Ground Leaning With Warmer Ambient Temps

A few of you have been mentioning engine stalling in ME and RF - and I'm sure other planes soon to follow. I have checked both and don't find anything really far out of whack. There are a couple things to remember: as temps warm up, density altitude increases (I know you know that). What may not be as obvious is that with warmer temps the idle mixture will also need to be adjusted leaner. That is something the mechanic will do during service to improve low RPM ops. In the interim it's possible you'll be more likely to get a rough idle or even an engine stall if you're not careful. Remember to lean well for all ground ops but especially when the temps go up. 

The POH method for mixture adjustment on the ground is to set the engine to 1200 RPM, and lean it out until the engine starts to stumble, then just slightly richen it up. The only 'too lean' on the ground is if you kill the engine. Don't bother messing with EGT on the ground. It means nothing at this power setting. You'll usually see the RPM increase when you're getting to that sweet spot of mixture adjustment on the ground. Much further lean and you'll kill the engine. 

When you land, make sure the student isn't trying to pull the throttle out of the firewall. I have seen that all too often. If you have an engine that is being picky with warmer temps and especially if they keep 'back pressure' on the throttle, they'll often pull the RPM even slower than the idle set just enough to cause an issue. All throttle movements should be gradual without yanking or smashing one way or the other. If the engine starts to sound rough as you slow down on the runway, nudge the throttle in just a tad (1000 RPM is fine). Obviously, you are balancing slowing down with potentially killing the engine but with proper management the latter shouldn't be an issue. 

Teach this scenario and help the students to be more aware of the dynamic nature of what's going on under the cowl. While your car has all the fancy engine controls to adjust air-fuel ratios, there is no such tech in most piston singles. They are as simple as a lawn mower. You, the pilot, are the tech. Use temp changes as a teachable moment to help the students understand that there is no single best setting for mixture that fits all scenarios. Don't stress on a perfect setting but do stress that the setting is dynamic. If you notice the engine starting to stumble a bit on the ground, give it some throttle. If you notice the mixture is excessively rich, lean it. Let us know of any anomalies and we'll always check to make sure all is well. 

Click on the video below to see an example of yanking the throttle too aggressively. 

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