Type Rating Knowledge — Cessna 172S NAV III

Vacuum system, standby instruments and stall warning

The glass 172 did not delete its vacuum pump — it narrowed the pump’s job to one instrument, the standby attitude indicator. Alongside it sits the only warning system on the aeroplane that needs no electricity at all.

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Vacuum system and the standby attitude indicator

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.

Redrawn from POH 172SPHBUS §7, Vacuum System and Instruments (Figure 7-9), Attitude Indicator, Vacuum Indicator and Low Vacuum Annunciation — structure restated, not traced.

What does the vacuum system on a C172S NAV III exist to do?

Operate the standby attitude indicator — and nothing else. Right, in the POH’s own words. One instrument, which is what makes a pump failure an inconvenience here and a serious event on an older aeroplane.

At what value does the LOW VACUUM annunciator appear, and what does it look like?

Below 3.5 in.hg of available vacuum from the engine-driven pump — an AMBER annunciator down the RIGHT side of the PFD. Right on all three: the threshold, the colour, and the side. Amber, not red — it is a caution, because one instrument is affected.

Two pieces of evidence, and they are not the same evidence

Vacuum is measured by a transducer that signals the engine display, and it appears as a reading on the EIS ENGINE page — down the left side of the PFD during engine start, down the left edge of the MFD in normal operation, and on the operating display in reversionary mode. That is the system-level number, and the LOW VACUUM annunciator is derived from it. But the instrument has its own indication too: a low-vacuum warning flag marked GYRO, on the face of the standby attitude indicator, which comes into view when vacuum is below the level the gyroscope needs to be reliable. Those are two different measurements. The annunciator speaks for what the pump is making; the flag speaks for what the gyro is actually receiving, after the filter and the plumbing. They usually agree, and the case where they do not is the case worth thinking about.

Where does the vacuum reading appear during normal cruise?

On the EIS ENGINE page along the left edge of the MFD. Right. During ENGINE START it is on the left of the PFD instead, and in reversionary mode the EIS bar moves to the left of whichever display is operating.

What is the GYRO flag on the standby attitude indicator telling you?

That vacuum at the instrument is below the level needed for reliable gyroscope operation — so the indication itself is not to be trusted. Right. It is an instrument-level flag, which is why it is separate evidence from a system-level annunciator upstream of the filter.

Vacuum system (POH 172SPHBUS §7, Vacuum System and Instruments)
Component	What it does
Engine-driven vacuum pump	One pump, on the engine accessory case. There is no standby pump.
Vacuum regulator	Holds the system at its working suction rather than whatever the pump can pull.
Vacuum system air filter	Filters the air the gyro runs on — upstream of the instrument.
Standby attitude indicator	The system’s only instrument. Vacuum-powered gyroscopic, centre panel below the MFD.
Vacuum transducer	Signals the engine display; read on the EIS ENGINE page.
LOW VACUUM annunciator	Amber, right side of the PFD, below 3.5 in.hg available vacuum.
GYRO flag	On the instrument face; in view when vacuum at the gyro is inadequate.

The engine-driven vacuum pump fails in cruise. What have you actually lost?

The standby attitude indicator. The PFD attitude comes from the AHRS and is unaffected. Right — you have lost your backup, not your primary. That is a reason to think about the flight, not a reason to fly on instruments you no longer have.

Someone tells you the modern C172S has eliminated vacuum system failures. What is the accurate correction?

The vacuum system is still fitted and still drives the standby attitude indicator — what changed is the CONSEQUENCE of a failure, not its existence. Right. §7 describes the pump and the instrument, and §3 carries an emergency procedure for the failure that supposedly cannot happen.

Stall warning system

A warning system with no electricity in it

The stall warning system is three things: an inlet in the leading edge of the LEFT wing, an air-operated horn near the upper left corner of the windshield, and the plumbing between them. No switch, no breaker, no bus, no vane, no reed. As the aeroplane approaches the stall, the low pressure on the upper surface of the wing migrates FORWARD around the leading edge and reaches the inlet. The resulting differential pressure draws air through the horn, and the horn sounds — 5 to 10 knots above the stall, in all flight conditions. Everything about the warning is a consequence of the aerodynamics that produce the stall, which is why it works at any weight and any bank angle without being told what those are.

INLET, L WING LE — Inlet in the leading edge of the LEFT wing. As the wing approaches the stall, low pressure on the upper surface moves forward around the leading edge and reaches this inlet.
PREFLIGHT SUCTION — The preflight check: a clean handkerchief over the vent opening, or a suction device, applied to draw air through the horn. The system is serviceable if the horn sounds.
PLUMBING — Plumbing from the wing inlet forward to the horn. Air, not wire.
AIR-OPERATED HORN — Horn near the upper left corner of the windshield. The pressure differential draws air THROUGH the horn, which is what makes the sound — 5 to 10 knots above the stall in all flight conditions.
ELECTRICAL SYSTEM — Drawn here deliberately UNCONNECTED. The stall warning system is pneumatic: no bus, no breaker, no switch. It works with the master off and it works after a total electrical failure.

Redrawn from POH 172SPHBUS §7, Stall Warning System — structure restated, not traced. C172S NAV III.

How much warning does the horn give, and under what conditions?

5 to 10 knots above the stall, in all flight conditions. Right, and the second half matters: the warning tracks the aerodynamics, so it does not need to know your weight, bank angle or load factor.

What physically makes the horn sound?

Low pressure at the wing inlet draws air THROUGH the horn, and the airflow through it makes the sound. Right — it is a whistle driven by suction, not a speaker driven by a switch. That is the whole reason it needs no power.

The silence you cannot detect in flight

Because the system has no electrical connection, it has no self-test and no annunciation. A blocked inlet — ice, an insect nest, or protective tape left on after a wash — removes the warning completely, and nothing tells you. There is no way to discover that in the air except by not hearing a horn you were expecting, at which point you are already at the speed that would have produced it. So the check is a ground check, and it is specified: apply suction to the system, either by placing a clean handkerchief over the vent opening and sucking, or by using some other suction device. The system is operational if the warning horn sounds. That is a real test of the whole path, not a look at the inlet.

How does the POH tell you to check the stall warning system on the preflight?

Apply suction at the inlet — a clean handkerchief over the vent opening, or a suction device — and listen for the horn. Right, and note what it proves: the inlet, the plumbing and the horn, end to end. Looking at the hole proves only the hole.

Why is a preflight suction check worth more than an inspection of the inlet?

Because it exercises the whole path — inlet, plumbing and horn — and the system gives no indication of its own health in flight. Right. A clear-looking inlet says nothing about a blocked line or a dead horn, and there is no annunciation to catch it later.