What happens if the static port is blocked?

With the static port blocked, the altimeter freezes at the altitude where the blockage occurred, the vertical speed indicator drops to zero and stays there, and the airspeed indicator becomes inaccurate, reading low in a climb and high in a descent. Opening the alternate static source restores a static reference from inside the cabin, usually with a small indication error that the flight manual lists.

What is the difference between variation and deviation on a magnetic compass?

Variation is the angle between true north and magnetic north at a given location; it changes with geography and is handled by the pilot using charts. Deviation is the error caused by the aircraft's own ferrous metal and electrical currents. A mechanic corrects deviation by swinging the compass, adjusting the compensating magnets, and recording the residual error on a compass correction card.

How do you swing a magnetic compass?

Position the aircraft on a compass rose away from steel structures, run the engines and switch on the electrical equipment normally used in flight, then align on north to adjust the north-south compensating screw and on east to adjust the east-west screw. Check south and west and remove half of any remaining error, then record readings every thirty degrees on a new correction card.

Which flight instruments are gyroscopic?

The attitude indicator, the heading indicator, and the turn-and-slip indicator or turn coordinator are the gyroscopic flight instruments. The attitude and heading indicators rely on rigidity in space, while the turn instruments rely on precession. Depending on the installation, the gyros are spun by an engine-driven vacuum or pressure pump, by a venturi on older aircraft, or electrically.

Why does a magnetic compass show a turn when the aircraft accelerates?

Acceleration error comes from magnetic dip. The compass card is weighted so that it hangs slightly tilted, and on an easterly or westerly heading a change in speed makes it swing. In the Northern Hemisphere, accelerating produces an indication of a turn toward north and decelerating an indication toward south. The error does not appear on north or south headings.

Aircraft Instrument Systems — FAA A&P Test Questions (ACS AM.II.H)

Aircraft instrument systems is one of the largest Airframe subject areas, and the written test leans on it heavily. You are asked which instruments run off the pitot-static system, what an altimeter actually senses, which gyroscopic property drives an attitude indicator versus a turn needle, and why a magnetic compass misreads during turns and speed changes. Almost every question is recall about how one instrument works or how it fails, so the payoff comes from learning each instrument's power source, its sensing element, and its characteristic error.

What ACS AM.II.H covers

This area follows the instrument material in FAA-H-8083-31 and splits into three groups. The pitot-static group comes first: the altimeter, airspeed indicator, and vertical speed indicator all use static pressure, but only the airspeed indicator also needs pitot ram pressure. The altimeter is an aneroid barometer that senses ambient static pressure and displays it as altitude, and the vertical speed indicator is a rate instrument built around a calibrated leak. Expect questions on blockages, such as a plugged pitot inlet or an iced static port, and on the alternate static source, which admits cabin air when the primary ports are blocked and typically makes the altimeter and airspeed indicator read slightly high in an unpressurized aircraft. Static system leak checks and altimeter tests follow 14 CFR Part 43, Appendix E. The gyroscopic group is second. Rigidity in space keeps the attitude indicator's gyro fixed while the aircraft maneuvers around it, and precession is the property the turn needle and the heading indicator depend on. Know which instruments are vacuum-driven and which are electric, and what a low vacuum indication means. The magnetic compass group is third, and it is where most candidates lose points. Magnetic dip, the vertical component of the Earth's field, causes the northerly and southerly turning errors and the acceleration errors. Deviation comes from the aircraft's own magnetic fields and is what you compensate for when you swing a compass and fill out the correction card. The rest of the area covers instrument range markings, case and panel installation, pressure and temperature instruments, and basic electronic display systems.

Where this sits on the test

ACS AM.II.H is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-31

Three traps candidates fall into

Candidates assume anything on the pitot-static system needs pitot pressure. Only the airspeed indicator uses ram air; the altimeter and the vertical speed indicator work on static pressure alone. That one fact answers several test questions, including every question about which instruments a blocked pitot line affects.
Rigidity in space and precession get swapped. Rigidity is why an attitude indicator's gyro stays put while the airframe rolls and pitches around it. Precession, the tilting that results when a force is applied to a spinning rotor, is the property that makes the turn needle deflect.
Variation and deviation are treated as the same error. Variation is the angular difference between true and magnetic north at a location, and no adjustment to the instrument removes it. Deviation is caused by ferrous metal and electrical fields in the aircraft itself, and swinging the compass is what corrects it.

7 free sample questions from ACS AM.II.H

AM.II.HTap an answer
Which three flight instruments are operated by the pitot-static system?
AM.II.HTap an answer
The altimeter, airspeed indicator, and vertical speed indicator each receive static pressure, but which one also requires pitot (ram air) pressure?
AM.II.HTap an answer
An aircraft altimeter measures aircraft altitude by sensing which of the following?
AM.II.HTap an answer
What is the primary purpose of the alternate static source provided on many aircraft?
AM.II.HTap an answer
Which gyroscopic property allows an attitude indicator's gyro to remain in a fixed position in space while the aircraft maneuvers around it?
AM.II.HTap an answer
Which gyroscopic property is used in the operation of the turn-and-slip indicator (turn needle)?
AM.II.HTap an answer
The magnetic compass error that produces northerly and southerly turning errors is caused primarily by which characteristic of the Earth's magnetic field?

