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 Loose fragments of cartilage and other tissue can interfere with the smooth motion of joints. 2  The knee may lock or stick during movement. 3  It may creak, click, snap or make a grinding noise (crepitus). 4  Pain may cause a feeling of weakness or buckling in the knee.