Inaccurate tach = grounded?

[poadeleted20]

Please elaborate on both what you believe to be Felix's "method" and how it would mask (leave undetected?) an ignition problem.

Felix appears to be saying he does the mag checks at or near idle RPM; the drop you get there if one plug isn't firing may not be as noticeable as at the recommended RPM for the check.

 

[gismo]

Felix appears to be saying he does the mag checks at or near idle RPM; the drop you get there if one plug isn't firing may not be as noticeable as at the recommended RPM for the check.

What you're saying is true if you're just looking for a drop in RPM but Felix is using an engine monitor and looking for EGT rise or fall on each cylinder. If a plug is fouled it will definitely show up there. Unfortunately he was't very clear about that in his last post.

 

[kmead]

I always do a full power mag check (38" or 42", whichever I'm in). I have found mags that check good at 2000 RPM but would die at checks performed at full power. The pressure fighting the spark in the cylinder is so much greater at high power setting. The part I never forget to tell people on the ground check, if the mag fails don't turn the good mag back on. Let it die then start it back up. A lot of people do LOP mag checks at altitude. This helps determine the health of mags in the worst environment. The warning is that the passengers may get scared when the engine goes quiet.

My tachometer accuracy tolerance is (+- 25 RPM) dating back to an old Hartzell SL and a Malibu SB. I have found many tachs off as far as 200 RPM. The best replacement I have found for the mechanical tach is the STC Horizon tach.

Regards, Kevin

 

[gismo]

I always do a full power mag check (38" or 42", whichever I'm in). I have found mags that check good at 2000 RPM but would die at checks performed at full power.

Man, your airport neighbors must love you. I'll bet your props aren't too fond of this either, but I do believe you are correct that there's quite a bit more stress on the ignition system at full power than at something like 1700 RPM.

 

[sba55]

What you're saying is true if you're just looking for a drop in RPM but Felix is using an engine monitor and looking for EGT rise or fall on each cylinder. If a plug is fouled it will definitely show up there. Unfortunately he was't very clear about that in his last post.

Yes - Thanks, Lance, that's what I was trying to say.

Ron, with an engine monitor, especially if you normalize the EGT display first (this also makes it much more sensitive), you can easily see the EGT rise. I don't do my mag checks quite at idle power, but even there, you can readily tell the EGT change.

Without the engine monitor, I'd agree with you that the RPM drop could be very hard to detect, especially with the RPM instruments in some planes. Looking at the EGTs, if possible, gives you a bit more information because it doesn't just tell you if one of your cylinders is having a problem, it tells you which one it is.

-Felix

 

[poadeleted20]

What you're saying is true if you're just looking for a drop in RPM but Felix is using an engine monitor and looking for EGT rise or fall on each cylinder. If a plug is fouled it will definitely show up there. Unfortunately he was't very clear about that in his last post.

That changes things. I was speaking to the majority who only have a tach.

 

[stingray]

I don't think Lance was advising a complete lack of any run-up--as much as he was saying it isn't necessary to do the mag check at high power settings.

I read this also from one of the magazines. It was not nesessary to go much over 1200 rpm's. Although I think it would matter on how often you fly the plane and is it owned or rented.

Dan

 

[Dan Thomas]

I always do a full power mag check (38" or 42", whichever I'm in). I have found mags that check good at 2000 RPM but would die at checks performed at full power. The pressure fighting the spark in the cylinder is so much greater at high power setting. The part I never forget to tell people on the ground check, if the mag fails don't turn the good mag back on. Let it die then start it back up. A lot of people do LOP mag checks at altitude. This helps determine the health of mags in the worst environment.

There's a reason why the POH calls for a runup at 1700 or 1800 or whatever. The manufacturer has determined the RPM at which maximum cylinder compression pressures are reached, and it'll be a little different for different installations. At lower than runup RPM throttle settings, there's not enough air available to reach max pressure (throttle valve is holding it back). At higher throttle settings, falling volumetric efficiency (drag caused by intake system components) also cuts into the air available and cylinder pressures drop. Since the spark has the most trouble when the pressure is highest (or mixture leanest), that's where we do mag checks. Bad mags might fire just fine at idle or 1200 RPM, and maybe even at cruise RPM.

We test spark plugs every time we clean them, and make sure they'll spark nicely at 130 psi. Compression pressures are seldom above that. And sometimes we find engines that run a tiny bit rough on both mags, smoothing out on one mag. That's a bad mag or bad plug, and the mag timings aren't fairly close together, with one mag firing a little sooner and raising the pressure enough to stop the other plug from firing. And yet, some engines have their mags timed several degrees apart, IAW the TCDS. Go figure.

Dan

 

[gismo]

There's a reason why the POH calls for a runup at 1700 or 1800 or whatever. The manufacturer has determined the RPM at which maximum cylinder compression pressures are reached, and it'll be a little different for different installations. At lower than runup RPM throttle settings, there's not enough air available to reach max pressure (throttle valve is holding it back). At higher throttle settings, falling volumetric efficiency (drag caused by intake system components) also cuts into the air available and cylinder pressures drop. Since the spark has the most trouble when the pressure is highest (or mixture leanest), that's where we do mag checks. Bad mags might fire just fine at idle or 1200 RPM, and maybe even at cruise RPM.

