Type Rating Knowledge — Cessna 172S NAV III

Electrical system

The C172S NAV III 28-volt electrical system: the alternator, main and standby batteries, the bus structure that keeps essential equipment powered when things fail, the switches that control it, and the annunciations that tell you it is failing.

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A 28-volt system with three power sources

Three sources, one 28-volt system

The C172S NAV III carries a 28-volt direct current electrical system. In normal operation a belt-driven 60-ampere alternator powers everything and holds the system at about 28 volts. A 24-volt main battery, mounted inside the engine cowling on the left firewall, starts the engine and backs the alternator up; both are controlled through the MASTER switch. A third source — a small standby battery between the firewall and the instrument panel — exists for one job only: keeping the essential bus alive if the alternator and the main battery have both failed. The system’s relays, the alternator control unit and the external power connector all live in a power distribution module on the left forward side of the firewall.

EXT PWR — External power receptacle — ground power for engine starts and servicing, connected on the main battery side.
MAIN BATT 24V — 24 V main battery, mounted on the left forward firewall.
ALT 60A — 60 A belt-driven alternator — the primary power source in normal operation; holds the system at 28 V.
STBY BATT — 24 V standby battery — powers the essential bus for at least 30 minutes when main bus voltage drops below 20 V.
PWR DISTRIBUTION — Power distribution module — alternator and battery power meet here and feed the two primary buses.
ELECTRICAL BUS 1 — Electrical bus 1 — one of the two primary distribution buses.
ELECTRICAL BUS 2 — Electrical bus 2 — the other primary distribution bus.
AVN SW 1 — Avionics master switch 1 — gates power from electrical bus 1 onto avionics bus 1.
CROSSFEED BUS — Crossfeed bus — ties the two primary buses together.
AVN SW 2 — Avionics master switch 2 — gates power from electrical bus 2 onto avionics bus 2.
AVIONICS BUS 1 — Avionics bus 1 — powers its share of the avionics behind avionics master switch 1.
ESSENTIAL BUS — Essential bus — keeps flight-critical equipment powered; fed from both primary buses and, in an emergency, the standby battery.
AVIONICS BUS 2 — Avionics bus 2 — powers its share of the avionics behind avionics master switch 2.

Redrawn from POH 172SPHBUS §7, Electrical System — structure restated, not traced. C172S NAV III with G1000 and GFC 700 (serials 172S10468 and on).

In normal flight, what is actually powering the electrical system?

The 60 A belt-driven alternator. Right. The alternator carries the whole electrical load in normal operation and holds the system at about 28 volts — the batteries just sit on charge.

Power distribution: the buses

Two primary buses, and the ones between them

Power reaches most equipment through two primary buses, ELECTRICAL BUS 1 and ELECTRICAL BUS 2. Connected between the primaries sit an essential bus — the flight-critical equipment — and a crossfeed bus tying the two sides together. Each primary bus also feeds an avionics bus through a circuit breaker and its AVIONICS BUS switch, so the avionics can be isolated during engine start or when external power is connected. The essential bus is deliberately a short list: on standby power alone the aeroplane cannot run everything — the transponder, for one, is not on it.

The essential bus is fed from…

Both primary buses and, in an emergency, the standby battery. Right — it sits between ELECTRICAL BUS 1 and 2 so either side can feed it, with the standby battery as the last-ditch source.

Master, standby battery and avionics switches

The MASTER switch: two poles, one rule

The MASTER switch is a two-pole rocker. The BAT side controls main battery power to the aeroplane; the ALT side controls the alternator. In normal operation both are ON together. The BAT side can be selected on its own, but the ALT side cannot be ON without BAT — the alternator is never the sole source. If the alternator fails, the MASTER switch can be set OFF to preserve main battery capacity for later in the flight — the flaps and the landing light at the far end of it — while, with the STBY BATT switch in ARM, the standby battery carries the essential bus.

Which statement about the MASTER switch is true?

The ALT side cannot be ON unless the BAT side is also ON. Right — the battery side can stand alone, the alternator side cannot. The alternator always works alongside the main battery, never instead of it.

STBY BATT and AVIONICS

The STBY BATT switch has three positions: ARM, OFF and a momentary TEST. Before engine start you TEST it and watch for the test lamp; during the start, ARM lets the standby battery help regulate and filter essential bus voltage; and in normal flight it stays in ARM so the standby battery charges and stands ready to take the essential bus automatically if the main sources fail. The AVIONICS switch is a two-pole rocker feeding AVIONICS BUS 1 and BUS 2. Both sides go OFF before turning the MASTER switch on or off, starting the engine, or connecting external power — transient voltages during those events can damage the avionics.

