Type Rating Knowledge — Cessna 172S NAV III

Lighting systems

Where every light is, which bus its breaker is on, and the two places the wiring surprises you — the map light that will not work until the NAV switch is on, and the one overhead switch that operates the rear dome light and the under-wing courtesy lights together.

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Lighting systems

Where the lights are

Exterior lighting is a short list, and it is worth knowing by position rather than by name. Navigation lights are on the two wing tips and on the TIP OF THE VERTICAL STABILIZER — three of them, not two. The landing and taxi lights are both in the LEFT wing leading edge. The flashing beacon is on top of the vertical stabilizer, and there is a strobe on each wing tip. Two courtesy lights are recessed into the lower surface of each wing to light the door areas. Everything except the courtesy lights is switched from the LIGHTS group on the lighted switch panel to the left of the PFD, where up is on.

ELECTRICAL BUS 1 — Primary bus 1. The POH groups the lighting breakers by bus, and BEACON and LAND sit on this one.
ELECTRICAL BUS 2 — Primary bus 2. TAXI, NAV and STROBE sit on this one — so a bus is not a set of lights that look alike, it is a set of breakers.
BEACON CB — Beacon circuit breaker, on the lighted breaker panel below the PFD. ELECTRICAL BUS 1.
LAND CB — Landing light circuit breaker. ELECTRICAL BUS 1.
PANEL LTS CB — The breaker the POH names for an INTERIOR lighting failure. Its bus is not stated in §7, so this diagram does not claim one.
TAXI CB — Taxi light circuit breaker. ELECTRICAL BUS 2.
NAV CB — Navigation light circuit breaker. ELECTRICAL BUS 2.
STROBE CB — Strobe light circuit breaker. ELECTRICAL BUS 2.
LIGHTS SWITCHES — The LIGHTS group on the lighted switch panel left of the PFD — BEACON, LAND, TAXI, NAV, STROBE. Up is on.
DOME/COURTESY SW — Push-button switch on the pilot’s overhead console. It is one switch for TWO sets of lights: the rear dome light and the under-wing courtesy lights.
DIMMING KNOBS — The DIMMING group on the switch panel: SW/CB PANELS, PEDESTAL, AVIONICS and STBY IND. Fully counter-clockwise is off.
BEACON — Flashing beacon on top of the vertical stabilizer.
LANDING LIGHT — Landing light in the LEFT wing leading edge, alongside the taxi light.
TAXI LIGHT — Taxi light, also in the left wing leading edge — but breakered on the other bus.
NAV LIGHTS — Navigation lights on the wing tips and on the tip of the vertical stabilizer.
STROBES — A strobe on each wing tip. Not to be used in cloud or overcast — reflected flashes can produce vertigo.
MAP LIGHT — Chart light and rheostat on the underside of the pilot’s control wheel. It only works once the NAV light switch is ON.
FLOOD LIGHTS — Two dimmable front crew flood lights in the overhead console, rotatable for directional lighting.
PANEL LED LIGHTING — Internally lit LED switch panel, breaker panel, engine controls, environmental panel and pedestal strips.
DOME + COURTESY — Rear dome light and the two courtesy lights recessed into the lower surface of each wing, lighting the door areas.

Redrawn from POH 172SPHBUS §7, Lighting Systems — Exterior Lighting / Interior Lighting. Structure restated, not traced. C172S NAV III.

Where are the landing and taxi lights installed?

Both in the leading edge of the LEFT wing. Right — they sit side by side. Which is exactly what makes their breaker allocation surprising, as the next question shows.

Which lights have their circuit breakers on ELECTRICAL BUS 1?

BEACON and LAND. Right, and the POH says so in exactly those words. TAXI, NAV and STROBE are the BUS 2 group.

The one switch that is not on the switch panel

There is a single exception to "the LIGHTS group switches the exterior lights", and it is a genuine trap on a night walk-around. The under-wing courtesy lights are controlled from the pilot’s OVERHEAD CONSOLE, and the rear dome light shares that same switch. It is a push-button: press once for on, press again for off. So the rear cabin light and the lights under the wings are one selection, not two — you cannot leave the courtesy lights on for boarding and the cabin dark, and turning the cabin light off on the ground removes the door lighting at the same moment.