I do agree that pre-ignition pressure will be higher at 1700 RPM than it is at idle and that the higher pressure requires a higher voltage to spark. But I can't see how the pre-ignition pressure would decrease as the throttle is opened further, raising the manifold pressure as long as the prop configuration was constant. It's certainly likely that the volumetric efficiency drops as the RPM increases but the increased MP has to be enough to get more air/fuel into the cylinders on each intake stroke. If it weren't the engine wouldn't produce the additional torque required to spin the prop faster.

And in any case, as I stated earlier, IME if one plug in a cylinder is carbon or oil fouled, it will clear in a few seconds at full power and if it's lead fouled it won't fire at fast idle.

 

[Hiperbiper]

I once did a Mag check with the carb heat on in my Grumman (O-235C2C) and got no disernable Mag drop. I noticed the carb heat out, pushed it back in,admonished myself and re-did the mag check. Normal mag drop once again! In essence I leaned the mixture inside the cylinder and made it easier for one set of plugs to fire the available fuel/air mix hence no drop.
IMPO the mag check must be carried out at the "worst case" conditions that the plugs will see in flight i.e. takeoff (Rich cooling mixture, high load and high ICP).Change the mixture, change the load or change the ICP (RPM) and you can have bad plugs pass the "mag check" with (non) flying colors.

I prefer to find my engine descrepincies on the hanger side of the Hold Short line...

JMPO

Chris

 

[gismo]

In essence I leaned the mixture inside the cylinder and made it easier for one set of plugs to fire the available fuel/air mix hence no drop.

You got that mostly correct except that carb heat makes the mixture richer not leaner and the richer mixture is what made for less RPM loss on one mag.

IMPO the mag check must be carried out at the "worst case" conditions that the plugs will see in flight i.e. takeoff (Rich cooling mixture, high load and high ICP).Change the mixture, change the load or change the ICP (RPM) and you can have bad plugs pass the "mag check" with (non) flying colors.

I prefer to find my engine descrepincies on the hanger side of the Hold Short line...

JMPO

Chris

There's definitely some validity to your concept but there's a couple downsides to a full power mag check such as prop abrasion (and deice boot damage in my case) as well as the potential for excessive CHTs on a hot day. My point is and has been that a mag check at 1700 RPM isn't significantly more likely to catch a real problem than one done at a less noisy and less damaging 1200 RPM. Neither 1200 nor 1700 provides anything close to the internal cylinder conditions as full power. Check your MP at 1700 and I suspect you'll find that it's around 20 inHg vs about 17 inHg at 1200 and 29 inHg at full throttle. Also IME a marginal plug or mag will be much more apparent with an extremely lean mixture than with any other fuel ratio as lean mixtures require more energy to ignite.

 

[Dan Thomas]

I do agree that pre-ignition pressure will be higher at 1700 RPM than it is at idle and that the higher pressure requires a higher voltage to spark. But I can't see how the pre-ignition pressure would decrease as the throttle is opened further, raising the manifold pressure as long as the prop configuration was constant. It's certainly likely that the volumetric efficiency drops as the RPM increases but the increased MP has to be enough to get more air/fuel into the cylinders on each intake stroke. If it weren't the engine wouldn't produce the additional torque required to spin the prop faster.

And in any case, as I stated earlier, IME if one plug in a cylinder is carbon or oil fouled, it will clear in a few seconds at full power and if it's lead fouled it won't fire at fast idle.

The mag will also generate a hotter spark at full static RPM than it will at 1700 simply because its little alternator is turning faster, so that mag problems are assumed to show up best at 1700. Full static isn't generally a good runup practice, since cooling can become an issue, as does prop blast in crowded areas and braking capacity on many airplanes.

We often use a brief full-power run to see if a fouled plug will clear. Oil-fouled plugs are common on the small Continentals (they seem to pump a lot of oil at idle when relatively cold, filling the bottom plugs) and lead fouling on all smaller low-compression engines on 100LL. Leaning during that clearing run helps.

Dan

 

[gismo]

The mag will also generate a hotter spark at full static RPM than it will at 1700 simply because its little alternator is turning faster, so that mag problems are assumed to show up best at 1700. Full static isn't generally a good runup practice, since cooling can become an issue, as does prop blast in crowded areas and braking capacity on many airplanes.

I'd like to see some data that supports the idea that 1700 is most likely to show up an ignition problem. Most marginal systems I've encountered on airplanes and other types of engines had the most trouble at full throttle max RPM. The energy available for each spark is limited by the inductance of the coil (non-CDI types) at RPM much above fast idle so there isn't necessarily more energy available above 1700. I agree with the rest about issues regarding full power runnups and contend that to some extent they apply to 1700 as well.

