Mount Cook AT72 at Nelson on Apr 9th 2017, unsafe gear

Last Update: December 4, 2019 / 21:14:04 GMT/Zulu time

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Incident Facts

Date of incident
Apr 9, 2017

Classification
Incident

Cause
Unsafe gear

Airline
Mount Cook Airlines

Flight number
NZ-5075

Departure
Auckland, New Zealand

Destination
Nelson, New Zealand

Aircraft Registration
ZK-MCY

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A Mount Cook Airlines Avions de Transport Regional ATR-72-212A on behalf of Air New Zealand, registration ZK-MCY performing flight NZ-5075 from Auckland to Nelson (New Zealand) with 66 passengers and 5 crew, was on approach to Nelson when the crew aborted the approach reporting an unsafe gear indication. The aircraft was diverted to Palmerston North (New Zealand) and burned off fuel. The aircraft landed safely on all three gear struts in Palmerston North and became disabled on the runway. The occupants disembarked onto the runway.

Ground observers reported there was a lot of smoke when the aircraft landed.

The airline reported the aircraft diverted to Palmerston North due to suspected nose gear trouble. The aircraft landed safely, but remains on the runway until it has been inspected by engineers. As result 9 other services to Palmerston North have been cancelled. The passengers are being taken to Wellington by bus and continue their journey from there.

On Apr 11th 2017 Australia's ATSB reported they are providing assistance (download of the CVR) to New Zealand's TAIC investigating the occurrence rated an accident. The ATSB reported during approach to Nelson the crew received an unsafe gear indication and diverted the aircraft to Palmerston North. During the landing roll the right main tyre burst, the aircraft was brought to a stop on the runway. There were no injuries.

On Apr 13th 2017 New Zealand's TAIC reported that the crew received an unsafe indication for the right main gear on both primary and secondary panels, discontinued the approach and worked the unsafe landing gear checklist followed by the gravity extension checklist. The unsafe indications remained however. The crew elected to divert to Palmerston North for the longer runway and worked the landing with abnormal landing gear checklist. The aircraft landed with the left main gear touching down first, the crew held the right main gear up as long as possible. Immediately after the right main gear touched down the outboard right main tyre burst. The aircraft was brought to a stop on the runway, the occupants disembarked through the main cabin door. The TAIC rated the occurrence a serious incident and opened an investigation.

The longest runway at Nelson 02R/20L is 1347 meters/4420 feet long, Palmerston North features a runway 07L/25R of 1900 meters/6240 feet length.

On Dec 4th 2019 the TAIC released their final report concluding the causes of the serious incident were:

- The unsafe condition of the landing gear was caused by the dual failure of the two landing gear locking springs to hold the right hand landing gear in a down and locked position.

- The two right main landing gear locking springs failed because of corrosion cracking, likely initiated by salt.

- The severe corrosion environment in which the aeroplane operated increased the likelihood of corrosion occurring.

- It could not be determined whether one locking spring failed before the other, or whether they failed simultaneously.

- The maintenance inspection programme for the locking springs would have been unlikely to detect the corrosion cracking in the locking springs prior to their failing.

- There was no required preventive maintenance of the locking springs to limit the extent of corrosion damage.

- The crew’s decision to divert to Palmerston North Aerodrome was sound and the preparation for the emergency landing was well conducted and in accordance with company procedures.

- The recycling of the landing gear several times before the emergency landing was outside documented procedures, and had the potential to exacerbate the condition.

The TAIC summarized the flight:

On 9 April 2017, an ATR72 aeroplane was on approach to land at Nelson with 71 persons on board. When the crew lowered the landing gear, they received an alert that the righthand main landing gear was not locked in the down position.

Unable to resolve the issue using the standard procedures, the crew diverted the aeroplane to the more suitable Palmerston North Aerodrome, where they made a successful landing. The landing gear did not collapse on landing and damage was limited to one burst tyre. Nobody was injured.

The TAIC analysed:

The right main landing gear did not lock down because the two locking springs had failed. The failure of either one or both springs was very likely to have been the cause of the unusual noises the cabin crew heard when the landing gear was lowered on approach to Nelson.

The failure of the locking springs to pull the over-centre lock into place meant that the two proximity position switches detected the abnormal landing gear position. This detection resulted in the circuit for the primary and secondary indication lights not being made. This was subsequently displayed to the pilots on the instrument panels, alerting them to the unsafe condition of the right landing gear.

The failure of both springs meant that the emergency checklist for an unsafe landing gear indication was ineffective in correcting the fault. The crew were therefore required to conduct a landing at a suitable aerodrome with the possibility that the landing gear would collapse on landing.

...

The aeroplane was operating in a corrosion environment that was classified as ‘severe’ according to the Federal Aviation Administration’s Advisory Circular 43-4B (AC 43-4B).

Metallurgy examinations of the locking springs found that both failed because of intergranular corrosion followed by stress corrosion cracking and fatigue. The springs finally broke in overload. The corrosion was likely initiated by salt depositing on the springs and over time reacting with the stainless steel.

...

The design of the main landing gear meant that, with both springs in a failed condition, a large sideways force when there was little or no weight (downwards force) on the wheels could have caused the landing gear to collapse. Once the aeroplane had landed and there was full weight on the wheels, the landing gear could not collapse. This explains why the right landing gear did not collapse on landing despite the two failed springs.

...

The aeroplane checklists stated that following a confirmed indication of an unsafe landing gear, pilots were directed to the ‘landing gear gravity extension’ checklist. If this was unsuccessful, pilots were further directed to the ‘landing with abnormal landing gear’ checklist.

In this occurrence, the flight crew were responding to a mechanical warning that the right main landing gear had not locked down. The flight crew, in attempting to lower the landing gear, recycled the landing gear, an action not included in the checklists for the aeroplane. Prior to this action, the Nelson tower controller had said that there appeared to be nothing untoward. This information was limited to the observation that the wheels appeared to be down, but the crew had no means of determining positively whether the landing gear was fixed in place.

...

While the flight crew’s actions to recycle the landing gear did not affect the outcome of this occurrence, these actions were outside the checklists contained in the flight crew operating manual procedures. In other failure cases, recycling might worsen the situation. The manufacturer confirmed this analysis and reiterated the need to follow the checklists.

...

The ‘landing with abnormal landing gear’ checklist directed that at touchdown both engines were to be shut down. This action results in the loss of a number of services, including reverse thrust, nose wheel steering and normal brakes. The loss of these services increases the landing distance and has the potential to make control of the aeroplane on the ground more difficult. The burst tyre on the right side and the associated skid mark suggested that the captain had to use heavy braking to prevent the aeroplane leaving the left side of the runway.

The manufacturer advised that, in the event of a main landing gear collapse, the propeller on that side would strike the ground with unfavourable results. A gear collapse would likely be sudden and give insufficient time for the crew to react and the propeller to slow before striking the ground. The checklist was therefore predicated on the worstcase scenario and considered appropriate.

Related NOTAM:
B1518/17 - RWY 07/25 CLSD DUE DISABLED ACFT. 09 APR 07:11 2017 UNTIL 09 APR 18:00 2017 ESTIMATED. CREATED: 09 APR 07:12 2017