PHAK · Chapter 8

Vacuum System and Failure

Master the aircraft vacuum system: how it drives gyro instruments, normal suction values, failure signs, and partial-panel procedures for safe IFR flight.

Source reviewReviewed by GroundScholar Editorial ReviewLast reviewed: 2026-08-05Primary FAA source

CFI's Whiteboard Explanation

Think of the vacuum system as a vacuum cleaner running off your engine — it sucks air through your attitude indicator and heading indicator, spinning their internal gyros at ~15,000 rpm. Normal suction is 4.5–5.5 in. Hg on the gauge.

The scary part: when the pump fails, the AI doesn't just quit — it slowly leans over and lies to you. In IMC, that can kill. That's why your turn coordinator is electric (different power source) and why you cross-check with airspeed, altimeter, VSI, and compass. Suspect failure? Cover the bad instruments, fly partial panel, tell ATC, and get to VMC.

Handbook Reference

PHAK Ch 8

7.vacuum-system-and-failure. Vacuum System and Failure

The vacuum system (sometimes called a suction or pressure system) powers the gyroscopic instruments in many light aircraft — most commonly the attitude indicator and the heading indicator. While the turn coordinator is also gyroscopic, it is usually electrically driven so that the pilot retains a bank reference if the vacuum system fails. This redundancy between vacuum and electrical sources is fundamental to instrument flight reliability.

How the System Works

In an engine-driven vacuum system, a vane-type pump mounted on the accessory case of the engine creates a pressure differential. Outside air is drawn through an inlet filter, through the instrument case (where it spins the gyro rotor by impinging on small buckets cut into the rotor's circumference), through the regulator, and finally overboard via the pump.

The major components are:

Engine-driven vacuum pump — typically a dry-vane pump that creates suction.
Suction relief valve (regulator) — maintains system suction at the value specified by the manufacturer, generally 4.5 to 5.5 inches of mercury (in. Hg) for most singles. Excess air is admitted through the valve to keep pressure constant.
Air filter — a central filter that prevents contaminants from reaching the instruments. A clogged filter is the most common cause of low suction.
Gauge (suction gauge) — displays system differential pressure in inches of mercury so the pilot can verify proper operation.
Plumbing and check valves — distribute suction to each gyro instrument.

As filtered air enters the sealed instrument case, it accelerates through a nozzle and strikes buckets machined into the rim of the gyro rotor, spinning it at roughly 10,000 to 18,000 rpm. The resulting rigidity in space and precession are then translated into pitch, bank, and heading information through the instrument's gimbals and linkages.

Preflight and In-Flight Checks

Before flight and during the runup, the pilot should:

Verify the suction gauge indicates within the green arc (typically 4.5–5.5 in. Hg).
Confirm the attitude indicator has erected within about 5 minutes after engine start and shows no more than slight bank or pitch deviation.
Confirm the heading indicator has been set to the magnetic compass and is not precessing abnormally (more than 3° in 15 minutes is suspect).
Listen for unusual pump noise and check for any annunciator warning lights, if installed.

Recognizing a Vacuum Failure

Vacuum failures are insidious because the affected instruments do not necessarily display obvious failure flags in legacy installations. Indications include:

A suction gauge reading below the green arc, or zero.
An attitude indicator that slowly rolls or pitches off as the gyro spins down, eventually showing a false attitude.
A heading indicator that drifts or freezes.
A red flag or annunciator (in newer installations equipped with vacuum warning systems).

The danger is the slow drift of the AI. A pilot following a failing attitude indicator in IMC may inadvertently enter a graveyard spiral. For this reason, the FAA recommends a regular instrument cross-check that includes the turn coordinator, airspeed indicator, altimeter, vertical speed indicator, and magnetic compass — the pitot-static and magnetic instruments — as a check on the gyros.

Partial Panel Procedure

If vacuum failure is suspected, the pilot should:

Cover or disregard the unreliable attitude and heading indicators (a sticky note or instrument cover works; many checklists call this out specifically).
Use the turn coordinator (electric) for bank information, combined with the airspeed indicator, altimeter, and VSI for pitch information.
Use the magnetic compass and timed turns for heading control.
Notify ATC, declare an emergency if necessary, and request vectors and a descent to VMC or to an airport with an approach that requires the fewest gyro instruments (e.g., an ASR or PAR if available).
Reduce workload — fly straight and level while troubleshooting.

Redundancy and Modern Systems

Because of the failure mode, many IFR-certified aircraft incorporate redundancy:

Standby vacuum systems that use engine manifold pressure as a backup source.
Dual vacuum pumps on twin-engine aircraft.
Electric attitude indicators as a backup to the primary vacuum AI.
Glass cockpits (e.g., G1000) that replace vacuum-driven gyros with solid-state AHRS (Attitude and Heading Reference System) units, eliminating the vacuum pump entirely. Even these installations typically retain a separate, battery-backed electric standby attitude indicator.

