Communications, Light Signals, and Runway Lighting Systems — FAA A&P Test Questions (ACS AM.II.I)
This Airframe subject area bundles three things a mechanic has to know cold: communication and navigation radio installations, the light gun signals a tower uses when radios fail, and the colors of runway and taxiway lighting. The test questions are mostly straight recall. What does a flashing red light mean to an aircraft on the ground, what frequency does a modern emergency locator transmitter use, what color are taxiway edge lights, and why must an antenna base be bonded to structure. Learn the colors and frequencies as a table and this area goes quickly.
Written by the AMTprep editorial team · Published · Last reviewed
Written against the FAA primary sources cited at the foot of this page.
What ACS AM.II.I covers
AM.II.I draws on the communication, navigation, and airport lighting material in FAA-H-8083-31. Radio installation is one block: antenna location and orientation, coaxial cable and connector care, and bonding. Bonding an antenna base to structure matters because the skin acts as the ground plane and the return path for radio frequency current; poor bonding degrades the radiation pattern, raises standing wave ratio, and lets precipitation static and electrical noise into the receiver. You should also know the frequency bands. VHF communication runs from 118 to about 137 megahertz, and VHF navigation occupies roughly 108 to 118 megahertz, so a frequency such as 112.1 megahertz belongs to a VOR.
Emergency locator transmitters are a second block. A modern ELT alerts search and rescue with a coded digital signal on 406 megahertz and also transmits on 121.5 megahertz as a homing signal. Questions cover the purpose of the unit, activation by an impact-sensing switch, inspection intervals, and battery replacement.
Light gun signals are third. A steady green clears an aircraft for takeoff on the ground and clears it to land in flight, flashing green means cleared to taxi on the ground, steady red means stop on the ground, and flashing red on the ground means taxi clear of the runway in use. Flashing white applies only on the ground and means return to the starting point on the airport.
Airport lighting closes the area. Runway edge lights are white, with the final section of an instrument runway shown amber as a caution zone. Threshold lights show green to an aircraft approaching to land and red as runway end lights from the other side. Taxiway edge lights are blue, and taxiway centerline lights are green.
Where this sits on the test
ACS AM.II.I is tested on the FAA Airframe written test, one of 1,739 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
- Blue and green get reversed on taxiway lighting. Taxiway edge lights are blue, while green taxiway lights are centerline lights. Runway edge lights are white, and on an instrument runway the final section is amber as a caution zone, which candidates often misread as a taxiway color instead.
- A flashing red light gun signal is treated as one signal with one meaning. On the ground it means taxi clear of the runway in use; to an aircraft in flight it means the airport is unsafe and do not land. Several test questions turn entirely on whether the aircraft is airborne or on the ground.
- Candidates still call 121.5 megahertz the primary distress frequency of a modern ELT. Satellites no longer monitor 121.5 megahertz for alerting. A current unit sends its coded digital alert on 406 megahertz and uses 121.5 megahertz only as the homing signal that search crews track on scene.
50 free sample questions from ACS AM.II.I
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What does a flashing red light signal from the control tower mean to an aircraft operating on the ground?
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Answer: Taxi clear of the runway in use
Why: In the FAA light-gun signal system, a flashing red light directed at a ground aircraft means 'taxi clear of the runway in use.' A steady red means stop, a flashing white means return to starting point on the airport, and a flashing green means cleared to taxi. These signals are the standard backup for radio communication failure.· FAA-H-8083-31 — Communications and ATC light-gun signals
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When installing or troubleshooting a VHF communications antenna, why is the bonding of the antenna base to the aircraft structure important?
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Answer: It provides a proper ground plane for the antenna
Why: A VHF antenna requires a low-resistance bond to the metal skin so the airframe acts as the ground plane that the antenna radiates against. Poor bonding raises resistance, degrades the radiation pattern, and causes weak transmission and reception. Bonding does not increase transmitter power or change the operating frequency.· FAA-H-8083-31 — Antennas, bonding, and grounding
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What is the primary purpose of an emergency locator transmitter (ELT) installed in an aircraft?
