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By T67M
#1737852
I was recently undergoing a renewal flight, and the instructor had some very strong opinions on the way to carry out a dead-cut check which didn't quite align with my previous instruction, and I'd appreciate other considered views. My previous understanding was as follows:

  1. Before taxi: "Drop-no-stop" checks in L and R positions at fast idle (1000-1200 RPM)
  2. Before flight: "Drop-but-smooth" checks in L and R positions at medium power (1800-2000 RPM)
  3. Before shutdown: "Drop-no stop" check in L and R positions, and a "drop-would-stop" check in the OFF position, both with the throttle closed (700-800 RPM)

The purpose of the before taxi check is to spot gross faults whilst still near the hangar and potential assistance.

The purpose of the before flight check is to ensure that the entire ignition system is working properly once fully warmed up, and that the engine is likely to provide adequate power without (for example) a fouled spark plug, or a mis-timed magento.

The purpose of the before shutdown check is two-fold, first to look for any new faults in the magentos, and also to ensure that the "off" position genuinely does result in both mags being "safe" in the hangar.

Whilst we agreed on most of this sequence, the instructor's view was that the "drop-would-stop" part of the before shutdown test was both superfluous and harmful to the engine. They asserted that having checked the "L" and "R" positions, there was no possible way that the "Off" position could not work, and further that running the engine with the magentos off and then re-energising them caused undue stress on the engine as I was "tell it to do two things at the same time". In an aircraft with two separate toggle switches for the mags, I might agree with the first part of this argument, however with a key-switch, the "Off" position is completely independent of the "L" and "R" positions and thus should (IMO) be checked separately. I also feel that the "drop-would-stop" check still results in the engine RPM remaining well above the cranking RPM during engine start, thus there should be no risk to the pistons or crankshaft, and it's unlikely even that the impulse coupling would retard the spark. The only potential risk I can see is that with the magentos off, unburned fuel enters a hot exhaust pipe which could result in a backfire.

Apologies for the long post, and thanks for reading thus far. I would be very interested in any comments on the way you do mags checks, and the pros-and-cons of the drop-would-stop part of the check.
User avatar
By GrahamB
#1737858
mick w wrote:Drop and stop is the only way my Aircraft can be stopped , apart from switching the Fuel off and waiting . :thumright:

But that’s not the same as a ‘drop-would stop-restore mags’ which is what the OP does.

Personally I don’t do that as a matter of course. Whilst understanding the reasons for doing it, I’d rather not risk the backfire, as the once or twice I’ve done it inadvertently on my engine it’s provoked a bang.
User avatar
By mick w
#1737860
GrahamB wrote:
mick w wrote:Drop and stop is the only way my Aircraft can be stopped , apart from switching the Fuel off and waiting . :thumright:

But that’s not the same as a ‘drop-would stop-restore mags’ which is what the OP does.

Personally I don’t do that as a matter of course. Whilst understanding the reasons for doing it, I’d rather not risk the backfire, as the once or twice I’ve done it inadvertently on my engine it’s provoked a bang.


Aye , too quick to answer , lost a couple of Car exhausts in my yoof doing just that . :lol:
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By Flyin'Dutch'
#1737865
Switching both magnetos off with the mixture still on unburned fuel air mixture will get into the exhaust manifold ready to be ignited with a bang when the magnetos get switched on again.

So best not do that.

In 40+ years of flying and being around small piston engines I have yet to encounter either myself or close quarters a magneto that remained 'live' with the switches in the 'off' position.

Treating any prop as 'live' is a good insurance policy; the amount of times they were not switched off by a previous user is more than a handful.
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By cockney steve
#1737866
ISTR watching youtube videos of the Shuttleworth Collection. Metallurgy was in it's infancy (as were all aeronautical technical disciplines) Despite the limitations, some of these aircraft have a fixed, wide-open throttle The sole means of regulating propulsive-power, is to switch the mags on-off repetitively, catching the very heavy wooden prop before the revs in those ancient spark-generators fell too low to produce a reliable spark. They're still performing 100 years later. I'd be wary of an instructor so ill-informed! :twisted:
@mick w Me too! but remember , there's a considerable volume trapped in a car exhaust, whereas the average SEP seems to have very short tuned-length extractor-stubs.I suspect it sounds loud , simply because it's right in front of you!.
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By Dave A
#1737868
I think your understanding is correct, Mike, and the Instructor is wrong. A "Before Shutdown" mags off check is the only way to ensure the magneto, and therefore the prop, is not still live. If the engine continues to run, there is a problem, and in my case, investigation showed a ground wire adrift from the dual magneto.

Secondly, the switching to 'off' is so momentary (or should be) that, as you say, the risk of any harm to the engine is negligible.

Dave A
User avatar
By flybymike
#1737869
Blew the entire exhaust system off my 1958 100E Ford Anglia when one of my mates turned off the ignition from the passenger seat and then switched it back on again.

(Central ignition key, long before steering locks)

Ooh, I was annoyed.....
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User avatar
By Flyin'Dutch'
#1737886
cockney steve wrote:ISTR watching youtube videos of the Shuttleworth Collection. Metallurgy was in it's infancy (as were all aeronautical technical disciplines) Despite the limitations, some of these aircraft have a fixed, wide-open throttle The sole means of regulating propulsive-power, is to switch the mags on-off repetitively, catching the very heavy wooden prop before the revs in those ancient spark-generators fell too low to produce a reliable spark. They're still performing 100 years later. I'd be wary of an instructor so ill-informed! :twisted:
@mick w Me too! but remember , there's a considerable volume trapped in a car exhaust, whereas the average SEP seems to have very short tuned-length extractor-stubs.I suspect it sounds loud , simply because it's right in front of you!.


That is typically on rotary engines where there are 2 inch exhaust stacks
User avatar
By GrahamB
#1737889
flybymike wrote:Blew the entire exhaust system off my 1958 100E Ford Anglia when one of my mates turned off the ignition from the passenger seat and then switched it back on again.

(Central ignition key, long before steering locks)

Ooh, I was annoyed.....

Lucky it didn’t blow the Anglia off the exhaust!
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User avatar
By rikur_
#1737891
@T67M - approach in our T67 syndicate is same as yours. Personally I've never experienced any detonation doing it. I presume the rationale is to check for a switch fault where the switch is making one or more contacts in the off position. I don't see why that should be impossible just because L & R positions work correctly.

The worse mag fault I've come across was a rental aircraft where the key could be removed in any position. Somewhat defeats the check of keys in pocket before touching the prop.
By johnm
#1737892
It depends on engine and installation but your third item is very dodgy. Our Lycoming check list has check for mag drop on both left and right mags at normal idle speed (1200 RPM) and shut down is then mixture to idle cut off and mags off when the engine has stopped.
Last edited by johnm on Tue Dec 31, 2019 1:57 pm, edited 1 time in total.
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User avatar
By Dodo
#1737893
rikur_ wrote:
The worse mag fault I've come across was a rental aircraft where the key could be removed in any position. Somewhat defeats the check of keys in pocket before touching the prop.


There was a fatality reviewed in a recent Flyer magazine when this happened.
By johnm
#1737899
I did a bit of checking courtesy of Google and found that the definitions can be misunderstood:

Dead Cut Check is one mag off at a time to make sure the engine will run on the other, a test failure implies that a dead cut will happen.

Live Mag check, both mags are switched off to ensure that neither is live with switch off.

Mag drop check one switched off at a time and drop in RPM noted and checked to be within limits.
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