I had a discussion with a fellow who maintains that he never does the check before shutdown on piston driven singles. He maintains that it does damage to the aircraft.
I made a mental note never to touch a propeller on his aircraft for ground handling, but walked away wondering if I had been improperly taught that the "P lead" check was an important safety consideration.
Could any AME tell me who is correct?
Mag Cutoff Check
Moderators: sky's the limit, sepia, Sulako, lilfssister, North Shore, I WAS Birddog
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Major Bedhead
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Jim la Jungle
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hi there,
well, I'm not an AME, but I can tell you that doing the live mag check ON THE GND is a safety measure. On the 337 that I fly, the mechanics have to write on the journey log after an inspection that they did so according to so and so service bulletin or AD (i'm not sure which).
If I remember correctly, in the 172/182 Poh, there's something about not doing a mag check IN THE AIR unless in an emergency.
I would suggest asking your AME what a live mag check could damage. Personnally, I always prefer an answer that says "no or yes BECAUSE" than a straight "yes or no" without details.
well, I'm not an AME, but I can tell you that doing the live mag check ON THE GND is a safety measure. On the 337 that I fly, the mechanics have to write on the journey log after an inspection that they did so according to so and so service bulletin or AD (i'm not sure which).
If I remember correctly, in the 172/182 Poh, there's something about not doing a mag check IN THE AIR unless in an emergency.
I would suggest asking your AME what a live mag check could damage. Personnally, I always prefer an answer that says "no or yes BECAUSE" than a straight "yes or no" without details.
Trapped in time, surrounded by evil, low on gas.
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Jaques Strappe
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Is doing a mag check a good idea? Absolutely but not how the schools seem to teach it.
I see newbies all the time winding it up to 1000 RPM just to switch the mags off and back on with a resulting backfire which can damage valves, connecting rods and crank.
Common sense should tell you to switch one mag off at a time just as you would in a run up. If there is no drop, then you should treat it like it has a live mag. This can be done at idle. No backfiring, no undue stress on the engine and it is safe.
Why schools don't teach this is beyond me.
I see newbies all the time winding it up to 1000 RPM just to switch the mags off and back on with a resulting backfire which can damage valves, connecting rods and crank.
Common sense should tell you to switch one mag off at a time just as you would in a run up. If there is no drop, then you should treat it like it has a live mag. This can be done at idle. No backfiring, no undue stress on the engine and it is safe.
Why schools don't teach this is beyond me.
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aero-singidunum
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I did made few $, replaceing mufflers that will be wide open, after dude will go with C172 to 1800 RPM, check left mag, swith to right mag, switch to off, KKAAAAAOUOOBBOOOOM,
there is another muffler on the ground, was not funny.
Schools do not teach, instructors have no idea (they never been shown how to do properly)
there is another muffler on the ground, was not funny.
Schools do not teach, instructors have no idea (they never been shown how to do properly)
The lower the rpm, the better. The "boom" is caused by unburned gas in the exhaust lighting off after you turn the mags back on again.
1700 rpm is indeed bad. But 700 rpm won't hurt the exhaust (or anything else).
P.S. Simply selecting "left" or "right" will tell you if the p-leads are connected, but it won't tell you if the switch is working in the "off" position.
Here's the AD, if you care:
76-07-12 BENDIX IGNITION SWITCHES: Amendment 39-2575 as amended by Amendment 39-3024. Applies to all aircraft employing magnetos and using Bendix ignition switches listed in the table below except switches identified by four digit date code (new) adjacent to the model number or a white dot (modified) on the support plate adjacent to the Bendix logo.
Rotary Action,
Bendix Switches Key or Lever Actuated
Switch Function Bendix (series) Part Numbers
Twist-to-Start 10-357XXX, 10-126XXX
Twist-to-Start/Push-to-Prime 10-357XXX, 10-126XXX
Push-to-Start 10-357XXX, 10-126XXX, 10-157XXX
Compliance required as indicated:
1. For switches subject to this AD, conduct the following checks within the next 100 hours' time in service and each 100 hours thereafter to detect possible switch malfunction:
(a) Observing regular ground run-up procedures, allow the engine to reach operating temperatures and perform a normal magneto check.
(b) With the engine at normal idle, rotate the switch key or lever through the "OFF" detent to the extreme limit of its travel in the "OFF" direction.
(c) If the engine stops firing, this indicates an airworthy switch.
