Vacuum System

Cessna Vacuum System: Quick Overview

The vacuum system powers gyroscopic instruments and is critical for maintaining situational awareness, especially in IFR conditions. Here’s what you need to know:

Components:

1. Vacuum Pump:
   • Engine-driven pump (typically a dry-type).  When the engine is running, it should be making suction.
   • Creates suction to spin gyros in attitude and heading indicators.
2. Gyroscopic Instruments Powered:
   • Attitude Indicator (AI): Displays pitch and bank.
   • Heading Indicator (HI aka, DG): Displays heading, must be aligned with the magnetic compass.
3. Vacuum Regulator:
   • Maintains proper suction pressure (usually 4.5–5.5 inches of mercury [Hg] in the Cessna 172).
4. Filter:
   • Removes contaminants to protect the system and instruments.
5. Suction Gauge:
   • Monitors vacuum pressure; typically located on the instrument panel.

Operation:

• Engine-driven pump creates suction.
• Airflow spins gyros at high speeds (10,000–15,000 RPM).
• Instruments interpret gyro motion to display attitude and heading.
• Air first enters:
  1. Through the vacuum air filter
  2. Then flows through the instrument spinning up the gyro
  3. Then flows to the vacuum pump
  4. Then flows overboard under the cowling through the exhaust port on a vacuum pump

Failure Modes:

1. Vacuum Pump Failure:
   • Loss of suction = loss of attitude and heading indicators.
   • Indicated by low suction on the gauge and erratic or failed gyroscopic instruments.
   • Partial-panel flying may be required—rely on turn coordinator, magnetic compass, and other non-vacuum instruments.
2. Blocked Filters or Lines:
   • Reduced or no suction.
   • Can lead to instrument errors or failures.

Preflight Check:

• Verify suction gauge reads within limits (4.5–5.5 inHg) during engine run-up.
• Ensure gyroscopic instruments are stable and operational.