11.2 Ignition, Magnetos, Carburetor vs Fuel Injection

ACS PA.I.G.K1c includes the ignition system that belongs to the powerplant. PA.I.G.K2 asks what fails, what still runs, and which handle you move. The highest-rate trap on this page is still the same folk theorem: “the magnetos need the battery.”

Dual magnetos do not drink from the bus

A magneto is a self-contained, engine-driven electrical generator. A permanent magnet and a coil produce high voltage that jumps the spark-plug gap. As long as the crankshaft is turning, each magneto can fire its plugs with the master switch off and the battery dead. That is why an electrical-system failure is not an engine failure.

Almost every PAR trainer has dual ignition: two magnetos, two harnesses, two spark plugs per cylinder. PHAK’s reasons are not decorative.

• Redundancy. One magneto, harness, or plug can fail and the other side still lights that cylinder.
• Better combustion. Two flame fronts burn the charge more completely and produce a little more power than one plug.

Normal flight is BOTH. The pre-takeoff mag check is how you prove each side still works before you need the redundancy.

The run-up: drop, difference, and a zero that is not a gift

A typical AFM tells you to stabilize at a published RPM — often 1,700 or 1,800 on a fixed-pitch trainer — then select RIGHT (or R), return to BOTH, then LEFT (or L), and return to BOTH. You are looking at three things:

1. The engine keeps running on each magneto alone.
2. The RPM drop on each side stays inside the AFM limit.
3. The difference between the two drops stays inside the AFM limit.

Those limits are AFM-specific. A common Cessna/Lycoming band is a maximum drop of about 125 to 175 RPM and a maximum difference of about 50 RPM. Some models say 150 and 50 at 1,800 RPM; others say 175. Memorize the table in that book, not a hangar number.

What the drop means:

A zero drop is not “a really good magneto.” It can mean the magneto you thought you turned off is still hot.

Impulse coupling, P-leads, and a hot mag

At starter-cranking speed a magneto may not spin fast enough for a fat spark, and a spark that is still on the normal (advanced) schedule can kick the propeller backward. An impulse coupling on at least one magneto retards the spark past top dead center and then snaps the magneto through its firing point so a start-worthy spark appears at cranking RPM. Once the engine is running, the coupling flies out and the magneto returns to its normal schedule. That is a starting device, not a cruise power add-on.

The wire from each magneto to the ignition switch is the P-lead (primary lead). With the key OFF, the switch grounds both P-leads so the magnetos cannot build a spark. With the key on LEFT, the right P-lead is grounded and only the left magneto fires — and the reverse for RIGHT. BOTH ungrounds both P-leads.

If a P-lead breaks or falls off, that magneto cannot be grounded. The engine may keep running when you select OFF at idle (a check some AFMs describe after the run-up). On the ground, a “hot” magneto plus fuel in a cylinder means moving the propeller by hand can start the engine. Treat every propeller as if the magnetos are hot. Mixture idle cutoff is how you normally shut the engine down; the key OFF is the ground on the mags, not a substitute for a mixture that is still at rich.

Carburetor ice, in systems language

Chapter 7 treated carburetor ice as weather. Here it is an induction-system problem. A float-type carburetor meters fuel into a venturi. Two cooling events happen at once: the venturi drops pressure and temperature, and fuel vaporizes and cools the metal still more. Moisture in the incoming air can freeze on the venturi and throttle plate without a cloud and without a freezing OAT. PHAK’s most-likely band is below 70 °F (21 °C) with relative humidity above 80 percent, but ice can form outside that band. Low power (a closed throttle in a descent) is the highest-risk power setting.

Carburetor heat routes unfiltered, exhaust-warmed air into the carburetor. Heated air is less dense, so power falls — PHAK notes a reduction of as much as about 15 percent — and the mixture goes richer. On a fixed-pitch propeller the first clue of ice is usually an RPM drop. On a constant-speed propeller RPM is held by the governor, so the first clue is usually a manifold-pressure drop.

Apply full carb heat and leave it on. If ice was present, RPM (or MP) drops further as the hot, thin air arrives, then rises as the ice melts and the engine smooths. Roughness for several seconds to a couple of minutes can be melted water going through; keep the heat on until it is smooth. If there was no ice, you get the drop and no later rise. Carb heat is also an alternate induction path if the filter ices or clogs.

Do not take off with carb heat on unless the AFM says to. You give away power and you are ingesting unfiltered air. Carb ice on takeoff is why you check heat in the run-up and then confirm cold/full-rich before you roll.

Fuel injection: different ice, different door

A fuel-injection system meters fuel to nozzles at each cylinder (or just ahead of the intake valve). There is no carburetor venturi, so you do not get classic carburetor ice. PHAK’s usual benefits: better distribution, crisper throttle response, more precise mixture, less evaporative icing, easier cold starts. Usual drawbacks: hot starts, vapor lock on hot ramps, and a harder restart after fuel starvation.

Injection is not immune to impact ice on the air filter or inlet. The published answer is alternate air — typically warmer, unfiltered air from inside the cowling, sometimes automatic, sometimes a cockpit control. Alternate air is not carb heat. Do not answer a fuel-injected induction-ice item with “pull carb heat.”

Scenario: Jordan’s zero-drop mag

Jordan’s AFM allows 175 RPM drop and 50 RPM difference at 1,800 RPM. Left drops 140, right drops zero. A passenger says that is the better magneto. Jordan does not launch. Zero drop is how a broken P-lead looks: the “off” magneto never stopped firing. After shutdown he treats the propeller as hot, writes it up, and lets maintenance find the P-lead. The battery was never the ignition system — and a dead battery in flight would not have stopped those magnetos either.