What happened
On 2 September 2016, an ATR 72-212A, registration EI-FAW, was performing a scheduled passenger flight from Edinburgh to Dublin. While executing an instrument landing system (ILS) approach to Runway 28, the aircraft descended below the altitude authorized by Air Traffic Control. This descent triggered a Minimum Safe Altitude Warning (MSAW) on the controller's radar screen. Upon being contacted by the controller, the pilot initiated a missed approach. The aircraft climbed to a safe altitude and subsequently landed at Dublin Airport without further incident. There were no injuries and no damage to the aircraft.
The investigation
The investigation focused on the descent profile and the cockpit environment during the approach. Investigators examined the aircraft's technical status, noting that the Travel Limitation Unit (TLU) was operating in manual mode due to a previously recorded defect. The investigation also reviewed the flight crew's experience and training records, specifically regarding the transition between different ATR 7/2 models. Data from the flight path showed that the aircraft's altitude dropped significantly below the required glideslope, reaching 1,082 feet when it should have been at 1,975 feet.
Findings
Several contributing factors led to the altitude excursion:
- The commander was distracted by the manual operation of the TLU to the point that flight path monitoring was neglected.
- Inappropriate pitch inputs applied to the control column caused the autopilot to disengage unexpectedly.
- The flight crew demonstrated a lack of familiarity with the specific features of the '-600' version of the aircraft.
- There was sub-optimal coordination and communication between the pilots during the approach.
- The aircraft was operating with a deferred maintenance item regarding the TLU's automatic mode.