Dangers of touching the prop

[RenegadeAV8R]

I am sure your instructors clearly explained what an impulse mag is.

I would say that most of today's instructor don't know much about the inner working of the Magnetos. Only one instructor showed me to only turn the propeller by standing behind the propeller, near the cowl; in case the engine starts. And that was at the conclusion of my private flight test. This instructor (and DFTE) was an older, experienced instructor, who is now retired.

I often say that today everybody knows who to use an electronic calculator; but nobody knows how to count anymore.

[Big Pistons Forever]

Show of hands: who here has personally found a defective switch with their testing every flight? That is, you had a drop on L, and a drop on R, but the engine kept running on BOTH because the swtich had an internal failure. I shall be interested to see the results, given the incredible over-testing.

I never turn the mags off with the engine running and neither do any of my students. I have never personally seen a mag switch that properly grounded out on the left and right position but left the mags live when it was in the "off" positions. One thing I emphasize is that as the students switch to the left and right positions as part of the shut down check; they give a tug on the key. It should not be possible to remove the key in any position other than off. I have had a switch fail this test as it was possible to remove the key with the switch in the left position. I grounded the aircraft with the snag "Mag switch unservicable".

In my opinion the idea of adjusting the prop after shut down is like most things in aviation. There is an acceptable level of risk if properly performed and an unlimited level of risk if the person turning the prop has no idea what may happen. As a general rule I do not turn the props if there is no good operational reason. However there is often perfectly legitimate reasons to adjust the prop. For example I must do this after ever flight in my Grumman, so the prop does not foul the tail of the other aircraft in the hanger and I pull the prop of my Nanchang through before every flight. All you have to do is not have any part of your body in the path of the prop, stand with your feet firmly planted and don't wrap your hand around the prop blade to reduce the risk to what is not zero , but IMO is low enough that it is reasonable

[mike123]

Something I have heard is if you need to reposition the prop (ie to attach the towbar) turn the engine backwards, as the mags will not fire in reverse.

I heard this too.

[Hedley]

I have never personally seen a mag switch that properly grounded out on the left and right position but left the mags live when it was in the "off" positions

Neither have I, in 35 years of operating piston aircraft engines. I can't even recall even HEARING about this particular failure mode EVER occurring to anyone else, despite how rumours love to travel in aviation.

Yet, people continue to damage their very expensive exhausts by performing this totally unnecessary test on EVERY flight, instead of every year as called for by the AD.

I say again, if you think a yearly AD should be tested every flight, why aren't you pulling your cowl and heater shround after EVERY FLIGHT and inspecting exhaust for cracks which could lead to deadly carbon monoxide in the cabin via the heat, as per the yearly AD?

Given that people are insisting on abusing their exhaust on every flight by performing a totally unnecessary OFF mag switch test, which we all know will stress and crack the exhaust, my notion of examining the exhaust for cracks after every flight makes quite a lot of sense.

PS the AD that all of you are frantically and insanely attempting to comply with on EVERY flight is
76-07-12. Click on this for the text:

http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... -07-12.htm

I must ask, what other yearly AD's do you attempt to comply with several times a day?

Do you swing your compass after every flight? Do you re-certify your ELT after every flight?

Given that people like to rest their headsets next the compass, my suggested compass swing after every flight makes at least as much sense as checking the OFF switch position every flight.

[Hedley]

Here's the incredibly ironic and funny part: despite every pilot in Canada frantically checking the OFF switch position every flight, I will wager that NOT ONE of their aircraft has AD 76-07-12 actually signed off in the tech logs.

If that isn't funny, I don't know what is.

What's a miscellaneous AD again? Bueller?

[RenegadeAV8R]

... AD 76-07-12 ...

Is this AD applicable to the Ignition Switch installed in the typical C150/152/172 found in flight schools?

[KK7]

Impulse coupling, that's the term I was looking for! It's been a little while since I was flying pistons and didn't feel like looking through the books.

Yes, the Islander has an impulse coupling, at least on the ones that have the 300hp IO-540s. I don't know about the 260 hp ones. Anyways, I don't know why it was so deadly, nobody lost a finger or anything, just abad cut and a bandage for a week or two. But there were quite a few... ... especially with one AME who was no longer allowed to work on the mags anymore. When I was flying them, I never had a reason to turn them, so I didn't, like any other piston powered aircraft.

[Hedley]

Is this AD applicable to the Ignition Switch installed in the typical C150/152/172 found in flight schools?

Sure is. And the funny thing is that NOT ONE of them has the AD signed off in the tech logs, despite all the frantic testing. And, that means that their C of A is not in force.

If that isn't funny, I don't know what is.

[Hedley]

Impulse coupling, that's the term I was looking for

The vast majority of light aircraft use impulse coupling magnetos
for starting. Shower of sparks is comparitively rare.

More trivia: most engines only have ONE implulse coupling magneto,
on the left side. When you are cranking, the switch grounds out the
right mag, so it can't fire BTDC and make the engine kick back.

