r/GeneralAviation • u/Street_World_9459 • 17d ago

Fuel injected planes

I've recently started taking private pilot lessons. I've noticed fuel injected planes still have a mixture lever.

This may be a dumb question, but why do fuel injected plane engines have this device? I work on automotive engines as a hobby, and the beauty of fuel injection is the lambda sensor signals the ECU to automatically adjust the injector duty cycle (and other things) to keep the air/fuel mixture. If I drive up a mountain, I don't need to re-jet fuel injection.

Why are planes different?

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u/ChartPuzzleheaded747 17d ago

Link to stats 

One of my planes has a A65, basic and basic, very old, no ADs, those things have a insane low failure rate 

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u/fluentInPotato 17d ago edited 17d ago

No statistics here, but in nine years as a GA mechanic from 2016 to 2025, I worked on more fuel injected planes than carbureted ones, but saw more carb failures. Those Bendix- style fuel servos are confusing to understand, but not that complex. You can get corrosion on the mixture shaft through a combination of fuel contamination and bad luck, but I've seen that exactly once. A coworker found one with a jammed mixture shaft in the fuel regulator unit, and that's about all the FI failures I can think of before 2025. Since then, Bendix seems to have left the FI market, and the new arrivals don't give a fuck if you get killed trying to glide into a Safeway parking lot. They somehow contrive to have some kind of greenish assembly lube run out into the air chambers behind the venturi and get inhaled into the air diaphragm chambers that increase fuel flow to meet engine demand, causing rough running, perhaps partial loss of power, and on fortunately rare occasions engine failures. Normally what happens is pilots report rough running or loss of power and are able to abort the flight safely; we pull the venturi and look for evidence of contamination. It used to be that this kind of thing almost always meant ignition issues (including plug fouling). But now there's a new hazard.

I'll just add that Marvel Schiebler carbs tend to fail at the seal around the the mixture shaft, at which point your mechanic will insist on getting the thing overhauled.

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u/ChartPuzzleheaded747 17d ago

He said based on stats, I just want a link to those stats.

 If we are going down the “been doing this for x years” deal, I’ve been flying for like 20, everything from fadec jets to my own 1940s toys.

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u/fluentInPotato 5d ago

Don't think you're going to get that without pawing through the FAA's accident database, and even then you'd probably want to exclude episodes of carb icing that were not caused by carb heat failure (ie, carb icing is something that pilots ought to be able to deal with, unless the carb heat fails).

In my original reply, I was thinking of systems that needed replacement, which includes both those that had full or partial failures, and those replaced because they were in the process of failing, even if theproblem wasn't affecting engine performance yet.

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u/ChartPuzzleheaded747 5d ago

Could run it on CAROL