Fuel, Air and Ignition

You’re four thousand feet above the ground and having the time of your life… when — without warning — the smooth purr of your engine chokes into a coughing fit that becomes sudden silence. The engine’s quit! Can you fix it… and how?

Fuel, Ignition, Air
Even the mechanically-challenged should remember that an airplane engine (or any engine, for that matter) needs three things before it will run: air, fuel, and a source of ignition. Combine proper amounts of fuel and air, ignite the mixture, and power results. It’s logical, then, that if you have an engine failure, you’ll try to restart it by adjusting the fuel, air and ignition controls. There’s not a whole heck of a lot else you can do from inside the plane, anyway.

Insider’s tip: Most lightplane engine failures result from fuel-related problems — fuel starvation or fuel exhaustion. For the record, starvation describes the scenario where fuel is available somewhere on board, but for some reason is not getting to the engine. Fuel exhaustion describes a scenario that is a bit more simplistic, and is best described by the phrase it tends to provoke: “I ran out of gas?”

The number of restart checklist steps, and the order in which you’ll do them, differs from one type of airplane to the next — and may even vary by options installed and model year. Read the Pilot’s Operating Handbook (POH) carefully for the particulars of your aircraft’s restart procedure… then commit it to memory.

NOTE: This same procedure will apply for engine roughness or partial power loss, as well as a total power failure.

Assuming you’ve continued to fly the airplane, have aimed for a place to land, and you’ve made your call for help on the radio, you’ll need to decide if you have enough altitude (read: time) to try to restart the engine. If the answer is “yes,” think Air, Fuel, Ignition.

AIRYour job here is to eliminate or bypass obstructions in the engine’s induction system.

  1. Apply carburetor heat. Application of full carb-heat may melt out the ice and restore airflow — even a short while after the engine has stopped producing power. Remember that if the engine is running — but running rough — the engine will momentarily run rougher when you pull the carb heat knob but will smooth out and improve performance if ice was indeed the problem, and if the heat is enough to melt it out. Important: If carburetor heat restores engine power, leave it on until you land. The conditions that cause the ice to form probably still exist.
  2. Use alternate air. Most fuel injected engines have an alternate air inlet, bypassing the normal filter system. In some airplanes it opens automatically if the air inlet gets plugged. In others it needs to be manually actuated. In still others it’s automatic with a manual backup. Important: If the activation restores power — leave it on (per the POH) — until you’re on the ground.

FUELMake sure it’s getting to the engine.

  1. Switch tanks. If your airplane has independently selectable fuel tanks, switch to another tank. This works if you’ve run a tank dry, if the tank’s vents or lines have become blocked, or if the fuel in the tank in use is contaminated.
  2. Use auxiliary pumps. If your airplane is equipped with a back-up fuel pump (common in low-wing designs and airplanes with fuel injected engines) … turn it on. Many older airplanes have a manual “wobble pump,” which can be used to push fuel into the engine. Be sure to follow all POH recommendations. Important: If the “aux pump” does not fix the problem, turn it off. Some other restart step may fix the engine, but the added flow from the aux pump will cause it to flood and if the problem is a loose line in the engine compartment you may simply be pumping fuel onto a hot engine.
  3. Adjust the mixture. Go to “full rich,” if not already there. If that doesn’t work, gradually lean the mixture to see if you can find a setting that allows the engine to fire. If you get near the idle cutoff position and the engine is still inoperative, return the mixture control to the approximate position it was in before the failure. That position will likely encourage engine operation if the problem is elsewhere.

IGNITIONIf the engine is still inoperative, you’ll need to try this last step

  1. Check that the magneto switch is in the “both” position. If not, put it there. If the mags are in the “both” position, then select a single magneto and see if that works. If not, try the other magneto.

Put It In Order
Remember, there’s a restart procedure in the airplane’s POH, and that should be the basis for your actions. If the order of troubleshooting steps listed in your POH is different from that listed above, follow them exactly as the POH lays them out. The technique above is intended to help you memorize the POH procedure:

EXAMPLE 1 (Normal Aspiration): With many carbureted engines like in most Cessnas, Pipers, etc., the restart steps are:

  1. Air. Apply full carburetor heat.
  2. Fuel. Verify fuel selector on BOTH.
  3. Fuel. Switch to the LEFT tank; if that doesn’t work, select the RIGHT tank.
  4. Fuel. Go to FULL RICH; LEAN as required.
  5. Ignition. Check magnetos on BOTH. Try individual mag positions as required.

EXAMPLE 2 (Fuel Injected Engines): With fuel injected engines in airplanes like those in late-model Beech Bonanzas and Mooneys:

  1. Fuel. Change fuel tanks.
  2. Fuel. Turn auxiliary pump on HI (LOW for rough-running engine). If power does not return turn the auxiliary pump OFF.
  3. Fuel. Go to FULL RICH; LEAN as required.
  4. Ignition. Check magnetos on BOTH. Try individual mag positions as required.
  5. Air. Activate alternate induction air (this is a manual back-up to an automatic alternate air system).

BOTTOM LINE: If you’ve got an engine problem that can be fixed from the cockpit, these are the only things you can do to make it work again. If you get through all those steps and the engine is still dead, well, hopefully you’ve complied with Jeff’s article this week and studied Paul’s article ‘‘When Your Airplane Becomes A Glider‘… we do what we can.

Next time, we’ll look at some engine-related “near misses” I’ve personally experienced in 3000+ hours of lightplane flying.