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Answer: Transmit a signal to aid search and rescue after a crash
Why: An ELT automatically transmits a distress signal on designated emergency frequencies to help search-and-rescue forces locate an aircraft after an accident; it is activated by the impact forces of a crash or manually. It is not a two-way voice radio and does not provide cockpit navigation displays.· FAA-H-8083-31 — Emergency locator transmitter (ELT)
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Modern ELTs required for civil aircraft transmit their primary digital distress signal on which frequency?
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Answer: 406 MHz
Why: Current ELTs transmit a digital distress signal on 406 MHz, which is monitored by the satellite-based search-and-rescue system and can carry a coded aircraft identifier for accurate location. The older 121.5 MHz signal is now used only as a low-power homing aid, and 243.0 MHz is a military frequency. The 406 MHz signal greatly improves location accuracy.· FAA-H-8083-31 — Emergency locator transmitter (ELT)
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Along the edges of a runway, the standard FAA runway edge lighting normally displays what color to a pilot during the landing approach and rollout?
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Answer: White
Why: Standard runway edge lights are white, marking the lateral limits of the usable runway surface. Blue lights mark taxiway edges, and green lights mark runway thresholds (when viewed from the approach end). Knowing the color code helps the mechanic verify that the correct lamps and lenses are installed during airfield-lighting maintenance references.· FAA-H-8083-31 — Runway and airport lighting systems
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On a runway equipped with edge lighting, what color do the runway threshold lights display to a pilot approaching to land?
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Answer: Green
Why: Threshold lights show green to an aircraft on approach, marking the beginning of the runway available for landing; the same fixtures appear red to traffic on the runway looking back, marking the end of the usable surface. Runway edge lights are white and taxiway edge lights are blue. This color logic is part of standard airport lighting conventions.· FAA-H-8083-31 — Runway and airport lighting systems
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What color are the edge lights used to outline a taxiway, as distinguished from the lights used to outline a runway?
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Answer: Blue
Why: Taxiway edge lights are blue, which distinguishes the taxi routes from the white edge lights of the runway. Some taxiway centerlines use green lights, but the edge lights are blue. This consistent color coding lets pilots and ground crews tell runways from taxiways at night and lets mechanics confirm correct lens colors during lighting maintenance.· FAA-H-8083-31 — Airport taxiway and runway lighting
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A radio navigation frequency of 112.1 MHz falls within the band used by a
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Answer: VOR navigation frequency in the VHF band.
Why: VOR stations operate in the VHF band from 108.0 to 117.95 MHz, so 112.1 MHz is a VOR frequency. ADF/NDB equipment works in the low/medium-frequency band well below 1 MHz, and an ILS localizer, though also in the 108–112 MHz range, uses only odd-tenth/odd-hundredth paired channels. (FAA-H-8083-31, Communications and Navigation Systems — VOR.)· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Compared with copper wiring, what are advantages of using fiber optic cable in an aircraft data system?
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Answer: Smaller size and weight, non-conductive, higher security, and higher bandwidth.
Why: Per FAA-H-8083-31, fiber optic cable is smaller and lighter than copper, is non-conductive (so it is immune to electromagnetic interference and creates no ground loops), is more secure because it is difficult to tap, and carries far greater bandwidth. Fiber is not, however, easy to install; its terminations require precise cleaving, polishing, and alignment.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A fiber optic cable carries light because it consists of a silica glass core surrounded by cladding that has
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Answer: a lower refractive index than the core.
Why: Per FAA-H-8083-31, a fiber consists of a glass core surrounded by cladding of a lower refractive index. That index difference produces total internal reflection at the core-to-cladding boundary, trapping the light within the core. Cladding of a higher refractive index would allow the light to escape, so it could not guide the signal.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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With respect to electromagnetic interference (EMI), fiber optic data cables are
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Answer: immune to EMI.
