Christopher Thorpe
Executive Manager, Operations
Aerotowing may appear to be a benign method of getting airborne in a glider, but there are inherent hazards, the most significant of which are tug upsets. In the last 10 years there have been 11 incidents in which gliders have become seriously out of position and in at least two of these incidents the tug pilot felt they were about to lose control. The majority of these occurred during a ‘boxing the slipstream’ exercise.
Generally speaking, the tug pilot is the person at greatest risk if an upset occurs. In the mid-1980s an educational campaign on safe aerotowing followed two fatal tug upset accidents between 1980 and 1984. A similar program was also run in the UK around that time. Since then, glider and tug pilot training has increased awareness of tow upset accidents and prevention. However, while the fatal accidents have stopped and tug upset incidents are now rare, some of that can be attributed to the protection afforded by the weak link in those cases where the glider has been flown well out of position. The following reports illustrate the dangers.
20 July 2020 – Potential Lateral Tug Upset
During the aerotow launch and at a height of about 1,500ft AGL, the student commenced an exercise to ‘box’ the tug’s slipstream. The exercise had been pre-briefed with both the student and tug pilot. The student commenced the exercise from the low tow position and moved the glider quite quickly to the right.
As the student began to transition into the high tow position while still out to the right of the slipstream, the tug pilot made a radio call to the crew in the glider advising that the glider was displaced too far to the right and that he was having control difficulties – specifically that he was being pulled into a roll to the left.
The instructor directed the student to return the glider to the normal low tow position but in so doing, the student allowed a significant bow to develop in the tow rope. The instructor took control of the glider but was unable to prevent the rope from quickly becoming taut, at which point the weak link broke.
As the towrope was re-tensioned, the tug pilot reported that this caused an excessive yawing movement, and the tug commenced an uncommanded roll to the left that he managed to counteract before it became too severe. Upon the weak link breaking, the rope sprang back and draped over the glider’s starboard wing. The student operated the glider’s release and the rope slipped off the wing and fell to the ground. Both the tug and glider landed safely back on the airfield.
The glider instructor advised that the student was unsure how far to the right the glider should be flown to avoid the slipstream. As a consequence, they flew far too wide and near the limit of the tug pilot’s ability to control the aircraft. When the tug pilot made a radio call to advise of his control difficulties, the instructor commanded the student to return to the normal towing position.
Bowed Rope
During this manoeuvre, a large bow developed in the rope while the glider was still slightly displaced laterally from the tug’s centreline. The instructor took over control but was unable to prevent the rope from quickly tightening and yawing the tug. The tug commenced an uncommanded roll to the left that was only prevented by the weak link breaking and by corrective control inputs made by the tug pilot.
The effect of lateral tension on the towrope may result in either a vertical stabiliser (fin) stall or a dynamic lateral upset of the tug. Both situations will result in the tug entering an uncommanded yaw, roll and nose pitch down. In either case, events would develop rapidly and probably take the tug pilot by surprise. For a fin stall to occur, the glider would need to apply a constant lateral force to the tail of the tug via the towrope, requiring the tug pilot to apply a constant rudder input to counteract that force.
If the tug pilot’s rudder input was nearing maximum and further lateral force was applied causing the tug to yaw further, the critical angle of attack of the fin may be exceeded, resulting in the fin stalling. In turn, this would result in a rapid loss of yaw control of the tug. With the lateral towrope force still being applied, the tug would roll and descend before the tug pilot could react.
For the dynamic lateral upset, divergence between the glider and the tug can induce a large, abrupt yaw of the towplane when the rope comes taut. The tug pilot won't have applied rudder to correct the yaw before it happens, and once the yaw and roll turns the towplane onto a further diverging flight path, the upset increases in magnitude. Further information on lateral tow upsets can be found in the GFA Aerotowing Manual at Section 10.3.
BOXING THE SLIPSTREAM
Boxing the slipstream is an exercise in control, resulting in the balancing of forces on the glider while on tow. The aim is to perform a square box outside the slipstream, pausing at each corner under control, and taking the smallest route outside the slipstream to safely carry out the task. In the context of aerotowing, the slipstream is the turbulent flow of air driven backward by the propeller of the tug.
