Certified Flight Instructor - Flight Instructor Airplane Practice Exam

Which instrument would be affected by excessively low pressure in the airplane's vacuum system?

1. Heading indicator

2. Airspeed indicator

3. Pressure altimeter

4. Vertical speed indicator

The correct answer is: Heading indicator

The heading indicator is affected by excessively low pressure in the airplane's vacuum system because it operates using a gyroscopic mechanism that relies on proper suction and movement within the vacuum environment. In aircraft, the vacuum system powers instruments like the heading indicator, attitude indicator, and sometimes the turn coordinator. When the vacuum pressure falls below necessary levels, these gyroscopic instruments can become unreliable or fail entirely. In contrast, the airspeed indicator, pressure altimeter, and vertical speed indicator operate based on differential air pressure or static pressure from the atmosphere rather than relying on a vacuum system. The airspeed indicator measures the difference between dynamic air pressure and static air pressure. The pressure altimeter relies on ambient air pressure to provide altitude readings, while the vertical speed indicator measures changes in static pressure. Therefore, any issues with the vacuum system primarily impact the performance of the heading indicator, thereby confirming why it is the correct choice in this scenario.