Type Rating Knowledge — Cessna 172S NAV III

Fuel system

Fifty-six gallons in two integral wing tanks, fifty-three of them usable, fed downhill to a reservoir and then pumped to the injection system. The selector positions and what each one is approved for, why the fuel gauges stop moving above 24 gallons, the auxiliary pump that will fly the aeroplane at maximum continuous power on its own, and the thirty-second limit that keeps a tank outlet covered.

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Tanks and capacities

Two integral tanks, and one number that is not the useful one

Fuel is carried in two vented INTEGRAL tanks, one in each wing — the tank is the sealed wing structure itself. Each holds 28.0 gallons, so total capacity is 56.0 gallons. But the number that matters for planning is USABLE fuel: 53.0 gallons in all flight conditions, because 1.5 gallons in each tank — 3.0 in total — is unusable. Note the phrase "in all flight conditions": the usable figure is the amount the aeroplane guarantees regardless of attitude, which is why it is lower than a level-flight measurement would give. Everything above the reservoir tank in the diagram works by GRAVITY: the tanks are in the wings, above the engine, and no pump is needed to get fuel down to the reservoir.

L TANK 28 GAL — Left integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable. Carries the check valve equipped overboard vent.
VENT LINE — Interconnecting vent line between the tanks, with the overboard vent under the left wing. Complete blockage decreases fuel flow and eventually stops the engine.
R TANK 28 GAL — Right integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable.
SELECTOR BOTH/L/R — Three-position fuel selector — BOTH for takeoff, climb, landing and any prolonged slip or skid; LEFT or RIGHT is for level cruise only.
RESERVOIR TANK — Fuel reservoir tank — everything above it is fed by gravity alone; it also receives the fuel return line.
AUX FUEL PUMP — Electrically driven auxiliary fuel pump — priming, vapour suppression, and enough flow for maximum continuous power if the engine driven pump fails.
SHUTOFF VALVE — Fuel shutoff valve, downstream of the auxiliary pump.
FUEL STRAINER — Fuel strainer — one of the five sampling points drained before flight.
ENG DRIVEN PUMP — Engine driven fuel pump — the normal supply for the injection system, needing no electrical power at all.
FUEL/AIR CONTROL — Fuel/air control unit (fuel servo) under the engine — meters fuel in proportion to induction air flow. The mixture control acts here.
FLOW SENSOR — Turbine type fuel flow transducer between the servo and the flow divider — the source of FFLOW GPH on the G1000 EIS.
FLOW DIVIDER — Fuel distribution valve (flow divider) on top of the engine — spring tension on a diaphragm and valve distributes fuel evenly to the nozzles.
INJECTOR NOZZLES — Air-bleed type injector nozzle in the intake valve chamber of each of the four cylinders.

Redrawn from POH 172SPHBUS §7, Fuel System, with capacities from POH §2 and TCDS 3A12 Rev 80 — structure restated, not traced. C172S NAV III.

You plan a flight with 56 gallons on board. How much fuel do you actually have?

53 gallons — three are unusable, 1.5 in each tank. Right, and planning on 56 is a three-gallon error in the direction that matters. The POH is explicit that fuel remaining once the indicator reads 0 cannot be safely used in flight.

Fuel quantity, U.S. gallons (POH 172SPHBUS §2 and Figure 7-5)
Fill state	Per tank	Total	Unusable	Usable, all flight conditions
Full	28.0	56.0	3.0	53.0
Reduced (to the filler tab)	17.5 usable	35.0	3.0	32.0

Why would anyone deliberately fill to the filler indicator tab instead of full?

To carry a heavier cabin load — reduced tanks give 17.5 gallons usable per side, trading fuel for payload. Right. It is a defined servicing option in the POH, not an improvisation, and the tab is the gauge for it.

A club briefing sheet says "172: 40 gallons usable". You are flying a 172S. What is true?

The sheet is describing an older 172 — the S has 53 usable. Right. 40 usable is the N and P standard figure. Thirteen gallons is not a rounding difference.

From tank to nozzle

Downhill, then pumped

The path is worth learning as two halves. In the first half gravity does everything: fuel flows from the wing tanks to the three-position selector valve — BOTH, RIGHT, LEFT — and on to the FUEL RESERVOIR TANK. In the second half it is pumped: from the reservoir through the electrically driven auxiliary fuel pump, through the fuel shutoff valve, through the fuel strainer, and to the ENGINE DRIVEN fuel pump. From there fuel reaches the fuel/air control unit under the engine, which meters it in proportion to induction air flow, then the flow divider on top of the engine, and finally an air-bleed injector nozzle in each cylinder’s intake chamber. The reservoir is the hinge between the two halves, and it is also where the fuel return line comes back.