Why: Per FAA-H-8083-31, because the signal is carried as light through a non-conductive (dielectric) glass fiber, electromagnetic interference cannot couple into it. The fiber is simply unaffected by EMI, which is a key advantage over copper wiring; it does not attenuate or otherwise reduce external EMI.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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When two fiber optic cable ends are aligned parallel and held close together but not actually touching, the connection is called
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Answer: an end-to-end coupling.
Why: Per FAA-H-8083-31, when two polished fiber ends are aligned parallel and held in close proximity without physically touching, the connection is an end-to-end coupling. A butt joint implies the faces are brought into actual contact, while a lateral gap describes a sideways offset rather than the small axial separation described here.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A recognized disadvantage of fiber optic cable in aircraft is that
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Answer: the end terminations are susceptible to environmental contamination.
Why: Per FAA-H-8083-31, the end terminations and connector faces of a fiber are susceptible to environmental contamination; dust, moisture, or oil on a polished face scatters or blocks light and raises insertion loss. Keeping the terminations clean is therefore a constant maintenance concern, whereas the glass fiber itself is durable and chemically inert.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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What type of light is normally used in an aircraft fiber optic system?
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Answer: Infrared.
Why: Per FAA-H-8083-31, aircraft fiber optic systems normally operate in the infrared band because silica fiber has its lowest attenuation and dispersion at those wavelengths. "Coherent" describes a property of laser output rather than a band of light, and diffused light would not couple efficiently into the narrow fiber core.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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In a fiber optic data system, the light signal is generated by
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Answer: a laser diode or an LED.
Why: Per FAA-H-8083-31, fiber optic data is launched by a semiconductor light source, either a laser diode or a light-emitting diode (LED), that can be switched on and off at high speed to modulate the optical signal. A halogen lamp or a strobe light cannot be modulated quickly enough nor focused efficiently into the small fiber core.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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High-intensity radiated fields (HIRF) are prevented from entering the electronics of a fiber optic system by
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Answer: using an opto-isolator coupling.
Why: Per FAA-H-8083-31, the glass fiber itself is a dielectric and carries no HIRF energy, so the vulnerability lies at the electrical interfaces. An opto-isolator coupling keeps the optical path electrically separated from the circuitry so high-intensity radiated fields cannot enter through the electronics. Grounding the fiber or shielding it with metal braid addresses copper conductors, not an inherently non-conductive fiber.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Continuity and signal loss of an installed fiber optic cable run are checked using
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Answer: a calibrated light source and an optical power meter.
Why: Per FAA-H-8083-31, continuity and attenuation of a fiber optic run are verified by injecting light from a calibrated source at one end and reading received power on an optical power meter at the other. A visual end-face inspection only checks cleanliness, not through-continuity, and an uncalibrated light source gives no reliable loss figure.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Light is guided along an aircraft fiber optic cable by
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Answer: total internal reflection at the core-to-cladding boundary.
Why: Per FAA-H-8083-31, light is confined to the fiber by total internal reflection at the core/cladding boundary, where rays striking the interface beyond the critical angle are reflected back into the core. Refraction would bend light out of the core, and dispersion is a signal-spreading loss mechanism, not the guiding principle.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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What is the principal disadvantage of a fiber optic data bus compared with copper wiring?
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Answer: It is comparatively expensive and difficult to install and terminate.
Why: Per FAA-H-8083-31, the chief drawback of fiber optic data buses is the cost and difficulty of installation, since precise terminations, special tooling, polishing, and trained personnel are required. Fiber actually offers excellent signal range, and once installed the system is reliable.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Which of the following is an optoelectronic device used in aircraft fiber optic systems?
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Answer: A laser diode that converts electrical energy into light.
