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PostPosted: Mon Aug 21, 2017 3:27 pm 
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Was wondering everyone's thoughts regarding a Cessna 172 -

Startup:

Master On - Engine On - Avionics On

Shutdown:

Avionics Off - Mixture Lean - Master Off

===================

When doing the live mag check before shut down, I have seen two variations:

1) Left, Back to Both, Right, Back to Both

2) Left, back to Both, Right, Back to Both, OFF , back to Both

- In variation 2) is that not essentially turning the engine off and back on with the avionics still on ?

There does not seem to be a clear consensus on this issue, just thought I would see everyone's point of view.

What would happen if you left the avionics on, turned off the engine and the master ?



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PostPosted: Mon Aug 21, 2017 3:42 pm 
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The reasons for not having the avionics on when you start the engine include 1) not having any unnecessary drain while the starter is engaged as it reduces the power available to start the engine and more importantly 2) reducing the exposure of the avionics to higher voltages that are generated by the alternator when it first starts spinning, it can spike until it starts to smooth out and regulate.

Switching the avionics off before you shut down the engine is less important because there is less chance of a high voltage surge as a result.

Note that switching the mags both off temporarily does not stop the engine from spinning and hence does not stop the alternator from charging so while you may be stopping the plugs from firing the engine will continue to happily spin for several seconds and of course will as a result happily continue to generate electricity if the alternator is 'on'.

So in answer to your two questions.
1 - no you are not turning off the ability of the engine to generate electricity by shutting off both mags briefly.
2 - very little would happen if you left the avionics on and shut the engine down except in the case where the battery had a bad cell or two and was producing less than 12v. In that case you could I suppose damage some poorly designed avionics by exposing them to less than 12v but it seems pretty unlikely to me.



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PostPosted: Mon Aug 21, 2017 4:07 pm 
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Usually the avionics are turned off before the live mag check. (Just had a look at one flight school's checklist, and that's how it's done).


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PostPosted: Mon Aug 21, 2017 4:29 pm 
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That seems to be the concensus in many checklists, I've seen it done with Avionics Off before Master Off & Master Off then Avionics Off, I'm just looking to understand if

Engine Off ---- >> Master Off ----- >> Avionics Off is in fact harmful :

Master Off results in no power any electrical component in the aircraft -

With the Master On, Avionics master off essentially removes their power requirements from the bus, isn't turning off the master and removing all power from bus essentially the same as [ Avionics off ---->> Master Off ] - why does avionics power need to be separately cut before everything else?

Am I missing something ?



CpnCrunch wrote:
Usually the avionics are turned off before the live mag check. (Just had a look at one flight school's checklist, and that's how it's done).



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PostPosted: Mon Aug 21, 2017 4:35 pm 
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The master switch isn't connected to the engine, so I don't see that it matters on or off for engine stop or live mag check. I leave mine on until after the engine stops because most of my engine gauges are electric and I prefer to have the beacon on with the engine running. Neither are actually required for the last few seconds before shutting down though. As for the avionics master, I don't have one so that means off after shutdown. All avionics built in the last 5 or more decades meet voltage spike requirements way beyond what a starter or alternator engaging are capable of delivering. You simply cannot damage your avionics by leaving them on during engine start. If you have a massive radio stack and multiple large EFISs then you might get a slightly easier start by turning it off but better would be to do the calculation and convince yourself whether or not your instruments are even 1% of your battery's CCA.

In any case NeckStrain, it seems that you are confusing a pre shutdown live mag check with a runup mag check. The live mag check can be done with the master switches in any position. The purpose of a pre shutdown live mag check is to make sure that with the switch in the off position, both mags are grounded (not live). So the only position you are interested in checking is OFF. No need to check L and R.

Spend a little more time with the FTGU engine and electrical system sections before you go flying again.