We often use a brief full-power run to see if a fouled plug will clear. Oil-fouled plugs are common on the small Continentals (they seem to pump a lot of oil at idle when relatively cold, filling the bottom plugs) and lead fouling on all smaller low-compression engines on 100LL. Leaning during that clearing run helps.

Ditto on clearing oil or carbon fouled plugs but IME once lead shorts out a plug no amount of running at full power with any mixture will clear it.

 

[Dan Thomas]

I'd like to see some data that supports the idea that 1700 is most likely to show up an ignition problem. Most marginal systems I've encountered on airplanes and other types of engines had the most trouble at full throttle max RPM. The energy available for each spark is limited by the inductance of the coil (non-CDI types) at RPM much above fast idle so there isn't necessarily more energy available above 1700. I agree with the rest about issues regarding full power runnups and contend that to some extent they apply to 1700 as well.



Ditto on clearing oil or carbon fouled plugs but IME once lead shorts out a plug no amount of running at full power with any mixture will clear it.

I went looking for the reference I had regarding the runup RPMs and cylinder pressures, and can't find it anywhere. I have WAY too many books here in my office and am getting old enough that remembering where I saw what is becoming difficult. At any rate, it was the only reference I've ever seen on the subject that spoke of cylinder pressures during runup. And it might have been wrong, anyway.

We've been able to clear lead-fouled plugs occasionally, but if we do we know the airplane is only good for that flight. We'll pull it in after the flight and clean the plugs. On some engines (notably the O-235) that run too cool all the time, we use the REM37BY plugs that have a shallow electrode well and extended electrodes so that the lead can't pile up and short the plug.
See the difference in the plugs: http://www.sacskyranch.com/faqsparkplug.htm
Scroll about one-third of the way down the page. We never get fouling with the REM37BY, and it's easy to see why.

Dan

 

[jmaynard]

On some engines (notably the O-235) that run too cool all the time, we use the REM37BY plugs that have a shallow electrode well and extended electrodes so that the lead can't pile up and short the plug.
See the difference in the plugs: http://www.sacskyranch.com/faqsparkplug.htm
Scroll about one-third of the way down the page. We never get fouling with the REM37BY, and it's easy to see why.

My O-200 has REM-37BYs in the lower holes, and I've never seen any evidence of lead fouling at all.

 

[Hiperbiper]

You got that mostly correct except that carb heat makes the mixture richer not leaner and the richer mixture is what made for less RPM loss on one mag.

There's definitely some validity to your concept but there's a couple downsides to a full power mag check such as prop abrasion (and deice boot damage in my case) as well as the potential for excessive CHTs on a hot day. My point is and has been that a mag check at 1700 RPM isn't significantly more likely to catch a real problem than one done at a less noisy and less damaging 1200 RPM. Neither 1200 nor 1700 provides anything close to the internal cylinder conditions as full power. Check your MP at 1700 and I suspect you'll find that it's around 20 inHg vs about 17 inHg at 1200 and 29 inHg at full throttle. Also IME a marginal plug or mag will be much more apparent with an extremely lean mixture than with any other fuel ratio as lean mixtures require more energy to ignite.

Lance,
You are correct of course, I should haved typed Richer not Leaner. I Boobed!
At to the "worst case you the engine will see' it is of course subjective; your RPM limit is the DI boots, mine is anything above 1800 and the tires are sliding. The point is you are doing a runup as close to full power as you are comfortable with for conditions to exercise the mags and engine at something as close to worst case as possible (max power).
IMPO if you aren't putting the whole propulsion system under as much stress as you can manage during a runup thean you might as well save the gas and not bother...


Again; JMPO


Chris

 

[sba55]

IMPO if you aren't putting the whole propulsion system under as much stress as you can manage during a runup thean you might as well save the gas and not bother...

Chris,

I agree with that sentiment. It comes down to what you can find during a runup - we've discussed that. I don't think it's very much. Personally, I switch to both mags at some point during taxi to observe the EGT change. That's it. I also do a mag change at cruise power LOP, and that will find problems 100s of hours before they become an issue. It will also find problems I'd never find on the ground.....

With appropriate instrumentation, there's no point in me doing a runup at all IMHO. There's all these piston guys who do it religiously without knowing why.

-Felix

 

[Sam Foster]

So a tach is minimum day vfr equipment per 91.205. To me that means an inop tach grounds the airplane. What about an inaccurate tach? Say one that is 3000-5000 rpm off and swings within that range? (bad cable). Is that considered inop? Should it ground the a/c?

Thanks,
Matt

I would say no. It is part of the day VFR required. You could contact the nearest FSDO and request a Special Flight Permit to take it somewhere to get fixed.

 

[MauleSkinner]

I would say no. It is part of the day VFR required. You could contact the nearest FSDO and request a Special Flight Permit to take it somewhere to get fixed.

Timely advice.

 

[James331]

3-5k off?

So it doesn’t work.

 

[Dan Thomas]

This thread is over 11 years old.

 

[Brad Smith]

I thought that the OP was trolling when I read the title. It's like asking, "my flaps are stuck at 20 degrees, can I still fly my plane?" But I guess my troll detection meter was off and it needs calibration.