A de Havilland Canada DHC-6 Twin Otter 300 was operating a cargo flight under VFR flight rules from Mestersvig Airbase (BGMV - Greenland) to a remote landing site (at coordinates 73º 24'N 24º 20'W) approximately 75 nm north of Mestervig Airbase.
Damaged flap (Source & © HCLJ)
It was the second flight to this remote landing site for the crew, the previous sectors had been operated without any problems. After an uneventful flight, the aircraft arrived at the selected landing area, where the crew flew a low pass over the area to assess the wind conditions. The pilot assessed that the wind was approximately 5 knots.
Just before the runway, the aircraft encountered a downdraft, pushing the aircraft down. The crew was unable to correct this in time and the right-hand main wheel contacted the ground. The aircraft bounced and touched down a second time, on the runway. During this landing, the aircraft rolled to the right to such a degree that the flap hinge on the right wing contacted the runway. Resulting in damage to the right aileron and flap. The pilot stopped the aircraft and performed the shutdown checklist. There were no injuries.
The accident was reported to the Havarikommissionen for Civil Luftfart og Jernbane (HCLJ - Accident Investigation Board Denmark), and an investigation was initiated.
As part of the investigation, a technical inspection of the aircraft was performed. It was determined that, during the first touchdown (short of the runway) the main wheel rubber shock absorber block had failed. This caused the aircraft to settle on the right wing flap hinge on the second touchdown resulting in damage to the right wing flap and aileron. The main wheel rubber shock absorber block was found to be an on-condition component (no hard time for replacement)
The failed main wheel rubber shock absorber block (Source & © HCLJ)
With the nearest weather station ~250 km (135 nm) from the accident site, it was difficult for the investigators to determine the exact weather (wind) conditions at the time of the accident. Based on the available data the investigators (with the aid of the Danish Meteorological Institute) determined that, as the landing site was sheltered by a mountain top to the north-west, and around this top, there were probably lee eddies that caused an "uneasy changing wind" and windshear conditions at the landing site. There may also have been "vertical rotors" in the shelter of the mountain, and these can be experienced as "downdraft". (see the attached drawing from the investigation report below)
Estimated wind directions on the area of the landing site, in the boxed area (Source & © HCLJ)
The HCLJ concluded in their preliminary report that it was likely that local topography combined with current wind conditions influenced the course of events.
The preliminary accident report from the HCLJ (in Danish), which served as the source for this blog, can be accessed by clicking on the .pdf file below;
** Editorial note **
V2 Aviation - Training & Maintenance has not been able to obtain an investigation report in English on this accident. This blog is therefore based on a translation of the original preliminary report in Danish. Should there be inconsistencies in the blog don't hesitate to get in touch with us. There are two possibilities to do that, via the comments function at the bottom of this page or the contact page of the website.
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