📖 The lesson
What a B1 needs to know about avionics
Sub-module 11.5.2 is examined at level 1 for B1.1: you must know what each system is for, how it is laid out and how it works in principle. The detail belongs to the B2 licence and Modules 13 and 5, but B1 engineers handle these systems every day — powering them, replacing LRUs within their privileges, recognising faults and working safely around antennas.
Autoflight (ATA 22)
- Autopilot — one or more flight control computers (FCCs) compare the selected target with the actual aircraft state from the air data and inertial reference units and radio receivers, and drive servo actuators (or, on fly-by-wire aircraft, send commands to the flight control computers). The inner loop keeps the aircraft stable in attitude; the outer loop steers it along the selected path.
- Servos drive the control system through clutches or torque limiters so that the crew can always overpower them; the autopilot disconnects through the disconnect button, by override force, or automatically on a failure, with an aural and visual warning.
- Mode control panel (MCP, Boeing) or flight control unit (FCU, Airbus) — the crew select speed, heading, altitude and vertical speed, and engage modes such as HDG, LNAV/NAV, VNAV, ALT HOLD, V/S, LOC, APP. Active modes appear as flight-mode annunciations on the PFD.
- Flight director (FD) — the same computations shown as command bars on the PFD so the pilot can fly them by hand.
- Autothrottle / autothrust — controls thrust to hold a speed or a thrust setting (e.g. for take-off, climb, go-around).
- Yaw damper and Mach trim — automatic stability functions (Chapter 12).
- Autoland — automatic approach, flare, landing and roll-out using ILS (or GLS) with radio altimeter data. Low-visibility categories (Cat II and III) require redundant, monitored channels: fail-passive (a failure disconnects the autopilot without a significant deviation) or fail-operational (the landing can be completed after a failure, using three or more channels).
Communications (ATA 23)
| System | Band and frequencies | Principle and use |
|---|---|---|
| VHF communication | 118.000–136.975 MHz, 8.33 kHz or 25 kHz channel spacing | AM voice, line-of-sight range (roughly 200 NM at high altitude). Usually three radios; one may carry VHF data link (VDL) for ACARS/CPDLC |
| HF communication | 2–30 MHz | Long range by sky waves reflected from the ionosphere; single-sideband (SSB) voice and HF data link. An antenna coupler tunes the antenna for each frequency. SELCAL alerts the crew when the ground calls |
| SATCOM | L-band (≈ 1.5–1.6 GHz) via geostationary or low-orbit satellites | Voice and data worldwide, including polar and oceanic regions; steerable high-gain antennas |
| CPDLC | Over VDL, HF data link or SATCOM | Controller–pilot data link communications: clearances and requests as text messages instead of voice — less congestion and fewer read-back errors |
| Audio system | — | Audio control panels at each crew station select which radios to hear and transmit on; flight and service interphone, cabin interphone, passenger address, crew and ground calls are routed through an audio management unit |
Safety — transmitting on the ground
Do not key an HF transmitter while anyone is near the HF antenna or during refuelling: the RF voltages are high enough to burn and to cause sparks. The same applies to SATCOM high-gain antennas and, above all, the weather radar.
Emergency locator transmitter (ELT)
- Transmits on 406 MHz to the COSPAS-SARSAT satellites — a digital message with the beacon's identity and often a GNSS position — plus 121.5 MHz for homing by search aircraft.
- Types: automatic fixed (AF), automatic portable (AP), automatic deployable (AD) and survival (S). Automatic ELTs are triggered by a g-switch (crash sensor); the cockpit remote switch allows ARM, ON and TEST.
- Maintenance: battery replacement by expiry date or after use, registration and coding of the beacon to the aircraft, and testing only as the manufacturer allows — the self-test, and 121.5 MHz tests only within the first five minutes of the hour for no more than three sweeps. An accidental activation must be reported to the rescue authorities straight away.
Cockpit voice recorder (CVR)
- Records the flight-crew audio channels and an area microphone in the flight deck onto crash-protected solid-state memory; newer installations also record data-link messages.
- Duration: at least 2 hours on older aircraft; large aeroplanes first certificated from 2021 must record 25 hours (EU rules and ICAO Annex 6).
- Painted bright orange with reflective strips and fitted with an underwater locator beacon (ULB) transmitting at 37.5 kHz for at least 30 days (90 days on newer units).
- A bulk-erase function works only on the ground (typically with the parking brake set); the recording must be preserved after an accident or serious incident. The flight data recorder (ATA 31) records flight parameters in a similar crash-protected unit.
