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Old 13th Jan 2017, 14:36
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An except from a recent Advisory Wire relating to this incident:

The Transportation Safety Board of Canada (TSB), with the assistance of Bombardier and of the landing
gear manufacturer, Messier-Bugatti-Dowty, has conducted a safety investigation on a Challenger 300 aircraft that experienced a directional pull to the right after landing. The aircraft exited the runway to the right, despite the effort of the crew in their unsuccessful attempt to control the uncommanded veer, about 4,300 feet from the threshold. The aircraft travelled 400 feet over frozen ground and came to a stop about 100 feet from the edge of the runway in a shallow snow bank. Skid marks left by the nose wheel tires on the runway showed that the nose wheel steering was incorrectly orientated.

No one was injured as a result of this occurrence and the landing gear did not collapse. Bombardier, Messier-Bugatti-Dowty and TSB representatives went on site for the investigation. The aircraft was recovered and inspected for damage under the TSB supervision. All the landing gears were replaced based on Messier-Bugatti-Dowty’s recommendation due to the drag load (g load) factor that the aircraft encountered during the excursion. All steering components were also removed and quarantined for future investigation at the Messier-Bugatti-Dowty facility. The aircraft has since returned to service without any occurrence of a failure of its Nose Wheel Steering System (NWSS).

The runway condition at the time of the occurrence was 90% bare and dry with 10% ice on the sides of the runway. The winds were from 120 at 3 knots and the outside temperature was - 29 C. The Transportation Safety Board of Canada (TSB) investigation conclusion has revealed, among other findings, that the accident was caused by a combination of two malfunctioning components:
The Electro Hydraulic Servo Valve (EHSV) of the steering manifold did not respond to the steering command from the Steering Electronic Control Unit (ECU).
The Steering Selector Valve (SSV) did not close, preventing the Nose Wheel Steering System (NWSS) from operating in a free castering mode.

Bombardier has conducted additional laboratory testing to determine the root cause for the failures of these components. Although we were not able to reproduce the root cause of the accident, we have confirmed that the design of the steering manifold (P/N 40750-101) can allow moisture to enter into the EHSV torque motor housing through the connector installation.

To prevent moisture ingress into the EHSV torque motor housing, a new design (see photos below) incorporating an O-ring seal between the connector and an EHSV valve cap has been developed. Sealant has been also applied to the mating surface and around the four mounting bolts for added protection. This new steering manifold (P/N 40750-103) is already being installed in production from aircraft 20384 and subs.

Operators should be aware of Ref. 1.3 Airworthiness Directive and schedule their aircraft for incorporation per the recommended compliance time.
As pointed out by HS-125, some strong similarities to
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