Air Nelson DH8C at Wellington and Blenheim on Feb 9th 2011, nose gear did not lower

Last Update: February 8, 2013 / 14:48:52 GMT/Zulu time

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Incident Facts

Date of incident
Feb 9, 2011

Classification
Accident

Airline
Air Nelson

Aircraft Type
De Havilland Dash 8 (300)

ICAO Type Designator
DH8C

New Zealand's Transport Accident Investigation Commission (TAIC) released their final report concluding the probable causes of the accident were:

- A faulty inhibit switch caused the loss of nosewheel steering when the aircraft was departing from Hamilton.

- The same faulty inhibit switch was the cause of the landing gear not extending normally when the aircraft was approaching Wellington.

- The Minimum Equipment List provision that had previously allowed a flight to commence without an operative nosewheel steering system appeared to be based on operational considerations only, and to have not considered the possibility of a related system failure.

- There was no defect with the alternate extension system that would have prevented any of the landing gear extending and locking down.

- The nose landing gear uplock did not release during the alternate extension procedure because the release handle had not been pulled hard enough or had not been held for as long as it took for the uplock to release.

- The training that Air Nelson gave its pilots on the alternate landing gear extension procedure did not include key information provided by Bombardier concerning the release handle forces, and there was no guidance in the QRH procedure.

- The simulator that Air Nelson used for training its pilots in the alternate landing gear extension procedure was not representative of the actual forces required to release the uplock.

- The similarity of text in adjacent steps of the QRH and the lack of numbered steps contributed to the first officer making minor errors in reading the landing gear non-normal procedures. However, these errors did not contribute to the accident.

The TAIC reported that prior to departure from Hamilton the nose gear steering malfunctioned. The minimum equipment list permitted departure with the nose wheel steering system inoperative.

The captain (34, ATPL, 5,549 hours total, 2,309 hours on type) and first officer (23, ATPL, 2,243 hours total, 1,315 hours on type) thus departed, the flight was uneventful until the gear was to be lowered on final approach to Wellington. After selecting the gear down none of the gear struts extended. The crew went around, actioned the relevant checklists and attempted an alternate gear extension, which brought both main gear struts into their extended locked position, the nose gear strut however did not extend remaining locked in the uplock when the nose gear release handle was pulled by the first officer although the nose gear doors immediately opened. The crew decided to divert to Blenheim where the aircraft landed with the nose gear up.

The aircraft received damage to the nose gear doors and surrounding structure, which collapsed during roll out, and three antennas at the lower fuselage.

A post flight inspection revealed that there was nothing wrong with the gear mechanism, had the nose gear left the uplock it would well have extended into its downlocked position. It was determined, that the crew did not pull the handle hard and long enough to release the nose gear out of its uplock.

The minimum equipment list (MEL) did not check for hydraulic pressure (failure of other systems) when permitting the dispatch of the aircraft with nose wheel steering inoperative. The aircraft manufacturer amended the MEL following the accident to require a check of the hydraulic pressure before dispatching the aircraft with nose wheel steering inoperative.

The TAIC analysed that the crew had not been made aware of how hard and how long the alternate gear release handle had to be pulled to release the nose gear out of its uplock, the simulator had required much lesser forces (about only 8.5kg/85N of force) and the checklist procedure did not advise about what forces would be needed. The airline modified their simulator to have the alternate gear handle reflect the forces needed on the real aeroplane and changed their procedure, which until the accident had required the crew to pull the nose gear alternate release handle in one movement to its full stop, now requiring the crew should pull the handle to the point, where the nose gear doors opened, wait 5 seconds, then pull the handle to the full stop. The background was, that it takes the nose gear doors about 2 seconds to fully open while the handle could be pulled to its full stop in less time, an average 30kg/300N of force was thus needed to release the nose gear from its uplock with a maximum of 65kg/650N.

The TAIC said: "the Training Manual also instructed pilots to pull the handle “in one continuous motion” when performing an actual alternate extension. Air Nelson trials showed that if the handle was pulled quickly, the uplock would release immediately, but the mechanical sequence valve could prevent the landing gear leg moving until the forward doors were open. In that case, an apparently strong pull on the handle might have been ineffective if it was relaxed too early and the uplock then re-engaged before the nose landing gear had begun to move down."

An inhibit switch was found intermittently faulty. This switch removes hydraulic power from the landing gear extension actuator as well as nose gear steering actuators. With the inhibit switch activated none of the gear legs would thus extend upon the gear being selected down. However, the inhibit switch would not affect the gear retraction as the hydraulic pressure forces the gear down and hydraulic pressure is removed upon gear retraction. If the gear is in the downlock position the mechanical downlocks would keep the gear in the extended position until the downlocks are released.

The TAIC found that a successful normal gear extension could have been achieved by bumping the inhibit switch. The crew however had no idea what caused the nose gear steering failure and thus was not aware the inhibit switch was the cause of their problem.
Incident Facts

Date of incident
Feb 9, 2011

Classification
Accident

Airline
Air Nelson

Aircraft Type
De Havilland Dash 8 (300)

ICAO Type Designator
DH8C

This article is published under license from Avherald.com. © of text by Avherald.com.
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