The auxiliary fuel pump is switched off and the engine driven pump has failed. Why does fuel still reach the reservoir?

Because the wing tanks are above it and feed by gravity — no pump is involved in that part of the system. Right, and it is the high wing’s quiet advantage. Getting fuel out of the tanks needs no power at all.

The selector: three positions, and what each is approved for

The selector is a three-position valve labelled BOTH, RIGHT and LEFT — note what is absent: there is no OFF position on the selector, and no crossfeed. BOTH is required for takeoff, climb, landing, and any manoeuvre involving prolonged slips or skids of more than 30 seconds. LEFT and RIGHT are reserved for LEVEL CRUISING FLIGHT ONLY. On BOTH, unequal flow from the two tanks can occur if the wings are not held exactly level, and the fix is to select the tank showing the HIGHER quantity until the two indications equalise, then return to BOTH. One refuelling detail follows from the same plumbing: park wings level in the normal ground attitude and put the selector on LEFT or RIGHT before filling, which ensures maximum capacity and minimises fuel crossfeeding between tanks through the interconnected airspace.

You notice a 6-gallon imbalance in the cruise with the selector on BOTH. What do you do?

Select the fuller tank until the indications equalise, then return to BOTH. Right — and returning to BOTH is the half people forget. LEFT or RIGHT is approved for level cruise only, so it is a temporary selection.

Which selector position is required for takeoff and landing?

BOTH. Right, and it is a §2 limitation rather than a technique — "takeoff and land with the fuel selector valve handle in the BOTH position".

Indication and totalising

The gauges stop moving before the tanks are full

Fuel quantity comes from one sensor per tank, displayed on the EIS in gallons. There is a quirk here that catches people, and it is in the POH in plain words: the SENSOR MEASUREMENT RANGE ENDS AT APPROXIMATELY 24 GALLONS, marked by the top of the green band. Above that level you can keep adding fuel and the indicator will not move — which means a full-tank indication and a nearly-full tank look identical. That is why a VISUAL CHECK of each wing tank is required before every flight, and why the POH tells you to compare the visual level with the indicated quantity to estimate usable fuel. At the other end, an empty indication is a red line and 0, with approximately 1.5 gallons of unusable fuel still in the tank. The indicators should not be relied on during skids, slips or unusual attitudes.

RPM SENSOR — Speed sensor on the engine tachometer drive accessory pad — sends a digital signal, not a mechanical cable drive.
FUEL QTY — One fuel quantity sensor per tank. Their measurement range ends at approximately 24 gallons, which is why a visual check of each tank is required.
OIL P / T — Oil pressure transducer on the forward oil pressure port, and the oil temperature sensor in the oil filter adapter.
EGT / CHT — A thermocouple in each cylinder head and in each cylinder’s exhaust pipe — four of each, which is what the LEAN page displays.
FUEL FLOW — Turbine type fuel flow transducer between the fuel servo and the flow divider — the source of FFLOW GPH, and of the calculated GAL USED and GAL REM.
LOW OIL SW — A SEPARATE low oil pressure switch. It drives the OIL PRESSURE annunciation directly, independently of the indicating chain — a second opinion by design.
ENGINE + AIRFRAME UNIT — The engine and airframe unit, forward of the instrument panel. It receives every engine and system sensor signal and feeds the displays — one box between all of them and you.
PFD — Primary flight display — carries the EIS strip during engine start, and the annunciation window at all times.
MFD — Multi-function display — carries the EIS strip in normal operation, with its ENGINE, LEAN and SYSTEM pages.

Redrawn from POH 172SPHBUS §7, Engine Instruments and Fuel Indicating System — structure restated, not traced. C172S NAV III with the Garmin G1000.

Both indicators read 24 gallons. How much fuel is in each tank?

At least 24 gallons — that is the top of the sensor range, so anything from 24 to full reads the same. Right, and it is exactly why a visual check of each tank is required before every flight. The gauge cannot tell 24 from 28.