Which three flight instruments are operated by the pitot-static system?

ACS code: AM.II.H

Correct answer: Airspeed indicator, altimeter, and vertical speed indicator

Rationale: The pitot-static system supplies ram (pitot) and static pressure to the airspeed indicator, altimeter, and vertical speed indicator. The attitude indicator, heading indicator, and turn coordinator are gyroscopic, and the magnetic compass is self-contained, so none of those rely on pitot-static pressure.

The altimeter, airspeed indicator, and vertical speed indicator each receive static pressure, but which one also requires pitot (ram air) pressure?

ACS code: AM.II.H

Correct answer: The airspeed indicator

Rationale: The airspeed indicator measures the difference between ram (pitot) pressure and static pressure, so it is the only one of the three connected to the pitot line. The altimeter and vertical speed indicator use static pressure alone, which is why a blocked static port affects all three but a blocked pitot tube affects only the airspeed indicator.

An aircraft altimeter measures aircraft altitude by sensing which of the following?

ACS code: AM.II.H

Correct answer: Ambient atmospheric (static) pressure

Rationale: An altimeter is an aneroid barometer that senses ambient static (atmospheric) pressure, which decreases with altitude. The difference between ram and static pressure is what the airspeed indicator senses, and the rate of change of static pressure is what the vertical speed indicator senses.

What is the primary purpose of the alternate static source provided on many aircraft?

ACS code: AM.II.H

Correct answer: To supply static pressure to the instruments if the primary static port becomes blocked

Rationale: An alternate static source gives the static instruments a backup source of static pressure if the primary external port becomes obstructed by ice or debris. It does not supply ram air, and moisture removal is handled by the system's drains, not the alternate static source.

Which gyroscopic property allows an attitude indicator's gyro to remain in a fixed position in space while the aircraft maneuvers around it?

ACS code: AM.II.H

Correct answer: Rigidity in space

Rationale: Rigidity in space is the gyroscopic property by which a spinning rotor maintains its plane of rotation relative to space, giving the attitude indicator a stable reference. Precession is the reaction of a gyro to an applied force, used by the turn instruments, and magnetic dip is a compass error, not a gyro property.

Which gyroscopic property is used in the operation of the turn-and-slip indicator (turn needle)?

ACS code: AM.II.H

Correct answer: Precession

Rationale: The turn needle relies on precession: as the aircraft yaws, a force is applied to the spinning gyro and the resulting precession deflects the needle in proportion to the rate of turn. Rigidity in space is the property used by the attitude and heading indicators, not the turn needle.

The magnetic compass error that produces northerly and southerly turning errors is caused primarily by which characteristic of the Earth's magnetic field?

ACS code: AM.II.H

Correct answer: Magnetic dip near the magnetic poles

Rationale: Turning and acceleration errors of the magnetic compass result from magnetic dip, the tendency of the compass card to tilt as the field lines angle into the Earth near the poles. Variation is the angular difference between true and magnetic north, and deviation is local error from aircraft metal and electrical fields.

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Frequently asked questions

What happens if the static port is blocked?: With the static port blocked, the altimeter freezes at the altitude where the blockage occurred, the vertical speed indicator drops to zero and stays there, and the airspeed indicator becomes inaccurate, reading low in a climb and high in a descent. Opening the alternate static source restores a static reference from inside the cabin, usually with a small indication error that the flight manual lists.
What is the difference between variation and deviation on a magnetic compass?: Variation is the angle between true north and magnetic north at a given location; it changes with geography and is handled by the pilot using charts. Deviation is the error caused by the aircraft's own ferrous metal and electrical currents. A mechanic corrects deviation by swinging the compass, adjusting the compensating magnets, and recording the residual error on a compass correction card.
How do you swing a magnetic compass?: Position the aircraft on a compass rose away from steel structures, run the engines and switch on the electrical equipment normally used in flight, then align on north to adjust the north-south compensating screw and on east to adjust the east-west screw. Check south and west and remove half of any remaining error, then record readings every thirty degrees on a new correction card.
Which flight instruments are gyroscopic?: The attitude indicator, the heading indicator, and the turn-and-slip indicator or turn coordinator are the gyroscopic flight instruments. The attitude and heading indicators rely on rigidity in space, while the turn instruments rely on precession. Depending on the installation, the gyros are spun by an engine-driven vacuum or pressure pump, by a venturi on older aircraft, or electrically.
Why does a magnetic compass show a turn when the aircraft accelerates?: Acceleration error comes from magnetic dip. The compass card is weighted so that it hangs slightly tilted, and on an easterly or westerly heading a change in speed makes it swing. In the Northern Hemisphere, accelerating produces an indication of a turn toward north and decelerating an indication toward south. The error does not appear on north or south headings.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

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