These are the commercial jet engines you see, like the Boeing 777, 747, Airbus A330 and A380s. Though there is a considerable reduction of the engine thrust, at these altitudes the lower drag associated with low air density means a more economical flight with a higher True Air Speed.
Crepitus in the knees may be caused by knee injuries such as meniscus tears. These are fairly common in people who play sports, jog, or run. A meniscus tear can cause crepitus as the joint moves. Chondromalacia patella is a dull ache behind the kneecap, usually caused by overuse or injury. There will be damage to the undersurface cartilage covering the kneecap.
In orthopedic medicine and sports medicine, crepitus describes a popping, clicking or crackling sound in a joint. Joint popping sounds may mean that air is moving in the joint, which is usually harmless. People most often notice crepitus in their knees, but it can also happen in other joints like the shoulder, elbow or neck. Crepitus with pain can be a sign of wear and tear or injury. If crepitus is painful, you should consult a doctor.