LOW FUEL, and the totaliser that does not know

Below 5 gallons indicated, held for more than 60 seconds, LOW FUEL L and/or LOW FUEL R appear in amber on the PFD with a tone, and the affected pointer and label turn steady amber; at the calibrated usable-empty level they go flashing red while the annunciation stays amber. The POH’s guidance is direct: takeoff is not recommended with both pointers in the yellow band or a LOW FUEL annunciation displayed. Separately, the SYSTEM page carries GAL USED and GAL REM, and these are a different kind of number entirely — they are CALCULATED from the fuel flow transducer since the last reset or pilot adjustment, they do not use the fuel quantity indicators at all, and the POH says outright that they give no indication of the actual fuel remaining in each tank. A totaliser tells you what the engine has burned; it cannot know about a leak, a mis-set adjustment or a cap left off.

GAL REM reads 22 and the tank indicators read about 8 gallons total. Which do you believe?

The indicators — GAL REM is calculated from fuel flow since the last pilot adjustment and knows nothing about what is actually in the tanks. Right, and the disagreement itself is information: a totaliser reading high against the gauges is what a leak or a missed adjustment looks like.

A red X appears through the TOP part of the fuel quantity indicator. What has failed?

Something associated with the LEFT fuel tank sensor. Right — top for left, bottom for right. It is a sensor or wiring failure, not a fuel state, and the tank is still full of whatever it was full of.

Pumps, venting and the limits

What the auxiliary pump is really for

The auxiliary fuel pump is electrically driven and has four jobs, only one of which is an emergency. Its primary use is PRIMING before start, through the injection system — and left ON for prolonged periods with the master on and the mixture rich while the engine is stopped, it will flood the engine. It suppresses fuel VAPOUR in hot weather; momentary use is normally enough, but continuous operation is permissible and it may be run continuously in the cruise. If fuel flow fluctuates by more than 1 GPH in a climb or cruise at high altitude on a hot day, switching it ON clears vapour from the system. And in the event of ENGINE DRIVEN FUEL PUMP FAILURE, it provides sufficient fuel to maintain flight at maximum continuous power. What it is NOT for is normal takeoff and landing: gravity and the engine driven pump supply adequate flow, and with a healthy engine driven pump the auxiliary pump causes only very minor mixture enrichment.

How much power can you sustain on the auxiliary fuel pump alone?

Maximum continuous power — the POH says it provides sufficient fuel for that. Right. This is not a limp-home capability: it is a genuine parallel supply, which is why an engine driven pump failure is an abnormal rather than an emergency.

Venting, and the 30 seconds that matter

Two abnormals here have nothing to do with pumps. The first is VENTING. The tanks are vented by an interconnecting vent line between them and a check-valve-equipped overboard vent in the left tank, protruding from the bottom of the left wing just inboard of the strut’s upper attachment point. Venting is essential: COMPLETE BLOCKAGE of the venting system results in decreasing fuel flow and eventual engine stoppage, because fuel cannot leave a sealed tank. The filler caps are vacuum vented as a back-up and will admit air if the overboard vent blocks. The second is UNCOVERING A TANK OUTLET. With a quarter tank or less, prolonged uncoordinated flight — a slip or a skid — can uncover the outlet and cause fuel starvation and engine stoppage. So the limits are specific: on one tank with a quarter or less, or with one tank dry, 30 SECONDS is the maximum slip or skid, and anything longer needs BOTH selected and more fuel over the outlet.

Why does a completely blocked fuel vent stop the engine?

Air cannot enter to replace fuel leaving the tanks, so fuel flow decreases and eventually ceases. Right, and the filler caps are vacuum vented as a back-up for exactly this. The gravity feed cannot pull fuel out of a sealed tank indefinitely.

Draining: five points, before every flight

The system has drain valves so the fuel can be examined for contamination and grade, and the POH names every one of them: each wing tank sump, the fuel reservoir tank sump, the fuel selector valve drain, and the fuel strainer sump — five points in total, drained with the sampler cup before each flight and after each refuelling. The reason the list is longer than a low-wing trainer’s is that the reservoir, the selector and the strainer are all low points where water can collect after it has left the tanks. Approved fuel is 100LL (blue) or 100 (green) grade aviation gasoline, and a grade check is one of the things the sample is for. If contamination is found it must be eliminated per the preflight checklist and Section 8, not simply drained until it looks clean. And if the next flight’s weight allows it, fill the tanks after each flight to prevent condensation — the water you are draining tomorrow gets in tonight.

How many fuel drain points does the POH name for a preflight sample?

Five — the two wing tank sumps, the reservoir tank sump, the selector valve drain and the strainer sump. Right, and the three beyond the wings are the ones people miss. They are the low points where water settles after it has left the tanks.