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.
1 Unlike a mechanical popping where this popping sensation is painless and intermittent, the crepitus caused by arthritis is oftentimes painful. 2  These symptoms are usually associated with other knee symptoms suggestive of arthritis, such as pain while walking, occasional swelling of the knee, stiffness, and so on.