He’s talking about switching momentarily to off when idling , before shut-down, not the pre-flight R/L mag check.
Michael wrote:
Unfortunately, Lycomings (in particular) are notorious for dumping fuel in the cylinders during cool down, hence why starting when still hot can be problematic.
On a few occasions, after shutting down an IO360-M1A (DA40) the HI FUEL PRESS caution displayed on the G1000. Opening the mixture released the pressure, but I suppose that dumped fuel into the cylinders?
Michael wrote:
Unfortunately, Lycomings (in particular) are notorious for dumping fuel in the cylinders during cool down, hence why starting when still hot can be problematic.Where does that fuel come from if you stop the engine with the mixture?
Patrick wrote:
Does that not depend on the aircraft and how the mag switch is wired?
If there are separate ground wires for the various switch positions, then you wouldn’t notice, but is that the case?
One of the nice features of the Super Cub type with an Armstrong starter, is that you can hand prop from behind with access to the mags. You start with the left mag only, and to be more cautious, after priming, you can switch fuel to off, turning it back on after start.
Getting into motor memory to avoid placing yourself in the propeller arc is a worthwhile habit, just as ensuring a good lookout. Unfortunately a lot of instructors are keen to show students how easy it is to ground handle 152s by pulling them around on the prop.
Michael wrote:
He’s talking about switching momentarily to off when idling , before shut-down, not the pre-flight R/L mag check.
Actually you can recognize it both ways.
When you check for magneto drop, and you don’t have any drop going from both to either left or right, then it is still running both mode, so one of the magneto’s doesn’t ground. A large drop isn’t good, no drop isn’t good either.
I once had this situation, where I did 500 hours inspection on magneto’s, reinstalled them, repaired one P lead (magneto shorting wire). Then the pilot with the aircraft commented that they are now worse then before, before I had only 50 drop on left, 0 drop on right., now I have drop on both magneto’s.
Which was because he was running that magneto “hot” before. During the magneto check he tought everything was fine, while he didn’t detect that the P lead was open, nor that he was unable to check the performance of the other magneto (as in one position it was running in both).
More obviouse and easier to detect is switching the magnetos off for a fraction of a second just, you will easily recognize if the engine wants to switch off or stays running (switch it back on before it dies).
Michael wrote:
He’s talking about switching momentarily to off when idling , before shut-down, not the pre-flight R/L mag check.
Airborne_Again specifically mentioned the mag check before take-off:
That should have been noticed at the mag check done before every takeoff!!
The distinction is important because while the pre-flight mag check during run-up is common practice, the idle-check before shut-down is not. I find it’s typically done e.g. in the US, while in Germany (schools, clubs, …) it is uncommon from my experience.
Jesse wrote:
Actually you can recognize it both ways.
When you check for magneto drop, and you don’t have any drop going from both to either left or right, then it is still running both mode, so one of the magneto’s doesn’t ground. A large drop isn’t good, no drop isn’t good either.
The question that is still open is: If you confirm there is a drop at L and a drop at R and you’re happy – does that automatically imply that the magnetos will also ground once the key is set to the OFF position? This depends, as Airborne_Again pointed out…
Airborne_Again wrote:
If there are separate ground wires for the various switch positions, then you wouldn’t notice, but is that the case?
… on the wiring. As far as I was told (when asking people why the idle-check before shut down is neccessary), the OFF switch is wired independently and hence, confirming a drop on L and R still leaves a chance of a broken wire from the OFF switch (and hence, a potentially live prop). What I don’t know is if the independent wiring is an OWT or not. Can anybody comment?
Patrick wrote:
I find it’s typically done e.g. in the US, while in Germany (schools, clubs, …) it is uncommon from my experience.
That’s interesting. It should be in the shutdown checklist. I just checked the DA40-180 “PARKING CHECK” and found “Ignition… Grounding check”
In Norway where I got my PPL there was emphasis on this item.
The full dead-cut check needs to be done below a certain RPM, otherwise you might blow up the exhaust. In my POH it is 1000rpm.
The normal mag check can be done at the normal ground idle of 1200rpm.
