Final answer:
During an airplane's ascent, gravitational potential energy decreases as it is converted into kinetic energy, facilitating a gain in altitude and acceleration.
Step-by-step explanation:
Just after taking off, an airplane continues to accelerate as it gains height. During this phase, one form of energy that decreases is gravitational potential energy. As the airplane ascends, the potential energy associated with its position relative to the Earth is converted into kinetic energy because the plane's engines are working to overcome gravity and increase the velocity of the aircraft as it gains altitude. Similar energy transformations occur in other contexts, such as a hybrid car charging its batteries through regenerative braking, water falling and turning a turbine to generate electricity, or a skydiver's potential energy converting into kinetic energy before opening their parachute.