both...left...both...right...both...Why?

kgruber

kgruber

Final Approach
Joined
Jan 3, 2007
Messages
5,094
Location
Western Washington
Display Name
Display name:
Skywag
I asked this same question 20 years ago on Avsig, and JD and RS didn't have an answer then.

Randy thought it was a hold over from 70 years ago and had something to do with a radial engine. John just scratched his head.

But, to this day, there is no logical reason to mag check in this manner. In fact, just: both...left...right...both is not only faster but gives better information, in that left and right are consecutive and the drop between them is more easily recognised. Also, less chance of leaving it on one mag.

Both...left...right...both!

Quicker, faster, and saves time.
 
Reason is that you need to provide the same "test" to both magnetos... namely starting from higher RPM and watching for the same "drop." If you simply switch from one to just the other one then you aren't actually applying the same test.

Subtle for sure but the checklist is written like that for a reason. Drop recover drop recover
 
Last edited:
How much time is actually saved?

Are we really in that much of a hurry doing the runup?
 
rocketflyer84 said:
Reason is that you need to provide the same "test" to both magnetos... namely starting from higher RPM and watching for the same "drop." If you simply switch from one to just the other one then you aren't actually applying the same test.

Subtle for sure but the checklist is written like that for a reason. Drop recover drop recover
Click to expand...

This is the way I see it.

both->left->right->both may give you quicker view of drop differential between both mags but, without returning to the the original position (both), you're not maintaining consistency between checks.
 
So....you guys seeing different results.....for each test sequence? :eek:

interesting......:rolleyes2:

When I do a mag check I'm looking for a couple of things.....

A. are each of the mags functioning independently?
B. is the drop for each mag about the same....?

I'm not seeing why it matters where we start from.....either higher rpm at "Both"or R or L. They are independent....it shouldn't matter.
 
Last edited:
Just came across something interesting...

The POH for the aircraft I'm getting current in (Cherokee 180) says....

"With the engine running at 2000 RPM, switch from both magnetos to only one and note the RPM loss; switch to the other magneto and again note the RPM loss."

It doesn't say to switch back to both. I suppose my primary training beat the other way into my head.
 
kgruber said:
But, to this day, there is no logical reason to mag check in this manner.
Click to expand...
Sure there is. When you check one side the spark plugs start to foul on the other. When you go to both again it cleans them up with a higher temperature.

But checking the closest switch position first saves your muffler if it's a dead mag. By planning to stop there you won't kill the engine and backfire it as you pass through. Instead, the engine will die giving you time to pull the throttle back to idle and letting the RPM come down before turning the key back to the operative mag. So, you are right about the flow: Both, left, both, right, both:
ignition_switch.jpg​

dtuuri
 
Last thing I'm trying to do on a runup is save time.

It's like people that just run through the checks like a race, might as well really save time and just skip the whole process. Strangly these folks are the same ones who will do a runup even after a 15min shutdown to pee.

When I select a mag I let it just run for a few seconds to listen to it, see the rpm drop, look at the CHT/EGT, FEEL the plane, the I go to the next mag, then both.
 
rocketflyer84 said:
Reason is that you need to provide the same "test" to both magnetos... namely starting from higher RPM and watching for the same "drop." If you simply switch from one to just the other one then you aren't actually applying the same test.

Subtle for sure but the checklist is written like that for a reason. Drop recover drop recover
Click to expand...


With a properly maintained system there is no "recovery." If you need to "recover" you need to taxi back and do proper maintenance.
 
dtuuri said:
Sure there is. When you check one side the spark plugs start to foul on the other. When you go to both again it cleans them up with a higher temperature.
Click to expand...


Nope. If your plugs are "fouling" during a mag test, there is some other reason. Lack of proper maintenance, improper leaning.

A normal good system will not foul the plugs in a mag check.
 
FlyingIsGood said:
Just came across something interesting...

The POH for the aircraft I'm getting current in (Cherokee 180) says....

"With the engine running at 2000 RPM, switch from both magnetos to only one and note the RPM loss; switch to the other magneto and again note the RPM loss."

It doesn't say to switch back to both. I suppose my primary training beat the other way into my head.
Click to expand...


GOOD! That is the way it should be done.
 
I think this post is all about the "points" right? nyak nyak ;P
 
rocketflyer84 said:
Reason is that you need to provide the same "test" to both magnetos... namely starting from higher RPM and watching for the same "drop." If you simply switch from one to just the other one then you aren't actually applying the same test.

