Type Rating Knowledge — Cessna 172S NAV III

Airframe and flight controls

What the Cessna 172S is made of and how it is flown: the semimonocoque fuselage and strut-braced wings that hold the fuel, the cable-operated primary controls and their certificated travels, the manual elevator trim, and the one flying control on the aeroplane that stops working when a circuit breaker opens.

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The structure

An all-metal, strut-braced, four-seat airframe

The 172S is an all-metal, four-place, high wing, single-engine aeroplane on tricycle landing gear, built for general utility and training. The fuselage is semimonocoque: formed sheet metal bulkheads, stringers and skin, where the skin itself carries load rather than merely covering a frame. The pieces of structure worth knowing by name are the front and rear carry-through spars the wings attach to, a bulkhead and forgings at the base of the rear door posts that carry the main landing gear, and fittings at the base of the forward door posts that take the lower end of the wing struts. Four engine mount stringers run forward from those same forward door posts to the firewall — which is why the door posts matter more than their name suggests.

LEFT WING — Externally braced left wing — front and rear spars, formed ribs and stringers under aluminium skin, containing the integral fuel tank.
RIGHT WING — Externally braced right wing, built the same way and carrying the other integral tank.
WING STRUTS — Wing struts — the external bracing. Their lower ends attach to fittings at the base of the FORWARD door posts, which is also where you push when moving the aeroplane by hand.
CARRY-THROUGH SPARS — Front and rear carry-through spars — the fuselage structure the wings attach to. The rear spars are partial span and attach to the fuselage only.
FORWARD DOOR POSTS — Forward door posts — they take the lower strut attachments AND the four engine mount stringers running to the firewall. Two major load paths meet here.
REAR DOOR POSTS — Rear door posts — a bulkhead and forgings at their base carry the main landing gear attachment.
FIREWALL / ENGINE — Firewall and engine mount. Four engine mount stringers run aft from the firewall to the forward door posts, so engine loads end up in the same structure as the struts.
MAIN GEAR — Tubular spring steel main landing gear struts — landing loads pass into the bulkhead and forgings at the base of the rear door posts.

Redrawn from POH 172SPHBUS §7, Airframe — structure restated, not traced. C172S NAV III.

What does "semimonocoque" mean about how the fuselage carries load?

The skin carries load along with the bulkheads and stringers. Right. The skin is structure, not cladding — which is why dents and skin damage are airworthiness items rather than cosmetic ones.

The wings are the fuel tanks

The wings are externally braced — that is what the struts are for — and each contains an INTEGRAL fuel tank: the tank is the wing structure, sealed, not a bladder or a separate can bolted inside it. Each wing is a front and rear spar with formed sheet metal ribs, doublers and stringers under an aluminium skin. The front spar carries the wing-to-fuselage and wing-to-strut fittings; the rear spars are partial span and attach only to the fuselage. Conventional hinged ailerons and single slot type flaps hang on the trailing edge. The empennage is conventional: vertical stabilizer, rudder, horizontal stabilizer and elevator, with the elevator trim tab actuator carried inside the horizontal stabilizer.

Where does the fuel live on a 172S?

In integral tanks that are part of the wing structure itself. Right — sealed integral tanks, one per wing. It is also why fuel reaches the engine by gravity as far as the reservoir: the tanks are above it.

Primary flight controls

Cables, and nothing else

The flight control system is conventional aileron, rudder and elevator surfaces, moved MANUALLY through cables and mechanical linkage: the control wheel drives the ailerons and the elevator, the rudder/brake pedals drive the rudder. There is no hydraulic boost, no fly-by-wire and no electrical involvement anywhere in the primary controls — which is worth stating plainly, because it means a total electrical failure costs you no flying control at all. The one exception is on the diagram below, and it is the flaps.

CONTROL WHEEL — Control wheel — ailerons in roll and elevator in pitch, both through cables and mechanical linkage with no boost of any kind.
RUDDER PEDALS — Rudder/brake pedals — rudder through cables, and the nosewheel steering and brakes through the same pedals.
TRIM WHEEL — Elevator trim wheel on the centre pedestal — forward trims nose down, aft trims nose up. Manual: no electric trim on this aeroplane.
FLAP LEVER — Wing flap control lever on the instrument panel, with mechanical stops at 10°, 20° and FULL, and a scale and pointer showing flap travel in degrees.
CABLES — Aileron control cables and mechanical linkage from the control wheel to each aileron.
CABLES — Elevator control cables and mechanical linkage from the control wheel to the elevator.
CABLES — Rudder control cables from the rudder bars to the rudder.
TRIM ACTUATOR — Elevator trim tab actuator, carried in the horizontal stabilizer and driven by the trim wheel.
FLAP MOTOR — Electrically driven flap motor — a large electrical load, on a circuit protected by the 10-ampere FLAP breaker on the left of the breaker panel.
AILERONS — Ailerons — up 20° ± 1°, down 15° ± 1° (TCDS 3A12). Balance weights in the forward spar.
ELEVATOR — Elevator — up 28° +1°/−0°, down 23° +1°/−0° (TCDS 3A12), with balance weights in the tip leading edge extensions.
RUDDER — Rudder — 16° 10′ ± 1° each side measured parallel to the waterline (TCDS 3A12), with a ground adjustable trim tab at the base of the trailing edge.
TRIM TAB — Elevator trim tab — up 22° +1°/−0°, down 19° +1°/−0° (TCDS 3A12).
FLAPS — Single slot type wing flaps — approved 0° to 10° for takeoff and 0° to 30° (FULL) for landing (TCDS 3A12).