Why: Per FAA-H-8083-31, a laser diode is an optoelectronic device because it converts electrical energy into coherent light. A Zener diode is a voltage-reference semiconductor and a triac is a power-switching device; neither emits or responds to light.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The light source used in an aircraft fiber optic transmitter is normally
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Answer: a laser diode or a light-emitting diode (LED).
Why: Per FAA-H-8083-31, the standard fiber optic transmitter uses a semiconductor source, either a laser diode or an LED, because each can be switched rapidly to encode data and emits at the infrared wavelengths the fiber carries best. A strobe or halogen lamp cannot be modulated at data rates or coupled efficiently into the core.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Fiber optic cable installed on an aircraft is commonly identified by a jacket color of
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Answer: orange, to distinguish it from copper wiring.
Why: Per FAA-H-8083-31, fiber optic cable is identified on aircraft by an orange jacket, warning technicians that the cable carries light and must not be sharply bent or stripped like ordinary wire. The color lets it be distinguished at a glance from copper electrical harnesses.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Common maintenance problems associated with aircraft fiber optic cable are
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Answer: kinking or excessive bending and contamination of connector end faces.
Why: Per FAA-H-8083-31, the typical fiber optic problems are kinking or exceeding the minimum bend radius, which fractures the glass or causes high bend loss, and contamination of the connector end faces, which scatters light. Glass fiber does not oxidize or corrode like metal conductors.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A fiber optic data bus on an aircraft
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Answer: can carry several data channels simultaneously on one fiber.
Why: Per FAA-H-8083-31, the very high optical bandwidth of fiber lets a data bus carry several channels or messages simultaneously, for example on different wavelengths of light. It is not limited to a single channel, and its low attenuation makes it suitable for long cable runs.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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When connecting a fiber optic cable to a line replaceable unit (LRU), the technician should
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Answer: mate it carefully to ensure precise core alignment and minimize light loss.
Why: Per FAA-H-8083-31, a fiber-to-LRU connection must be mated very carefully so the core faces align precisely, because any lateral or angular offset, gap, or contamination directly increases light loss. The delicate ferrule and fiber must not be over-torqued, so optical alignment, not high torque, governs a good connection.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Compared with copper cable, the advantages of fiber optic cable include that it is
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Answer: non-conductive, higher bandwidth, more secure, and smaller and lighter.
Why: Per FAA-H-8083-31, the genuine advantages of fiber over copper are that it is non-conductive (immune to EMI/HIRF), offers much higher bandwidth, is harder to tap so more secure, and is smaller and lighter. Fiber is not more rugged, nor is it easier to manufacture and assemble than copper.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A single-mode fiber optic cable is characterized by the fact that
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Answer: its core diameter is dependent on the wavelength of the light used.
Why: Per FAA-H-8083-31, single-mode fiber has a core only a few wavelengths across, so its core diameter is tied to the wavelength of the light used; that small size permits only one propagation mode. Allowing several modes to propagate simultaneously describes multimode fiber, not single-mode.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A semiconductor that emits photons and releases additional photons when stimulated by incoming photons is a
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Answer: laser diode.
Why: Per FAA-H-8083-31, a laser diode both emits photons when forward-biased and, through stimulated emission, releases further photons when struck by incoming photons, which is the defining lasing action. A power diode merely rectifies current and a Zener regulates voltage; neither interacts with light this way.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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In a fiber optic network, a star topology
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Answer: provides direct point-to-point links on dedicated fibers from a central hub.
Why: Per FAA-H-8083-31, a star topology routes a dedicated fiber from a central hub or coupler out to each unit, providing direct point-to-point links from that common center. A continuous closed loop describes a ring topology, and a single shared trunk passing node-to-node describes a bus topology.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A fiber optic coupling that uses a lens with an integral LED, compared with a plain end-fire (butt) coupling, is
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Answer: more efficient at coupling light into the fiber core.
