Flight Training - November 2011 - 20

PREFLIGHT»

HOW IT WORKS

MAGNETOS
The engine’s primary electrical source
INTERNAL COMBUSTION ENGINES like those we operate

BY IAN J. TWOMBLY

on our aircraft are so named because fire in the cylinders is what provides power. To create fire we must have an ignition source, which in our case is spark plugs. But where do spark plugs get the electrical current? In the vast majority of aircraft engines, it’s through a magneto or pair of magnetos. At its most basic, the magneto is a circular magnet that rotates to provide magnetic flux to an iron core wrapped in wire. That generates a strong electrical current, which
ROTOR POLE SHOE

is then transferred to the spark plugs via ignition leads. The reason aircraft engines use magnetos, and not more modern ignition sources, is because they are highly reliable, and self-contained. If the electrical system in the airplane fails, the engine will continue to run because magnetos provide the spark. That’s also one reason why there are two. This provides redundancy and more efficient ignition (an rpm drop when you check each magneto is a good example of this).

BREAKER POINTS

HOW IT WORKS
• The magnet, or rotor, has a minimum of two poles on opposite ends of the circle. The rotor is connected to the engine via the accessory drive, which causes it to rotate. • Two pole shoes fit snugly in to the larger housing, collecting magnet flux from the rotor, and sending it through direct contact to the iron core of the coil. • Inside the coil is an iron core that is surrounded by two large windings of wire. The initial wire layer has about 200 windings of heavy gauge wire. It’s covered with an insulation layer, and then 13,000 wraps of very small gauge wire is wound around that. The first winding is attached to the breaker points, and the second is attached to the high-tension terminal. • A small piece of plastic attached to the rotor called the breaker cam opens and closes the breaker points. This is what controls the flow of electricity, and allows for short, powerful pulses of power, rather than a constant stream. • The distributor gear assembly rotates quickly to provide the ignition leads with the current. It gathers this current through a carbon brush.

COIL DISTRIBUTOR GEAR

PLUS Watch the magneto in motion.

WHEN IT DOESN’T WORK
From the cockpit, it’s often difficult to diagnose magneto problems. That’s because spark plug issues often manifest themselves as a magneto failure. Too large of a drop in rpm on the magneto test, an engine that loses power in flight, or one that begins to run rough, all call for a mechanic to check the system.
STEVE KARP

20 /

FLIGHTTRAINING.AOPA.ORG

http://FLIGHTTRAINING.AOPA.ORG

Flight Training - November 2011

Table of Contents for the Digital Edition of Flight Training - November 2011

Flight Training - November 2011
Contents
President’s Perspective
Right Seat
Letters
Know Where to Go
This Weekend
Success Stories
AOPA Action Member Products
News
ASI News Tech Tip
Since You Asked
How It Works
News
Final Exam
Checkride
Flying Carpet
Follow the River
When the Plan Goes Out the Window
Technique
Weather
Flight Lesson
Time for a Change?
Career Advisor
Tech Talk
Teaching the T&G
Add an Hour
The CFI Bookshelf
Advertiser Index
Debrief
Flight Training - November 2011 - Flight Training - November 2011
Flight Training - November 2011 - Cover2
Flight Training - November 2011 - Contents
Flight Training - November 2011 - 2
Flight Training - November 2011 - 3
Flight Training - November 2011 - President’s Perspective
Flight Training - November 2011 - 5
Flight Training - November 2011 - Right Seat
Flight Training - November 2011 - 7
Flight Training - November 2011 - Letters
Flight Training - November 2011 - 9
Flight Training - November 2011 - Know Where to Go
Flight Training - November 2011 - 11
Flight Training - November 2011 - This Weekend
Flight Training - November 2011 - Success Stories
Flight Training - November 2011 - AOPA Action Member Products
Flight Training - November 2011 - News
Flight Training - November 2011 - ASI News Tech Tip
Flight Training - November 2011 - 17
Flight Training - November 2011 - Since You Asked
Flight Training - November 2011 - 19
Flight Training - November 2011 - How It Works
Flight Training - November 2011 - News
Flight Training - November 2011 - 22
Flight Training - November 2011 - 23
Flight Training - November 2011 - Final Exam
Flight Training - November 2011 - Checkride
Flight Training - November 2011 - Flying Carpet
Flight Training - November 2011 - 27
Flight Training - November 2011 - Follow the River
Flight Training - November 2011 - 29
Flight Training - November 2011 - 30
Flight Training - November 2011 - 31
Flight Training - November 2011 - 32
Flight Training - November 2011 - 33
Flight Training - November 2011 - When the Plan Goes Out the Window
Flight Training - November 2011 - 35
Flight Training - November 2011 - 36
Flight Training - November 2011 - 37
Flight Training - November 2011 - Technique
Flight Training - November 2011 - 39
Flight Training - November 2011 - Weather
Flight Training - November 2011 - 41
Flight Training - November 2011 - 42
Flight Training - November 2011 - Flight Lesson
Flight Training - November 2011 - 44
Flight Training - November 2011 - Time for a Change?
Flight Training - November 2011 - Career Advisor
Flight Training - November 2011 - 47
Flight Training - November 2011 - Tech Talk
Flight Training - November 2011 - Teaching the T&G
Flight Training - November 2011 - Add an Hour
Flight Training - November 2011 - The CFI Bookshelf
Flight Training - November 2011 - Advertiser Index
Flight Training - November 2011 - 53
Flight Training - November 2011 - 54
Flight Training - November 2011 - 55
Flight Training - November 2011 - Debrief
Flight Training - November 2011 - Cover3
Flight Training - November 2011 - Cover4
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