A passenger asks you to switch off the rear dome light while the aircraft is parked at night. What else goes out?

The under-wing courtesy lights that illuminate both door areas. Right — they share the one overhead console switch. Worth knowing before you kill the cabin light with people still boarding.

Exterior lighting — position, switch and bus (POH 172SPHBUS §7)
Light	Position	Switched from	Breaker bus
BEACON	Top of the vertical stabilizer	LIGHTS group, switch panel	ELECTRICAL BUS 1
LAND	Left wing leading edge	LIGHTS group, switch panel	ELECTRICAL BUS 1
TAXI	Left wing leading edge	LIGHTS group, switch panel	ELECTRICAL BUS 2
NAV	Both wing tips and the fin tip	LIGHTS group, switch panel	ELECTRICAL BUS 2
STROBE	One on each wing tip	LIGHTS group, switch panel	ELECTRICAL BUS 2
Courtesy (2 per wing surface)	Recessed in the lower wing surfaces	Overhead console push button (shared with the rear dome light)	Not stated in §7

You are climbing through a broken layer at night with the strobes and beacon on. What does the POH tell you to do?

Do not use the strobes and flashing beacon in cloud or overcast — reflected flashes can produce vertigo and loss of orientation. Right, and it is a NOTE in §7 rather than a limitation. The mechanism is the light reflecting from water droplets and particles, which is worst at night.

Interior lighting, and the dimmer that means "automatic"

Interior lighting is four dimmers in the DIMMING group on the switch panel, plus a rheostat on the control wheel. SW/CB PANELS dims the internally lit switch and breaker panels; PEDESTAL dims the LED strips in the throttle/flap panel and above the 12 volt outlet; STBY IND dims the standby airspeed indicator, attitude indicator, altimeter and the magnetic compass. AVIONICS is the interesting one. Rotated fully counter-clockwise — the OFF position — it does not turn the displays off. It hands lighting control to the displays’ own internal photocells, and that is the position the POH recommends for day and for lower light levels. Rotating it clockwise out of off takes manual control of every avionics lighting level, which is what you want at night as your eyes dark-adapt.

What happens when the AVIONICS dimmer is rotated fully counter-clockwise to off?

The avionics displays use their internal photocells to set their own lighting levels — the recommended setting for daylight. Right. "Off" here means "off manual", not "off". The displays are still lit; they are just deciding for themselves.

Which instruments does the STBY IND dimmer light?

The standby airspeed indicator, attitude indicator and altimeter, and the non-stabilized magnetic compass. Right — the four things that are still there when the glass is not, which is a good way to remember what that dimmer is for.

When a light does not come on

The POH gives an order to work in, and it starts with the boring answer: whatever the system, the most probable cause of a light failure is a burned out bulb. If it is not that, check the breaker — PANEL LTS for an interior lighting failure, and the light’s own breaker for an exterior one, so LAND LT for the landing light. If a breaker has opened and there is no obvious sign of a short circuit — no smoke, no odour — turn the affected lights OFF, reset the breaker, and turn the lights back on. If it opens a second time, do not reset it again until maintenance has been performed. That last sentence is the whole point of the procedure: one reset is a test, two is a decision you are not equipped to make in flight.

The panel lighting has failed. Which breaker does the POH send you to?

PANEL LTS. Right — one breaker covers the interior lighting case, while exterior failures send you to that light’s own breaker.

A lighting breaker opens, you reset it, and it opens again. What now?

Leave it. Do not reset again until maintenance has been performed. Right. The single permitted reset is conditional on there being no smoke or odour, and a second opening has answered the question the reset was asking.

Someone tells you the landing light on their 1999 steam-gauge 172S is on ELECTRICAL BUS 1. What is the problem with that?

That aeroplane has a single primary bus — the two-bus grouping is a NAV III architecture, so the statement is about a bus that is not there. Right, and this is #327’s rule in miniature: the model name stayed the same while the number underneath it changed. Their POH is 172SPHUS.

Lighting systems