Only having one impulse coupling magneto means that only one
spark plug is firing, in each cylinder, as compared to 2 when it is
running normally.

Our Maule has, somewhat unusually, two impulse coupling magnetos,
which probably explains why it's TCM IO-360 is so easy to start
in comparison to some other engines which are complete pigs to
hot start.

But back to the majority of the fleet, which only have impulse coupling
on the left mag. Some day, you are going to fly an aircraft which
does not have the industry-standard Bendix ignition switch - it might
have individual switches for magneto on/off like the Citabria or Decathlon
or Maule or PT-19 Cornell. The list goes on, and on.

If you are starting an aircraft which has such manual control over
magneto on/off (ie no Bendix switch) then be sure to select ONLY
left (impulse couling) mag on, for starting. After starting, turn on
the right (non-impluse coupling) mag.

If you do not do that, you may get kickbacks if the right (non-impulse
coupling) mag has enough RPM to fire at the normal 20 to 30 degrees
before top dead center.

A little systems knowledge really pays off. I know I'm a ridiculous
gearhead - I've been working on piston engines since the 1970's - but
there are some operational details that pilots should be aware of.

[iflyforpie]

Show of hands: who here has personally found a defective switch with their testing every flight? That is, you had a drop on L, and a drop on R, but the engine kept running on BOTH because the swtich had an internal failure. I shall be interested to see the results, given the incredible over-testing.

I have never personally seen a mag switch that properly grounded out on the left and right position but left the mags live when it was in the "off" positions

Neither have I, in 35 years of operating piston aircraft engines. I can't even recall even HEARING about this particular failure mode EVER occurring to anyone else, despite how rumours love to travel in aviation.

Well, I have personally experienced TWO of them, both on aircraft at my field, and one that I fly the most regularly. This only in ten years (logged my first flight hour ten years ago next month), and both within a year of each other.

First was on the neglected 172 I have mentioned elsewhere. An old Bendix mag switch with the OFF L R BOTH and no start. Would drop in L, drop in R, and continue to run in OFF. I took it apart (I actually showed pictures of it on here) but I never found anything conclusive. Switch replaced, tested serviceable. It is not affected by the AD.

Second was on our 206. 45 years of wear I guess, but when I turned it to OFF (which I have been taught both in flight school and in mechanic's school), it would run until you jiggled the key, again with normal drops on L and R. Our other pilot was completely oblivious to it since she always did L and R. I tested it with the leads off in the shop just to make sure it wasn't pre-ignition or something and sure enough, blue sparks.

Yet, people continue to damage their very expensive exhausts by performing this totally unnecessary test on EVERY flight, instead of every year as called for by the AD.

Don't blame the practice, blame the method by which it is carried out. Years and thousands of hours doing this and I have found very few cracked exhausts or made fewer big bangs from doing this. More apocalyptic explosions come from people overpriming on that difficult fuel injected hot start than a proper live mag check. You have to do it at idle, and you can't leave it for five seconds before turning them on. A65 to R-1340 I've been taught this, and done this, and had no problems whatsoever. Maybe a Kinner or a Ranger is different, but certainly no opposed engine.

In fact, you can turn BOTH off at substantially higher RPMS and not blow the exhaust manifold to kingdom come by retarding the throttle until you can count blades before turning the mags back on. Not something I would recommend trying, but if I have an engine act up and do an in-flight mag check, I am sure not going to wait for the engine to quit before turning things back on if I've discovered I only have one mag.

Our Maule has, somewhat unusually, two impulse coupling magnetos,
which probably explains why it's TCM IO-360 is so easy to start
in comparison to some other engines which are complete pigs to
hot start.

Another trick with the Continental is that you can 'purge' them of vapour lock by running the boost pump with the throttle in and the mixture idle cutoff. It sends the fuel to the EDP, FACU, and then back the return lines, pushing the fuel vapour with it. The vapour in the spider and fuel injection lines is then simply purged into the engine when you start. Lycomings, you have to sacrifice a virgin to the aviation gods, touch your nose, wiggle your toes, and use the exact right combination of throttle, mixture, and fuel flow to get it started.

[Hedley]

Well, I have personally experienced TWO of them

You definitely win the prize!

our 206. 45 years of wear

I suppose, after all that time, it was paid for!

I must ask: do you sign off this misc AD in the airframe log?

What was the TIS on each? Are we talking 20,000?

I have found very few cracked exhausts

Now the tables are turned. I am looking at replacing BOTH
of the very expensive exhausts on a couple of 172's. Guess why?

PS Yes, the Continental fuel injection has a wonderful return
line which is lacking in the Lycoming fuel injection.

[Bushav8er]

TC, taking their mandate of Public Safety seriously, is proposing a new safety regulation - it is planning to ban piston propulsion systems in aircraft.