To avoid the propeller slipstream, the glider only needs to be displaced about half the glider’s wingspan from the tug’s centreline – usually when the glider’s nose is pointing just outside the wings of the tug. This exercise must always be performed at a safe height (about 1,000ft AGL), and while maintaining tow rope tension. On completion, the tug pilot is to be advised that the exercise is completed.
Should a significant bow appear in the rope, release the cable immediately rather than attempt to ‘fly it out’. Invariably, attempts to ‘fly out’ the bow results in broken weak links and ropes draping over wings or otherwise contacting the glider. For further information, refer to Operational Safety Bulletin No. 01/12 – Boxing The Slipstream.
7 June 2021 – Potential Vertical Tug Upset
During an aerotow and at approximately 4,000ft (2,920ft AGL), the solo glider pilot climbed out of station and started to lift the tail of the tug. The tug pilot was about to activate the tow release when the glider pilot released from tow. Both aircraft landed safely, and a debriefing was held with the Duty Instructor.
The flight was the pilot’s third solo after 17 dual training flights, having recently returned to gliding after a break of several years. The glider pilot reported that the tug climbed unexpectedly, then descended, at which point the glider unexpectedly climbed, resulting in the glider being significantly out of station above the tug. The glider pilot also reported that he had misidentified the tow release handle, and when he attempted to release from tow, he may have inadvertently pulled the wheel brake control, which is a similar shape to the release handle but a different colour.
Mistaken Release
After realising that the tow cable had not released, the glider pilot then identified and operated the cable release control. The tug pilot, who is the club’s Tugmaster and an experienced sailplane pilot, advised he was about to release the glider when the tow rope was released by the glider pilot. The tug pilot reported that the air was particularly smooth, and that the combination had climbed well above the temperature inversion when the incident occurred. The tug pilot stated there was no environmental turbulence at any time during the launch.
Investigation by the CFI concluded that the glider pilot probably operated what he thought was the release handle and most likely performed a climbing clearance turn to the right. By the time he realised that the cable had not released, he then identified and operated the actual release control, by which time the glider was most likely well above the tug and pulling the tail of the tug upwards and to the right.
Tug upsets are serious and have caused the deaths of a several tug pilots around the world. If the glider is allowed to climb rapidly behind the tug, it can very quickly become impossible to prevent it accelerating upwards in a slingshot action, rather like a winch launch, and tipping the tug over into a vertical dive. Once that has happened, the tug pilot will only be able to recover provided there is sufficient height.
Glider Displacement
Downward displacement of the glider below the slipstream is quite acceptable, but upward displacements are much more critical. The glider pilot must release immediately if the glider is going high and the tendency cannot be controlled, or the pilot loses sight of the tug. The circumstances which make tug upsets more likely are:
• a light pilot flying close to the minimum cockpit weight
• an inexperienced pilot – particularly wire launch pilots with little recent aerotow experience
• glider with a belly or CG hook
• an all-flying tailplane, or a glider with very light elevator forces
• short rope
• turbulent conditions
Vertical upset can also arise when the glider releases if the glider turns before the pilot has confirmed that the rope has separated. A tug upset is less likely to occur if the glider pilot avoids transitioning above or below the slipstream prior to release. If towing in low-tow, then the glider pilot should release from low-tow and vice versa.
Essential Checks
It is essential to check that, prior to release, the airspace is clear (a) to the right where the glider is just about to turn, and (b) to the left and below where the tug is just about to descend. The glider pilot must then ‘Locate, Identify, Operate’ the tow release. The release should not be operated until it has been positively located and identified as the one required. This eliminates any possibility of error in selection of the wrong control.
The same principle applies to all ancillary controls. When ready, the glider pilot will pull the release, and must observe the rope falling away before beginning their clearance turn to the right, while simultaneously applying normal targeted scan. The release should be operated while the towrope is still under some tension, and the tug pilot, after feeling the release, should check that the glider has in fact released and begin a descending turn to the left.
Post release actions should then be carried out and transition from launching pilot to soaring or landing pilot. For further information on tug upsets, please refer to Section 10.3 of the GFA Aerotowing Manual.