(d) If the engine continues to run with the switch in the extreme "OFF" direction indicating a malfunctioning switch, prior to the next flight accomplish Part III outlined in Bendix Service Bulletin No. 583, dated April 1976, for Repair and Replacement or use an alternate method approved by Chief, Engineering and Manufacturing Branch, Eastern Region.
2. The aircraft may be flown in accordance with FAR 21.197 to a place where these modifications can be accomplished.
3. The checks required by this AD may be performed by the pilot.
4. Upon submission by an operator with substantiating data, an FAA Maintenance Inspector subject to prior approval of the Chief, Engineering and Manufacturing Branch, FAA Eastern Region may adjust the compliance times specified in this AD if the request contains substantiating data to justify the increase for the operator:
(NOTE: If the engine continues to run when complying with paragraph 1 and repair or replacement cannot or will not be accomplished immediately, the magneto (primary circuit) should be grounded in accordance with Bendix Service Bulletin No. 583, dated April 1976.)
Amendment 39-2575 was effective April 14, 1976.
This amendment 39-3024 is effective August 30, 1977.
1700 rpm is indeed bad. But 700 rpm won't hurt the exhaust (or anything else).
P.S. Simply selecting "left" or "right" will tell you if the p-leads are connected, but it won't tell you if the switch is working in the "off" position.
Here's the AD, if you care:
76-07-12 BENDIX IGNITION SWITCHES: Amendment 39-2575 as amended by Amendment 39-3024. Applies to all aircraft employing magnetos and using Bendix ignition switches listed in the table below except switches identified by four digit date code (new) adjacent to the model number or a white dot (modified) on the support plate adjacent to the Bendix logo.
Rotary Action,
Bendix Switches Key or Lever Actuated
Switch Function Bendix (series) Part Numbers
Twist-to-Start 10-357XXX, 10-126XXX
Twist-to-Start/Push-to-Prime 10-357XXX, 10-126XXX
Push-to-Start 10-357XXX, 10-126XXX, 10-157XXX
Compliance required as indicated:
1. For switches subject to this AD, conduct the following checks within the next 100 hours' time in service and each 100 hours thereafter to detect possible switch malfunction:
(a) Observing regular ground run-up procedures, allow the engine to reach operating temperatures and perform a normal magneto check.
(b) With the engine at normal idle, rotate the switch key or lever through the "OFF" detent to the extreme limit of its travel in the "OFF" direction.
(c) If the engine stops firing, this indicates an airworthy switch.
(d) If the engine continues to run with the switch in the extreme "OFF" direction indicating a malfunctioning switch, prior to the next flight accomplish Part III outlined in Bendix Service Bulletin No. 583, dated April 1976, for Repair and Replacement or use an alternate method approved by Chief, Engineering and Manufacturing Branch, Eastern Region.
2. The aircraft may be flown in accordance with FAR 21.197 to a place where these modifications can be accomplished.
3. The checks required by this AD may be performed by the pilot.
4. Upon submission by an operator with substantiating data, an FAA Maintenance Inspector subject to prior approval of the Chief, Engineering and Manufacturing Branch, FAA Eastern Region may adjust the compliance times specified in this AD if the request contains substantiating data to justify the increase for the operator:
(NOTE: If the engine continues to run when complying with paragraph 1 and repair or replacement cannot or will not be accomplished immediately, the magneto (primary circuit) should be grounded in accordance with Bendix Service Bulletin No. 583, dated April 1976.)
Amendment 39-2575 was effective April 14, 1976.
This amendment 39-3024 is effective August 30, 1977.
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Jaques Strappe
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HZ2P
That Ad is a "every 100 hr " check the way I read it. You are right in saying that just selecting one mag from the other does not check the off position but I would not do that for each and every shut down. It also only refers to rotary switches. For toggles, like in my Citabria, that logic would not apply.
JigglyBus
I'll bite, why can you not say muffler? Mine has an annual recurring Ad on it.
That Ad is a "every 100 hr " check the way I read it. You are right in saying that just selecting one mag from the other does not check the off position but I would not do that for each and every shut down. It also only refers to rotary switches. For toggles, like in my Citabria, that logic would not apply.
JigglyBus
I'll bite, why can you not say muffler? Mine has an annual recurring Ad on it.
Haha.... my bad actually.
I've never considered a heat exchanger a muffler, but you and the other fella are quite correct. Muffler indeed. I've just never heard it refered to that way before. But, like you said, apparently that is what it is called.
I was aware that there were a few sound cancelling mufflers around but had never seen one on a 172.....
Learn something new every day.
ps. Reminder to self, remove foot from mouth.
