Airspeed is a function of both pitot and static.
If a vacuum is not
bled over to the pitot side when performing altimeter checks the airspeed indicator will most likely be pegged out high around 5000' above field level. With the crossbleed valve open the airspeed indicator will remain at 0 knots and the aircraft can be taken to service altitude without fear of damaging the airspeed system.
The IVSI control valve should also be left open. The IVSI is used to verify altitude rate of change. As a general rule, never exceed 6000' a minute going up or down.
- Close all other valves before starting.
- Charge the vacuum tank with the electric pump if it is available. Leave the pump running, it will be needed. Unlike the pitot system, the static takes a considerable amount of suction to reach higher altitudes. Hand pumping a system to 35K is not fun.
- Slowly begin to open the static control valve. The altimeter can be driven to the desired value. The higher altitudes require opening the control valve more on the way up. Keep the IVSI within the 6000’ rate.
- Verify the airspeed indicator remains at 0 knots.
- Watch your hoses. Anyone stepping on or tripping over a hose might make it fall off the test port. If an airspeed indicator gets pissed from being driven to fast, an altimeter or air data computer will go into a rage if it is dropped from 40K to 0 all at once.
- Keep in mind that both pitot and static are being evacuated. If a small leak is noted in the 35K+ range it is most likely with the test connections or the tape on the opposite side static port. Newer aircraft that have the static port incorporated in the side of the pitot probe do not usually have this problem. I have found that static and pitot test adapters from
provide a very tight connection.
- The system can be lowered by slowly opening the static vent valve. Again, do not exceed the 6000’ rate.