Chapter 5 - The Fuel System

How do high-wing and low-wing aircraft usually differ in fuel supply?

High-wing aircraft can often use gravity feed from the wing tanks to the carburettor, while low-wing aircraft usually need a fuel pump because there is insufficient fuel head for reliable flow.

What are the main components of a typical light aircraft fuel system?

Fuel tanks, vents/expansion spaces, sumps/drain cocks, fuel selector/shut-off valve, filters, fuel pumps, priming pump, fuel gauges, and fuel lines.

Why do aircraft fuel tanks need vents?

To maintain atmospheric pressure above the fuel as it is used, preventing a partial vacuum and fuel starvation.

Why are fuel tank vents often forward-facing?

To maintain positive air pressure above the fuel.

What is the purpose of sumps in fuel tanks?

To collect impurities heavier than fuel, such as dirt, sediment, and water.

Why is there unusable fuel in each tank?

Because the fuel outlet is above the level of the sump, leaving some fuel below the outlet.

What is the purpose of a fuel strainer/drain valve?

To trap contaminants and allow fuel to be drained for inspection.

Why is the auxiliary fuel pump usually switched ON before starting?

To prime the fuel lines and provide fuel pressure for starting.

How can the pilot check that the auxiliary fuel pump is working before start?

By observing the required increase in fuel pressure on the fuel pressure gauge.

When is the auxiliary fuel pump commonly switched ON?

For take-off, landing, changing tanks, low flying, and as a backup if fuel pressure is low.

What is the purpose of the priming pump?

To pump a measured amount of raw fuel directly into the induction manifold near one or more cylinders for starting.

Why is priming needed more in cold conditions?

Because fuel is reluctant to vaporise, making the mixture too lean for starting.

Why is little or no priming usually needed for a hot engine?

Because vaporisation is improved when the engine is warm.

What danger can result from too much priming?

The mixture may become too rich and there is a danger of starting a fire in the engine.

Why must the primer be locked when the engine is running?

To prevent excessive fuel being drawn into the cylinders through the priming line.

What can happen if the primer is not locked during engine operation?

The engine may run roughly or stop due to an over-rich mixture, especially at low power settings.

How are most light aircraft fuel gauges operated?

Electrically, requiring the master switch to be ON.

When are aircraft fuel gauges most accurate?

In straight-and-level flight attitude.

Why should fuel gauge readings be cross-checked?

Because they are not always accurate and should be compared with flight plan fuel calculations and actual consumption experience.

How should fuel quantity be visually checked during pre-flight?

By removing the fuel caps, visually checking contents, preferably dipping the tanks if a dipstick is provided, and replacing the caps securely.

What does correct leaning of the mixture affect?

Fuel consumption.

What can happen if correct leaning procedures are not followed?

Fuel consumption may be as much as 20% higher than stated figures.

What must be checked when using the fuel selector valve?

That the correct tank is selected, the selector is in the correct detent, and the valve selection matches the intended selection.

When should fuel tank selection be checked?

Before start-up, during take-off and landing DVAs, and during periodic operating checks.

Why is it poor practice to run a tank dry before switching tanks?

The fuel pump may draw air into the fuel lines, causing vapour lock and possible fuel stoppage.

What should be done before changing fuel tanks if an electric boost pump is fitted?

Switch on the electric fuel boost pump to help guarantee fuel pressure to the carburettor.

What should you do, when you switch fuel tanks?

Monitor fuel pressure

What are two possible causes of sudden or unexpected loss of power?

Lack of fuel to the engine and carburettor icing.

What actions should be taken if incorrect fuel selection causes power loss?

1. Close the throttle

2. Select the correct fuel tank

3. Turn the electric fuel boost pump ON

4. And, set the mixture to FULL RICH.

What actions should be taken if carburettor causes power loss?

Apply full carburettor heat.

How does Avgas generally differ from Mogas?

Avgas is more leaded and has a higher octane or performance rating.

Why is Avgas less volatile than Mogas?

To reduce vapour lock problems at altitude and meet aircraft safety requirements.

What is the only grade of Avgas available for light aircraft in New Zealand according to the text?

100LL.

What colour is 100LL Avgas dyed?

Blue.

What colour is Mogas 91?

Purple/bronze or red/orange.

What colour is Mogas 96?

Yellow.

What colour is Jet A1 kerosene?

Straw coloured or clear.

What do figures at or below 100 in fuel grading refer to?

Octane rating.

What do figures above 100 in fuel grading refer to?

Performance number.

What does the higher number in a two-number fuel grade measure?

Power potential compared with a reference fuel before detonation with a full-rich mixture.

What does the lower number in a two-number fuel grade measure?

Power possible before the same fuel, leaned out, would detonate.

Why can Avgas contribute to spark plug fouling?

Because grades of Avgas contain lead deposits, especially during extended low-power operation.

How can pilots reduce lead fouling during long descents?

Operate the engine at the high end of the normal cylinder head temperature range or periodically increase power.

