Nextjet ATP at Vilhelmina on Apr 6th 2016, runway excursion, subsequent departure despite not airworthy

Last Update: April 5, 2017 / 19:44:19 GMT/Zulu time

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Photo of Nextjet SE-LLO, BRITISH AEROSPACE ATP — Nextjet SE-LLO, BRITISH AEROSPACE ATP (Photo credit: Anna Zvereva / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Apr 6, 2016

Classification
Report

Cause
Runway excursion

Airline
Nextjet

Flight number
2N-773H

Departure
Hemavan, Sweden

Destination
Vilhelmina, Sweden

Aircraft Registration
SE-LLO

Aircraft Type
BRITISH AEROSPACE ATP

ICAO Type Designator
ATP

A Nextjet British Aerospace ATP, registration SE-LLO performing flight 2N-773H from Hemavan to Vilhelmina (Sweden) with 19 passengers and 4 crew, landed on Vilhelmina's runway 28 following an uneventful and stable ILS approach, touch down occurred on center line in the touch down zone. The crew immediately selected reversers and applied reverse thrust. The aircraft began to drift right, the right main gear departed the paved runway surface about 400 meters after touchdown and went over soft ground for about 155 meters at a maximum of 2.5 meters off the edge of the paved surface before the aircraft returned onto the runway. About 500 meters after touchdown the aircraft contacted a runway edge light causing damage to the right hand wing flap. The crew reported they had almost run off the runway and taxied to the apron.

A subsequent inspection of wheel tracks confirmed the aircraft had been off the runway and took out a runway edge light.

The operator's technician had already left the aerodrome, the captain therefore performed walk around inspection and did not spot any damage to the aircraft. The crew contacted flight operations and reported the occurrence stating they had nearly run off the runway. Following the discussion flight operations did not object to the aircraft departing for the next sector to Stockholm (Sweden) with passengers.

The following day, after the return flight had landed at Stockholm, damage to the right hand wing flap was discovered, which rendered the aircraft not airworthy.

Sweden's Haverikommission (SHK) released their final report separating the runway excursion as first incident and the continuation of flight as second incident and concluding the probable causes of the serious incidents were:

The first incident was caused by:

- Asymmetrical reverse thrust.
- The braking action was probably worse than what was indicated by the friction coefficients.

The second incident was caused by:

- Continued flight was prioritised in the crew’s assessment of the incident during landing.

- Shortcomings in the company’s systematic safety management with regard to maintenance checks and inspections.

The SHK analysed that the approach was stabilized until touchdown. Following selection of reverse thrust the reverse thrust however was assymetrical with the right hand engine delivering higher reverse thrust than the left engine, the aircraft changed heading to the right, opposite rudder was applied. The SHK wrote: "The reason for the asymmetric reverse thrust has not been clarified. In previous investigations of SHK have, however, some deviations when setting the engine controls (rigging) been identified, which may cause the reverse thrust to be asymmetric with equal position on the power levers. Given there was no crosswind and that no other external disturbances have been established, there is a high probability that the change in heading to the right – out towards the right runway edge, after touchdown – was caused by the asymmetrical engine power."

With respect to the performance data used the SHK analysed:

At the time of landing, the runway was covered with 5 mm of slush (see Figure 12). The performance data used at the time of the incident show that the crew had not made any corrections for the contaminated runway; they based their calculations solely on the reported friction coefficients. Contact with the operator and performance data provider has revealed that this was consistent with the data used in the operator's manuals.

According to SHK, however, this procedure is not fully consistent with the provisions of Regulation (EU) 965/2012, which requires that performance calculations take into account whether or not the runway is contaminated with slush.

With respect to the continuation the SHK analysed that the crew correctly recognized that they had veered off the runway and had hit a runway light, reported the occurrence to tower, who dispatched a vehicle for a runway inspection. The excursion was confirmed by the vehicle. However, as the crew had reported the occurrence themselves tower did not report the result of the inspection to the crew.

While looking for the technican, doing the walk around and contacting operations the mental image of the crew changed, they believed now they had only come close to the runway but not beyond the runway edge, there was no damage. The captain reported he was focussed on gear and tyres believing damage would be there, therefore did not spot the damage that the knocked over runway light had caused to the trailing edge flap.

The SHK assessed the damage: "In the present case, the damage was localised to the right wing flap, which constitutes a vital part of the aeroplane's flight control system. This part of the aeroplane's structure is subjected to strong aerodynamic forces every time it is deployed during flight, whereby the strain on the unit and its attachments increases. This can also entail the risk of the existing damage increasing in terms of their scope or spread."

The SHK analysed with respect to the walk around inspections:

After the incident, five PW inspections were conducted, i.e. performed by means of pilots walking around the aeroplane and looking for any damage or other discrepancies. The damage to the aeroplane was not discovered on any of these occasions, despite the fact that such inspections are intended to discover damage of this exact nature.
Collisions with birds or objects, leakage, etc. are normally defects which can arise – or which may have arisen – within a shorter time perspective, for example during the previous flight.

It has not been possible to discern the reason why the damage was not discovered during the inspection which took place immediately after the incident. However, it is possible that the inspection was focused to such a great extent on damage to the wheels or landing gear that other parts of the aeroplane did not receive appropriate checks.