6.7 Communication, Navigation, Antennas, and Avionics Inspection

Communication and Navigation Equipment

A mechanic does not repair the inside of avionics boxes (that is for repair stations / certificated avionics technicians), but the Airframe ACS expects you to install, mount, inspect, and operationally check the system as installed.

Communication is dominated by VHF voice radios working in the aviation band 118.000-136.975 MHz with 25 kHz (and increasingly 8.33 kHz) channel spacing; HF radios provide long-range/oceanic comms, and SATCOM serves transports.

Navigation systems you must recognize:

The maintenance focus is equipment security, cooling, electrical connections, circuit protection, placards, antenna condition, and an operational (functional) check per the installation data, plus logging the work correctly.

Avionics boxes are typically line-replaceable units (LRUs) mounted in a rack that provides cooling air, alignment, and the connector. The mechanic's tasks are removing/installing the LRU, ensuring adequate cooling (a clogged cooling path overheats and shortens equipment life), checking grounding and shielding of the harness, confirming circuit protection (correct breaker/fuse rating), and performing the operational check the maintenance manual specifies.

Installing new avionics is major work that requires approved data (an STC, a 337, or manufacturer data) and proper weight-and-balance and electrical-load updates — you cannot simply bolt a radio in and sign it off.

Antennas, Coax, Bonding, and Static Wicks

Antennas must be the right type for the band (a VHF blade, a GPS patch, a DME/transponder L-band stub), securely mounted to structure, electrically bonded to a good ground plane, and sealed against moisture. The feed is coaxial cable (coax) with matched connectors (BNC, TNC, N); coax must be the correct impedance, routed without sharp bends, protected from chafing, and kept dry. The classic intermittent-avionics culprits are poor bonding, a chafed or pinched coax, a loose/corroded connector, or moisture intrusion — not a faulty radio.

Bonding and grounding tie metal parts together to a common potential, controlling static buildup, lightning current paths, and radio noise. Measure bonding resistance to spec; corrosion under a bond strap raises resistance and creates noise.

Static dischargers (static wicks) on trailing edges bleed off precipitation static (P-static) charge into the air so it does not arc and blanket the radios with noise; they are inspected for security, condition, and count. Antenna installation also requires attention to structural reinforcement, drag/airload, and not blanking other antennas. These small details — bonding, coax routing, sealing — are where most avionics-installation discrepancies actually live.

When terminating coax, the connector must be the matched impedance (usually 50 ohms for aviation comm/nav) and assembled so the center conductor and shield are clean, tight, and not shorted; a sloppy connector raises VSWR (voltage standing-wave ratio) and cuts radiated power. Antennas are mounted to reinforced structure with the proper sealant and gasket, oriented per the installation drawing, and located to avoid blanking (one antenna shadowing another) and to keep required separation between transmitting antennas.

After installation the system gets an operational check and, where required, a ground or flight check of accuracy.

Transponders, ADS-B, ELTs, and Avionics Safety

The transponder replies to ATC secondary-radar interrogation: Mode A (4-digit identity code), Mode C (pressure altitude), and Mode S (selective, data-capable). S. controlled airspace (Class A/B/C and specified airspace). 411**, each every 24 calendar months. 225 if the GPS position source is degraded.

The Emergency Locator Transmitter (ELT) transmits a distress signal (modern units 406 MHz with 121.5 MHz homing). Maintenance rules: inspect/test per 14 CFR 91.207, replace the battery when half its useful life has expired or after 1 cumulative hour of transmission, and test only during the first 5 minutes of any hour while monitoring 121.5 MHz to avoid false alerts.

Avionics safety is graded:

• Electrostatic discharge (ESD): ground yourself and use ESD-safe handling; static can destroy circuit cards invisibly.
• RF energy: do not transmit (or run radar) near personnel or fuel; high-power systems and radar emit hazardous radiation.
• Live electrical work: de-energize, follow circuit-protection limits, and never substitute a wrong-rated breaker/fuse.
• Stay inside your scope — internal repair of TSO'd avionics is a repair-station task.

Aircraft equipped with the optional ADS-B In function in the U.S. are able to receive free FIS-B weather and TIS-B traffic. The position source feeding ADS-B must meet accuracy/integrity standards, so a GPS or antenna fault can knock the aircraft out of compliance even when the transponder itself works. Static dischargers are counted and checked at each inspection because losing them returns the P-static noise they were installed to cure.

A mechanic returning radios or antennas to service confirms the bonding straps and ground plane are clean and tight, since poor grounding is a leading cause of weak transmissions and navigation error. After any work that could affect the static system or transponder, the appropriate 24-month tests under 14 CFR 91.411 and 91.413 must be current before the aircraft flies under IFR or in transponder-required airspace.