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.
Patella maltracking, or patellofemoral pain syndrome, is an imbalance in the gliding of the kneecap that aggravates cartilage behind the knee. Rather than gliding vertically as the knee bends, a maltracking patella is pulled to the outward side as it moves upward in the groove. This may result in pain on the outward and front sides of the knee, clicking sounds, swelling, and pain from both activity and long bouts of sitting.