FAA Ground School Practice Test

Which instruments will become inoperative if the static vents become clogged?

1. Airspeed only

2. Altimeter only

3. Airspeed, altimeter, and vertical speed

4. Vertical speed only

The correct answer is: Airspeed, altimeter, and vertical speed

When static vents become clogged, the impact on instruments is significant because the static pressure used by multiple flight instruments is affected. The airspeed indicator, altimeter, and vertical speed indicator all rely on static pressure readings to function correctly. The airspeed indicator uses both static pressure and dynamic pressure (from the pitot tube) to determine true airspeed. If the static port is blocked, it cannot accurately measure the ambient air pressure, which can lead to erroneous speed readings. The altimeter relies solely on static pressure to determine altitude. A blockage in the static vent would prevent it from accurately sensing changes in pressure associated with altitude, causing it to freeze at the altitude it was at when the blockage occurred. Similarly, the vertical speed indicator measures the rate of change in static pressure. If the static vent is clogged, the instrument cannot detect changes in air pressure as the aircraft ascends or descends, rendering it inoperative as well. Therefore, all three instruments—airspeed indicator, altimeter, and vertical speed indicator—become inoperative if the static vents are clogged, making the correct answer comprehensive.