What happened
On 21 December 2004, a DHC-8-311, registration G-BRYU, was preparing for a scheduled passenger flight from Edinburgh to Manchester. The aircraft was heavily loaded, with a mass of 18,186 kg and a centre of gravity positioned at the forward limit. Due to the passenger and baggage configuration, the pitch trim had been set toward the aft limit.
During the takeoff roll at 0723 hrs, the pilot encountered extremely high control forces in pitch when attempting to rotate at the calculated speed of 109 knots. The resistance was so significant that the pilot initially suspected the forward centre of gravity or a potential elevator jam, and was considering aborting the takeoff. However, through sustained and increasing back pressure on the control column, the aircraft eventually achieved a very slow rotation and became airborne. Once the aircraft had stabilized in flight, the control forces returned to normal, allowing the crew to continue the flight to Manchester, where they landed without further incident. There were no injuries and no fatalities among the 4 crew and 49 passengers.
The investigation
The investigation examined the aircraft's de-icing history and physical condition. The aircraft had been treated with Type II 75/25 de-icing fluid at 0400 hrs, well within the five-hour holdover period. While the pilot noted an excessive amount of fluid on the fuselage, a post-flight inspection revealed that while de-icing fluid was present on the forward tailplane, the elevator hinges and spring tabs were dry.
Investigators looked into whether the elevator spring tabs had been restricted. The investigation considered two primary possibilities: that the tabs had frozen while the aircraft was parked overnight and remained frozen despite the de-icing process, or that the tabs had become frozen due to the rehydration of de-icing fluid residue.
Findings
- The primary cause of the high pitch control forces was the freezing of the elevator spring tabs.
- This restriction was likely caused by either incomplete de-icing of the tabs or the freezing of rehydrated de-icing fluid residue.
- The aircraft was operating within its valid de-icing holdover time.