Flight carrying 59 passengers between Newquay and Heathrow made a precautionary landing at Exeter last November because of control issues relating to the ailerons
Investigators have determined that a break in the left aileron cable of a De Havilland Canada DHC-8 operated by Flybe, caused the control yokes to have to be offset by up to 40° to keep the wings level, which resulted in the pilots reporting difficulty in maintaining right turns.
The aircraft – which was flying from Newquay Airport to London/Heathrow last November, diverted to Exeter Airport where it made an uneventful landing.
The turboprop, G-FLBE (c/n 4261) was a ten-year-old example that had accrued approximately 22,400 flying hours and was carrying 59 passengers and four crew at the time of the incident.
The report by the Air Accidents Investigation Branch found that shortly after take-off in a strong crosswind, the pilots noticed that both control yokes were offset to the right in order to maintain wings-level flight.
The crew described the conditions during the departure as “quite rough with a lot of drift”. At the acceleration altitude of 1,000ft, the captain engaged the autopilot (AP) and passed control to the co-pilot who made a right turn, using the AP, towards the reporting point DAWLY.
Investigators said the co-pilot felt that the aircraft “struggled” to maintain the right turn and that he subsequently informed the captain there were issues with the controls. After taking control again, the commander elected to level the aircraft at FL200 and noted that the control deflection increased.
The captain then contacted the company operations to seek engineering advice. The report said the response did not help his understanding, so he informed operations of his intention to divert to Exeter, as this was the location of the firm’s engineering base.
After receiving a routing towards the EX NDB, the pilots declared a PAN to Exeter air traffic control and consulted the quick reference handbook but found no checklists that they deemed relevant.
The cabin wasn’t ready in time for a direct approach and so the commander decided to join the hold. As the airspeed reduced, the outboard spoilers became active and the crew recalled that the yoke deflection decreased.
The captain deselected the AP earlier than normal and recalled a slight pull to the right on the yoke, but felt the aircraft was completely controllable. The report said the approach and landing were uneventful.
When investigators inspected the aircraft, they found that the lower-left aileron cabled failed where it passed over a pulley mounted on the rear wing spar. The AAIB said the broken cable was dirty and left a residue on a cloth when it was wiped. This build-up was also present on other aircraft in the operator’s fleet.
The cable had been fitted to the aircraft for six years and had flown approximately 13,000 hours – it was visually inspected on five occasions with the last being 10 months prior to the incident.
The manufacturer identified low tension, vibration and contamination as the main causal factors for wear in the control cables. The airframer published its first advice to operators in 2004 with a polymer coating being introduced in 2015. The modification was not mandatory.
Flybe had installed the modified cables on 24 of its aircraft and was in the process of completing the work on the remaining 30 examples when the cable failed on G-FLBE. An inspection carried out following the failure on the unchanged turboprops, revealed 18 aileron cables and several pulleys that required replacement.
The operator was in the process of investigating the cause of the unresponsive aileron on the incident aircraft when they ceased trading. Consequently, the AAIB could not identify the cause.
The report concluded that the most probable reason for the aileron cable breaking was its strength had been reduced as a result of wear leading to failure of the individual wires within the cable. The accumulation of dirt could have penetrated the strands and formed an abrasive compound which investigators say could accelerate the normal rate of wear.
The AAIB says further investigation is required to determine if there is a wider safety issue.