Aviation Electronics

Possible Causes

• •Failed transponder
• •Antenna fault
• •Incorrect mode/code
• •Low transmit power

Diagnostic Steps

1. 1.Verify transponder is in ALT or ON mode
2. 2.Check transponder antenna connection at belly of aircraft
3. 3.Confirm correct squawk code
4. 4.Have transponder output power tested — minimum 125W for Mode C

Possible Causes

• •Faulty encoding altimeter
• •Incorrect wiring to transponder
• •Gray code error
• •Pressure altimeter out of calibration

Diagnostic Steps

1. 1.Compare transponder altitude readout to calibrated altimeter
2. 2.Have encoding altimeter tested and calibrated
3. 3.Check connector and wiring between altimeter encoder and transponder
4. 4.Verify transponder reports correct altitude to ATC during flight check

Possible Causes

• •Antenna shielded by airframe
• •Antenna damaged or corroded
• •Cable loss excessive
• •Wrong antenna location

Diagnostic Steps

1. 1.Inspect antenna for cracks, corrosion, or physical damage
2. 2.Check coax from transponder to antenna for continuity and low loss
3. 3.Verify antenna has clear view of sky — should be on belly of aircraft
4. 4.Have antenna VSWR measured

Possible Causes

• •Avionics bus fuse or circuit breaker tripped
• •Bus voltage low
• •Faulty avionics master switch
• •Short circuit on bus

Diagnostic Steps

1. 1.Check avionics master switch and circuit breaker
2. 2.Measure bus voltage with avionics master on — should be 27–29V (28V aircraft)
3. 3.Remove units one at a time to find shorted unit
4. 4.Inspect bus bar and wiring for signs of heat damage

Possible Causes

• •Display backlight failure
• •Contrast setting wrong
• •Temperature extreme
• •Display hardware fault

Diagnostic Steps

1. 1.Check contrast and brightness settings in unit menu
2. 2.Allow unit to warm up in cold conditions
3. 3.Inspect display for physical damage
4. 4.Check for moisture behind display bezel

Possible Causes

• •Short circuit in wiring
• •Overloaded circuit
• •Failing unit drawing excess current
• •Undersized circuit breaker

Diagnostic Steps

1. 1.Measure current draw of circuit — should not exceed 80% of breaker rating
2. 2.Disconnect units one at a time to find faulty unit
3. 3.Inspect wiring for chafing against airframe
4. 4.Check connector pins for pushed-back or shorted contacts

Possible Causes

• •Microphone or PTT switch fault
• •Antenna or feedline fault
• •Low transmitter output power
• •Incorrect frequency selected

Diagnostic Steps

1. 1.Verify correct frequency is selected and radio is not in monitor or intercom-only mode
2. 2.Check microphone connection and PTT switch — use a known-good headset to isolate
3. 3.Verify antenna connection at radio and at antenna base
4. 4.Have transmitter output power measured — minimum 4W for 25W rated radio

Possible Causes

• •Squelch set too high
• •Volume set too low
• •Antenna fault
• •Audio panel routing issue

Diagnostic Steps

1. 1.Check volume and squelch settings
2. 2.Verify audio panel is routing COM audio to headset — switch COM1/COM2 selection
3. 3.Listen for background noise with squelch fully open — no noise at all suggests antenna or receiver fault
4. 4.Try second COM radio if equipped to isolate whether the fault is the radio or the audio system

Possible Causes

• •Antenna or feedline open circuit
• •RF final stage failure
• •PTT keying the audio but not the RF stage

Diagnostic Steps

1. 1.Check antenna coax continuity from radio to antenna
2. 2.Verify antenna is not shorted or open at the connector
3. 3.Have radio bench tested — sidetone with no RF output indicates a failed PA stage

Possible Causes

• •Loose antenna connection
• •Corroded connector
• •Chafed coax at a bend
• •Loose connector pin at radio

Diagnostic Steps

1. 1.Wiggle antenna cable while monitoring reception — change indicates intermittent connection
2. 2.Inspect all connectors for corrosion, particularly at the antenna base where moisture collects
3. 3.Check coax routing for tight bends or chafe points against airframe structure
4. 4.Inspect radio connector for pushed-back or loose pins

Possible Causes

• •Antenna fault
• •Weak VOR signal
• •Receiver fault
• •Station under maintenance

Diagnostic Steps

1. 1.Verify VOR station is active and not NOTAMed out of service
2. 2.Check aircraft position relative to station — signal weakens at low altitude and long range
3. 3.Perform VOR receiver check per FAR 91.171 — maximum allowable error is ±4° ground check, ±6° airborne
4. 4.Inspect VOR antenna (typically on top of fuselage) for damage or corrosion

