Detection of system malfunctions or failures.

Bottom line

Malfunction of aircraft systems, engines, flight controls, or automation is a common threat, and threats may appear suddenly and limit the time available for analysis. With a partial static system blockage, the altimeter, airspeed indicator, and VSI may operate normally during takeoff, climb, and level-off at cruise altitude, with no indication of malfunction until the airplane begins a descent. If a static pressure problem exists after confirming it with the alternate static source, the alternate source should be used during the remainder of the flight.