Final answer:
When an Alternate Static Air Source is used, the flight instruments that use static pressure, such as the altimeter, will indicate higher altitude readings due to the slightly higher air pressure inside the cabin compared to the outside air at the same altitude. This error occurs because the instruments interpret the higher pressure as being at a lower altitude.
Step-by-step explanation:
If an Alternate Static Air Source Is Used, you might expect your instruments to Indicate higher than the actual altitude. The alternate static source is often located within the aircraft cabin or another protected area where the air pressure is slightly higher than outside at the same altitude due to the aircraft's structure acting somewhat like a pressure vessel. This higher pressure from the alternate source causes the altimeter airspeed indicator and vertical speed indicator to register slightly erroneous readings.
For instance the altimeter would show a lower altitude than true altitude because the instrument will interpret the higher pressure as being at a lower altitude where the pressure should be higher.
Examples in Flight Situations:
- Altimeter - The primary effect will be that the aircraft will seem to be at a lower altitude than it actually is.
- Airspeed Indicator - It may also read slightly higher than the actual airspeed.
- Vertical Speed Indicator - May show a climb or descent when the aircraft is actually in level flight.