Possible Causes

• •Receiver frequency not set to ILS frequency
• •Antenna fault
• •Aircraft not within localizer coverage area
• •Receiver calibration drift

Diagnostic Steps

1. 1.Verify ILS frequency is set correctly — localizer frequencies are 108.10–111.95 MHz odd tenths only
2. 2.Confirm aircraft is within the localizer coverage sector (±10° of runway centerline, within 18 nm)
3. 3.Check VOR/LOC antenna for damage
4. 4.Compare to a known-good ILS approach at another airport to isolate aircraft vs. ground system

Possible Causes

• •Glideslope receiver fault
• •GS antenna fault
• •Not within glideslope coverage
• •ILS frequency not tuned

Diagnostic Steps

1. 1.Verify ILS frequency is selected — glideslope is paired automatically with localizer frequency
2. 2.Check glideslope antenna (typically under fuselage, forward of main gear) for damage
3. 3.Verify aircraft is within glideslope coverage — GS is only captured within 10 nm and ±0.7° of glidepath
4. 4.Test glideslope on the ground using an IFR 4000 or similar ramp tester

Possible Causes

• •Signal too weak
• •Antenna fault
• •Receiver fault
• •VOR station off air

Diagnostic Steps

1. 1.Check that VOR or ILS station is within range and the frequency is correct
2. 2.Verify antenna connection — a disconnected antenna is the most common cause of a persistent nav flag
3. 3.Check receiver self-test function if available
4. 4.Try tuning a closer or stronger VOR station to verify receiver is functional

Possible Causes

• •Battery expired
• •Antenna disconnected or damaged
• •Unit internal fault
• •G-switch activated and discharged battery

Diagnostic Steps

1. 1.Check battery replacement date on label — ELT batteries must be replaced per manufacturer schedule and after any activation
2. 2.Verify antenna connection — ELT will not pass self-test without antenna
3. 3.Perform self-test only during the first 5 minutes of each hour and for maximum 3 audible sweeps per FAA guidance
4. 4.If battery is current and antenna is connected, return unit to authorized repair station

Possible Causes

• •G-switch sensitivity to hard landings or turbulence
• •G-switch stuck in activated position
• •Water in unit triggering activation
• •Battery connector vibration

Diagnostic Steps

1. 1.Check ELT control panel or remote switch — reset to armed position if activated
2. 2.Inspect G-switch for corrosion or debris
3. 3.Contact the appropriate rescue coordination center immediately if inadvertent transmission occurred — do not wait
4. 4.Document and report inadvertent activations to the FCC as required

Possible Causes

• •Antenna damaged in impact
• •Battery depleted
• •Unit damaged beyond function
• •Antenna cable severed

Diagnostic Steps

1. 1.Check antenna connection — impact may have disconnected or damaged the antenna
2. 2.Verify battery is not expired — a depleted battery may not sustain transmission
3. 3.If possible, manually activate using the remote switch or control panel
4. 4.Move the ELT away from the wreckage to improve antenna clearance if safe to do so

Possible Causes

• •Pitot tube blocked by ice, insects, or covers left on
• •Pitot heat failure
• •Pitot line leak or blockage
• •Airspeed indicator instrument fault

Diagnostic Steps

1. 1.Verify pitot cover was removed before flight
2. 2.Check pitot heat circuit breaker and verify pitot heat is functional on the ground
3. 3.Inspect pitot tube opening for insect nests, debris, or ice
4. 4.Check pitot line for kinks, blockages, or moisture traps between tube and instrument

Possible Causes

• •Incorrect altimeter setting (Kollsman window)
• •Static port blocked
• •Altimeter instrument fault
• •Alternate static source selected

Diagnostic Steps

1. 1.Verify correct local altimeter setting is dialed in
2. 2.Check static ports (typically on fuselage sides) for blockage — insects and ice are common causes
3. 3.Verify alternate static source is not selected — alternate static typically reads slightly high
4. 4.Compare to a second altimeter if equipped

Possible Causes

• •Static line blockage
• •VSI instrument fault
• •Moisture in static line

Diagnostic Steps

1. 1.Check static ports for blockage
2. 2.Drain moisture from static lines if equipped with drain valves
3. 3.Perform pitot-static system check per FAR 91.411 if instruments are used for IFR flight
4. 4.Replace VSI if static system checks out and other instruments read correctly

Possible Causes

• •One static port blocked
• •Independent pitot-static systems with a fault on one side
• •Alternate static source selected on one side

Diagnostic Steps

1. 1.Cross-check instruments against each other and against GPS-derived altitude and groundspeed
2. 2.Check both static ports independently
3. 3.Verify both systems are on separate independent pitot-static systems and cross-check connections
4. 4.Pitot-static system must be tested per FAR 91.411 every 24 calendar months for IFR operations