WHY DO WE SHUT DOWN WITH MIXTURE??

[gasman]

Why not shut down with the mags grounding so you know if you have a hot mag or elec ign that is not OFF! Mixture location means nothing if you have no fire. Many times an engine will fire a cyl or two when the prop is moved and the key in your pocket and the mixture at full cutoff, but you had a hot mag and didn't know it.

So my question is..... what bad thing will happen if we shut down by turning off the ignition and watch the engine die and then pulling the mixture control to full lean.

[Geico266]

Fuel / air mixture washes the oil off the inside of the cylinders.

[AlexPeterson]

I don't know specifically about our aircraft engines in this regard, but a lot of air cooled engines will let you know the exhaust is still hot by making a loud "bang" in the exhaust if you are putting unburned fuel into it after shutting off the spark.

[gasman]

We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?

[Norman CYYJ]

The engines in the Champs and Cubs never had mixture controls in them to worry about. Shut off the mags and the engine shut down.

[RV8N]

Cars don't have magnetos that are alive if not grounded and you can't carelessly turn the prop either.

While you could shut off the engine with the mag switch, it is feasible that the p-lead could break at some point afterward (during maintenance or an oil change). Then the mag would be hot AND have a fuel charge... a very bad combination. The only way to guarantee you won't have both a spark and fuel is to eliminate the fuel until the engine starves.

I think the broken p-lead was a much bigger issue back in the '30s. It seems I've seen uninsulated p-leads on some antiques... must have been easier to break.

Karl

P.S. I used to rent a Cub and one day I was the first to fly it after it came out of 100 hr inspection. I taxied to the end of the runway and did a mag check. To my surprise, one of the mags was totally dead. I taxiied back and told the owner about it. He couldn't believe it because he had flown it 30 minutes before and it was fine. We ran the engine and the mag was dead (internal ground?). While this is the opposite of a broken p-lead, I will alway shut down with mixture and only turn the prop with extreme caution.

[Kevin Horton]

Originally Posted by gasman

We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?

Why would it be safer to leave a combustible mixture in the cylinders? If someone moves the prop, and a mag is live, a spark could cause a cylinder to fire, and the prop could do significant injury. Cars don't have props, nor mags, so they don't have this problem.

[John Clark]

Originally Posted by gasman

We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?

Modern automotive fuel injection does, in fact shut off the fuel as soon as the switch is turned off. The fuel continues to flow into the cylinders when the mags are turned off in most aircraft engines. But not very much. So my answer to the question is "because we can." Why take the risk on washing the oil into the sump? On the hot mag issue, you should shut down with the mags (I do it about once a week) to check that everything is working properly. I usually do it when the engine is going to be started again, like after fueling with a taxi to the hangar.

John Clark
RV8 N18U "Sunshine"
KSBA

[n8zg]

I was taught to always do a ground check before shut-down.

Mixture Rich
Throttle to Idle
Mags OFF momentarily and right back on BOTH
Engine should stop firing, then resume normal Idle
Mixture ICO
Mags OFF


It demonstrates that immediately before shut-down, the mags will ground as expected, but leaves no fuel in the intake that could fire (or diesel) later.

[Stephen Lindberg]

The reason Cubs, Champs, and the like are shut down with the mag switch is that their Stromberg carbs have no mixture control. The few carbs that have a mixture control (Cessna 140) do not have an idle cutoff position, in other words, the mixture can be leaned for altitude but not leaned enough to kill the engine on the ground.

The proper technique is (1) switch the mags off and (2) simultaneously open the thottle wide. The wide open throttle eliminates the suction in the carburetor throat so that fuel is not sucked through the idle passage. The engine induction system and cylinders therefore fill with plain air and not air/fuel as the engine spins down.

You can't do this with a larger engine because they have a carburetor equipped with an accelerator pump. The accelerator pump shoots fuel into the carburetor throat when the throttle is opened, thereby negating the reason for the whole process, which is to fill the cylinders with plain, non-combustible air.

You don't need a hot mag to fire a hot engine. Simply moving the prop can be enough to set off a combustible charge in a cylinder. I have seen this happen twice. Each time a pilot was moving the propeller on a hot engine. The mags were not "hot". Each pilot (both experienced) got away uninjured but shaken. They each had suddenly found themselves standing right next to a spinning propeller. One guy flattened his back against a hangar door while the engine coughed and farted right in front of him!

Some engines, when hot, will start firing while being cranked even though the mags are off and the mixture is still in idle cut-off. This was very common on a DC-3 I used to fly. Broken p-lead not required. There was enough residual heat in the engine and enough residual fuel in the induction system to start it running just by cranking, although usually not enough fuel to allow the engine to accelerate. But plenty of energy to kill you if you moved that propeller on the ground and got in the way. I have heard that hot engines can "cook off" on the ground, in other words, the prop can move all by itself after the engine has been shut down. That I have never seen, but I believe it could happen.