Why: Per FAA-H-8083-31, a lens-type coupling with an integral LED uses the lens to collect and focus the source's diverging light into the fiber core, so it captures more of the emitted power than a bare end-fire coupling. Concentrating the light into the acceptance cone makes the lens arrangement more efficient.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Signal loss (attenuation) in an optical fiber is caused mainly by
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Answer: absorption, scattering, and reflection.
Why: Per FAA-H-8083-31, optical power is lost mainly through absorption by impurities in the glass, scattering off microscopic density variations, and reflection at joints, connectors, and end faces. These mechanisms attenuate the guided signal as it travels along the fiber.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The acceptance angle of an optical fiber is measured between
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Answer: the longitudinal axis of the core and the outermost acceptable ray.
Why: Per FAA-H-8083-31, the acceptance angle is measured between the longitudinal axis of the core and the outermost ray that can still enter and be guided by total internal reflection. The full cone of acceptance is twice this angle, spanning both sides of the axis.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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In an aircraft fiber-optic data cable, light is confined to the core because the surrounding cladding has a
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Answer: lower refractive index than the core.
Why: FAA-H-8083-31 explains that an optical fiber consists of a core surrounded by cladding of a lower refractive index. That lower index produces total internal reflection at the core-to-cladding boundary, keeping the light trapped inside the core as it travels down the fiber.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Light is carried along an aircraft fiber-optic cable primarily by
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Answer: repeated total internal reflection along the fiber.
Why: FAA-H-8083-31 describes that light propagates through an optical fiber by repeated total internal reflection. Each time a ray strikes the core/cladding boundary beyond the critical angle it reflects back into the core, zig-zagging along the length of the fiber rather than refracting out of it.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The light sources used in aircraft fiber-optic systems typically emit light in which region of the spectrum?
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Answer: the infrared region of the spectrum.
Why: FAA-H-8083-31 notes that fiber-optic transmitters use light-emitting diodes or laser diodes operating in the infrared region, where glass fiber attenuation is lowest. Infrared has a longer wavelength and lower frequency than visible light.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A major advantage of a fiber-optic data link over a copper-wire link on an aircraft is that the fiber-optic cable can
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Answer: carry multiple data signals at the same time.
Why: FAA-H-8083-31 points out that the high bandwidth of optical fiber allows it to carry large amounts of data, and multiple signals can be transmitted simultaneously, for example on different wavelengths. Fiber is not limited to a single signal at a time.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Fiber-optic systems on aircraft are capable of transmitting data in
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Answer: both directions at the same time.
Why: FAA-H-8083-31 explains that fiber-optic links support full-duplex operation, sending data in both directions at the same time by using separate fibers or different wavelengths for each direction. Restricting transmission to one direction at a time would be half-duplex.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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For high-bandwidth, high-speed fiber-optic data transmission over long runs, which type of optical fiber is preferred?
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Answer: Single-mode fiber.
Why: FAA-H-8083-31 distinguishes single-mode from multimode fiber. Single-mode fiber has a very small core that supports only one propagation path, eliminating modal dispersion and preserving pulse shape, which makes it the choice for high-bandwidth, high-speed, long-distance links. Multimode fibers spread the pulse and limit usable bandwidth.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Compared to the speed of light in free space, the speed of light traveling within an optical fiber is
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Answer: always less than the speed of light in free space.
Why: FAA-H-8083-31 notes that light slows when it enters the denser glass medium of the fiber. Because of the fiber's refractive index, its speed in the fiber is always less than the speed of light in free space; the higher the index, the slower it travels.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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In an installed aircraft fiber-optic system, the most common practical cause of signal attenuation (loss) is
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Answer: poorly terminated or contaminated connectors.
Why: FAA-H-8083-31 emphasizes that in an installed fiber link the dominant practical loss comes from poor termination of connectors, where end-face misalignment, gaps, and surface contamination scatter and reflect light. Cleanliness and proper termination are critical when working with optical fiber.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The Aircraft Communications Addressing and Reporting System (ACARS) is used to
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Answer: send and receive digital aircraft status and report messages.
