Preflight and runup... every time?

[MuseChaser]

By “mag check” at shutdown, I presume you folks mean turning them both off (switch it off) and verifying the engine stops. Because I don’t think checking each one separately proves they are both off when the switch is off.

Kind of. Don't switch to "off" long enough to actually stop the engine... just long enough to verify by ear that it WOULD stop the engine. It's a quick, momentary contact with the off position, then back to "on." If you leave it in "off" until the engine almost dies then turn it back to on, you could cause a backfire.

Edit: @Bill Jennings beat me to it. To answer Salty's next post, a runup mag check checks for operational mags. A pre-shutdown check checks for correctly grounded P-leads.

 

[MooneyDriver78]

By “mag check” at shutdown, I presume you folks mean turning them both off (switch it off) and verifying the engine stops. Because I don’t think checking each one separately proves they are both off when the switch is off.

So preflight makes sure they’re grounded and can be turned off individually, the only thing the post flight checks if the switch turns both off when in the off position. That’s gotta be pretty rare failure, I personally always rotate the prop backwards to avoid the issue.

 

[Craig]

Anyone care to list a reason (other than time) for not doing a complete pre-flight before every flight regardless of how long between shutdown and the next flight?

Do you do a complete preflight before you leave the hangar and then do another after getting to the fuel pump? Got to use some common sense and intelligently cautious attitude. Stuff can and does break at various times, but an informed pilot knows what need to be checked based on overall history of the aircraft and what the prior operations were since the last preflight/thruflight or quickturn.

 

[smv]

So preflight makes sure they’re grounded and can be turned off individually, the only thing the post flight checks if the switch turns both off when in the off position. That’s gotta be pretty rare failure, I personally always rotate the prop backwards to avoid the issue.

Preflight mag check is NOT intended to test the ability to turn off the mags. It is to ensure they are both functional and each pulling their own weight.

 

[MooneyDriver78]

Preflight mag check is NOT intended to test the ability to turn off the mags. It is to ensure they are both functional and each pulling their own weight.

But you are testing the ability to turn them off (RPM drop) in addition to them being functional and all plugs are operating well.

 

[smv]

But you are testing the ability to turn them off (RPM drop) in addition to them being functional and all plugs are operating well.

The purpose of the preflight check is not to see if you can turn them off, it is a necessary step in the process. When you ground a mag during runup, you are testing the operation of the ungrounded mag. You should get a predictable drop in RPM. Too much? You got a problem with one of the ignition paths on the still-operating mag. No drop? Perhaps you did not really ground the mag not under test, but another possibility that you likely would not have caught up to this point is that you were not actually firing from the grounded mag from the beginning and were operating on just the one mag all along.

 

[Dan Thomas]

Preflight mag check is NOT intended to test the ability to turn off the mags. It is to ensure they are both functional and each pulling their own weight.

Almost but not quite. Mags that get worn enough can go rogue and start sending sparks to the plugs at the wrong times. If those plastic distributor gears wear enough they'll slip and the distributor gets all mixed up. Carbon tracking in the distributor cap does it, too. When a plug fires on an intake stroke, the incoming mixture ignites and the flame shoots out the still-open intake valve and ignites the whole works in the manifold, and the engine gets really rough and the power goes to nearly nothing. If you can't shut that rogue mag off, you're going to land somewhere really inconvenient.

P-leads and properly-operating mag switches are important. It's not just the mags and plugs we're testing. I really wish groundschool was more comprehensive.

 

[PaulS]

Almost but not quite. Mags that get worn enough can go rogue and start sending sparks to the plugs at the wrong times. If those plastic distributor gears wear enough they'll slip and the distributor gets all mixed up. Carbon tracking in the distributor cap does it, too. When a plug fires on an intake stroke, the incoming mixture ignites and the flame shoots out the still-open intake valve and ignites the whole works in the manifold, and the engine gets really rough and the power goes to nearly nothing. If you can't shut that rogue mag off, you're going to land somewhere really inconvenient.

P-leads and properly-operating mag switches are important. It's not just the mags and plugs we're testing. I really wish groundschool was more comprehensive.

Yeah, and I wish airplane engines were more modern...

 

[chemgeek]

Do you do a complete preflight before you leave the hangar and then do another after getting to the fuel pump? Got to use some common sense and intelligently cautious attitude. Stuff can and does break at various times, but an informed pilot knows what need to be checked based on overall history of the aircraft and what the prior operations were since the last preflight/thruflight or quickturn.

Of course not. But one should re-check the fuel for water after filling the tanks. It would be easy to "assume" after flying a successful flight leg that another walk-around and engine inspection is not necessary, but I don't think that is a good idea. If you don't look, you won't know if you have a blown exhaust gasket, developing oil leak, a loose fairing or baffle, or some other problem without taking a look. Of course, 98% of the time, the walkaround is unremarkable. It's the 2% of the time that you go "what the h*** is that?" that it is so worthwhile. If you are an owner and are familiar with your plane, it's amazing how quickly even the smallest odd thing stands out during a quick walkaround.

 

[ateamer]

What do commercial piston operators do?

 

[asicer]

It wasn't until I discovered I had damaged the oil cooler that I found the source of the leak. Replacement cooler took care of that. My point is that if I would have taken my friends advice I may have suffered a complete failure of the cooler in flight.

I've known a leaking oil cooler to seal itself via sludge

 

[Daleandee]

I've known a leaking oil cooler to seal itself via sludge

Yeah that works on the old cars I used to drive. Don't like doing it that way on the plane ...

 

[Dan Thomas]

Yeah, and I wish airplane engines were more modern...

We all do, but few of us want to pay the huge price for stuff that gets past certification. In the meantime, we need to know how to operate the existing stuff.