Air Nelson DH8C at Blenheim on Sep 30th 2010, nose gear extended partially only
Last Update: November 1, 2012 / 23:07:33 GMT/Zulu time
Incident Facts
Date of incident
Sep 30, 2010
Classification
Accident
Airline
Air Nelson
Aircraft Type
De Havilland Dash 8 (300)
ICAO Type Designator
DH8C
The failure of the nose landing gear to extend fully was most likely caused by debris in the hydraulic fluid blocking orifices within the nose landing gear extend/retract hydraulic actuator. The debris probably came from damaged seals within the actuator.
- The damage to the seals within the extend/retract hydraulic actuator could have initially been caused by a manufacturing defect in the seals or by an incorrect assembly technique. The damage may have been exacerbated by debris in the hydraulic fluid that originated from excessive wear in another actuator in the system, the one that opened and closed the forward nose landing gear doors.
- Two instances of unusual operation of the nose landing gear had been reported in the 5 weeks prior to the failure to extend at Woodbourne. The cause of the earlier events was probably the same as that for the failure to fully extend.
- With the nose landing gear stuck in a partially extended position, light from the taxi light was likely detected by the sensor for the down-lock verification system, causing it to give a false green light.
- The false green light on the verification system misled the pilots of ZK-NEB into believing that the nose landing gear was fully down and locked.
- The verification system for checking if the landing gear is down and locked on the Dash 8 series of aircraft is not reliable enough for pilots to place total trust in it when trying to establish the status of the landing gear.
- Had the pilots known that the nose landing gear was not down and locked and then tried the alternate extension procedure, that action would have been unlikely to succeed because debris would still have been blocking the restrictor within the hydraulic actuator.
- Assuming that the reason for the nose landing gear not fully extending was debris in the hydraulic fluid, it is possible that cycling the landing gear up and down again would have succeeded in getting all of the landing gear down and locked. However, that action was not recommended by the aeroplane manufacturer at the time.
- The aural warnings that not all of the landing gear was locked down were genuine warnings. Pilots must respect warnings. In this case, the pilots should have responded by performing a go-around, which would have given them more time to consider the situation.
- The pilots did not use all of the available resources to confirm the nose landing gear position. If they had asked the controller to confirm the status of the nose landing gear, it is likely that they would have taken further action in an attempt to get the nose landing gear locked down.
- The captain should have explained to the flight attendant the reason for the go-around, even though he believed that the situation had been resolved. The flight attendant would have been better prepared should the landing have been less favourable.
- The captainÂ’s predetermination of what was going to be required to get the landing gear down and locked interfered with the proper following of the QRH checklist; nevertheless, all of the items were covered.
Following two go arounds at the planned destination Nelson due to weather the captain (ATPL, 12,040 hours total, 1,066 hours on type) determined Blenheim was the better diversion aerodrome than the planned alternate Christchurch. The aircraft positioned for a straight visual approach to Blenheim's runway 24, the gear selected gear down and received green indications for the main gear struts, the nose gear however indicated red (not locked down) and amber (gear doors open). The crew went around advising tower of a gear problem and requesting emergency services on stand by and joined a traffic circuit to work the checklists leaving the gear extended. The check list required to check the gear status verification lights located in the flight deck floor. Much to the surprise of the captain, who was sure the nose gear had not fully extended, the verification lights showed three greens. The checklist stated that if any of the two displays showed a green for the gear strut, the gear strut was down and locked. The crew therefore advised the gear problem had been resolved and positioned for another approach to runway 24. The crew also did not inform the flight attendant about the possible gear up condition. Tower used binoculars to check the gear status and had the impression in the distance all three struts were down, but subsequently got occupied with coordination tasks and did not check again.
Although they had three greens the first officer (CPL, 1,167 hours total, 780 hours on type) asked whether they should conduct a low approach to have the gear inspected from the ground, the captain responded "a green is a green". The crew assumed that one of the proximity sensors of the primary system was malfunctioning and the alternate indication was correct, but anticipated that due to the unsafe indication on the primary system they would get all gear related warnings during final approach.
While extending the flaps to 15 degrees the crew received a landing gear horn, later followed by the GPWS "too low gear" aural alert as expected by the crew, the crew discarded these alerts. The flight attendant heard the alerts from the cockpit, but knew those alerts would usually be silenced immediately. She became puzzled when the alerts were not silenced, however was sure she would have heard from the flight deck if there was any real problem.
After main gear touch down the captain lowered the nose but realised the nose would lower further than expected and progressively increased back pressure on the elevator to cushion nose touch down while applying brakes pressure. The flight attendant, although hearing unusual screeching sounds, did not perceive the attitude of the aircraft as unusual and thus did not see any need to alert the passengers to take their heads down or anticipate an emergency evacuation.
The tower controller watching the landing through the nose gear was collapsing after touchdown and immediately cleared emergency services onto the runway.
The captain decided to not evacuate and instructed the cabin to remain seated and the first officer to shut down the engines, then he went to the cabin and explained what had happened. The occupants subsequently disembarked. No injuries occurred.
The TAIC reported that debris from a damaged seal in the nose gear's hydraulic actuator blocked orifices in the nose gear actuator. The nose gear strut had accumulated 13,340 hours, the manufacturer stated the average life time of the strut without replacement was 30,000 hours.
The TAIC reported that there had been two occurrences in the 5 weeks prior to the accident flight on the accident aircraft where the nose gear would not fully extend, the gear was selected up and down and went into position in all these occurrences. Maintenance investigated the occurrences but could not determine any causes for the intermittent gear extension faults.
The TAIC analysed that had the accident crew selected the gear up and down again, the fault would most likely have cleared due to pressure changes in the system. This however was strongly discouraged by the manufacturer. Had the crew attempted an alternate gravity/manual extension, this would NOT have cleared the fault as the present hydraulic pressure in the actuator would not have changed and thus removed the blockages.
The aircraft manufacturer's checklist required following an unsafe gear indication to conduct an alternate gear extension. The airline had adopted the checklist by introducing the check of the alternate verification lights. In case the lights were not illuminated the checklist as produced by the manufacturer would resume.
The TAIC analysed that the verification indication system was not reliable. The taxi light mounted on the nose gear strut triggered the proximity sensor of the verification system causing the nose gear showing safe and thus misleading the crew.
The TAIC further analysed, that regardless of the checklist being used the outcome would have been the same, the aircraft would have landed without safe nose gear.
The TAIC analysed that the illuminated taxi light on the nose gear as well as stray light could cause the alternate verification sensor to trigger, the manufacturer had overlooked that possibility during the design. As a result, the operator changed the checklists to ensure the taxi lights are off when checking the gear verification lights.
The TAIC nonetheless continued that there were at least two other cases where the verification system had indicated green although the nose gear had not been extended, even without taxi lights on. Water ingress and chafing of wires were the causes, the TAIC thus concluded that "reliability of the verification system for the Q300 (and Dash 8 series) did not warrant the trust placed in it by the manufacturer".
The TAIC analysed that both pilots did not actively respond to the two alerts (gear horn and GPWS "too low gear"). Although both pilots rightfully believed these alerts were false caused by a faulty sensor, neither stated that very fact and neither ensured they were in agreement about proposed actions being acceptable.
The TAIC analysed that had the crew reacted as required by the GPWS "too low gear" by initiating a go-around, it was most likely the controller would have detected the nose gear was not in position and thus allowed the crew to recognize the indication was genuine permitting the cabin to be prepared for the nose gear up landing, the only difference that possibly could have occurred but would not have changed the outcome.
Incident Facts
Date of incident
Sep 30, 2010
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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