This work is licensed under a Creative Commons Attribution 4.0 International License
Philip Plane
Recently I had an incident where the towplane lost power and waved me off at low level. I turned back and landed on the runway without further incident.
This all worked out well for a number of reasons. I am very current, doing a lot of takeoffs and landings as a full time instructor. I probably pull the low level launch failure on students three or four times a season, so I'm familiar with the options close to the airfield if the launch fails. The conditions were very calm, just a light breeze. The glider was a Duo Discus, which is very forgiving at low speed. All these things contributed to a good outcome.
Even with all these factors in my favour, the turnback was not perfect. It was easy to bleed off speed in the turn and difficult to recover the speed when it was noticed.
There are many accidents that happen when gliders are turning close to the ground. The two classics are the turn from base to finals, and the low level launch failure turn back to the airfield. In mountainous areas thermalling close to terrain and turning close to the ridges are significant contributors to accident statistics.
Given that we do the base to final turn most flights, why do pilots continue to make basic mistakes?
The effect of wind is very apparent at low level and this can lead to quite powerful visual illusions.
When flying into wind at a constant airspeed the groundspeed is low and this can lead to lowering the nose.
Downwind the groundspeed is high and this may result in the nose attitude being raised.
When flying across the wind the effect of drift is most noticeable. A suitable reference point on track must be chosen. In order to track towards this reference point, the appropriate amount of drift must be offset and coordinated flight maintained.
When turning from into-wind to downwind an illusion of slipping into the turn will occur and likewise, when turning from downwind into the wind, an illusion of skidding out of the turn will occur. The strength of this illusion increases in proportion to the wind strength. Reference to the yaw string during low-level flight is vital.
Looking down the low wing while turning at altitude the wing tip appears to travel backwards across the ground. At low altitude (200 - 400 ft at glider approach speeds) the wing appears to travel forward over the ground. The altitude this happens varies with speed and bank angle and is called the pivotal altitude. This may lead to a tendency to yaw the glider around the turn with the rudder.
At low altitude as the glider descends the horizon appears to rise. This may lead to a tendency to raise the nose
To reduce these undesirable tendencies it is best to look over the nose, not down the wing. Looking over the nose you can see the attitude of the glider, the angle of bank, and the yaw string. The ground isn't going anywhere, so it can be seen adequately with peripheral vision.
When the glider is descending into wind at low altitude, as is normally the case on finals before landing, there is likely to be less wind close to the ground. As the glider descends into the reducing headwind it loses energy. The sink rate increases and the airspeed decreases. Maintaining steady airspeed can only be achieved by lowering the nose and increasing the sink rate, and/or reducing the airbrakes.
When the glider is ascending downwind the same loss of energy occurs. The worst case is doing a competition finish downwind, then turning into wind to land.
Energy is lost in both directions. This is the opposite of dynamic soaring.
Always fly the glider within the limitations of the pilot, the glider, and the conditions.
If the pilot is inexperienced, not current, or just cautious, it is best not to perform turns at low level. Fly a conservative circuit that has the final turn completed above 3-400ft. In the case of soaring close to terrain, if the pilot is not 100% confident that they have plenty of margin they should be cautious and back off even if this means they don't get the climb they were after.
Some gliders have benign handling and are reluctant to stall/spin. If this is not the case it is best be more cautious.
A smooth stable day with little wind is very different to an unstable windy day. What is acceptable on an easy day might be completely inappropriate on a less predictable day.
To avoid getting caught by these low level gotchas we must be aware of the potential problems and train to do the correct actions.
Upper air demonstrations are not adequate as they do not show the visual illusions or the effect of windshear.
Every landing gives to opportunity to observe the illusions and experience the windshear. If the pilot knows to look for the problems they are less likely to sneak up and catch the pilot by suprise.
To avoid having problems while turning at low level it is important to fly accurately. Maintaining the safe speed near the ground as we were all taught at the start of our flying. Keeping the glider coordinated as we turn as we were all taught at the start of our flying. This is just basic airmanship.
A good scan is important. The attitude, angle of bank, path of the glider, traffic, rate of climb/sink, airspeed, etc must all be continuously monitored. Getting fixated on any one thing can lead to missing important changes in other areas.
By maintaining airspeed and coordination, keeping the glider flying smoothly, we can safely handle the glider close to the ground.
This work is licensed under a Creative Commons Attribution 4.0 International License