I've never considered a heat exchanger a muffler, but you and the other fella are quite correct. Muffler indeed. I've just never heard it refered to that way before. But, like you said, apparently that is what it is called.
I was aware that there were a few sound cancelling mufflers around but had never seen one on a 172.....
Learn something new every day.
ps. Reminder to self, remove foot from mouth.
Last edited by JigglyBus on Wed May 25, 2005 2:49 pm, edited 1 time in total.
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Right Seat Captain
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I know a fella who did just that. He never checked the 'off' position, and paid the price for it with stitches in his hand and a pretty good scare.hz2p wrote: P.S. Simply selecting "left" or "right" will tell you if the p-leads are connected, but it won't tell you if the switch is working in the "off" position.
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Jaques Strappe
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I hate to ask what he did. I was taught from the age I could walk, not to mess about at the front end of an airplane and always treat it like the mags are live.I know a fella who did just that. He never checked the 'off' position, and paid the price for it with stitches in his hand and a pretty good scare.
Anyone who grew up around horses will also tell you not hang around the back end of a horse unless you want a hoof in your nuts.
Why is it so hard to exercise common sense?
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wha happen
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Major Bedhead
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Right Seat Captain
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In some older POHs, it unfortunately recommends turning the prop a few times prior to starting to get the oil moving around. However I believe this is the reason it no longer recomends this in Cessna POHs.Jaques Strappe wrote: I hate to ask what he did. I was taught from the age I could walk, not to mess about at the front end of an airplane and always treat it like the mags are live.
Jim la Jungle -I can sympathize with your thoughts. I read somewhere that manufacturers have to list limitations and procedures in the AFM but there is no requirement to divulge why the limitations or procedures are there. Something to do with proprietary information. Macy's does not tell Gimbals what it is doing sort of thing. Pisses me off to no end.
The average pilot, despite the somewhat swaggering exterior, is very much capable of such feelings as love, affection, intimacy and caring.
These feelings just don't involve anyone else.
These feelings just don't involve anyone else.
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Jaques Strappe
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I have always wondered where that came from. I would see guys doing that in the morning. I asked one guy why and he told me that it pumped oil through the system, sort of a pre-lube. I have a hard time believing that pulling a prop through a couple of compression strokes builds enough oil pressure to move anything. I just figured it was one of those urban legends of aviation.In some older POHs, it unfortunately recommends turning the prop a few times prior to starting to get the oil moving around. However I believe this is the reason it no longer recomends this in Cessna POHs.
Hydraulic lock is not just a radial-engine phenomena.
If you don't pull some blades through before starting a radial engine, sooner or later you will blow the bottom jugs off.
It is quite possible to hydraulic lock a flat engine, usually through over-priming. Liquid fuel doesn't compress very well. I know of at least one C-210 engine that failed (broken con rod) and it was blamed on exactly this. You wouldn't think the starter would have enough oomph to bend a connecting rod, but there you have it.
Other Cessna flight manuals I have read over the decades also warn about this, and recommend pulling through by hand to clear the cylinders after over-priming.
Now, no one ever over-primes, hm?
If you don't pull some blades through before starting a radial engine, sooner or later you will blow the bottom jugs off.
It is quite possible to hydraulic lock a flat engine, usually through over-priming. Liquid fuel doesn't compress very well. I know of at least one C-210 engine that failed (broken con rod) and it was blamed on exactly this. You wouldn't think the starter would have enough oomph to bend a connecting rod, but there you have it.
Other Cessna flight manuals I have read over the decades also warn about this, and recommend pulling through by hand to clear the cylinders after over-priming.
Now, no one ever over-primes, hm?
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fougapilot
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Ahhh, the old Hydraulic lock. 
Usualy have one every year on my Yak. The engine manufacturer recommends pulling the prop 9 blades before the first stard of the day. If the airplane has been sitting for long, I prefer pulling 15. Quite often there will be nothing, but once in a while, she will spit out a cup of oil or so, and once a year I end up having to remove the plugs to drain half a liter from the bottom cylinder. You can only imagine how important the mag check becomes...
D
Usualy have one every year on my Yak. The engine manufacturer recommends pulling the prop 9 blades before the first stard of the day. If the airplane has been sitting for long, I prefer pulling 15. Quite often there will be nothing, but once in a while, she will spit out a cup of oil or so, and once a year I end up having to remove the plugs to drain half a liter from the bottom cylinder. You can only imagine how important the mag check becomes...D
