Runup's and mag drops...

Maddog1974

Maddog1974

Well-Known Member
So I have a question about doing a runup on a C-172 with a Lycoming 180 horse engine. It involves the following service instruction and concerns doing a runup and checking the mags.

Service Instruction 1132B

There is a debate between the IA and the Instructors on the line. Currently/previously we would start and warm up in a normal manner and taxi to a runup area. Once there we would obviously bring up the RPM's so that we could check engine instruments, smooth run, and the magneto drop off. In the past we have had problems with drops greater than 150 rpm's and we would return and squawk the aircraft.

The mechanic we have has found this service bulletin and has presented it to us with an unusual procedure. In his opinion we should, once in the runup area, do the following (very simplified) procedure:
  1. Increase RPM to 2000
  2. lean mixture for peak RPM
  3. retard throttle to 2000 rpm
  4. check mags

I have the idea that this is not correct because we are changing the test to receive the results we require. To me the service bulletin gives a procedure to leave the mixture full rich, increase the RPM to 1800 (2000 max), and then check the mags. If during the initial test you get a drop of greater than 175 and no more than 50 between the two then you should continue to the leaning procedure. Once you do this you should see a drop of less than 175 and 50 between and smooth operation you may continue the flight.

My questions would be as follows:
  1. What procedure would you interpret in SI-1132B
  2. If you see a drop of greater than 50 RPM in the initial mag drop, would you continue to step E(1)(d)?

We do typically see high density altitude on a regular basis during the summer months but usually don't have problems with the mag drop. We have had in the past problems with certain aircraft having a high or inconsistent mag drop during the runup but it may be isolated to specific aircraft during the warm months here. I should note that if we see a bad drop we would typically do a spark plug clearing procedure but would still see the same or greater drops on the mag check.

Any suggestion or comments would be extremely helpful.

I am trying to find ways of making suggestion to the IA without stepping on his toes and letting him "own" the solution for this but I am finding it difficult to come up with suggestions due to my lack of knowledge.

Thanks!
 
Go with his solution. As pointed out in the SB, if with the mixture leaned out your mag drop is good it is a sign of an over rich mixture not an ignition problem. Very common to have to do that in a hot environment with some engine setups.
 
I see that except that the SB says to begin with the mixture rich then in section E(1)(c) it states that you might see an abnormally high drop due to high mixture and propeller loading. However, it still states that the drop "must not exceed 50 RPM between magnetos."

If i go with his procedure then I may not see an indication of error between the magnetos. Also, we have 7 other aircraft with the same engine, with varying number of hours of operation, on which we typically do not use this procedure.
 
We always leaned the engine for peak RPM's before taxi (at 1200 RPM's), the mixture rich for the runup. If the mag drop was excessive during the runup, we'd runup to 2000RPM...lean for peak RPM for 45 seconds to burn off any excess carbon, etc.

If you're in a hot (or high) environment, you should probably lean the engine for runup as well (similar to leaning for takeoff).

This is with 2003-brand new 172SP's.
 
SurferLucas said:
We always leaned the engine for peak RPM's before taxi (at 1200 RPM's), the mixture rich for the runup. If the mag drop was excessive during the runup, we'd runup to 2000RPM...lean for peak RPM for 45 seconds to burn off any excess carbon, etc.
Click to expand...

What he said.
 
Yeah, we made is standard at UND to taxi with the engine leaned for peak RPM. After start, brought RPM to 1200, leaned for peak, back to 1000. Then taxi as needed, did the runup check, and only went full rich right before take off.

Apparently Cessna ok'd the 1200 rpm lean for us, saying the difference would be marginal for a runup if we had leaned at 2000 rpm. So that's what we did, took 10 seconds and reduced engine problems A LOT.
 
We actually do lean the mixture for taxi operations every time we start the engine. The 1200 RPM thing is now in the POH as a part of normal operation. After start, flaps, and avionics, RPM's 1200, lean for peak, retard to 1000 and finish start checklist or taxi.
 
I don't know how to turn the direction this thread has gone but I thought maybe if I just post a couple of notes here it might help.

We do have a very good checklist, start and taxi procedure in place. We have high density altitude at the airport for almost 75% of the year. The airport is at 20' MSL, the average monthly temperature ranges from 29 to 37 degrees celcius with an average monthly humidity of 50%. For example this moment the current conditions at OEJN are as follows:

OEJN 281500Z 30008KT CAVOK 32/22 Q1003 NOSIG

With a field elevation of 48' msl that gives me a density altitude of 2684', no offense but, to say we need to lean the mixture before taxi is kinda...well, yeah, we have to!

Again, the 1200 RPM then lean for taxi item is in the POH having been included in a update.

Once startup, lean, taxi checklist complete, etc, etc, then we move to the runup area and continue. At this point the mixture will return to full rich, engine is increased to 1800 and we observe RPM's, check the engine gauges, and then perform the mag drop.

If the drop is excessive or uneven, greater than 50 between, then we always do a spark plug burn off, cleaning, whatever you call it. This is standard for our instructors before we go turning it over to the maintenance department. If we don't then the IA takes us and he sits in the airplane with us while we do the runup through the checklist and we clean the plugs.


Again, the problem is with this bulletin...I read that it represents a step by step procedure in that we begin the runup to mag drop with the mixture full rich. If the drop is excessive then the mixture may be leaned to reduce the loading associated with density altitude, engine wear, high mixture setting, or propeller loading.

The question is if you get an uneven drop before you lean to retry the mag drop, what would you do?

If the mags are evenly dropping with 1800 rpm and a full rich mixture then that is fine, unless the drop is more than 175 rpm. Then I would lean for peak rpm's and set the throttle between 1800 and 2000 and then retry the mag drop.

Please reread the SB and then give me your thoughts on how you would deal with the following occurrences and lets assume that the mixture is rich, RPM is steady and smooth at 1800:

The mag drop exceeds 175 but is no greater than 50 between the two.

The mag drop is greater than 175 and is greater than 50 between the two.

The mag drop is less than 175 but is greater than 50 between the two.

What would you do next for each of these issues?
 
Just do the mag check with the mixture leaned. Our procedure in the 172R in hot, humid climates was to go full power and lean for 13 GPH while checking static RPM, then throttle back to 1800 with the mixture still leaned and perform the mag check. We would then take off with the mixture still leaned. Had very few problems with fouled plugs and bad mag drops, and very few engine related problems in general.
 
Roger said:
Just do the mag check with the mixture leaned. Our procedure in the 172R in hot, humid climates was to go full power and lean for 13 GPH while checking static RPM, then throttle back to 1800 with the mixture still leaned and perform the mag check. We would then take off with the mixture still leaned. Had very few problems with fouled plugs and bad mag drops, and very few engine related problems in general.
Click to expand...

Thanks
 
A reminder.. The 172R only has 160 HP vs the 180 in your plane. Might need more than 13 GPH for those.
 
Roger said:
A reminder.. The 172R only has 160 HP vs the 180 in your plane. Might need more than 13 GPH for those.
Click to expand...

yeah, we use the LEAN page on the G1000 to get peak rpm and egt's and then reduce the power for taxi or runup (if above 2000). In the particular airplane we are seeing a drop of 180 on one side and drop of 100 on the other that will reduce when the engine is leaned.

I guess though that I should ignore that and go with the procedure that gives me the desired outcome, eh? throttle 2000, peak EGT/RPM, check the Mags.
 
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