Failure indications

Electrical bus — overview

A typical light aircraft has:

• Battery (Pb-acid or Pb-gel, 12 V or 24 V; 24 V common in larger types)
• Generator or alternator (engine-driven, charges battery and feeds the bus in flight)
• Voltage regulator / controller — keeps bus voltage constant (typ. 14.0–14.3 V on a 12 V system) and protects the electrical system against overload through a regulator/controller
• Master switch and alternator switch (often separate; many AFMs require alt off on start)
• Fuses or circuit breakers for each circuit

Power supply in general: an aircraft's electrical power supply is ensured by two systems — the battery and the generator (or alternator).

Basic electrical units

→ The unit for electrical power is Watt; the unit for voltage is Volt. P = U × I (watt = volt × ampere).

Marking of electrically driven instruments — "DC"

Electrically driven flight instruments (e.g. electric attitude indicator, electric turn indicator, GPS displays) are often marked "DC" (Direct Current) — usually as a label or symbol on the instrument. This marking shows:

• The instrument is fed from the 12 V or 24 V DC bus.
• On master off or alternator failure with empty battery the instrument quits.
• Vacuum-driven instruments (standard attitude indicator, directional gyro) are NOT marked "DC" and are independent of the electrical system.

Master switch inadvertently OFF in flight

If the master switch is inadvertently switched off in flight, all electrically powered devices may fail:

• Avionics (radio, transponder, GPS, EFB connection).
• Electric flaps, electric gear.
• Electric instruments (fuel gauges, electric turn indicator, electric heating).

BUT: engine operation with magneto ignition is not affected — the engine continues because the magnetos generate their ignition voltage independently of the electrical bus (see Ignition lesson). Pilots typically notice the master-off state through lost radio and instrument indications and can restore by switching back on.

Starter — no response

If the pilot notices that the starter does not respond (no rotation, no click) when activated, the most probable cause is that the master switch is not switched on:

• The starter motor is fed via the master + starter solenoid from battery power.
• No master = no power = no starter.

→ First check on starter problems: verify master switch ON.

Alternator/generator fails in flight

When the alternator/generator fails in flight, nothing happens at first, as long as the battery provides enough power:

• The battery supplies all electrical loads alone.
• Remaining time typically 20-60 minutes depending on load (all loads on: 30 min; reduced loads: up to 90 min).
• The pilot notices via negative ammeter and/or voltmeter (< 12 V falling).

Almost discharged battery — starter behaviour

If the battery is almost discharged, on activating the starter the pilot may encounter the situation that the engine cannot fully turn over:

• The starter rotates the engine only slowly or unevenly.
• Compression strokes not overcome → no start.
• → Solution: external boost (jumper battery or GPU), or recharge.

Fully charged battery — ammeter reading

As soon as the battery is fully charged, the ammeter normally shows zero in cruise. Reason:

• A fully charged battery does not accept charge current.
• The generator only supplies the actual consumer current.
• Ammeter (in the "charge current" variant) then reads ≈ 0 A.

(A differential ammeter showing net current to/from the battery reads 0 = "battery neither charging nor discharging".)

Effect of electrical failure — what stays, what quits

On electrical failure:

• Magneto ignition continues → engine runs.
• Mechanical instruments (altimeter, ASI, vacuum AI, vacuum DG, magnetic compass) continue.
• Radio and navigation equipment as well as electrically driven gyros are affected by an electrical-system failure.

Normal battery and generator failure

Even with the generator (alternator) failed, with normal battery charge, the pilot can still extend electrically powered flaps — the battery has enough reserve for flap operation (typically 5-10 A for a few seconds).

→ Pilot can land normally but should minimise loads.

Pitot-heat function check — ammeter

An increase of the ammeter reading by about 5 A after switching on the pitot heat indicates that the pitot heat is operating normally. Reason:

• Pitot heat draws typically 5-10 A.
• Current rise = heater operating.
• No rise = heater defective or fuse tripped.

Generator function check — voltmeter

The proper functioning of the generator can be checked with the voltmeter, if voltage rises by about 2 V after engine start (from battery 12.4 V to 14.0-14.4 V generator level on a 12 V system). If voltage does not rise → generator faulty or regulator faulty.

Failure indications table

Response to alternator failure

1. Try the AFM reset procedure: typically alt switch off, brief wait, on again.
2. Success: continue, monitor.
3. No success:
   • Switch off non-essential loads (landing lights, pitot heat unless in icing, strobes, cabin lights, some radios).
   • Squawk normal.
   • Mayday/Pan-Pan not required (not an immediate emergency, but land as soon as practical).
   • Battery reserve depending on load 20–60 min (very type-specific, AFM gives figures).

Static electricity and refuelling

When refuelling the pilot must observe:

• No open fires in the immediate refuelling area.
• No smoking (smoking ban).
• Apply ground wires — prevents static charging during fuel flow by electrically bonding aircraft to fuel truck.

Static dischargers

Static dischargers are small spikes or brushes on trailing edges of wings, ailerons, and tail surfaces that discharge static charges during flight:

• In flight the aircraft is electrostatically charged by friction with air particles, rain, or ice.
• Static charging can cause radio interference ("P-static") and in extreme cases sparks.
• Static dischargers conduct the charge into the surrounding air through corona discharge (small pointed shape with high field gradient).