Understanding the vacuum system — its components, normal indications, and failure signatures — is essential not only for legacy IFR flying but also for recognizing why modern AHRS-based panels were designed the way they are.

FAA PHAK Chapter 8 decoder

No rule requires a gyro for VFR — which is why a vacuum failure is a workload problem in VMC and an equipment problem under IFR

Source checked: FAA PHAK Chapter 8: Flight Instruments and 14 CFR §91.205; Last verified: 2026-08-05; date_retrieved: 2026-08-05.

As verified 2026-08-05, 14 CFR §91.205 requires no gyroscopic instrument of any kind for VFR flight, day or night — the only heading reference either list names is the magnetic direction indicator at §91.205(b)(3). All three gyroscopic instruments appear only in the IFR list: the rate-of-turn indicator at §91.205(d)(3), the pitch and bank indicator at (d)(8), and the direction indicator at (d)(9). The handbook explains how the pump spins those gyros; it never says which of them the rule demands. The first table below adds that layer, the second reads a vacuum failure against the three failures it is most often confused with, and the locator answers where in the chapter each fact lives.

Power source → required equipment → what a pump failure does

Instrument	Power source	Required by §91.205?	After a vacuum failure
Attitude indicator	Vacuum or pressure system in most light singles	IFR only — 14 CFR §91.205(d)(8), "gyroscopic pitch and bank indicator (artificial horizon)". Absent from the VFR day (b) and VFR night (c) lists. Also required for night vision goggle operations, §91.205(h)(5).	Leans off slowly as the rotor spins down, with no flag in legacy panels. The instrument that lies rather than quits.
Heading indicator	Vacuum or pressure system	IFR only — §91.205(d)(9), "gyroscopic direction indicator (directional gyro or equivalent)".	Drifts, then freezes. Heading control reverts to the compass and timed turns.
Turn coordinator / turn-and-slip indicator	Electrical system in most light singles (PHAK page 8-16)	IFR only — §91.205(d)(3), "gyroscopic rate-of-turn indicator", except on the two aircraft types with a third attitude instrument system described in (d)(3)(i)-(ii).	Unaffected. Splitting the power sources is exactly why this instrument survives — it is the remaining bank reference.
Slip-skid indicator (the ball)	None — gravity and inertia	IFR only — §91.205(d)(4), "slip-skid indicator".	Unaffected. It needs no power of any kind, vacuum or electrical.
Magnetic compass	None	Every operation — §91.205(b)(3), "magnetic direction indicator", carried into VFR night by (c)(1) and into IFR by (d)(1). The only heading reference the rule requires for VFR flight.	Unaffected.
Airspeed indicator, altimeter, vertical speed indicator	Pitot and static air pressure	ASI §91.205(b)(1) and altimeter (b)(2) for VFR day; (d)(5) upgrades the altimeter to a sensitive altimeter for IFR. §91.205 names no vertical speed indicator in any list.	Unaffected — a different system entirely, with its own failure modes.
Generator or alternator	Engine-driven	IFR only — §91.205(d)(7), "generator or alternator of adequate capacity".	Not a vacuum component, but it is what keeps the electric turn coordinator alive — which makes its failure the mirror image of this one.

Four failures that are routinely confused, side by side

Failure	Instruments lost	Instruments retained	IFR equipment rule
Vacuum or pressure pump fails	Attitude indicator, heading indicator	Turn coordinator, ball, magnetic compass, all pitot-static instruments	§91.205(d)(8) and (d)(9) are no longer met — the classic partial panel.
Alternator or generator fails and the battery depletes	Turn coordinator and any other electrically driven gyro	Vacuum attitude and heading indicators for as long as the engine turns the pump, ball, compass, pitot-static	§91.205(d)(3) and (d)(7) are no longer met — the mirror-image failure.
Static ports block	Altimeter and VSI freeze; the ASI misreads with altitude	Every gyro, the ball, and the compass	§91.205(b)(2) and (d)(5) are no longer met; the gyros are untouched.
Both vacuum and electrical power lost	All three gyroscopic instruments	Ball, magnetic compass, airspeed indicator, altimeter, VSI	§91.205(d)(3), (d)(8) and (d)(9) are all unmet — compass, clock, airspeed and altimeter only.

Where it is in the chapter — with page numbers

Page numbers below are the printed folios in 10_phak_ch8.pdf, the chapter PDF the FAA links from its Chapter 8: Flight Instruments page, retrieved 2026-08-05.