Why: Per FAA-H-8083-31, ACARS is a digital data-link system, not a voice system. It sends and receives short digital messages such as aircraft status reports, OOOI times, and maintenance and performance data between the aircraft and ground stations. HF and direct voice calls are handled by the normal radio communication systems.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Which system is used on modern aircraft to automatically report in-flight faults to a ground engineering and monitoring station?
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Answer: An aircraft communications addressing and reporting (data-link) system.
Why: In-flight faults are relayed to the operator's ground engineering and monitoring station over the aircraft communications addressing and reporting (data-link) system, which automatically transmits maintenance and fault messages from the aircraft. A terrain-warning system and the on-board engine/crew alerting display do not provide air-to-ground fault reporting.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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ACARS is an acronym for which of the following?
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Answer: Aircraft Communications Addressing and Reporting System.
Why: ACARS stands for Aircraft Communications Addressing and Reporting System, a datalink that exchanges short messages between the aircraft and ground stations. The first word is Aircraft, which distinguishes the correct expansion from the distractors (FAA-H-8083-31, Communications systems).· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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A TCAS/Mode S equipped aircraft has its protected airspace breached by an aircraft equipped with a Mode A only transponder. What alert does the Mode S aircraft receive?
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Answer: A TA with a visual indication and a 'Traffic, Traffic' aural alert
Why: A Mode A only aircraft reports no altitude, so TCAS cannot compute a vertical resolution and can only issue a traffic advisory. The Mode S aircraft therefore receives a TA shown visually together with the 'Traffic, Traffic' aural alert, but no resolution advisory. A climb or descend (RA) command requires Mode C/S altitude reporting from the intruder, which is absent here.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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For maximum power transfer between an aircraft VHF transceiver and its antenna, the output impedance should be matched to a characteristic impedance of
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Answer: 50 ohms
Why: Per FAA-H-8083-31, aircraft VHF communication transmitters, their coaxial feed lines, and antennas are standardized to a characteristic impedance of 50 ohms. Matching the transmitter output to this 50-ohm value provides maximum power transfer and a low standing-wave ratio (SWR) on the line. A mismatched impedance reflects power back down the line, raising the SWR and reducing radiated output.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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Aircraft radio communications are limited to line-of-sight propagation at frequencies
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Answer: above about 100 MHz
Why: FAA-H-8083-31 explains that VHF and higher-frequency radio waves are not refracted back by the ionosphere and travel essentially in straight lines, so their range is limited to line-of-sight (the radio horizon). At roughly 100 MHz and above, propagation becomes line-of-sight, which is why VHF communication range is limited. Lower HF bands can be returned by the ionosphere as sky waves and reach beyond the horizon.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The horizontal radiation (polar) pattern of an aircraft ATC transponder antenna is
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Answer: omnidirectional
Why: FAA-H-8083-31 notes that the ATC transponder must reply to interrogations arriving from any bearing, so its antenna is designed with an essentially omnidirectional horizontal radiation pattern. A directional pattern would leave the aircraft unable to respond to ground-station or traffic-collision-avoidance interrogations approaching from certain directions.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The usable range of VHF communication between an aircraft and a ground station is limited primarily by what factor?
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Answer: Radio line of sight between the antennas
Why: VHF signals travel in essentially straight lines and are not reliably refracted by the ionosphere, so they cannot follow the curvature of the Earth. Communication range is therefore limited to radio line of sight between the antennas, which is why range increases with the height of both the aircraft and the ground station.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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The DME squitter (the stream of randomly spaced pulse pairs) originates from which source?
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Answer: The ground-based DME station
Why: Squitter is the stream of randomly spaced pulse pairs transmitted by the ground DME station to maintain a constant transmitter duty cycle and keep its receiver gain stable when few aircraft are interrogating. The airborne interrogator only sends interrogation pulses, so the squitter originates at the ground station, not the aircraft.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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By the standard radio-frequency band designations, the range 300 kHz to 3 MHz is known as which band?
