If the air is rising faster than the rate of sink , the aircraft will climb . At lower speeds an aircraft may have a worse glide ratio but it will also have a lower rate of sink . A low airspeed also improves its ability to turn tightly in centre of the rising air where the rate of ascent is greatest . A sink rate of approximately 1.0 m/s is the most that a practical hang glider or paraglider could have before it would limit the occasions that a climb was possible to only when there was strongly rising air . Gliders ( sailplanes ) have minimum sink rates of between 0.4 and 0.6 m/s depending on the class . Aircraft such as airliners may have a better glide ratio than a hang glider , but would rarely be able to thermal because of their much higher forward speed and their much higher sink rate . ( Note that the Boeing 767 in the Gimli Glider incident achieved a glide ratio of only 12:1 . ) The loss of height can be measured at several speeds and plotted on a " polar curve " to calculate the best speed to fly in various conditions , such as when flying into wind or when in sinking air . Other polar curves can be measured after loading the glider with water ballast . As mass increases , the best glide ratio is achieved at higher speeds . ( The glide ratio is not increased . ) Soaring animals and aircraft may alternate glides with periods of soaring in rising air . Five principal types of lift are used : thermals , ridge lift , lee waves , convergences and dynamic soaring . Dynamic soaring is used predominately by birds , and some model aircraft , though it has also been achieved on rare occasions by piloted aircraft
Gimli Glider
Gimli Glider
Gliding flight