an ATR 72-201, registration EI-REH, was performing a scheduled passenger flight from Dublin to Kerry Airport. While executing an Instrument Landing System (ILS) approach to Runway 26 in instrument meteorological conditions, the aircraft's autopilot struggled to maintain the glide path. The flight crew observed the aircraft pitching down with an excessive descent rate, eventually triggering the Enhanced Ground Prolam Proximity Warning System (EGPWS) with both 'Terrain Caution' and 'Terrain Warning' alerts.
In response to the terrain threat, the pilot initiated a go-around. A second attempt at an ILS approach was made, but because the glideslope signal remained erratic, the crew transitioned to a non-precision localizer/DME approach. The aircraft landed safely without further incident.
The investigation
The AAIU examined flight data recorder information, EGPWS logs, and air traffic control recordings. The investigation focused on why the autopilot failed to maintain the vertical profile and why the aircraft descended to a minimum altitude of approximately 1,922 feet, well below the minimum safe altitude.
Investigators also reviewed the aircraft's maintenance history and the cockpit environment. The crew was engaged in a line check, with the commander flying from the right-hand seat. The investigation also looked into the capabilities of Kerry Tower, which at the time operated using procedural control without a radar display.
Findings
- The primary cause of the unstable glideslope signal was a missing Glideslope Antenna Deflector from the inside of the aircraft's radome. This component likely detached during a recent repair of the radome, and there was no subsequent check to ensure it had been reinstalled.
- The flight crew experienced a loss of situational awareness, becoming preoccupied with the erratic autopilot performance and the 'twitching' glideslope display. This led to a failure to cross-check the aircraft's altitude against its distance from the runway.
- The crew's focus on the technical malfunction resulted in 'attention tunneling,' where the monitoring of the aircraft's vertical path relative to the terrain was neglected.
- The lack of a radar display at Kerry Tower meant the controller could not independently identify the low altitude of the aircraft, relying instead on the pilot's position reports.