Croatia DH8D at Zurich on Sep 27th 2013, nose gear up landing

Last Update: August 31, 2015 / 14:03:01 GMT/Zulu time

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Photo of Croatia Airlines 9A-CQC, De Havilland Dash 8 (400) — Croatia Airlines 9A-CQC, De Havilland Dash 8 (400) (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Sep 27, 2013

Classification
Accident

Airline
Croatia Airlines

Flight number
OU-464

Departure
Zagreb, Croatia

Destination
Zurich, Switzerland

Aircraft Registration
9A-CQC

Aircraft Type
De Havilland Dash 8 (400)

ICAO Type Designator
DH8D

A Croatia Airlines de Havilland Dash 8-400, registration 9A-CQC performing flight OU-464 from Zagreb (Croatia) to Zurich (Switzerland) with 60 passengers and 4 crew, was on approach to Zurich's runway 14 when the crew went around reporting they had an unsafe nose gear indication, the nose gear appeared to be blocked. The aircraft entered a hold at 9000 feet, then declared Mayday reporting they needed to land without nose gear. Zurich Airport halted all traffic in preparation for the emergency. About 40 minutes after aborting the first approach the aircraft landed on runway 14 safely without nose gear and came to a stop on the runway on main gear and nose section of the aircraft, emergency services reported everything appeared under control from the outside, people could disembark slowly. The crew advised both engines were shut down, a few minutes later the crew advised all passengers had disembarked, everything was safe on board. Tower annotated to the crew: "Excellent job, well done".

Other traffic resumed a few minutes after landing, traffic was halted for about 15 minutes.

On Sep 30th 2013 Switzerland's SUST released notice, that an accident investigation has been opened and stated: "During the IFR-approach on runway 14 on Zurich Airport the nose landing gear didn’t extend. A missed approach procedure was initiated and in the holding trouble shooting was performed. After all it was not possible to extend the nose landing gear. A prepared emergency landing on runway 14 of Zurich Airport with extended main gear and retracted nose landing gear was performed."

On Oct 15th 2013 the Swiss SUST released a brief preliminary report stating that after the aircraft had been towed off the runway it was attempted to release the nose gear using the alternate gear extension. The force on the lever was gradually increased until it reached 543N (122lb/55kg), then the nose gear extended. It was subsequently discovered that the cover plates protecting the two weight on wheel sensors had fractured at the lower left and right lug attachment points, pivoted up and was squeezed into the nose gear extension mechanicsm between the upper and lower drag strut. After the cover plate was removed the landing gear was selected up and down several times and functioned normally, the force requires to achieve an alternate extension reduced to 335N (75lb/34kg).

Laboratory analysis showed the fracture surface in very poor condition, where the fracture surface was still intact a mixture of dimple fractures due to static overload and welding porosity was found as well as other indications of welding defects.

One safety action was taken by the operator and a safety recommendation released by SUST as result of the investigation so far.

On Aug 31st 2015 the SUST released their final report concluding the probable causes of the accident were:

The accident is attributable to the fact that the flight crew was not successful in extending the nose landing gear and then had to conduct a landing with extended main landing gear and retracted nose landing gear.

The following factor was identified as the direct cause of the accident:

- The WOW cover plate on the nose landing gear was broken on both lower lugs so that it was bent upwards around the upper tab mounts and was trapped in the nose gear mechanism, which prevented the extension.

The crack on the lower WOW cover plate lug is attributed to the following factors:

- The way the WOW cover plate was mounted led to lateral forces on the WOW cover plate and mechanical stress in the area of the upper and lower lugs during operation of the nose landing gear steering.

- There were weld defects in the area of the lower lugs.

The SUST analysed: "Despite repeated attempts, the flight crew was not successful in extending the nose landing gear using the normal or alternate procedures. The checklist for the alternate procedure (cf. Annex 3) explicitly states that the force to be applied on the release handle can be up to 41 kg (approximately 400 N). The FOSL, which the crew also consulted, stipulates that it may be necessary to apply an even greater force and that the necessary forces in the air are probably greater than those on the ground or in a simulator. In view of this evidence it can be assumed that the crew attempted to extend the nose landing gear with the maximum force they could apply. Since there is no evidence that the crew made errors in applying the procedure, it can be concluded that it was not possible to extend the nose landing gear due to a technical fault. On the ground, after the landing with retracted nose landing gear, it was possible to extend the nose landing gear using the alternate procedure by applying a force of 543 N, corresponding to 55 kg (122 lb), on the release handle. This is significantly higher than the aforementioned maximum force of 41 kg (90 lb; approximately 400 N) according to the checklist. After the damaged WOW cover plate was identified and removed, it was possible to extend the nose landing gear using the alternate procedure with a force of 334 N on the release handle; extending and retracting the nose landing gear using the normal procedure also functioned smoothly. These findings lead to the conclusion that on the ground after the landing, the damaged WOW cover plate, which had been bent upwards, prevented the extension of the nose landing gear to the extent that the force required on the release handle increased significantly compared to the normal state."