Page 8-15 — Gyroscopic Flight Instruments: Where the gyro discussion starts — rigidity in space and precession, before any power source is named.
Page 8-16 — Sources of Power: The paragraph that actually answers "which instruments does the vacuum system drive": vacuum or pressure powers the heading and attitude indicators while the electrical system powers the turn coordinator. Also the "usually between 4.5 "Hg and 5.5 "Hg" figure.
Page 8-17 — Figure 8-20, Typical vacuum system: The schematic: filter, pump, relief valve, suction gauge, and the two instruments in the airflow path.
Page 8-18 — Attitude Indicator: The vacuum-driven artificial horizon and its limits.
Page 8-19 — Heading Indicator: The vacuum-driven directional gyro and its precession error.
Page 8-23 — Magnetic Compass: The one direction reference that needs no power at all — the fallback after a vacuum failure.

Aircraft-specific limit: which gyro sits on which power source is an installation decision. Some aircraft run every gyro on vacuum or pressure, some reverse the split, and a glass panel may have no vacuum pump at all. The aircraft POH or AFM, its equipment list, and its abnormal procedures are the authority for a specific airplane; a handbook and regulation summary is not.

Vacuum system and partial panel FAQ

Where in the PHAK can I find vacuum systems, and what instruments do they drive?

FAA PHAK Chapter 8, Flight Instruments. In the chapter PDF the FAA links from that page (10_phak_ch8.pdf, retrieved 2026-08-05), the Gyroscopic Flight Instruments section opens on page 8-15 and the "Sources of Power" heading — the paragraph that names the instruments — is on page 8-16. It states that vacuum or pressure systems provide the power for the heading and attitude indicators while the electrical system provides the power for the turn coordinator. Figure 8-20, "Typical vacuum system", is on page 8-17; the attitude indicator follows on 8-18 and the heading indicator on 8-19.

What chapter of the PHAK covers partial panel failures?

The same one: Chapter 8, Flight Instruments. The handbook does not use a "partial panel" heading, which is why the topic is hard to find by name — it is spread across the Sources of Power discussion on page 8-16, where the FAA explains that most aircraft carry at least two sources of power so that one bank reference survives, and the Magnetic Compass section on page 8-23. In practice partial panel means the instruments in the right-hand column of the failure matrix above: the electric turn coordinator, the ball, the magnetic compass, and the pitot-static group.

Which instruments does the vacuum system drive?

In a typical light single, the attitude indicator and the heading indicator. The turn coordinator is gyroscopic too, but it is normally electrically driven so that a pump failure cannot take every bank reference at once — PHAK page 8-16 says so directly. The slip-skid ball and the magnetic compass need no power source at all, and the airspeed indicator, altimeter and VSI run on pitot and static air pressure. Installations vary: some aircraft run every gyro on vacuum or pressure, others reverse the split, so the aircraft POH or AFM is the authority for a specific airplane.

Is it legal to keep flying after a vacuum failure?

Under the operating rule, it depends entirely on whether the flight is VFR or IFR. 14 CFR §91.205 requires no gyroscopic instrument at all for VFR day or VFR night — the only heading reference either list names is the magnetic direction indicator at §91.205(b)(3). All three gyroscopic instruments appear only in the IFR list, at (d)(3), (d)(8) and (d)(9), so an IFR flight that loses the pump no longer meets the required-equipment rule. That is separate from whether the aircraft may be dispatched with the equipment already inoperative, which §91.213 and the aircraft POH or MEL govern, and separate again from the pilot in command’s emergency authority under §91.3.

What is normal vacuum suction, and does the PHAK give a number?

PHAK page 8-16 says the amount of vacuum or pressure required for instrument operation varies, but is usually between 4.5 inches of mercury and 5.5 inches of mercury. Read that as a typical range, not a limit: the same paragraph points the pilot at the marked normal range on the aircraft’s own suction gauge, and some installations add a warning light that illuminates when suction drops below the acceptable level. The green arc on the gauge in front of you, not the handbook range, is the number that matters.

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Oral Exam Questions a DPE Might Ask

Q1Which instruments are typically driven by the vacuum system, and which are not?

The attitude indicator and heading indicator are usually vacuum-driven. The turn coordinator is electric, and the pitot-static instruments (airspeed, altimeter, VSI) and magnetic compass operate independently of the vacuum system — providing redundancy if vacuum fails.

Q2How would you recognize a vacuum system failure in flight?

The suction gauge would read below the green arc (normally 4.5–5.5 in. Hg), and the attitude and heading indicators would slowly drift or give inconsistent readings. A cross-check against the turn coordinator, altimeter, airspeed, and compass would reveal the disagreement.

Q3If you lost vacuum in IMC, how would you handle it?

I'd cover the unreliable AI and HI, fly partial panel using the turn coordinator, airspeed, altimeter, VSI, and magnetic compass, declare an emergency with ATC, and request vectors to VMC or to the nearest suitable airport for an approach that minimizes gyro dependence.

Related FAR References

FAR § 91.205

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