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.
While it may sound like something is terribly wrong with the joint, in most cases joint noise is nothing to worry about. Crepitus is the medical term that refers to joint noises, such as popping and cracking and creaking, particularly in the knees. Such joint noise can be quite disturbing and cause concern, especially when it shows up suddenly. In most cases, these noises are not indicative of any underlying problem.