What can happen if a higher fuel grade than recommended is used?

It can increase the chances of plug fouling and may cause corrosion of exhaust valves and seats in older engines.

What can happen if a lower fuel grade than recommended is used?

It increases the risk of detonation, especially at high power settings.

Why is the age of fuel important?

Time-expired fuel loses anti-detonation qualities and its octane rating falls.

Why should Mogas not be used unless specifically authorised?

It has lower quality control, is more volatile, may cause vapour lock, plug fouling, detonation, and lower power output.

What is Avtur?

Aviation turbine fuel, a kerosene-type fuel used in turbine engines.

Why must Avtur not be used in piston engines?

It can be disastrous and must not be used unless the piston engine is specifically designed for it.

What distinctive smell does Avtur have?

Kerosene.

What type of engine is a conventional turbocharged engine?

A piston engine that uses Avgas, not Avtur.

How are Avtur fuel decals usually labelled?

The word Avtur in white letters on a black background.

How are Avgas decals usually labelled?

The word Avgas and/or the octane rating, such as 100LL, in white letters on a red background.

What is the most common fuel contaminant?

Water.

Why is water especially dangerous in fuel?

It can block fuel passages and cause engine failure.

Why does water gather at low points in the fuel system?

Because water is denser than fuel.

How can water enter aircraft fuel tanks?

Through leaking ground tanks, fuelling trucks, unsecured or worn fuel caps, wet conditions, and condensation.

How can full fuel tanks overnight reduce contamination risk?

They minimise the airspace above the fuel, reducing condensation.

What are the disadvantages of keeping aircraft fuel tanks full when the aircraft is not being used?

The aircraft may need to be partially defuelled before the next flight to reduce weight, and as temperatures rise, fuel expansion can cause overflow, creating a potential fire hazard.

Why must extra care be taken with fuel drums?

They can easily become contaminated with water, dirt, rust flakes, and other matter.

How should fuel drums for storage be initially filled?

To the brim, then stored on their sides to cover the inside of the bungs with fuel.

Why should drums be stored on their sides?

To help prevent the ingress of air.

What should be fitted when refuelling from drums?

Pumping equipment with an approved in-line filter that absorbs moisture and removes particulate matter.

Why should a chamois leather strainer generally be avoided?

It can create a static electricity hazard, especially synthetic chamois.

Before using fuel from an unknown drum to refuel an aircraft, what should you check?

Check that it is the correct type and grade of fuel and check its date of expiry.

Besides water, what other impurities can cause fuel problems?

Rust, sand, dust, and micro-organisms.

What does water look like in a fuel sample?

Small globules at the bottom of the container.

What may other fuel impurities look like in a sample?

Darker-coloured sediment at the bottom of the container.

What should be done if a fuel sample check is positive for water or contamination?

Empty the container safely, carry out a further check, and seek guidance if contamination remains.

What should be done if there is a large amount of water in the tanks?

A maintenance engineer should be called to clear it.

In some remote situations, what may be done to help remove water from tanks?

Drain all tanks until water is removed and rock the wings to help water gravitate to the water trap.

After fuel checks, what must be ensured?

All drain valves are properly closed and show no signs of leakage.

What can a leaking drain valve cause?

A fire hazard and much higher in-flight fuel consumption than expected.

When should fuel checks generally be carried out?

Before the first flight of the day, after each refuel, and whenever contamination is suspected.

What should be checked before refuelling an aircraft?

That the fuel being loaded is the correct type and grade.

What should happen to passengers and casual observers during refuelling?

They must be kept clear.

Where should torches be switched on when checking fuel contents?

Well away from the filler caps.

Why should the fuelling nozzle not rest on the intake neck of the tank?

It can cause cracking around the intake over time.

What should be done after refuelling is complete?

Ensure filler caps are refitted and properly locked in place.

Why must refuelling documentation be completed?

It may be vital if the aircraft becomes overdue on its next flight.

What should each dipstick show?

The registration letters of the aircraft for which it is calibrated.

During pre-flight fuel management, what must be ensured?

That the aircraft has the correct type and grade of fuel on board and that it is free of impurities.

What should be done to confirm sufficient fuel before flight?

Calculate required fuel, visually check fuel level, and dip the tanks if necessary.

What should be checked about fuel caps during pre-flight?

They are secure, not leaking, and vents are clear and unobstructed.

What can happen if fuel tank caps are not secure?

Fuel can be siphoned out very quickly in flight.

Why is extra care needed with high-wing aircraft fuel caps?

They may not be easily visible from the ground.

What should be done when selecting a new tank in flight?

Move the selector valve firmly and positively into the correct detent.

What is preferred near the ground if the aircraft allows it?

Select and draw fuel from more than one tank at the same time.

When changing tanks in flight, what should be confirmed first?

That there is fuel in the tank about to be selected.

What should be done after switching on the electric boost pump during a tank change?

Monitor the fuel pressure during the transfer and after switching the pump off.