A self-launching glider can give the pilot much more freedom in exchange for a more complex and expensive aircraft. First, a self-launching glider allows the pilot to fly from airports without a towplane or tow pilot. Second, the engine can be used to avoid off-field landings and extend the flight. In theory, when low in a self-launching glider, simply start the engine and climb to the next source of lift. This second advantage has pitfalls and dangers of its own and has led to many accidents due to engine failure and/or improper starting procedures. Engines on self-launching gliders generally are less reliable than those on airplanes and are susceptible to special problems. For instance, in the western United States, summer thermals often extend to altitudes where the air is cold. The self-launching engine can become cold soaked after several hours of flight, and may take more time to start or may fail to start.
Overreliance on the engine may result in a false sense of security. This can lead pilots to glide over unlandable terrain, something they might not normally do. If the engine then fails when needed most, the pilot has no safe place to land. Some accidents have occurred in which the engine starting system was actually fully functional, but in the rush to start the engine to avoid landing, the pilot did not perform a critical task, such as switching the ignition on. Other accidents have occurred in which the engine did not start immediately, and while trying to solve the starting problem, the pilot flew too far from a suitable landing area. For a self-launching glider with an engine that stows in the fuselage behind the cockpit, the added drag of an extended engine can reduce the glide ratio by 50 to 75 percent. [Figure 11-17]
The critical decision height to commit to an engine start on a self-launching glider is typically higher than the critical decision height for a nonpowered glider. This is due to a combination of the time needed to start the engine and extra drag during the starting process. It may take anywhere from 200 feet to 500 feet of altitude to extend and then start the engine. Whereas a pure glider may commit to landing at 1,000 feet AGL, the pilot of a self-launching glider probably opts for 1,500 feet AGL, depending on the glider and landing options should the engine fail. In this sense, the self-launching glider becomes more restrictive.
Cross-country flight can also be done under power with a self-launching glider, or a combination of powered and soaring flight. For some self-launching gliders, the most efficient distance per gallon of fuel is achieved by a maximum climb under power followed by a power-off glide. Check the GFM/ POH for recommendations.
Another type of glider features a sustainer engine. These engines are not powerful enough to self-launch but are able to keep the glider airborne if lift fails. Sustainers can only produce enough power to overcome the glider’s sink rate, and the higher sink rates can easily overwhelm the climb rate capability of many sustainer powerplants. The sustainer engine is typically less complex to operate than their self-launching counterparts, and can eliminate the need for a time-consuming retrieval. Pilots flying with a sustainer are susceptible to the same pitfalls as their self-launching counterparts.