Executive summary
On 9 February 2011 a Bombardier DHC-8-311 aeroplane (known as a "Q300" operated by Air Nelson Limited departed from Hamilton Aerodrome on a scheduled flight to Wellington Aerodrome. On board were 2 pilots, a flight attendant and 41 passengers.
Prior to taking off from Hamilton, the nosewheel steering malfunctioned because an "inhibit switch" in the cockpit was faulty. The faulty switch caused a loss of hydraulic pressure to the nosewheel steering. The nosewheel steering system was considered non-essential, so in accordance with the approved Minimum Equipment List, the aeroplane departed Hamilton with the system inoperative. The trip towards Wellington was uneventful.
The nosewheel steering hydraulic power came from the extend side of the landing gear hydraulic system. On the approach to Wellington, none of the landing gear extended when it was selected down. The pilots carried out a go-around to give them time to perform the relevant procedures provided in a Quick Reference Handbook (QRH). The Q300 was fitted with an alternative system for lowering the landing gear when the normal system failed. The "Alternate Gear Extension" procedure succeeded in getting the main landing gear to extend, but not the nose landing gear.2 That remained locked in its retracted position.
There was nothing mechanically wrong with the alternate landing gear extension system. The nose landing gear did not extend because the pilots did not pull hard enough on the handle that should have released the uplock. If the uplock had released, the nose landing gear would have lowered under gravity and locked down.
The pilots decided to divert to Woodbourne Aerodrome and to land with the nose landing gear retracted. No-one was injured in the landing. The damage to the aeroplane was confined to the area around the nose landing gear and the lower forward fuselage.
The Minimum Equipment List appeared to have considered the operational consequences only of allowing a Q300 to depart with inoperative nosewheel steering. The link between a failure of the nosewheel steering and a potential failure in the hydraulic system, which would affect the landing gear, did not appear to have been considered. The manufacturer has since amended the Minimum Equipment List to require a check of the hydraulic system pressure before allowing a departure with the nosewheel steering inoperative.
The operator's pilots were not made aware through their training of how hard one had to pull the handle to release the nose landing gear uplock. A much lesser pull was required when practising the procedure in the operator's flight simulator and the Alternate Gear Extension procedure did not, at the time, give any guidance as to the force required.
Air Nelson modified its flight simulator so that the forces were more typical of those found on the actual aeroplane, and provided its pilots with more technical information on the Alternate Gear Extension procedure. The aeroplane manufacturer provided all operators of the Q300 with a more comprehensive description of the Alternate Gear Extension procedure and provided options for pilots to consider should the procedure be unsuccessful.
A recommendation was made to the Director of Civil Aviation that he liaise with Transport Canada to make other National Aviation Authorities aware of this incident and of the desirability of flight simulators closely representing the actual forces required for an alternate landing gear extension.
Although not contributory to the accident, the report discusses the design of checklists and how they can lead to pilots making errors or missing important items during times of high workload. The Transport Accident Investigation Commission (the Commission) had commented on this issue in a previous inquiry. A recommendation was made to the Director of Civil Aviation regarding the format of QRHs.
The Commission made findings about the cause of the nosewheel steering and landing gear extension failures, about crew training in alternative procedures, and the importance of having well designed QRHs.
The Commission also identified the following key lessons:
- in their simulator training pilots should be taught how to perform emergency and non-normal procedures as robustly and rigorously as if the procedures were being performed on the actual aircraft
- pilots should be informed of flight simulator characteristics that differ from those in the aircraft to ensure that pilots are not misled during actual flight operations
- QRHs should be designed to minimise the potential for error as they are used by pilots during times of high workload and, potentially, high stress when dealing with emergencies.
Related Recommendations
On 14 December 2012 the Commission recommended to the Director of Civil Aviation that he urge operators to adopt QRH checklist formats that reduce the possibility of misreading or omitting a procedural step
On 14 December 2012 the Commission recommended to the Director of Civil Aviation that he liaise with Transport Canada to make other National Aviation Authorities aware of this incident and of the desirability of Dash 8 flight simulators closely representing the actual forces required for a landing gear alternate extension.