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DHC6, Tiree UK, 2017

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Summary
On 7 March 2017, a DHC-6-300 left the side of the runway after touchdown in what the crew believed was a crosswind component within the Operator's crosswind limit. The Investigation concluded that the temporary loss of control of the aircraft was consistent with the occurrence with a sudden gust of wind above the applicable crosswind limits and noted the reliance of the crew on 'spot' winds provided by TWR during the final stages of the approach.
Event Details
When March 2017
Actual or Potential
Event Type
Runway Excursion, Weather
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft DE HAVILLAND CANADA DHC-6
Operator Not Recorded
Type of Flight Public Transport (Passenger)
Origin Glasgow International Airport
Intended Destination Tiree Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Landing
LDG
Location - Airport
Airport Tiree Airport
General
Tag(s) Non Precision Approach,
PIC less than 500 hours in Command on Type,
Monitored Approach
RE
Tag(s) Directional Control,
Significant Crosswind Component,
Off side of Runway
WX
Tag(s) Strong Surface Winds
Outcome
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Operation
Safety Recommendation(s)
Group(s) None Made
Investigation Type
Type Independent

Description

On 7 March 2017, a DHC6-300 (G-BVVK) being operated by a UK regional airline on a scheduled passenger flight from Glasgow to Tiree left the side of the runway soon after touching down at destination after a final approach conducted in day VMC. It travelled a short distance over grass before entering and coming to a stop on the intersecting runway near its intersection with the landing runway. The aircraft was undamaged and the occupants were uninjured and it was subsequently taxied to the Terminal.

Investigation

An Investigation was carried out by the UK AAIB. It was noted that the 55 year old-Captain had accumulated 10,680 total flying hours of which just 152 were on type.

It was established that the excursion during landing on runway 23 (see the extract -*from AIP aerodrome chart below) had occurred after a VOR/DME approach made in gusty southerly wind conditions. The Captain had taken over as PF from the First Officer on gaining visual reference with the runway shortly before reaching the approach MDA of 450 feet in accordance with the Operator's Monitored Approach Procedure. Both pilots described the touch down as "normal" and the Captain advised that they had considered that "the required control inputs were consistent with the crosswind". However, "after touchdown the left wing suddenly lifted as the aircraft decelerated and the aircraft veered to the left (and) despite the application of full right rudder and left aileron, the aircraft continued to veer left for two or three seconds before resuming a more normal attitude". It was then stopped using normal braking and the crew concluded that the temporary loss of directional control had been caused by a sudden strong gust of wind.

Part of the Aerodrome Chart for Tiree current at the time (north is at the top). [Reproduced from the UK AIP]

The crew reportedly initially believed that whilst the aircraft had departed the 30 metre defined runway width, it had remained on the asphalt surface at the side of the runway. It was subsequently discovered that there were wheel marks on the adjacent grass which corresponded to the aircraft ground track during the excursion (see the illustration reproduced below). As a result of the initial determination, the aircraft was immediately taxied uneventfully to the assigned parking stand with the airport RFFS in attendance and shut down normally.

A sketch of the excursion track (not to scale). [Reproduced from the Official Report]

The Investigation reviewed the recorded wind velocity during and prior to the landing and the extent to which such information had been passed to the crew by the Airport FISO, Tiree being without an ATC service. The wind direction at the anemometer just north of the Terminal recorded at 1 minute intervals was found to have remained within the range 165° to 170° and the mean (maximum gust) wind speeds recorded at the same site and at the same intervals ranged between 28.5 (32.4) knots 6 minutes prior to the landing and 21.0 (25.4) knots. At the approximate time of landing, this site recorded the wind as from 167° at a mean (maximum gust) speed of 24.7 (30.62) knots. Data from an additional wind sensor positioned alongside the runway 23 touchdown area were not recorded but it was this sensor which was used by the FISO as the source of their wind readings passed to the crew by radio during the approach.

UK regulatory requirements for the passing to approaching aircraft of the maximum wind gust up to 10 minutes after its occurrence were noted to apply only if such a gust exceeded the current mean speed by 10 knots or more. It was noted that on first contact with Tiree, the crew had been advised of a surface wind of 160° at 24-35 knots and about 5 minutes prior to landing that it was currently 160° at 27 knots. The crew requested further wind checks two minutes before landing and just prior to touchdown and were respectively given (spot) winds of 170° at 25 knots and 170° at 23 knots. The DHC6 AFM was noted to include the "demonstrated crosswind" of 25 knots which was "not considered limiting" on a dry runway but it was found that the Operator's policy was "to consider 25 knots, including gusts, as a limit".

The Investigation noted that given the 500 foot cloudbase at Tiree Airport, there had been "no viable approach to a more into-wind runway than a VOR/DME approach to Runway 23". The crew "considered that the aircraft response during the approach and landing was as they would expect for a crosswind of around 25 knots and they described the landing as normal" whereas the recorded meteorological data showed that the surface wind at the airport "was gusting to approximately 32 knots". A sudden gust from the left would be consistent with the aircraft weathercocking into wind as the controls temporarily lost their authority.

The formally-stated Conclusion of the Investigation was as follows:

"Given the information they received, the crew believed the wind was within their limitations for landing. However, it is highly likely that the aircraft was affected by a strong gust of wind from the left during the landing roll. The event was sudden and briefly overcame the controls, which were reduced in effectiveness due to the low airspeed. As a result, the aircraft veered sharply left and departed the paved surface. There were no injuries and no damage to the aircraft or the airfield infrastructure."

It was note that "Tiree Airport has a policy of increasing the RFFS readiness state in marginal weather conditions (and) with the low cloudbase, strong crosswind and poor visibility they had done so for this incident flight".

The Final Report of the Investigation was published on 10 August 2017. No Safety Recommendations were made.

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