‘Clearing the engine’ means briefly advancing the throttle while in an idle power scenario, such as a simulated engine failure, or a power-off landing. Some instructors I’ve flown with have done this, and others have not, so I’d like to better understand the reasons for the practice.
I was unable to find any reference to this practice in the FAR’s, the AIM, or the PHAK. The current AFH refers to it during the discussion of a steep spiral (10-4):
Operating the engine at idle speed for any prolonged period during the glide may result in excessive engine cooling, spark plug fouling, or carburetor ice. To assist in avoiding these issues, the throttle should be periodically advanced and sustained for a few seconds. Monitoring cylinder head temperature gauges, if available, provides a pilot with additional information on engine cooling. When advancing the throttle, the pitch attitude should be adjusted to maintain a constant airspeed and, preferably, this should be done when headed into the wind.
Though this is included in the section on steep spirals, it should apply to any power-off scenario, including power-off approaches and simulated engine failures.
A follow-up question: how long is a ‘prolonged period’? As luck would have it, an online sample chapter of Barry Schiff’s Flying Wisdom, The Proficient Pilot, Volume 3 has the following:
During prolonged glides, be certain to “clear” the engine by applying power every 30 to 45 seconds. Many pilots believe that this prevents the spark plugs from fouling, but “clearing” usually does not do that because the power application is too short. The most important reason for applying occasional power is simply to confirm that the engine is still running. After all, an idling engine and a windmilling propeller are almost indistinguishable.
So every 30-45 seconds is what Barry recommends as an engine clearing interval. He continues:
Applying power periodically during power-off descents also helps to keep the engine warm and provides a modicum of carburetor heat. (Carburetor ice can form during prolonged idling even when carburetor heat is applied because there often is not enough heat being generated by an idling engine to prevent ice from forming.)
During prolonged idling, fuel can condense and form small puddles in the induction lines of some carbureted engines, especially those using autogas. Adding power periodically also clears out this fuel before it can accrue sufficiently to cause the engine to falter.
When “clearing” the engine during a glide, do not jab the throttle or apply a large amount of power. Instead, move it gently, especially when operating engines with counterweighted crankshafts.
This expands on the statement from the AFH, providing a bit more detail and some additional reasons for clearing the engine.
Another question: how much power is needed? Interestingly, older versions of the AFH have the following(from FAA-H-8083-3B):
The engine should be cleared periodically by briefly advancing the throttle to normal cruise power
It’s unclear why the procedure changed in FAA-H-8083-3C from briefly advancing the throttle to normal cruise power to the throttle should be periodically advanced and sustained for a few seconds. To be fair, advancing to normal cruise power seems pretty drastic. Perhaps the intent of the ‘normal cruise power’ procedure was to clear potentially fouled spark plugs, but was changed in recognition that it often takes much longer than a few seconds to do so. In other words, if you can’t actually clear the spark plugs this way, then you don’t need to advance the throttle all the way to cruise power.
To summarize, clearing the engine:
(4) is a point of disagreement between the AFH and the Barry Schiff book, and I’m not sure I understand (3), as the idea of a puddle in an induction line doesn’t make sense to me. Those two points may merit additional research.
As always, this page is perpetually a work in progress. Please feel free to contact me with comments or corrections. I also welcome pull requests with proposed changes.