11.   Flying at lower temperatures could result in fuel-starved engines and malfunctioning hydraulic systems as fluids vital to aircraft operations thicken.

12.   Furthermore, Airbus plans to have a decentralized hydraulic system with pressurized reservoirs of fluid close to where it is needed to move flaps and slats.

13.   Generally, the cable or an electronic signal from the computer connects to a hydraulic system that actually does the work.

14.   However, shuttles have descended successfully with one APU in a standby mode and, in at least one instance, with a gushing hydraulic system leak.

15.   If both hydraulic systems are lost, the plane crashes.

16.   If flying parts disabled the hydraulic system, pilots would not have been able to steer the aircraft.

17.   In a hydraulic system, the pressure is equal throughout.

18.   In each instance, Alaska mechanics reported replacing the hydraulic line and servicing the hydraulic system, FAA records obtained by The Chronicle show.

19.   In the Pittsburgh case, investigators believe that a malfunction in the hydraulic system made the rudder swing all the way to one side and stick there.

20.   Investigators have also concluded that because of a computer software malfunction, an emergency backup for the hydraulic system did not work.