Redrawn from POH 172SPHBUS §7, Flight Controls / Trim Systems / Wing Flap System, with surface travels from TCDS 3A12 Rev 80 — structure restated, not traced. C172S NAV III.

How much aileron travel does the type certificate allow?

Up 20°, down 15°. Right — and the asymmetry is deliberate. Less down-travel on the rising wing reduces the drag that causes adverse yaw.

Certificated control surface travels (TCDS 3A12 Rev 80, Model 172S)
Surface	Travel
Ailerons	Up 20° ± 1°, down 15° ± 1°
Elevator	Up 28° +1°/−0°, down 23° +1°/−0°
Elevator trim tab	Up 22° +1°/−0°, down 19° +1°/−0°
Rudder	16° 10′ ± 1° each way (parallel to the waterline)
Wing flaps	Takeoff 0°–10°; landing 0°–30° +0°/−2°

You are holding right rudder to stay coordinated in the cruise. How do you trim it out?

You cannot — the rudder trim tab is ground adjustable only. Right. The tab at the base of the rudder trailing edge is bent on the ground by a mechanic. In flight, rudder trim on a 172S is your right foot.

The control lock, and where its flag sits

The control lock is a shaped steel rod with a flag on it, there to stop wind buffeting damaging the controls while the aeroplane is parked. It goes through the hole in the top of the pilot’s control wheel shaft and into the hole in the shaft collar on the instrument panel, which secures the ailerons NEUTRAL and the elevator slightly trailing edge down. Properly installed, the flag lies over the ignition switch — so you cannot reach the switch without dealing with the flag, which is the entire design intent. In high or gusty winds a separate lock should also be fitted over the vertical stabilizer and rudder.

Why does the control lock flag sit over the ignition switch?

So you cannot operate the ignition switch without removing the lock first. Right — a physical interlock made of a flag. Any locking device must be removed before starting the engine, and this is how the aeroplane insists.

Trim and flaps

Elevator trim: manual, and which way is which

The airframe’s own trim system is manual: the vertically mounted trim wheel on the centre pedestal drives the elevator trim tab through an actuator in the horizontal stabilizer. FORWARD rotation of the wheel trims NOSE DOWN; aft rotation trims nose up. The tab moves up 22° and down 19° at the certificated limits. Note what this module is NOT claiming: on an aeroplane with the GFC 700 AFCS installed — which the anchor aeroplane has — there is ALSO manual electric trim and autotrim, driving the same tab through a pitch trim servo, with limits of its own (electric trim maximum operating speed 163 KIAS). Those belong to the avionics module. The wheel is what you have either way, and it is what you use when the electric side misbehaves: the POH’s autopilot or electric trim failure drill is grasp the wheel, hold the A/P TRIM DISC button, and adjust the elevator trim control MANUALLY.

You roll the manual trim wheel forward. What happens?

The aeroplane trims nose down. Right — forward is nose down, aft is nose up, in the same sense as pushing the wheel. Worth having in your hands before an approach.

Flaps: single slot, electric, and stopped at three notches

The flaps are single slot type, extended and retracted by the wing flap control lever on the instrument panel. The lever runs in a slotted panel with mechanical stops at 10°, 20° and FULL, and to pass the 10° and 20° stops you move the lever to the right to clear them — the notches are how you set a flap position by feel. A scale and pointer to the left of the lever shows flap travel in degrees. The circuit is electrical, driven by a flap motor that POH Section 3 describes as a large electrical load, and it is protected by a 10-ampere breaker labelled FLAP on the left side of the circuit breaker panel.

You are at 105 knots and want the first stage of flap. Legal?

Yes — VFE is 110 KIAS for flaps up to 10°. Right. The 172S has two flap limit speeds: 110 KIAS to 10°, then 85 KIAS from 10° to FULL. The white arc starts at 85 because that is the limit for the range beyond 10°.

Flap-related limits (POH 172SPHBUS §2)
Limit	Value
VFE, flaps up to 10°	110 KIAS
VFE, flaps 10° to FULL	85 KIAS
Approved takeoff flap range	UP to 10°
Approved landing flap range	UP to FULL (30°)
White arc on the airspeed indicator	40–85 KIAS
FLAP circuit breaker	10 amperes

How much flap may be used for takeoff?

Up to 10°. Right — the approved takeoff range is UP to 10°, and 10° is the setting the short-field technique uses.