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6.2.4 Ignition Systems
Principles of Magneto Ignition
The basic requirement of any ignition system is to deliver an intense electrical current to the spark plugs. This ignites the fuel/air mixture in each cylinder. Since the valves are closed during ignition, the pistons are forced down because of the now rapidly expanding gasses, producing work and in turn rotating the crankshaft and thus the propeller.
Construction and Function
A magneto is simply a permanent magnet that rotates within a conductor and coil to generate an alternating electrical current. This current is completely independent of the aircraft’s electrical system. The mechanically or, engine driven magneto, generates the required electrical energy to the spark plugs in each cylinder – Igniting at just the right time. This takes place when the starter is engaged and the crankshaft begins to turn and it continues to operate whenever the crankshaft is rotating.

The distributor, which consists of a rotor that spins inside the non-conductive distributor block and makes contact with terminals embedded within the block. Each terminal is connected to a spark plug. The rotor (carrying a high voltage charge from the magneto circuit) comes in contact with each terminal, and the current is conducted to the applicable plug in the correct sequence.Most modern aircraft make use of a dual ignition system with two individual magnetos. Separate sets of wires and spark plugs improve the redundancy and reliability of the ignition system. Both magnetos operate independently to fire their own spark plug within their allotted cylinders. Combustion of the fuel-air mixture is therefore improved with dual spark ignition and results in a slightly higher power output. In the event of one magneto failing, the other will be unaffected. This redundancy allows the engine to continue somewhat normal operation, although engine RPM can be expected to be slightly reduced, resulting in a lower power output. Operation of the magnetos are controlled in the cockpit through various ignition switch positions:
R (right)
L (left)
Purpose and Principle of Impulse Coupling
In order to produce a spark in the plugs, the magneto spins a magnet within an iron coil core. This generates an alternating current within the coil and produces up to 20 000 volts which is used to fire the spark plugs. In order for sparks to be effective, the magnet needs to be rotating at speeds of at least 500 R.P.M. Anything below this results in weaker sparks and reduces engine start-up efficiency. A premature power stroke known as kick-back results from normal magneto timing set for a higher R.P.M setting. Ultimately, this can lead to the crankshaft being forced in the wrong direction. On startup (during low R.P.M operation) the ignition therefore needs to be delayed.

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This start-up problem is solved through the use of Impulse coupling. This device works in two ways. The magneto uses spring weights and a spring-loaded coupling which prevents the magneto from turning at first. Once the spring is fully wound it releases the magnet at an accelerated rotational velocity. The benefits of this are two-fold. First, it accelerates the rotation of the magnet producing a higher voltage or, better spark, and second the ignition spark is delayed during start-up. Once the engine is running, the centrifugal force of the flyweights ensures the impulse coupling is disconnected and does not interfere during normal operations.

Serviceability Checks
The spark plug is a useful indicator of engine condition. At each Mandatory Periodic Inspection (M.P.I) the plugs are removed for inspection and testing. Normal engine operation is indicated by a light grey coating of the end of the plugs. Excessive wear may indicate detonation. Black sooty-like deposits will appear in cases where mixture has not been sufficiently leaned. Whereas engines operated with too lean a mixture will leave white powdery traces behind. Black oily deposits suggest excessive oil consumption. If hard brittle deposits are found lodged in the spark plug gap, it means fuel lead is not being removed during combustion. If disregarded, these deposits can build up sufficiently causing the plugs to ground without a spark. This often results in a “mag drop” which can be identified by a rough running engine and an excessive loss in R.P.M. When detected on the ground during magneto checks, the flight should be aborted immediately.
Operational Procedures
The ignition system can be identified faulty during the pre-takeoff run-up checks. This is done by observing the reduction in R.P.M that occurs between when the LEFT and RIGHT ignition are selected independently. The maximum allowable reduction and ‘total drop’ limits are listed in the POH. If the engine stops running when switched to one magneto or if the rpm drop exceeds the allowable limit, the aircraft should not be flown until the system is serviced and the problem is corrected.
Possible causes of an unacceptable mag drop could be the result of fouled spark plugs, damaged wires between the magneto and the spark plugs, or incorrect timing. “No drop” in R.P.M is abnormal and considered another cause for concern. In this case, the aircraft should not be flown and sent in for immediate inspection.
“No drop” is an indication that one of the magnetos is not grounding and can result in a premature start, simply by turning the propeller. Even with the battery and master switches OFF, the engine can fire and turn over if the ignition switch is left ON and the propeller is moved. This happens because the magneto is self-exciting. If this occurs, the only way to stop the engine is to move the mixture lever to the Idle Cut-Off (I.C.O) position, then have the system checked by a qualified AMO. Ensure the magneto switch is turned to the OFF position after flight and be extremely cautious when in the vicinity of the propeller.

