Type Rating Knowledge — Cessna 172S NAV III
Vacuum system, standby instruments and stall warning
Prefer to read? Open the reference page →
1 / 21
The system that was supposed to have gone
A glass panel replaces spinning gyros with solid-state sensors, so the vacuum system should be gone. On this aeroplane it is not. POH §7 opens the section with what it is for: "The vacuum system provides the vacuum necessary to operate the standby attitude indicator." Five components — one engine-driven vacuum pump, a vacuum regulator, the standby attitude indicator itself, a vacuum system air filter and a vacuum transducer. What changed between a steam-gauge 172 and this one is not whether there is a pump, but how much depends on it. Here, exactly one instrument does.
- CABIN AIR IN — Air drawn into the system by the pump — the working fluid of a vacuum instrument is ordinary air.
- VACUUM AIR FILTER — The system air filter. It sits UPSTREAM of the instrument, so it is the one component that can starve a healthy pump’s gyro.
- STANDBY ATTITUDE IND — Vacuum-powered gyroscopic attitude indicator on the centre panel below the MFD. The NAV III did NOT delete the vacuum system — this is what it kept it for.
- GYRO FLAG — Low-vacuum warning flag on the face of the standby attitude indicator, coming into view when vacuum is below the level needed for reliable gyro operation.
- VACUUM REGULATOR — Regulator holding the system at its working suction rather than whatever the pump can pull.
- VACUUM TRANSDUCER — Transducer providing a vacuum signal to the engine display — this is why vacuum is a number on a screen rather than a separate gauge.
- ENGINE-DRIVEN PUMP — One engine-driven vacuum pump on the accessory case. One pump, no standby — its failure is a single point of failure for the standby attitude indicator.
- EIS VACUUM READING — Vacuum indication on the EIS ENGINE page — left of the PFD during start, left of the MFD in normal operation, and on the operating display in reversionary mode.
- LOW VACUUM — Amber LOW VACUUM annunciator down the right side of the PFD, displayed when available vacuum from the engine-driven pump falls below 3.5 in.hg.