At the altitude where the Pitot tube becomes blocked, the airspeed indicator remains at the existing airspeed and doesn't reflect actual changes in speed
At altitudes above where the Pitot tube became blocked, the ASI displays a higher-than-actual airspeed increasing steadily as altitude increases
At lower altitudes, the ASI displays a lower-than-actual airspeed decreasing steadily as altitude decreases
Pitot Tube Drain Hole Blockage:
Would cause ASI to malfunction
Pitot tube and drain hole become blocked:
Would cause ASI to malfunction
Static Port Blockage:
Would cause ASI, altimeter, and VSI to malfunction
If in a real emergency, malfunctions can be corrected with alternate air or breaking the glass on a Pitot static instrument (VSI)
If the static ports become blocked, the ASI would still function but could produce inaccurate indications
At the altitude where the blockage occurs, airspeed indications would be normal
At altitudes above which the static ports became blocked, the ASI displays a lower-than-actual airspeed continually decreasing as altitude is increased
At lower altitudes, the ASI displays a higher-than-actual airspeed increasing steadily as altitude decreases
The trapped pressure in the static system causes the altimeter to remain at the altitude where the blockage occurred
The VSI remains at zero
On some aircraft, an alternate static air source valve is used for emergencies
If the alternate source is vented inside the airplane, where static pressure is usually lower than outside static pressure, selection of the alternate source may result in the following erroneous instrument indications:
Altimeter reads higher than normal
Indicated airspeed (IAS) reads greater than normal
VSI momentarily shows a climb
Common Training Aircraft Pitot-Static System Characteristics:
Piper Arrow:
Composed of a heated Pitot tube on the lower left wing
Two static ports are located on each side of the fuselage
Alternate static air (below pilot control yoke) provides static pressure from inside the cabin
Cessna 172:
Composed of a heated Pitot tube on the lower surface of the left wing
An external static port is located on the lower left side of the forward fuselage
Pitot Tube consists of a heating element, a 5-amp switch/breaker, and associated wiring
Alternate static (below throttle) provides pressure from inside the cabin
