There are really two things you are checking:
1) p-leads to the magneto, and
2) switch contacts
They actually have nothing to do with each other.
If you can select L, and see an RPM drop, then
go back to both, and then select R, and see an
RPM drop, then you know that your p-leads and
magnetos are grounding out perfectly via the
switch contacts for L and R, and that they are not
grounded out on BOTH.
However the nightmare scenario is that the switch
might be faulty in the OFF position. If you do test
the OFF position with the engine running, please do
it at absolute minimum RPM for a very brief moment.
The higher the RPM, and the longer you leave it in
the OFF position, the more unburned combustible
fuel-air mixture is going to be in the exhaust, which
is going to light off when you turn the mags back on.
I am sure that we have all witnessed student pilots
doing the click-click-BOOM method of magneto testing
during a warmup. No one ever listens, but if you ever
accidentally select OFF during a runup and kill the engine,
just let it die, and restart it after the gas evaporates out
of the exhaust.
Now onto AD's. Lycoming and Continental really don't
have much to do with accessories such as magnetos
and airframe items such as ignition switches.
The governing AD is from 1976 (reallly) before most
people here were born. It's still in effect, but its in the
misc section so no one ever complies with it, or even
knows it exists:
http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... -07-12.htm
Note that it specifies checking the switch every 100
hours. Checking it 100x as frequently as that (every
flight) is a little over the top, but if it makes everyone
feel like part of the team, and if it makes the FTU checklist
longer, hey, why not?
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.
There is another (Canadian) AD on all aircraft requiring annual
inspection of the exhaust for cracks, to avoid carbon monoxide
entering the cabin via the shroud heat.
Using the same logic as testing the switch, I think that after
every flight, everyone should remove their engine cowls and
shroud and inspect their exhaust for cracks. After every flight,
instead of the annual inspection requirement in the AD.
Same logic as checking the switch every flight. And frankly,
far more important. Carbon monoxide poisoning will kill
you. If you don't play with the prop, a hot mag is unlikely
to kill you.
PS For IFP:
