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.
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Written against the FAA primary sources cited at the foot of this page.
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.
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.
Which three flight instruments are operated by the pitot-static system?
The altimeter, airspeed indicator, and vertical speed indicator each receive static pressure, but which one also requires pitot (ram air) pressure?
An aircraft altimeter measures aircraft altitude by sensing which of the following?
What is the primary purpose of the alternate static source provided on many aircraft?
Which gyroscopic property allows an attitude indicator's gyro to remain in a fixed position in space while the aircraft maneuvers around it?
Which gyroscopic property is used in the operation of the turn-and-slip indicator (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?
When a magnetic compass is 'swung' and the compass correction card is filled out, what compass error is being compensated for?
An aircraft equipped only with an electric turn coordinator and otherwise air-driven (vacuum) gyro instruments loses the vacuum pump in flight. Which instrument would still be expected to operate normally?
On an electronic horizontal situation indicator (EHSI), which group of formats is typically available to the pilot?
On an electronic attitude director indicator (EADI), the moving runway symbol moves downward during the final stages of an approach. To recapture the correct profile, the aircraft must
During an instrument approach, the glideslope pointer deflects below the center mark. This indicates that the aircraft is positioned
On a typical electronic flight instrument system (EFIS), engine parameters are normally displayed on the
On an attitude director indicator (ADI), which feature is fixed to the instrument case and serves as the reference datum?
On an EADI, the flight director command bars display
Which electronic flight instrument display includes a rising runway symbol during an ILS approach?
On an autopilot coupled to the radio navigation receivers, the guidance inputs used for approach and en-route tracking are
On a color EFIS, the flight director command bars are conventionally displayed in which color?
An EFIS uses two control panels. What is their purpose?
On an electronic flight instrument system (EFIS), which information is presented on the attitude director indicator (ADI/EADI)?
On an EFIS attitude director indicator (ADI), in addition to pitch and roll attitude, the display normally presents
If the glideslope pointer on an instrument display is below the center mark, the aircraft is
On an EFIS installation, the weather radar return is normally displayed on the
The attitude director indicator (ADI) of an electronic flight instrument system primarily displays
On a navigation display in weather radar mode, the most intense precipitation of a severe storm is shown as
On an EICAS-equipped airplane during normal flight, the lower display normally presents
On an EFIS installation, radio altitude is normally displayed on the
On an EFIS attitude director indicator, in addition to pitch and roll, the display typically includes
An electronic flight instrument display unit is made up of which of the following?
A basic electronic flight instrument system (EFIS) consists of
On a color primary flight display/navigation display, ILS deviation and computed guidance are conventionally shown in
A modern electronic horizontal situation indicator (navigation display) will display which of the following?
What does the abbreviation EFIS stand for?
What does the abbreviation EICAS stand for?
A fiber-optic data bus link in an aircraft digital system is capable of:
With respect to data transmission, the ARINC 629 data bus operates as a:
The signal in an ARINC 629 data bus system is carried over:
A serial ARINC 429 data bus transmits data words using:
One of the recognized numeric data formats used in the ARINC 429 data word is:
A group of data bits that are all transmitted at the same instant, each on its own line, is referred to as:
The ARINC 429 data bus transfers data using which encoding method?
Which of the following aircraft data buses is bidirectional?
Data is transferred on the MIL-STD-1553B data bus using:
The general arrangement of an ARINC 629 data bus installation includes the:
A simplex data transmission system consists of:
Because the ARINC 429 data bus is unidirectional, establishing a two-way (bidirectional) communications link between two units requires:
A liquid crystal display (LCD) used in an electronic flight instrument must be driven by what type of voltage?
Under what condition does a light-emitting diode (LED) used as an instrument indicator produce light?
How does the operation of a liquid crystal display (LCD) instrument behave at low ambient temperatures?
What is a primary advantage of a liquid crystal display (LCD) over a light-emitting diode (LED) display in aircraft instruments?
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.
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.
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.
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.
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.
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.
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.
ACS code: AM.II.H
Correct answer: Deviation caused by aircraft magnetic fields
Rationale: Compass swinging adjusts the compensating magnets and records residual deviation, the error caused by the aircraft's own magnetic and electrical fields. Variation is a charted geographic value not corrected by swinging, and dip-related turning errors are inherent to the instrument and cannot be removed by swinging.
ACS code: AM.II.H
Correct answer: The turn coordinator
Rationale: Because the turn coordinator in this setup is electrically driven, it continues operating when the vacuum pump fails. The attitude indicator and heading indicator are air-driven gyros that depend on the vacuum system, so both become unreliable once suction is lost. Splitting power sources this way provides redundancy.
ACS code: AM.II.H
Correct answer: VOR, map, plan, and weather radar.
Rationale: The EHSI is the navigation display of an electronic flight instrument system. It presents lateral navigation in selectable formats such as VOR, map, and plan, and it can overlay weather radar returns on the map. Engine thrust and raw ADF/NDB data are not the EHSI's defining navigation formats, so those choices are incorrect.
ACS code: AM.II.H
Correct answer: fly down.
Rationale: On the EADI the moving runway symbol rises toward the aircraft symbol as the airplane descends correctly onto the glideslope. If the runway is moving downward, the airplane is high relative to the desired path, so the pilot must fly down to recapture the proper approach profile. Holding altitude or correcting laterally would not address a high vertical position.
ACS code: AM.II.H
Correct answer: above the glideslope.
Rationale: The glideslope pointer shows the position of the beam relative to the aircraft. When the pointer deflects below center, the glidepath is below the airplane, which means the airplane is above the glideslope and must descend. The localizer provides separate lateral guidance, so it is not the correct reference for a vertical deflection.
ACS code: AM.II.H
Correct answer: engine indicating and crew alerting display (EICAS/ECAM).
Rationale: Engine parameters are presented by the engine indicating and crew alerting system (EICAS/ECAM), usually on a dedicated engine and warning display. The EHSI is the navigation display and the EADI is the attitude display, so neither presents engine data.
ACS code: AM.II.H
Correct answer: The miniature aircraft symbol.
Rationale: On an ADI the miniature aircraft symbol is fixed to the case as the datum, while the artificial horizon and pitch scale move behind it to display the aircraft's attitude. Because the horizon line and pitch bars move, they cannot be the fixed reference.
ACS code: AM.II.H
Correct answer: the required path with respect to the actual path.
Rationale: Flight director command bars compute the steering commands the pilot must follow, showing the required flight path relative to the aircraft's actual path. The pilot maneuvers to center the bars, so the bars indicate where the aircraft should go rather than simply reporting current heading.
ACS code: AM.II.H
Correct answer: Electronic attitude director indicator (EADI).
Rationale: The rising runway symbol appears on the electronic attitude director indicator (EADI) during an ILS approach, climbing toward the aircraft symbol as the airplane descends toward the runway. The EHSI is the navigation/plan display and the EICAS/ECAM shows systems and engine data, so neither carries this attitude-related cue.
ACS code: AM.II.H
Correct answer: ILS and VOR.
Rationale: A radio-coupled autopilot steers the aircraft using radio navigation signals, taking the ILS (localizer and glideslope) for approach guidance and VOR for en-route tracking. ADF bearing data is not used to directly couple the autopilot for tracking and capture, so options that include ADF are incorrect.
ACS code: AM.II.H
Correct answer: Magenta.
Rationale: On a color EFIS the flight director command bars are conventionally shown in magenta, the color reserved for computed and commanded guidance. Green and red are used for other functions, so those colors do not match the standard EFIS color convention.
ACS code: AM.II.H
Correct answer: One selects the type of EFIS display, and the other selects the source of the information being displayed.
Rationale: An EFIS uses two panels: a display control panel to select the display format and modes, and a source-select (switching) panel to choose which symbol generator or sensor feeds the displays for redundancy. The other choices misdescribe the panels, since neither is merely a brightness control nor a single-panel arrangement with a passive backup.
ACS code: AM.II.H
Correct answer: Flight director command bars, slip indicator, radio altitude, and autoland status.
Rationale: The attitude director indicator is the attitude display, so it presents flight director command bars, the slip/skid indicator, decision-height/radio-altitude data, and autoland/approach status. Magnetic heading, selected heading, waypoints, and weather radar are navigation-display items shown on the horizontal situation indicator (HSI/EHSI). See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: decision height, autoland status, radio altitude, and altitude.
Rationale: Beyond pitch and roll, the ADI presents approach guidance such as decision height, radio altitude, and autoland/approach status. The compass rose is part of the navigation display (HSI), so any option pairing the ADI with a compass rose describes the wrong instrument. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: above the glideslope.
Rationale: The glideslope pointer shows the position of the beam relative to the aircraft. With the pointer deflected below center, the beam is below the aircraft, meaning the aircraft is above the glideslope and must descend to intercept it. Localizer deflection is a separate lateral indication. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: horizontal situation indicator (navigation display).
Rationale: Weather radar returns are overlaid on the navigation display (horizontal situation indicator/EHSI), where they appear relative to the aircraft track in a plan view. The attitude director indicator is the attitude display and does not present a plan-view weather picture. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: pitch and roll attitude.
Rationale: The attitude director indicator is the attitude instrument, displaying aircraft pitch and roll against an artificial horizon. Waypoints, heading, and weather radar are navigation-display items shown on the horizontal situation indicator, so options listing those are incorrect. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: red areas with black surrounds.
Rationale: Weather radar codes return intensity by color, with the heaviest precipitation of a severe cell shown in red; areas with no significant return appear black around it. Green denotes light returns, so a green or blue core would not represent the most severe cell. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: secondary engine parameters and system data.
Rationale: On an Engine Indicating and Crew Alerting System, the upper display carries primary engine indications and crew alert messages, while the lower display in normal flight shows secondary engine parameters and system data. The maintenance/synoptic format is selected separately and is not the routine in-flight content. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: attitude director indicator.
Rationale: Radio altitude is part of the approach/attitude information presented on the attitude director indicator, where it supports the decision-height and autoland displays during an approach. The horizontal situation indicator is the navigation display and EICAS shows engine and system data, so neither is where radio altitude is normally read. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: autoland status, decision height, flight director bars, and slip indicator.
Rationale: As the attitude display, the ADI adds approach and guidance data to pitch and roll: autoland status, decision height, flight director command bars, and the slip indicator. The compass rose and heading data belong to the navigation display, so any option that includes a compass rose describes horizontal-situation-indicator content rather than the ADI. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: Mode control panel, raster and stroke generator display, microprocessor, and data inputs.
Rationale: An electronic flight instrument display is built from a mode/control panel for selections, a CRT or LCD driven by combined raster and stroke generation, a processor to format the picture, and the sensor data inputs. Items such as a separate air data computer or weather radar processor are sources that feed the system, not parts of the display unit itself. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: attitude director indicator, horizontal situation indicator, and symbol generators.
Rationale: A basic EFIS comprises the two display tubes — the attitude director indicator (attitude) and the horizontal situation indicator (navigation) — driven by symbol generators that build the images from sensor data. The mode control panel and radio/distance magnetic indicator are associated equipment rather than the core display-plus-generator set. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: magenta.
Rationale: On a color glass-cockpit display, ILS deviation pointers and computed/selected guidance are shown in magenta, the convention used for commanded or selected navigation information. Green and red serve other purposes in the color coding, so they do not match the ILS guidance convention. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: Waypoints, ILS steering information, distance-to-go, and magnetic heading.
Rationale: A modern horizontal situation indicator presents flight-plan waypoints, ILS steering information, distance-to-go, and magnetic heading in a plan/map format. The glideslope readout and flight director data are attitude-display items, so the option grouping those with the navigation display is incorrect. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: Electronic Flight Instrument System.
Rationale: EFIS stands for Electronic Flight Instrument System, the CRT/LCD-based replacement for traditional electromechanical attitude and navigation instruments. The other expansions referring to fire indication or flight information are not the accepted meaning of the abbreviation. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: Engine Indicating and Crew Alerting System.
Rationale: EICAS is the Engine Indicating and Crew Alerting System, used to display engine parameters and present crew alert messages on a glass-cockpit airplane. The other expansions mis-state the words and are therefore incorrect even though they sound similar. See FAA-H-8083-31, Aircraft Instrument Systems.
ACS code: AM.II.H
Correct answer: bidirectional data transfer
Rationale: Per FAA-H-8083-31, a fiber-optic data bus can carry light signals in either direction along the same fiber, making it capable of bidirectional data transfer. It is not limited to a single channel or to one direction only.
ACS code: AM.II.H
Correct answer: half-duplex bus
Rationale: Per FAA-H-8083-31, ARINC 629 is a half-duplex bus: terminals share a common medium and take turns transmitting and receiving rather than doing both at the same instant. Full-duplex operation would require simultaneous two-way transmission, which 629 does not provide.
ACS code: AM.II.H
Correct answer: a twisted pair of wires or fiber optics
Rationale: Per FAA-H-8083-31, ARINC 629 carries its signal on a single shared twisted pair of wires (or alternatively on fiber optics), which allows many terminals to share one transmission line. It does not use a separate wire pair or coaxial cable for each data source.