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6.5.6Turn indicator- rate gyro
A rate gyro can only move on one axis, and springs are used to keep it from toppling, due to its mounting, any angular movement will apply a force to the gyro and it will move about its axis.

– purpose and function
The purpose of a turn indicator is to show that the aircraft is turning i.e. changing direction. Most turn indicators will have markings to show the pilot when he is doing a standard rate turn (3 per second)
The turn indicator makes use of a rate gyro, and its property of precession, if the aircraft changes direction a force is applied to the gyro and it indicates a turn.

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– effect of speed
To maintain a standard rate turn of 3 per second the bank angle required will increase with an increase in airspeed.

– presentation
Older style turn indicators have a needle that can move left or right depending on the direction of turn and markings to line the needle up with that indicate a standard rate turn.

Newer style turn indicators or turn co-ordinators (more on these later) of an aircraft graphic that rolls left and right and markings to indicate to the pilot when he is doing a standard rate turn.

A good diagram of the different instrument presentations:
– turn co-ordinator
The turn co-Ordinator is operates the same as the turn indicator, but the gyro is mounted at an angle and the gyro can therefore sense the aircraft rolling, therefore there is very little delay in the instrument indicating a turn.

– limited rate of turn indications
Most turn indicators and turn co-ordinators will only indicate a rate of turn just past a standard rate turn, any rate faster than this will not be indicated.

– power source
Turn co-ordinators in General aviation aircraft are powered by the vacuum pump.

– balance indicator
The balance indicator or balance ball is used to aid the pilot in keeping the aircraft in co-ordinated flight.

– principle
In co-ordinated straight and level flight the ball is kept in the middle of the tube by gravity, if the aircraft was in un-co-ordinated flight (this means the nose of the aircraft is not pointing in the direction that the aircraft is travelling) the ball would be forced out to the side of the tube, in order to keep co-ordinated flight or “the ball in the middle” we simply apply rudder on the same side the ball is deflected, we use the saying “step on the ball”.

– presentation
The balance indicator is a simple curved glass tube that is filled with a fluid and has a small ball inside the tube. Usually it is part of the turn indicator or turn co-Ordinator.

A good diagram to put here:
– pilot’s serviceability checks
During the taxi is when we check the serviceability of the balance indicator, if we turn to the left, the balance ball should move out to the right and vice versa.

6.5.7Attitude indicator- earth gyro
An earth gyro is a gyro that is mount horizontally with its axis vertical and has freedom of movement in 2 planes.

– purpose and function
The attitude indicator hives the pilot an instant indication of the attitude of the aircraft, its I a small but accurate representation of what the pilot sees outside the aircraft. The attitude indicator uses the gyroscopic property of rigidity, the gyro maintains its orientation in space and the aircraft pitches and rolls around the gyro, it therefore moves the small aircraft on the display of the instrument.

– presentations
The attitude indicator has a round face, this is divided equally up into 2 sections, a blue section representing the sky and a brown section representing the ground. It has markings on it to show degrees of pitch up and down, and degrees of roll left and right.