They state that this will have the following benefits:

- all hand propping accidents occur with piston engines,
- most aircraft accidents occur to aircraft in the light, private piston powered, under 12500 lbs., category
- avgas is detrimental to the environment additionally, fewer oils would be required,
- piston engines have been around since the early 1900's and are 'old technology',
- it would eliminate a large portion of the CARs thereby reducing costs,
- the media would focus on real news items as the term "...a small airplane crashed..." would no longer be necessary.
- it would permit industry to explore anti-gravity and levitation devices,

Due to the decrease of aircraft in the industry, it is revising the Flight Duty Time regulations for 704 and 705 operators to allow them to 'economically employee pilots' on a 24 hour basis. They state that this is in keeping with Safety as "...new aircraft are automated anyway".

[iflyforpie]

I must ask: do you sign off this misc AD in the airframe log?

No, none of our switches are applicable. Even if they were, I am just doing a function check for a warm and fuzzy feeling which in my experience doesn't damage the engine. (Do it at idle, not 1000 RPM, not 800RPM, idle stop. Maybe people's idle mixtures are too rich or idles too high).


The only mag switch AD we have is on our 172. AD93-05-06 Mag switch lubrication. It was done before I started working here and it has another 1000 hours to go before the next accomplishment is required.

What was the TIS on each? Are we talking 20,000?

The 206 only had 7000 hours. The 172 (the one with the failed mag switch, not ours) had 3000.

[KK7]

Some day, you are going to fly an aircraft which
does not have the industry-standard Bendix ignition switch - it might
have individual switches for magneto on/off like the Citabria or Decathlon
or Maule or PT-19 Cornell. The list goes on, and on.

If you are starting an aircraft which has such manual control over
magneto on/off (ie no Bendix switch) then be sure to select ONLY
left (impulse couling) mag on, for starting. After starting, turn on
the right (non-impluse coupling) mag.

If you do not do that, you may get kickbacks if the right (non-impulse
coupling) mag has enough RPM to fire at the normal 20 to 30 degrees
before top dead center.

Yes, this is also the case for the Islander, switches for mags, which is probably why the impulse coupling was front and center in my memory for this. Started on left mag then as the engine was picking up the right mag is switched on.

[iflyforpie]

The Islander also had another mag cutoff in the pilot entry door since the left prop is right behind it. I remember doing mag timing on the left engine of one and I couldn't get the synchronizer to work. Of course I just had the leads and plugs out and just turned the switches on instead of disconnecting the P-leads, the front door propped open against the nose. Took me a while to figure it out.

[cgzro]

Show of hands: who here has personally found a defective switch with their testing every flight? That is, you had a drop on L, and a drop on R, but the engine kept running on BOTH because the swtich had an internal failure. I shall be interested to see the results, given the incredible over-testing.

No but after a mag overhaul last year I was doing my runup and was certain all was well with minimal drop. When I came back and did about a 700 RPM off test .. no dice. Turns out the ground lead to the left mag was not re-connected. I was sure I had a tiny bit of drop but because the mags were new I was very happy that it seemed minimal when infact it was no drop.

I do know a colleague that hurt his hand turning a prop on a defective switch. He had somehow managed to remove the key in the 'almost' off position leaving the L mag on.

Like a gun you gotta treat it as live all the time to really be safe.

[Hedley]

I was sure I had a tiny bit of drop

Hm. You should see at least 40 to 50 rpm drop. Even if you can't see it on the wiggling needle on the tachometer, you should be able to hear the pitch of the engine drop.

Engines are in fact incredibly musical. Far more so than a stupid drum kit which you will find in any rock band, which has it's motto, "If you can't play well, play loud".

Here's an F1 engine playing "God Save The Queen", starting at 0:58:



Richard Bach - I doubt anyone here remembers him - once wrote that he and his Seabee at 2400 RPM used to sing Greensleeves together

[Shiny Side Up]

Engines are in fact incredibly musical. Far more so than a stupid drum kit which you will find in any rock band, which has it's motto, "If you can't play well, play loud".

Hence why these guys introduced a two stroke engine into their act.




Pure awesome.

[iflyforpie]

I pick music in keys that go with my cruise RPM. I'll give the prop a tweak if it is off.

And sometimes when I come back in in the Skymaster on a nice summer day, I will reduce power on the front engine to 1750 RPM (A6) and keep the back one at 2600 (E7) coming into the circuit. Makes a perfect fifth harmony. Yes, I know I am a nerd...

[Hedley]

Music and (esp powered) flight are both applied physics

[J31]

To add for AD 76-07-12 Applies to batch of Bendix ignition switches manufactured in the early 70's. They are Rotary Action, Twist-to-Start, Twist-to-Start/Push-to-Prime, and Push-to-Start.

From http://www.ercoupe.net/ads/76_07_12.html

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.

Also note the check is for the extreme "Off" position and is done ONLY at idle and is ONLY required every 100 hours.

I cringe at the flight schools teaching their students to momentary turn the mag switch off at 1700-1900 RPM during the runup or shutdown!?

If you must check that your switch is turning off properly do it only at idle.

I prefer an annual/100 hour check with a continuity meter connected to the P leads. That way you can wiggle the switch in the off position to truly test it. I found a few switches that had intermittent failures to ground out in the off position. Of course make sure you check the affected switches as per AD 76-07-12.