Shutdown Procedure

[Avio]

For the cub it's recommended that the engine shutdown begins with turning the magnetos off first then the fuel cut-off. But for most other props, it's always the other way round, that is, mixture cut-off then mags off. Isn't this contradictory? For other props shouldn't we follow with mags off first then mixture cut-off ?

Avio

[Bogusheadbox]

Someone who has the manual for a CUB may be able to confirm to better detail.

However, as far as i am aware on the props i have flown (carb and fuel injected), putting the mixture to ICO (idle/cut off) or fuel off before mags off is purely a safety concern (as is a dead cut before shut down).

If you have a faulty mag then that ignition system may remain live even after the key is turned off and removed. Makes sense that if you starve the engine of fuel, have a defective and live magneto and some poor soul leans on the propeller, they won't loose and arm.

A small problem with this is that when starving the engine of fuel, you will lean the mixture to a point (for a very short time) where detonation may be possible and a tiny percentage of wear may be increased. However this is not considered relevant in consideration to safety.

Again with the dead cut, another safety feature I am required to do whilst engine is runnings and just prior to shut down, is to turn both mags off and on again at idle for a very brief moment - just long enough to hear the engine want to stop. This is purely to ensure that at that point of time when shutting down the engine, no fault in the mags have been detected and they will not (should not) remain live after engine is shut down. Again, this can cause additional wear and tear on an engine for that brief moment.

So as far as i know, fuel off before mags off is purely a safety thing...... unless there is some other reason mentioned in the aircraft flight manual.

[skunker]

From the actual A65 manual:

7. Stopping

a. Under conditions of normal usage the engine should have cooled sufficiently during the glide and taxiing period to permit turning ignition switch "OFF" without additional idling. However, if there is an excessive amount of taxiing, the engine should be allowed to run two or three minutes about 800-1000 RPM before stopping.

[Tako_Kichi]

P.75 of the A2A Cub manual (P.81 of the PDF document) covers engine shutdown in detail and explains why it is recommended to kill the engine using the magnetos rather than by cutting off the fuel in the case of the Cub.