245092413343100A good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– interpretation
A good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– operating limitations
Most attitude indicators are limited to 100° in roll, left and right and 60 degrees in pitch up and down
– power source
The attitude indicator in most light aircraft are powered by the vacuum pump, but they can also be electrically powered.

– pilot’s serviceability checks
During the taxi the attitude indicator would be checked during a turn and it should not indicate a roll or a bank as the aircraft is yawing, if any roll or bank is shown the attitude indicator could be faulty.

Heading indicator
– directional gyro
A directional gyro has its axis oriented in the horizontal plane and the gyro in the vertical plane, it only has one plane of movement.

– purpose and function
The purpose of the heading indicator is to make turns onto specific headings and maintaining straight flight without having to deal with the numerous errors present on the compass.
– presentation
23729549472300A good diagram to put here:
(Taken from
– use with magnetic compass
The heading indicator is aligned to the compass by the pilot when the compass is indicating an accurate heading. This needs to be done relatively often as the heading indicator gyro suffers from an error called drift.

– setting mechanism
On the front of the instrument is a knob, this is used to set the heading according to the gyro, to use it, the knob needs to be pushed in and rotated, this will rotate the heading card.

– apparent drift
Due to the rotation of the earth, and the gyro maintaining its orientation in space, the heading indicator suffers from apparent drift. Therefore the heading indicator needs to be re-aligned with compass every 10 to 20 minutes.

– transport wander
Transport wander creates errors in the heading indicator due to the aircrafts movement over the earth. Therefore, the heading indicator needs to be re-aligned with compass about every 15 minutes.

– operating limitations
The heading indicator, due to its errors such as apparent wander and transport wander needs to be re-aligned with the compass every 10 to 20 minutes during flight.

– power source
In most general aviation aircraft, the heading indicator is powered by the vacuum pump. They can also be electrically powered.

– pilot’s serviceability checks
During a turn while taxiing the pilot will check that the heading moves in the correct direction and roughly the correct amount, i.e. a 90° turn should move the heading indicator 90°.

Magnetic compass
– construction and function
The compass consists of a sealed case filled with fluid, there is a hardened steel pin, which the compass rests on, using stone jewel, this arrangement reduces friction to a very small amount. The compass consists of two magnets mount to a compass card with the appropriate gradutions, the magnets are mounted below the mounting point of the pin and stone jewel.

A good diagram to put here:
(Taken from:

– earth’s magnetic field
The earth’s magnetic field runs from the like pole to the south pole, theses invisible lines of magnetism are called lines of flux.

20914486760800A good diagram to put here:
(Taken from:
– variation and deviation
Variation is the difference between true North and magnetic North, magnetic North is not located in the same spot as true north, therefore depending on where the aircraft is flying there will be a difference between true North and magnetic North. Variation can be found on an aeronautical navigation chart. This value needs to be applied to the true direction to get magnetic direction.
We use a saying “West Best. East Least”, what this mean is that if the variation is easterly i.e 14°E and our true direction is 090° we need to subtract the variation from the true heading:
090° – 14° = 076°, our magnetic heading is now 076°
For a westerly variation i.e. 10°W, we need to add the variation to the true heading:
245° + 10° = 255° our magnetic heading is now 245°
Deviation is the difference between compass heading and magnetic heading, this error is caused by other magnetic sources, such as, the aircrafts structure, live electrical wires etc.

Deviation is reduced as much as possible by performing a maintenance procedure called a compass wing. This is performed by an appropriately rated maintenance engineer. This procedure produce a compass correction card which tell you the difference between magnetic and compass heading for 8 different headings including the 4 cardinal points.

An example of a good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– turning, acceleration errors
Various errors are caused in the compass by the aircraft accelerating and turning.

When turning onto a northly heading, due to the magnetic dip of the compass the compass tends to show a more northerly heading than the actual magnetic heading, to compensate for this when turning on the compass onto a northerly heading the pilot must roll wings level about 15°.

