Aviation safety investigations & reports

Grob Twin Astir, VH-IKL, Stirling Range WA, 9 July 1990

Investigation number:
Status: Completed
Investigation completed


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The accident occurred on the first gliding flight of the day. The air temperature was approximately two degrees Celsius and a shallow low-level layer of moist air was present. Shortly after takeoff on a dual tow, and at approximately 300 ft above the ground, what appeared to be ice formed on the outside of the canopies of both gliders causing a complete loss of forward vision. Although the pilots were able to see the ground through their clear view panels (left hand side) visibility was severely restricted. The pilots of both gliders released their tow ropes (short tow first) and attempted emergency landings. The long tow glider turned left through 90`, sighted a suitable paddock, turned left through 270` and completed a successful landing. The short tow glider (VH-IXL) turned left through 240`, sighted the airfield and had commenced a right turn to line up for a landing when it struck the top branches of a tree, crashing near the base of the tree. The Bureau of Meteorology reported that the most likely cause of the loss of visibility was the rapid deposition of hoar frost on the canopy after the cold aircraft (having radiated energy all of the previous night) ascended from a 200 to 300 ft thick, cold but relatively dry band of air into an inversion layer of warm and relatively moist air. In the Bureau's opinion, it would be virtually impossible to routinely give prior warnings of such an unusual phenomenon, or for aircrew to be able to determine the state of the atmosphere from a purely visual observation. It was considered that the time of the occurrence was significant. The two layers would have mixed together as the sun continued to rise thus diminishing the potential for the occurrence. The air temperature had risen to six degrees Celsius within 75 min of the accident. The pilots were aware of the standard procedure to attempt to land straight ahead following a low level tow release. However this procedure assumes forward visibility and on this occasion visibility was severely restricted. Both pilots were forced to manoeuvre in an attempt to sight a clear landing area through their clear vision panels. The procedure worked for one aircraft and it almost worked for the second. Consideration was given, by at least one of the pilots, to jettisoning the canopy however, this was not done because of the possibility of tail damage and complete loss of control.

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[Download  PDF: 25KB]
General details
Date: 09 July 1990   Investigation status: Completed  
Time: 645    
Location: Stirling Range    
State: Western Australia    
Release date: 02 December 1991   Occurrence category: Accident  
Report status: Final   Highest injury level: Serious  

Aircraft details

Aircraft details
Aircraft manufacturer Grob - Burkhart Flugzeugbau  
Aircraft model G103  
Aircraft registration VH-IKL  
Serial number N/K  
Type of operation Gliding  
Damage to aircraft Destroyed  
Departure point Stirling Range WA  
Departure time N/A  
Destination Stirling Range WA