ACS code: AM.II.H
Correct answer: time division multiplexing
Rationale: Per FAA-H-8083-31, a serial ARINC 429 link sends words one after another in sequence over a single pair, which is time division multiplexing. Frequency-division or wave-division techniques separate signals by frequency and are not used by the serial 429 bus.
ACS code: AM.II.H
Correct answer: BCD (binary coded decimal)
Rationale: Per FAA-H-8083-31, BCD (binary coded decimal) is one of the numeric data formats carried in the ARINC 429 word, alongside BNR. ISO is a character/alphanumeric standard and is not one of the core numeric data formats of the 429 word.
ACS code: AM.II.H
Correct answer: parallel data
Rationale: Per FAA-H-8083-31, when a group of bits is transmitted simultaneously, each on a separate line, that is parallel data. Serial transmission sends bits one at a time on a single line, and a sync signal is a timing pulse rather than a data group.
ACS code: AM.II.H
Correct answer: bipolar return-to-zero
Rationale: Per FAA-H-8083-31, ARINC 429 transfers data using bipolar return-to-zero encoding, in which each bit returns to a null (zero) level between high and low states. Manchester bi-phase is used by MIL-STD-1553, and plain non-return-to-zero lacks the null state that 429 uses.
ACS code: AM.II.H
Correct answer: ARINC 629
Rationale: Per FAA-H-8083-31, ARINC 629 is the bidirectional bus, allowing terminals both to transmit and receive on the shared line. ARINC 429 is unidirectional (single source to multiple sinks), so it is not bidirectional.
ACS code: AM.II.H
Correct answer: Manchester bi-phase encoding
Rationale: Per FAA-H-8083-31, MIL-STD-1553B transfers data using Manchester bi-phase encoding, in which each bit carries its own clock transition at mid-bit, giving self-clocking and good noise immunity. Plain or bipolar non-return-to-zero encodings lack this embedded clocking and are not used by 1553B.
ACS code: AM.II.H
Correct answer: data bus cable, current-mode coupler, and stub cable
Rationale: Per FAA-H-8083-31, a typical ARINC 629 installation consists of the main data bus cable, a current-mode coupler that inductively links each terminal to the bus, and a stub cable connecting the coupler to the LRU. The coupling is inductive (current mode), not voltage mode.
ACS code: AM.II.H
Correct answer: one transmitter and one or more receivers
Rationale: Per FAA-H-8083-31, a simplex system provides one-way transmission from a single transmitter to one or more receivers, the same single-source, multiple-sink arrangement used by ARINC 429. Multiple transmitters into one receiver, or a controller/terminal pair, describe other topologies rather than simplex.
ACS code: AM.II.H
Correct answer: two separate data buses
Rationale: Per FAA-H-8083-31, ARINC 429 carries data in only one direction, so two-way communication between two units requires two separate buses, one for each direction of data flow. A single bus can carry data only one way, and four buses are more than the two actually needed.
ACS code: AM.II.H
Correct answer: Alternating current (AC).
Rationale: Liquid crystal cells are driven with alternating current. A sustained DC voltage causes electrolytic deterioration of the liquid crystal material, so the cells are energized with an alternating voltage that keeps the average DC level at zero, preserving the display over its service life.
ACS code: AM.II.H
Correct answer: When it is forward biased.
Rationale: A light-emitting diode is a semiconductor PN junction that emits light only when it is forward biased, allowing current to flow so electrons and holes recombine and release photons. With no bias or with reverse bias, no recombination current flows and no light is produced.
ACS code: AM.II.H
Correct answer: It is slower to update as the displayed data changes.
Rationale: At low temperatures the liquid crystal becomes more viscous, so the molecules realign more slowly when the drive field changes. The display therefore responds sluggishly and is slow to update, which is why many LCD instruments include heaters to maintain an acceptable response time in cold conditions.
ACS code: AM.II.H
Correct answer: It consumes less electrical current.
Rationale: An LCD only modulates existing light rather than generating it, so each cell draws very little current, whereas an LED must be driven hard enough to emit light directly. The main advantage of the LCD over the LED is therefore its much lower current consumption.
That is a free sample of the 542 questions tagged to ACS AM.II.H. The rest, the adaptive engine that serves you more of whatever you keep missing, and the timed exam simulator come with a free account.
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