What happened
On 16 December 2018, a Piper PA-34-200T Seneca II, registration G-IFLP, was conducting a private night flight from Leeds Bradford Airport to Oxford Airport. While climbing through instrument meteorological conditions, the aircraft encountered freezing rain, which caused rapid ice build-up on the wings and windscreen.
The pilot experienced significant control difficulties, particularly regarding pitch stability. While attempting to manage the aircraft and operate the pneumatic de-icing boots, the pilot requested a descent from FL90. However, the air traffic controller, not perceiving the urgency of the situation, instructed the aircraft to maintain its altitude.
As the ice continued to accumulate, the aircraft descended to FL86, resulting in a loss of separation with an Airbus 319. Radar data showed the minimum separation between the two aircraft dropped to 4.5 nm and 600 ft, failing to meet the required 5 nm or 1,000 ft standard. During the event, another pilot on the frequency attempted to alert the controller that G-IFLP was losing control. After approximately four minutes, the de-icing system successfully cleared the ice, and the aircraft returned to its cleared level.
The investigation
The investigation examined radar and radio recordings from the air traffic service provider, as well as the aircraft's de-icing system capabilities. The analysis focused on the pilot's ability to communicate during the emergency and the controller's response to the request for descent. The investigation also reviewed the meteorological forecast, which had predicted moderate icing and severe icing within embedded cumulonimbus clouds for the area.
Findings
- The aircraft descended below its assigned altitude due to control difficulties caused by ice accumulation on the airframe.
- The pilot's capacity to communicate was severely limited because they were manually flying the aircraft without an autopilot while simultaneously managing the de-icing system.
- The air traffic controller did not recognize the severity of the pilot's request for descent, leading to the instruction to maintain altitude.
- The minimum separation between the two aircraft fell below the required regulatory standards.