Radio navigation (ATA 34)
| System | Frequencies | Principle |
|---|---|---|
| VOR | 108.00–117.95 MHz | The ground station transmits a reference 30 Hz signal and a variable 30 Hz signal whose phase depends on direction; the phase difference gives the radial (magnetic bearing from the station). Shown with TO/FROM on the HSI, RMI or ND |
| DME | 962–1213 MHz (UHF), paired with VOR/ILS channels | The aircraft interrogator sends pulse pairs; the ground transponder replies after a fixed delay; the time gives slant range in nautical miles |
| ADF | 190–1750 kHz (LF/MF) | A loop and a sense antenna find the relative bearing to an NDB or broadcast station. Subject to night effect, coastal refraction, thunderstorm and mountain effects |
| ILS | Localiser 108.10–111.95 MHz (odd tenths); glideslope 329.15–335 MHz; markers 75 MHz | Lateral and vertical guidance from the difference in depth of modulation of 90 Hz and 150 Hz tones |
| MLS | 5031–5091 MHz (C-band) | Time-referenced scanning beams allow curved and segmented approaches; little used today, largely superseded by satellite-based approaches |
ILS approaches are classified as Category I (decision height not lower than 200 ft, RVR not less than 550 m), Category II (DH 100 ft, RVR 300 m) and Category III (DH below 100 ft or none, with lower RVRs), each requiring progressively more capable and better-monitored aircraft equipment and maintenance.
Area navigation, FMS and satellite navigation
- Area navigation (RNAV) — flying any desired path within the coverage of navigation aids rather than from beacon to beacon, using position from DME/DME, VOR/DME, GNSS or inertial systems. Performance-based navigation (PBN) specifies the accuracy required (e.g. RNAV 1, RNP 0.3); RNP adds on-board performance monitoring and alerting.
- Flight management system (FMS) — a flight management computer (FMC) with control and display units (CDU/MCDU). It blends sensor data into a best position, holds the flight plan, computes lateral (LNAV) and vertical (VNAV) guidance and performance (speeds, fuel, altitudes), and auto-tunes the navigation radios. Its navigation database is updated every 28 days (the AIRAC cycle) using a data loader (Chapter 19).
- GNSS — GPS (USA), GLONASS (Russia), Galileo (EU) and BeiDou (China). The receiver measures its range to at least four satellites (three for position plus one to solve its own clock error). RAIM checks integrity using extra satellites. Satellite-based augmentation systems (SBAS) — WAAS (USA), EGNOS (Europe), GAGAN (India) — broadcast corrections and integrity data for approaches with vertical guidance; GBAS supports GLS precision approaches.
Surveillance: transponder and TCAS
- ATC transponder (secondary surveillance radar) — replies on 1090 MHz to ground interrogations on 1030 MHz. Mode A replies with a four-digit identity code (4096 codes; 7500 unlawful interference, 7600 radio failure, 7700 emergency); Mode C adds pressure altitude from the encoding altimeter or ADC; Mode S uses a unique 24-bit aircraft address for selective interrogation, enhanced surveillance data and ADS-B (the aircraft broadcasts its GNSS position on 1090 MHz extended squitter).
- TCAS II (ACAS II) works independently of ground radar, interrogating nearby transponders and tracking their range, bearing and altitude. It gives traffic advisories (TA) and resolution advisories (RA) in the vertical plane only, coordinated with the other aircraft's TCAS through Mode S, and shown on the ND and PFD with aural alerts.
Weather radar, radio altimeter and inertial systems
- Weather radar — an X-band (about 9.3 GHz) or C-band pulsed radar in the nose radome with a stabilised flat-plate antenna. It shows precipitation as green, yellow, red (and magenta) levels; Doppler processing detects turbulence and predictive windshear. Never transmit on the ground near people, fuel or other aircraft, or in a hangar: the radiation is harmful and can ignite fuel vapour. Use test or standby, and follow the AMM safe-distance rules.
- Radio altimeter — a low-power FMCW radar at 4.2–4.4 GHz with separate transmit and receive antennas under the fuselage, measuring height above the terrain, displayed typically below 2 500 ft. It feeds GPWS/TAWS, autoland and flare, TCAS inhibit logic and configuration warnings.
- Inertial navigation / reference systems (INS/IRS) — accelerometers measure acceleration and gyros measure rotation; integrating once gives velocity, twice gives position, with no external signals. Older INSs used a gimballed, gyro-stabilised platform; modern strapdown IRSs fix ring-laser or fibre-optic gyros to the aircraft and compute attitude mathematically. An ADIRU combines air data and inertial reference. The system must align on the ground (about 10 minutes, longer at high latitudes) with the aircraft stationary; position errors then grow at roughly 1–2 NM per hour, so the FMS blends in GNSS and radio updates.
ARINC reporting and avionics test equipment
ACARS (Aircraft Communications Addressing and Reporting System) is the digital data link, via VHF, HF or SATCOM, that sends OOOI times (out, off, on, in), position and engine reports, fault and maintenance messages to the operator's maintenance control in real time, and brings weather, clearances and load data to the crew. "ARINC" is also the organisation whose standards define aircraft data buses (ARINC 429, 629 and 664/AFDX) and equipment form factors.
| Test equipment | Typical use |
|---|---|
| Digital multimeter, milliohmmeter / bonding meter | Voltage, current, continuity; bonding and grounding resistance |
| Insulation tester ("megger") | Wiring insulation — with LRUs disconnected, as high voltage damages electronics |
| Air data (pitot-static) test set | Leak checks and instrument checks |
| Ramp test sets | Functional checks of VOR, ILS, DME, transponder, TCAS and ELT by simulating ground stations or targets |
| RF power / VSWR meter, time-domain reflectometer | Transmitter output, antenna and coaxial cable condition, locating cable faults |
| Data bus analyser, data loader, oscilloscope | ARINC 429 traffic, software and database loading, signal waveforms |