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ASN Wikibase Occurrence # 195650
Last updated: 16 May 2020
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Time:4:20 UTC
Type:Silhouette image of generic ask1 model; specific model in this crash may look slightly different
Schleicher ASK 21
Owner/operator:Nijmeegse Aeroclub
Registration: PH-733
C/n / msn: 21185
Fatalities:Fatalities: 0 / Occupants: 2
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:Malden -   Netherlands
Phase: En route
Departure airport:Gliderfield Malden
Destination airport:Gliderfield Malden
Investigating agency: Dutch Safety Board
Members of the gliding club had intended to conduct flights on Ascension Day, shortly after dawn – an annual tradition called ‘sunrise flying’. Sunrise on the day was at 05.31, but because visibility was too poor for flying at time, the first launch was delayed. At around 06.00 the weather appeared to have improved to the point that flying would be possible. This was related to the fact that horizontal visibility was now 4 kilometres, and the fact that ground mist and clouds were no longer visible. Horizontal visibility was ascertained by the visibility of the Erasmus building in Nijmegen, located about 4 kilometres to the north of the airfield. Before taking the decision to fly, the duty instructor and the instructor of the flight concerned discussed the weather conditions at length and consulted weather information from the Royal Netherlands Meteorological Institute (KNMI). Both were convinced that visibility was adequate to begin flying.
The instructor and the student pilot were the first to takeoff.
The student was flying the glider. Use was made of a winch launch, and as they climbed during take-off the instructor noticed that at a height of about 100 metres they were flying through mist and scraps of cloud. This reduced visibility, because of which he took over control of the glider. The instructor decided not to abort the winch launch because the glider climbed above the mist shortly after. At that moment there was a blue sky and the sun was shining.
The instructor disconnected the glider from the winch cable at a height of about 400 metres. The winch operator had noticed that during the winch launch PH-733 had passed through occasional shreds of cloud, but because he had been able to see the glider more or less continuously he had decided not to abort the winch launch.
The crew of the glider had no ground visibility after the winch launch. A circling flight path was initiated in the hope that as soon as they regained ground visibility they would also be able to see the glider site. At a height of about 200 metres the glider re-entered the mist layer. It was only at a height of about 50 metres that the crew regained ground visibility. The glider was then too far south of the glider site to be able to land there.
Flight data shows that the flight had lasted for 4 minutes and 36 seconds and that PH-733 had flown at a maximum height of 400 metres. The glider crashed 0.7 kilometres west-south-west of the glider site.
KNMI data showed that there had been a weak northerly flow of humid air, whose structure was stable early in the morning. Mist and haze were present in many places. The upper ceiling of the humid layer of mist and low clouds was estimated at an altitude of about 300 metres. This mist and haze did not evaporate everywhere at the same time; occasional banks of mist and cloud drifted over the airfield from the north shortly after, and perhaps even during, the glider’s take-off. Although the wind speed on the ground was weak, 3 knots, the wind speed at a height of 300 metres was about 10 knots from a direction of 010°.
This wind was blowing mist and haze from the north over the glider site relatively quickly. It is quite possible that between the moment that the decision was taken to commence flying and the moment of the actual launch, a new bank of mist had been blown onto the glider site from the north. The absence of ground visibility from the crew’s perspective was partly the result of the reflection of bright sunlight on the minute water droplets of the mist bank.
Because this reflection is absent when viewing from the ground, it is possible that the visibility perceived when looking up from the ground was better than it was when looking down from the air.
The northerly wind also meant that the glider drifted southwards as it circled. The fact that the captain thought he was above the airfield can be explained by his perception of minimal wind speed on the ground. He was, however, insufficiently aware of the prevailing upper winds.


Revision history:

25-May-2017 06:56 Chieftain Added
25-May-2017 09:58 gerard57 Updated [Source]
25-May-2017 11:45 RobertMB Updated [Aircraft type, Registration, Cn, Operator]
27-May-2017 05:25 GlidingWolf Updated [Time, Phase, Damage, Narrative]
15-Aug-2017 19:53 harro Updated [Destination airport, Source, Narrative]
13-Dec-2017 13:21 harro Updated [Source, Narrative]

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