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PostPosted: Mon Aug 21, 2017 4:39 pm 
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I understand the fear and myth of possibly low voltage to the avionics during engine start when it draws a lot of power - I just cannot comprehend how there is a risk of that happening during shut down requiring avionics off before everything else.

Thank you for the detailed answer!


ahramin wrote:
The master switch isn't connected to the engine, so I don't see that it matters on or off for engine stop or live mag check. I leave mine on until after the engine stops because most of my engine gauges are electric and I prefer to have the beacon on with the engine running. Neither are actually required for the last few seconds before shutting down though. As for the avionics master, I don't have one so that means off after shutdown. All avionics built in the last 5 or more decades meet voltage spike requirements way beyond what a starter or alternator engaging are capable of delivering. You simply cannot damage your avionics by leaving them on during engine start. If you have a massive radio stack and multiple large EFISs then you might get a slightly easier start by turning it off but better would be to do the calculation and convince yourself whether or not your instruments are even 1% of your battery's CCA.



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PostPosted: Sat Sep 02, 2017 1:14 pm 
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This seems misunderstood.

The reason you shut the avionics off first is to prevent the system from over voltage.

When the field that is keeping the master solenoid closed collapses (master turned off )it can induce huge voltage into the buss system. Shutting the avionics master off before shutting the master off protects th expensive bits from this surge of back emf. Modern aircraft have a diode on the master to prevent this. In all the years I've been in aviation I've never seen radios cooked because the avionics were left on when the master was shut down.

Most electronics in an AC (that you would find at an ftu) wont blow because of a low voltage situation. They will do funny things and then turn off but when full voltage is applied they will come back to life.

How about the procedure stated above, l mag - both - r mag - both - off - both.

What you are describing is two separate checks. The first four actions are a mag drop test, the last two are a live mag test.

I come from a maintenance background and we mostly do mag drop tests at a mid power setting and live mag tests at idle.



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PostPosted: Sat Sep 02, 2017 1:42 pm 
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Quote:
I come from a maintenance background and we mostly do mag drop tests at a mid power setting and live mag tests at idle.


Me too and it is amazing how many instructors do not understand such basic proceedures.

I had a couple of instructors who would do the live mag check at the same RPM as the mag check and they would leave the mags off long enough that the engine would back fire with all the fuel that was pumped into it while they had the mags off.

In fact they ruined an exhaust system on one of my 150's.


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PostPosted: Sat Sep 02, 2017 2:24 pm 
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Quote:
When the field that is keeping the master solenoid closed collapses (master turned off )it can induce huge voltage into the buss system.
That's what the suppression diode (9) prevents. Nobody uses a contactor without a diode to remove the back emf.

Without the diode the back emf will cause arcing across the battery master switch, killing it in short order.

Attachment:
IMG_2148.PNG
IMG_2148.PNG [ 130.67 KiB | Viewed 784 times ]


Lots of aircraft don't have avionics masters, and even amongst those that do there are plenty of electronic systems (engine monitors, electronic tachs) that are not individually switched and survive being connected to the bus while the battery master is energized and deenergized.


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PostPosted: Sat Sep 02, 2017 3:55 pm 
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Cat Driver wrote:
Quote:
I come from a maintenance background and we mostly do mag drop tests at a mid power setting and live mag tests at idle.


Me too and it is amazing how many instructors do not understand such basic proceedures.

I had a couple of instructors who would do the live mag check at the same RPM as the mag check and they would leave the mags off long enough that the engine would back fire with all the fuel that was pumped into it while they had the mags off.

In fact they ruined an exhaust system on one of my 150's.


Good point.

I've had an engine quit on a mag drop test. That's a tough one because you have to be with it enough to counter the instinct to turn the mags back on to both!

If your engine loses ignition during a mag drop test let it die. Resist the urge to turn the mags back on.

The idea behind this is that while your fuel is not being ignited, the exhaust is filling up with an air fuel mixture. If you introduce ignition to this situation you will get a bang and it will blow a hole right into your wallet.



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