Subtle for sure but the checklist is written like that for a reason. Drop recover drop recover
Click to expand...
This.

'Gimp
 
kgruber said:
With a properly maintained system there is no "recovery." If you need to "recover" you need to taxi back and do proper maintenance.
Click to expand...
Not correct, one mag will not produce the same RPM as 2 on any aircraft I have ever flown (carbed or injected), there is always a drop, and always a recovery to the reference RPM after going to BOTH.

You need to see both the drop and the recovery to ensure the system is fully functional. If I saw no drop at all I'd be concerned about not being able to shut off the mags (or at least one of them). In my Yak this would mean using the firewall shutoff to shut the engine down (pressure compensated carb, no mix control) since we cage the motor with the mag switch.

The mag check verifies that they are each working and also by default shows you can ground the side not being checked (OFF) individually, essentially it wrings out all elements of the ignition system.

'Gimp
 
kgruber said:
With a properly maintained system there is no "recovery." If you need to "recover" you need to taxi back and do proper maintenance.
Click to expand...

That's simply incorrect per certainly every piston airplane I've every flown.

A lack of RPM drop is also considered a 'fail' on the test. It may indicate that you have an electrical issue such a short that's not allowing you to 'ground' the magnetos and stop those sparks from firing in the cylinder. I've seen airplanes where this has occurred... you could switch the ignition to 'off' and the engine would still keep running normally--a potentially deadly safety issue if someone turns the prop after 'shutdown' and there's fuel in the cylinders.
 
Last edited:
AcroGimp said:
Not correct, one mag will not produce the same RPM as 2 on any aircraft I have ever flown (carbed or injected), there is always a drop
Click to expand...

Obviously. But it is completely unnecessary to to return to both to measure the difference in the drop.
 
kgruber said:
Wrong. Either way you test will reveal a lack of drop.
Click to expand...

You can check your mags however you want to, your method will some-what work, but is not optimal.

From an technical point of view, without returning to both, you are looking for a subtle and smooth RPM drop on the first mag with hot, clean, functioning plugs, and the second one immediately after its plugs have been not firing and cooling in an abnormal environment for some arbitrary period of time.

Just feel like stirring the pot today, or what?

The reason for returning to BOTH with a rotary switch (and an
equivalent pause with both magnetos ON for a toggle switch
system) is primarily to let the engine stabilise at normal rpm.
This gives each magneto a common starting point, making
comparison of the two rpm drops valid. Also, the short period
on BOTH lets the oil and fuel burn off the first set of spark
plugs that were shut down.
Click to expand...
 
ElPaso Pilot said:
You can check your mags however you want to, your method will some-what work, but is not optimal.

From an technical point of view, (I love this part!) without returning to both, you are looking for a subtle and smooth RPM drop on the first mag with hot, clean, functioning plugs, and the second one immediately after its plugs have been not firing and cooling in an abnormal environment for some arbitrary period of time. Huh? I read that three times??

Just feel like stirring the pot today, or what?
Click to expand...

Stir away, but there is no reason to return to both. And if there were, it would also indicate reason to return to base for maintenance.
 
kgruber said:
A normal good system will not foul the plugs in a mag check.
Click to expand...
Yeah, on a good day I'm sure that's right. However plugs are not operating optimally on one mag and in time will foul sooner or later. So, to make sure it doesn't happen Lycoming has you go back to BOTH:

For both fixed and rotor wing installations, do not operate on a single magneto for too long a period; a few seconds is usually sufficient to check drop-off and to minimize plug fouling.​

dtuuri
 
kgruber said:
Stir away, but there is no reason to return to both. And if there were, it would also indicate reason to return to base for maintenance.
Click to expand...

Whatever. :rolleyes:

You're mind's made up. Do it your way.
 
dtuuri said:
Yeah, on a good day I'm sure that's right. However plugs are not operating optimally on one mag and in time will foul sooner or later. So, to make sure it doesn't happen Lycoming has you go back to BOTH:

For both fixed and rotor wing installations, do not operate on a single magneto for too long a period; a few seconds is usually sufficient to check drop-off and to minimize plug fouling.​

dtuuri
Click to expand...

I'm sure that is just great for a rental, that runs around on full rich ALL the time. And, when there is a mag drop problem it is FULL POWER to "Burn" the deposits off.

