Baltic DH8D at Riga on Sep 17th 2016, nose gear problems lead to landing without nose gear

Last Update: June 13, 2018 / 17:16:04 GMT/Zulu time

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Photo of Air Baltic YL-BAI, De Havilland Dash 8 (400) — Air Baltic YL-BAI, De Havilland Dash 8 (400) (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Sep 17, 2016

Classification
Accident

Airline
Air Baltic

Flight number
BT-641

Departure
Riga, Latvia

Destination
Zurich, Switzerland

Aircraft Registration
YL-BAI

Aircraft Type
De Havilland Dash 8 (400)

ICAO Type Designator
DH8D

An Air Baltic de Havilland Dash 8-400, registration YL-BAI performing flight BT-641 from Riga (Latvia) to Zurich (Switzerland) with 63 passengers and 4 crew, was climbing out of Riga's runway 18 when the crew reported problems with the nose gear, stopped the climb at 5000 feet and entered a hold to work the related checklists. The nose gear, that had not fully retracted and became stuck half way into the wheel well, could now neither be fully retracted nor extended. After a hold of about 100 minutes trying to release the nose gear the crew performed a low approach which verified the nose gear was barely visible and definitely not in a position for landing. The crew prepared the aircraft for a nose gear up landing and landed on runway 36 about 2.5 hours after departure, keeping the nose up as long as practicable, then lowering the nose onto the runway surface. The aircraft came to a stop on both main gear and the nose. There were no injuries.

The runway remained closed for about 4 hours and reopened at about 14:00L.

The airline reported after departure a nose gear problem was detected which prompted the return to Riga.

On Jun 13th 2018 Latvia's Transport Accident Incident Investigation Bureau (TAIIB) released their final report concluding the probable cause of the serious incident was:

Root cause

Root cause is insufficient retention of the bushings in the LLL body.

Direct cause

Excessive clearance in the mechanism by virtue of corrosion of LLL hole and wear of bushing OD.

Possible contributing causes

- Moisture and runway de-icers penetration in the joint, salt based (potassium formate and sodium acetate) cadmium plating dissolving and base metal corrosion progression.

- Insufficient tightness applied in fastening bushings with LLL hole.

- Possibility that the upper lock link pin failed simultaneously with the LLL pin or when the lugs of the lower drag strut came into contact with the upper lock link.

The TAIIB reported the damage to the airframe was confined to the nose gear doors, the fuselage remained undamaged. In addition there were scratches on the runway.

The TAIIB analyed that the crew actions, in particular passenger briefings, communication with ATC, communication with maintenance, were appropriate and useful.

The TAIIB analysed the failure scenario:

After cleaning and detailed inspection in the NLR of drag strut assembly it was stated:

- The damage to the components is largely confined to the locking mechanism;

- The upper and lower drag strut are undamaged, except for the connections to the locking mechanism and some surface damage;

- The LLL, LLL pin and upper lock link pin are the parts that completely failed;

- The LLL apex pin, upper lock link pin and the stop bolt were unable to rotate. For the LLL apex pin this is because the pin is heavily deformed;

- The failed pins are highly deformed and the fracture surfaces indicate that failure occurred by shear overload;

- All fracture surfaces of the LLL are largely covered with dimples indicative for overload;

- All damage observed in the drag strut assembly agrees with overloading of the locking mechanism when it locked at the minimum drag strut angle. When the aircraft lands and puts load on the NLG with NLG in this condition, this will transfer all load through the locking mechanism, which is not designed for carrying this load.

- Locking of the mechanism at this position is only possible if the total distance between the turning points is decreased;

- It is likely that the corrosion of the LLL hole for the LLL pin and wear of the bushings OD has decreased the total distance between the turning points of the locking mechanism. Over time this would inevitably result in locking of the mechanism at the minimum drag strut angle;

- Insufficient retention of the LLL bushings will result in bushing movement and subsequent sealant loss. This sealant loss allows moisture ingress, accelerates corrosion and LLL wear. The resultant wear will elongation the LLL bushing holes and this resulted in the premature engagement of the LLL in the drag strut assembly;

- The top fracture surfaces of the LLL shows two optical transitions. The first transition occurs at 50-100 ìm crack depth and corresponds to the transition from fatigue to overload The fatigue crack growth rate at the transition is very slow based on the striation spacing (in the order of 100 nm per cycle). The second transition occurs at 250-400 ìm crack depth, but dimples are present before and after the transition. It is therefore concluded that the second transition is not related to a fatigue beach mark. It is expected that the lug of the LLL failed by overload, because the fatigue crack length and crack growth rate at the transition are both small and all fracture surfaces are largely covered with dimples.

- The stop bolt shows a clear deformation on the side that comes into contact with the LLL;

- The other side is rough, but not deformed;

- The stop bolt is unable to rotate after failure of the drag strut assembly. This indicates that a high load was transferred from the LLL to the stop bolt and that it was in contact with each other when a high load was introduced to the LLL. This means that the locking mechanism was locked when a high load was introduced;