What does setting the STBY BATT switch to OFF actually do?

Disconnects the standby battery — it neither charges nor steps in automatically after a failure. Right, and that is why OFF is not a normal flight setting: an electrical failure with the switch OFF leaves the essential bus with no automatic backup.

Monitoring and annunciations

Volts: 28 normal, 24.5 low, 32 high

Bus voltage for the main and essential buses shows at the bottom of the EIS bar on the left of the PFD or MFD, labelled M and E under VOLTS. With the alternator running, both read about 28.0 volts. Below 24.5 volts the value turns red and the LOW VOLTS annunciation appears — the alternator is not supplying what the aeroplane needs. Above 32.0 volts the value turns red and HIGH VOLTS appears — the alternator is overvolting, and the ALT side of the MASTER switch goes OFF immediately.

The LOW VOLTS annunciation appears when main bus voltage falls below…

24.5 volts. Right — below 24.5 volts the alternator is not carrying the load. Expect it transiently during low-RPM taxi with a high electrical load; persistent LOW VOLTS in flight means shed load and land as soon as practicable.

Amps: which way the current flows

Battery current shows on the same EIS bar, labelled M and S under BATT — main battery under the M, standby under the S. A positive value, shown in white, means the battery is charging; a negative value, shown in amber, means it is discharging. If the standby battery ever has to discharge, steady state should be less than 4.0 amps. The STBY BATT annunciator comes on when the standby battery discharges at more than 0.5 amps for over 10 seconds — the alternator and main battery are not supplying what the essential bus needs, and if it cannot be resolved, the flight should be terminated as soon as practicable.

In cruise, the M BATT ammeter shows a negative (amber) value. What is happening?

The main battery is discharging — the alternator is not carrying the load. Right. Negative means current is flowing OUT of the main battery. Either the alternator has failed or the load exceeds its output — and the battery is now a countdown timer.

Circuit protection and external power

Breakers you can pull, and breakers you cannot

The circuit breaker panel sits below the pilot’s control wheel. Every breaker on the ESSENTIAL BUS, AVIONICS BUS 1 and AVIONICS BUS 2 can be pulled out for emergency electrical load management — though using a breaker as a switch shortens its life. The breakers on ELECTRICAL BUS 1, ELECTRICAL BUS 2 and the CROSSFEED BUS cannot be opened at all. Inside the power distribution module, three push-to-reset breakers protect the bus feeders, and the standby battery has its own fast-blow fuse. The alternator has an electronic guard too: on an overvoltage or other alternator fault, the ACU automatically opens the ALT FIELD breaker and stops alternator output. One reset is allowed; if it opens again, leave it out and have it fixed on the ground.

Which circuit breakers can be pulled in flight for load-shedding?

Those on the essential bus and the two avionics buses. Right — those are the pullable ones, for emergency electrical load management. The primary bus and crossfeed breakers are sealed against it.

External power

An external power receptacle, integral to the power distribution module, sits on the left side of the cowl near the firewall — for cold-weather starts and long maintenance sessions. Both AVIONICS switches go OFF before external power is connected, and avionics work needs a regulated, filtered 28 VDC source. After any engine start on external power, the aeroplane must prove its own battery works: with the external source disconnected, check that the M BATT ammeter shows a positive charge and the LOW VOLTS annunciator stays out. If either check fails, the main battery gets serviced or replaced before flight.

You started the engine on external power. After disconnecting it, what must you verify?

M BATT ammeter positive and the LOW VOLTS annunciator out. Right — a battery too flat to start the engine may also be too flat to be trusted. Positive charge with no LOW VOLTS proves the aeroplane’s own system is carrying itself.

Electrical system key numbers (C172S NAV III, GFC 700)
Item	Value
System voltage	28 V DC
Alternator	60 A, belt-driven
Main battery	24 V, left firewall
Standby battery	24 V, between firewall and instrument panel
Standby feed trigger	M BUS VOLTS below 20 V
Essential bus on standby battery	At least 30 minutes
LOW VOLTS annunciation	Below 24.5 V
HIGH VOLTS annunciation	Above 32.0 V
Standby steady discharge (max normal)	Less than 4.0 A
STBY BATT annunciator	Discharge above 0.5 A for 10 s