When turning onto a southerly heading the compass card assembly lags the current aircraft heading and therefore when turning onto a southerly heading using the compass, the pilot must pass the required heading.

Turning errors are increased as the aircraft gets closer to either pole, therefore the closer the aircraft to the pole the greater the correction that needs to be applied by the pilot when doing turns using the compass.

When the aircraft accelerates and decelerates an apparent turn is caused on the compass, this is because of the magnetic dip and pendulous mounting of the compass card assembly, these errors are only cause on easterly and westerly headings. When accelerating on a easterly r westerly heading the compass shows and apparent turn towards North, when decelerating it shows an apparent turn towards south. A mnemonic, ANDS or Accelerate North/Decelerate South. will help remember these errors.

– precautions when carrying magnetic items
No magnetic or electronic items should placed near the compass, this can cause large errors on the heading showed and can cause pilots to get lost when navigating using the compass.

– pilot’s serviceability checksThe pilot should check serviceability of the compass before taxi, this can be done by checking that the compass is showing the correct general direction. During the taxi the pilot should check that the compass indicates a turn, and just before take-off, after lining up the compass can be checked against the heading of the runway.

6.5.10Engine instruments
– principles, presentation and operational use of:
– oil temperature gauge
The oil temperature gauge measures the temperature of the engine oil was it has passed through the oil cooler, the gauge has a green normal band, and sometimes a yellow caution band and a red max oil temperature limit. The oil pressure gauge should be checked regularly during flight.

– oil pressure gauge
The oil pressure gauge shows the health of the engine oil system, it has a maximum and minimum pressure. A hotter oil will have a subsequent lower pressure but should still be in the green range.

An example of a good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– cylinder head temperature gauge
The cylinder head temperature gauge shows the temperature the one or more engine cylinder heads, this is a good indication of the engine is getting too hot. The gauge has a normal green range and a red minimum and maximum markings
– exhaust gas meter
Also known as an EGT gauge or exhaust gas temperature gauge. This indicates the temperature of the exhaust gas just as it exits the cylinder head, it is usually measured on the hottest cylinder. The exhaust gas temperature gauge aids the pilot in setting the fuel mixture of the engine.

– manifold pressure gauge
The manifold pressure gauge measures the pressure of the air inside the engine intake manifold. Most commonly found on piston engine aircraft with variable pitch propellers. This gauge aids the pilot in setting the power output of the engine.

An example of a good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– fuel pressure gauge
The fuel pressure gauge shows the pressure of the fuel system after the fuel pump. The gauge has a green range and a maximum and minimum fuel pressure indicated by red makrs.

– fuel flow gaugeThe fuel flow gauge shows the fuel that is consumed by the engine per hour. It assists the pilot in setting power and aids in fuel calculations.
– fuel quantity gauge(s)
The fuel quantity gauge indicates the amount of fuel in each tank. Most aircraft have one gauge per a tank. They are usually graded in US Gallons or litres.

– tachometer
The tachometer indicates the engine and/or propeller RPM, in an aircraft with a fixed pitch propeller, the pilot uses the tachometer to set engine power.

An example of a good diagram to put here:
(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)

6.5.11Other instruments
principles, presentation and operational use of:
– vacuum gauge
The vacuum gauge shows the vacuum in the instrument vacuum system in inches of mercury. The gauge has a green normal range. The vacuum gauge will be the first indication if the pump has failed and the then pilot will know that certain instrument will be unreliable.

An example of a good diagram to put here:

(Image source:
– voltmeter and ammeter
A voltmeter indicates the voltage of the aircraft electrical system. The ammeter shows how load in Ampere hours is being placed on the Alternator or battery.
– warning indicators
Some aircraft are equipped with red, amber or green lights that will illuminate if a system has malfunctioned, or the is low fuel pressure or quantity or if a system is off and or failed, if in the case of a green light this indicates a system is functioning correctly.

An example of a good diagram to put here:
(Image source:

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