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Answer: The Medium Frequency (MF) band
Why: By the standard radio-frequency band designations, 300 kHz to 3 MHz is the Medium Frequency (MF) band, which includes the NDB and AM broadcast ranges. Low Frequency (LF) covers 30 to 300 kHz and Very High Frequency (VHF) covers 30 to 300 MHz, so neither label fits this range.· FAA-H-8083-31 — Communications, Light Signals, and Runway Lighting Systems
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- Aircraft Instrument Systems (AM.II.H)
- Aircraft Electrical Systems (AM.II.K)
- Ground Operations and Servicing (AM.I.F)
Frequently asked questions
- What does a flashing red light from the control tower mean?
- It depends on where the aircraft is. To an aircraft on the ground, flashing red means taxi clear of the runway in use. To an aircraft in flight, flashing red means the airport is unsafe and the pilot should not land. Steady red means stop for an aircraft on the ground, and means continue circling and give way to other aircraft for one in flight.
- What color are taxiway lights compared with runway lights?
- Taxiway edge lights are blue and taxiway centerline lights are green. Runway edge lights are white, and on an instrument runway the last portion of the runway shows amber to a departing pilot as a caution zone. Threshold lights appear green to an aircraft approaching to land and red when seen from the runway side as runway end lights.
- What frequency does an ELT transmit on?
- A modern emergency locator transmitter broadcasts a coded digital distress message on 406 megahertz, which satellites receive and pass to search and rescue along with the beacon's registration identity. The same unit transmits a lower-power analog signal on 121.5 megahertz that search crews and aircraft use to home in on the beacon during the final stage of a search.
- Why does a VHF antenna have to be bonded to the airframe?
- The skin under a blade or whip antenna acts as the ground plane and the return path for radio frequency current, so the antenna base needs a clean, low-resistance, metal-to-metal connection to structure. Poor bonding weakens transmission, distorts the radiation pattern, raises standing wave ratio, and lets precipitation static and electrical noise into the receiver as interference.
- What is a frequency of 112.1 MHz used for?
- That frequency falls in the VHF navigation band of roughly 108 to 118 megahertz, which is assigned to VOR stations and, in its lower portion, to instrument landing system localizers. VHF communication radios operate above it, starting at 118 megahertz. Recognizing which band a given frequency belongs to is a common written-test question in this subject area.
All 40 ACS areas
General
- Fundamentals of Electricity and Electronics
- Aircraft Drawings
- Weight and Balance
- Fluid Lines and Fittings
- Aircraft Materials, Hardware, and Processes
- Ground Operations and Servicing
- Cleaning and Corrosion Control
- Mathematics
- Regulations, Maintenance Forms, Records, and Publications
- Physics for Aviation
- Inspection Concepts and Techniques
- Human Factors
Airframe
- Metallic Structures
- Non-Metallic Structures
- Flight Controls
- Airframe Inspection
- Landing Gear Systems
- Hydraulic and Pneumatic Systems
- Environmental Systems
- Aircraft Instrument Systems
- Communications, Light Signals, and Runway Lighting Systems
- Aircraft Fuel Systems
- Aircraft Electrical Systems
- Ice and Rain Control Systems
- Airframe Fire Protection Systems
- Rotorcraft Fundamentals
- Water and Waste Systems
Powerplant
- Reciprocating Engines
- Turbine Engines
- Engine Inspection
- Engine Instrument Systems
- Engine Fire Protection Systems
- Engine Electrical Systems
- Engine Lubrication Systems
- Ignition and Starting Systems
- Engine Fuel and Fuel Metering Systems
- Reciprocating Engine Induction and Cooling Systems
- Turbine Engine Air Systems
- Engine Exhaust and Reverser Systems
- Propellers
Primary sources
Drill ACS AM.II.I inside the full Airframe bank
Adaptive engine, timed exam simulator, ACS-code remediation, and oral prep — everything you need to pass.