The Fuel System

Question 1

Fuel Types

Answer 1

AVGAS 100/130: Aviation gasoline, coloured green, normally used in aircraft piston engines
AVGAS 100LL (low lead): A suitable alternative to AVGAS, coloured blue
MOGAS: Coloured red, and unsuitable for use in aircraft
AVTUR: Jet fuel, clear or straw-coloured, will damage an aircraft piston engine

Question 2

MOGAS

Answer 2

In an aircraft engine it will cause a lower power output and an increased risk of detonation
More volatile and will vaporise more readily
Will boil at about 15,000ft compared to AVGAS at 30,000ft

Question 3

Octane Rating

Answer 3

The resistance of the fuel to detonation

The higher the rating, the more the mixture can be compressed and heated without detonating

Question 4

Fuel/Air Mixture

Answer 4

Ratio of fuel to air in the cylinder for combustion

Question 5

Chemically Correct Mixture

Answer 5

The mixture which results in no excess fuel or air present in the cylinder after combustion takes place
Approx ratio = 1:16

Question 6

Leaning Using an EGT Gauge

Answer 6

Each marking on the gauge = 25 degrees F starting at 1000 degrees F
Pull mixture out until the needle on the EGT gauge wont rise any more (max EGT) and then enrich by 2 markings

Question 7

Detonation

Answer 7

An explosive and instantaneous combustion of the mixture in the cylinder
Can severely damage the piston, valves, spark plugs and cause engine failure
Usually occurs in all cylinders

Question 8

Symptoms of Detonation

Answer 8

High CHTs and rough running

Question 9

Causes of Detonation

Answer 9

Mixture being too hot

1. Over-lean mixture (no excess fuel to evaporate and cool the cylinders)
2. Time expired or wrong fuel octane rating
3. Low airspeed (less air cooling)
4. Unnecessarily high power settings
5. Excessive manifold pressure with a low RPM

Question 10

Remedial Action For Detonation

Answer 10

Enrich the mixture
Reduce power settings
Increase airspeed

Question 11

Pre-Ignition

Answer 11

Uncontrolled ignition of the mixture before the spark plugs fire on the compression stroke
Forms a progressive-burn
A problem in one cylinder

Question 12

Causes of Pre-Ignition

Answer 12

1. Carbon deposits
2. Hot spots in the cylinder
3. Overheated spark plugs
4. High power settings and a lean mixture

Question 13

Symptoms of Pre-Ignition

Answer 13

Rough running and high CHTs with possible backfiring

Question 14

Remedial Action For Pre-Ignition

Answer 14

Enrich the mixture
Decrease power
Increase airspeed

Question 15

Purpose of The Fuel System

Answer 15

Store fuel
Provide fuel to the engine via the carburettor
Provide flow of fuel continuously and in correct amounts
A means to monitor the fuel load and usage during flight

Question 16

Types of Fuel Tanks

Answer 16

Aluminium (C-152, C-172, etc): A metal rigid structure
Rubber cells/bladder
The whole wing (big boys): Integral/wet wing design

Question 17

Fuel Tanks

Answer 17

Store fuel for the flight
Different aircraft have different numbers of tanks
Have a sump with a drain point
May contain baffles to prevent movement of the fuel within the tank during flight

Question 18

Fuel Tank Vents

Answer 18

Provides positive pressure at all times
Allows atmospheric pressure to be retained as altitude increases
Ensures that fuel will always be capable of being drawn from the tank to the engine
Used as an overflow drain should pressure become too high at any stage
Should be clear of obsructions

Question 19

Rich Mixtures

Answer 19

When the ratio of fuel to air is increased
Mixtures richer than 1:8 wont burn (flooding)
Burns cooler as the un-burnt fuel evaporates and cools the cylinders

Question 20

Lean Mixtures

Answer 20

Ratio of fuel to air is reduced

Mixtures leaner than 1:18 will not burn

Question 21

Mixture Strength

Answer 21

Affects the temperature of combustion and exhaust gases

Question 22

Mixture Versus Power Output

Answer 22

As mixture is leaned the power output will increase (increasing TAS in level flight)
Leaning past the best power mixture will reduce the TAS and the fuel flow

Question 23

Best Economy Mixture

Answer 23

The point at which TAS begins to reduce faster than the fuel flow
Very close to peak EGT

Question 24

Best Power Mixture

Answer 24

The mixture that gives the max TAS

Question 25

Leaning at Altitude

Answer 25

As altitude increases, pressure reduces and the weight of the air entering the carburettor reduces
If the fuel flow is not adjusted, the mixture will become too rich

Question 26

Consequences of an Over-Rich Mixture

Answer 26

Reduced power
Rough running
High fuel consumption
Fouling of the spark plugs and lead deposits
Black exhaust smoke is often an indication

Question 27

Consequences of an Over-Lean Mixture

Answer 27

High CHTs and detonation

Blue exhaust smoke is often an indication

Question 28

Components of the Fuel System

Answer 28

Fuel Tanks
Fuel Tank Vents
Fuel Lines
Fuel Selector
Fuel Strainer (Gascolator)
Fuel Pumps
Fuel Mixture Control

Question 29

Fuel System Mixture Control

Answer 29

Mixture control linked to carburettor or fuel injection system

Question 30

Fuel Drain

Answer 30

Drains in the bottom of the tanks and below the strainers

Allow to check for contaminants

Question 31

Fuel Selector Valve

Answer 31

Allows the fuel flow from the tanks, or either tank separately, to be turned on and off

Question 32

Fuel Quantity Gauge

Answer 32

Operated via a float in the fuel tank

Question 33

Fuel Pump

Answer 33

A mechanical engine-driven pump for normal operation

A backup electric pump for takeoff, landing and engine failure is also available

Question 34

Fuel Vaporisation

Answer 34

The fuel will expand in the lines and the fuel flow to the engine will be interrupted

Question 35

Risk of Vaporisation Increased By

Answer 35

1. Volatility
2. High temperatures
3. Low atmospheric pressure

Question 36

Boost Pump

Answer 36

Helps to prevent vapour locks
Acts as a:
- Primer to purge the fuel lines of vapour prior to start
- Substitute in case the engine-driven pump fails