I just LOVE that advice, and am so glad I don't have to fly a scrounged out POS rental.

Any remotely well maintained engine will give a good mag check, my way, or the rote method ......which has no legitimate reason. Even a (actually "the") NDPER agreed 20 years ago.

And, as for saving time? That is what an airplane is for. When working 135 we were ready at the end of the runway, and sometimes had to wait only for the gyros to erect.
 
Put me squarely in the both...left...both...right...both camp.

Going back to both between the two, and taking the time (just a few seconds) to confirm that the RPM completely recovered ensures that nothing fouled during the first check.
 
The actual reason, if you look at it carefully, is to keep you from flying on one magneto. It is awfully easy to go both-left-right-left without going back to both. Try it sometime and see how easy it is to mess up.

Jim
 
Ok, to contribute to this discussion some, let me share my secret with y'all.
After the mag check, I do the carb heat check. If the RPMs are not where they are supposed to be, something is amiss and I would investigate. Simple enough?
 
I used to see students rushing the runup. Both-right-left-both, and sometimes only back to right. Too much hurry. And sometimes they'd accidentally go to Off and then reflexively back on again and make that big bang that fractures expensive mufflers and sometimes even blows the intake elbow gasket out of the cylinder that happens to have open exhaust and intake valves (valve overlap at TDC) at the instant of the bang. They'd yank the carb heat on for one or two seconds and push it off.

The problem with all the rushing is more than busted mufflers and possible carbon monoxide poisoning. An engine that is running too rich will suffer an increasing RPM drop on one mag, if the pilot gives it time. Nice to know that so that if you lose a mag in flight you know you might need to lean the engine lest it load up and quit. The O-540-J3C5D in the 182RG will do that at high DA. Two seconds for carb heat checks that it's heating alright, but doesn't tell you if you've picked up ice during the warmup and taxi. Many times I've pulled the carb heat and left it on for a little longer and found the RPM rising after it initially fell, a dead giveaway that we're in carb icing conditions. Even in the middle of summer you can get that, even in a Lycoming, and even more so in a Continental.

An experienced operator will know his airplane and engine and the conditions that lead to carb ice (if he isn't running fuel injection), but most of us are a long way from that level of experience and expertise. We are smarter to be careful.

Dan
 
If your sequence is both-left-both-right-both then you might be doing it wrong, at least with respect to a 172.
 

Attachments

  • Capture.PNG
    Capture.PNG
    42.6 KB · Views: 39
This plug clearing explanation makes no sense. When you move from both to the right mag, you are grounding the left mag off. If you then go straight to left, you are turning the left mag back on and grounding the right mag off. The time spent running on one mag is no different than if you went back to both after checking the right mag. That's what can cause fouling - time spent running on one plug. Someone please explain how going straight from right to left increases the time spent running on a single plug.
 
weirdjim said:
The actual reason, if you look at it carefully, is to keep you from flying on one magneto. It is awfully easy to go both-left-right-left without going back to both. Try it sometime and see how easy it is to mess up.
Click to expand...
And this is why I go to the farthest from "BOTH" mag first.
 
asicer said:
If your sequence is both-left-both-right-both then you might be doing it wrong, at least with respect to a 172.
Click to expand...

Yes, those 172's are very finicky about which mag you check first. :lol:
 
At the end of the day just follow the POH. Are you going to destroy the engine by going both-left-right-both? No... but the POH procedures were written that way for a reason. Deciding to make up ones own procedures contrary to the POH is generally not the smartest idea.

Lycoming maintenance manuals describe the procedure as:

Switch from both magnetos to one and note the drop-off; return to BOTH until the engine regains speed and switch to the other magneto and note the drop-off, then return to BOTH. Drop-off must not exceed 175 RPM and must not exceed 50 RPM between magnetos. Smooth operation of the engine but with a drop-off that exceeds the normal specification of 175 RPM is usually a sign of propeller load condition at a rich mixture. Proceed to step E.(1)(d).

For those trying to argue that this both-left-right-both move is some sort of time saving measure then I'd sure hate to see what the rest of your pre-flight looks like! v:eek:
 
I usually do my mag checks and prop cycling on the roll. I go from both - L - R - both because I can do it without looking at the switch. It's never caused me any problems and I don't figure I'll change my procedures based on anything I've read here. It's funny what grown people will draw lines and argue about.
 
You must log in or register to reply here.
Back
Top