Magneto operation and constant speed propellers

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CerroTorre

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CerroTorre
There is a question in the commercial pilot oral exam preparation that is a little bit confusing to me.

“While in flight in an airplane with a controllable pitch propeller, how will the failure of a single magneto affect rpm indication in the cockpit?”

Will this not indicate as simply a drop in manifold pressure and constant RPM? The power output of the engine will decrease so won’t the governor simply decrease the pitch of the propeller, thus maintaining propeller RPM? So no change in RPM?

or what am I missing in the chain of events? Thanks all.
 
u ever do a runup and check the mags? what are you supposed to monitor when you do the mag check?
 
Well there is an RPM drop with a fixed pitch propeller of course. And we do one prior to every single flight. That’s easy.

But the question is asking about an in-flight failure with a constant speed propeller where power output is reflected by the manifold pressure.
 
Or, is the answer simply “there is no change in rpm indication“.
 
First, the HP drops, and the RPM.
Next, the governor changes the pitch to recover the set RPM.
Now, the RPM is back, but the MP is lower due to the higher RPM flowing more air, even as the HP is lower.

The OP has it right. No change in RPM.

Practical result, the plane starts to descend slowly, as the air speed drops from the lower HP. Looking for the cause, you find the changed MP, and increase throttle to get back the cruise speed that you wanted and flight planned.
 
I can understand how, during runup (in both fixed pitch and CS prop aircraft) at the lower RPM/power setting, we are
watching for the RPM drop as the indication of the reduced power since we are below the power output where the governor is activating to maintain propeller RPM.

But the question seems to be in reference to in-flight power settings.
 
Thanks geezer.

This might be one of those times where my overactive brain overthinks the question. They are just testing my understanding of the systems.
 
CerroTorre said:
There is a question in the commercial pilot oral exam preparation that is a little bit confusing to me.

“While in flight in an airplane with a controllable pitch propeller, how will the failure of a single magneto affect rpm indication in the cockpit?”

Will this not indicate as simply a drop in manifold pressure and constant RPM? The power output of the engine will decrease so won’t the governor simply decrease the pitch of the propeller, thus maintaining propeller RPM? So no change in RPM?

or what am I missing in the chain of events? Thanks all.
Click to expand...

Just think of it as if you slightly pulled back on the throttle. What indications would you see? If the prop is within the governor range, RPM will remain constant and MP will drop. If it is below the governor range, both will drop.
 
Yeah sarangan, understood. The way the question was posed made me think they were looking for an answer that there would be a change in RPM at cruise or in-flight power settings - when there is none (or none besides a momentary adjustment if/as the governor lags a bit maybe).

I was inferring something that was not stated in the question. Which led to confusion where there isn’t any.

KISS principle often works on test questions just like in real life :)
 
Realistically speaking in practice you would likely have a noticeable roughness as well
 
geezer said:
Practical result, the plane starts to descend slowly, as the air speed drops from the lower HP. Looking for the cause, you find the changed MP, and increase throttle to get back the cruise speed that you wanted and flight planned.
Click to expand...
That's what a lot of pilots would do. Just open the throttle to get the airspeed back. They don't stop to wonder why the power decreased, so they don't do any diagnostics. Carb ice is another way to get caught; the airplane slows down while the RPM stays constant, and by the time the pilot realizes something is wrong, the ice situation has gotten pretty bad and maybe the carb heat won't fix it before the engine dies.
 
CerroTorre said:
There is a question in the commercial pilot oral exam preparation that is a little bit confusing to me.

“While in flight in an airplane with a controllable pitch propeller, how will the failure of a single magneto affect rpm indication in the cockpit?”

Will this not indicate as simply a drop in manifold pressure and constant RPM? The power output of the engine will decrease so won’t the governor simply decrease the pitch of the propeller, thus maintaining propeller RPM? So no change in RPM?

or what am I missing in the chain of events? Thanks all.
Click to expand...
Should be no change in RPM most of the time. Could happen though if you are at such a low power setting that you are 'out of the governor.'
 
Strictly speaking (and pedantically), if they really mean "controllable pitch propeller" rather than constant speed propeller, the answer is probably a little more complicated, and more akin to what happens with a fixed pitch prop until the pilot adjusts the prop pitch manually. But then I've never seen a controllable pitch prop rather than a true constant speed prop, so what would I know?
 
I did consider going down that rabbit hole as well woywoyboy. :)
 
woywoyboy said:
Strictly speaking (and pedantically), if they really mean "controllable pitch propeller" rather than constant speed propeller, the answer is probably a little more complicated, and more akin to what happens with a fixed pitch prop until the pilot adjusts the prop pitch manually. But then I've never seen a controllable pitch prop rather than a true constant speed prop, so what would I know?
Click to expand...
Seems to me I saw a controllable pitch prop not long ago. It wasn't infinitely controllable so to speak. But there were a some fine/coarse changes you could make while the engine was running.
 
If you have an electronic tach, a little red light comes on.
 
I’m not convinced the MP will drop. It’s not the same as pulling back the throttle. You aren’t restricting air into the intake. If anything, the MP might go up because the engine isn’t sucking in as much air. But I don’t think it would be enough to move the MP.

tell me why I’m wrong.
 
Look over my shoulder… controllable pitch prop. Gonna see a rpm loss.
 
Salty said:
I’m not convinced the MP will drop. It’s not the same as pulling back the throttle. You aren’t restricting air into the intake. If anything, the MP might go up because the engine isn’t sucking in as much air. But I don’t think it would be enough to move the MP.

tell me why I’m wrong.
Click to expand...

Yep I'm with Salty on this one. I can confirm that at runup RPM the MAP does in fact increase a bit but that is different.

Been awhile since I have done an inflight mag check but I don't think the MAP is going to change much if at all. The EGTs on the otherhand...
 
Agree, I think MP will stay the same. EGTs will go up if ROP, down if LOP. Airspeed or altitude will go down.

Carb ice will cause a loss of airspeed or altitude and most telling, a loss of MP.
 
eman1200 said:
u ever do a runup and check the mags? what are you supposed to monitor when you do the mag check?
Click to expand...

Two things. During runup the prop is not in the governing range. There is no lower pitch for it to use to prevent RPM loss. At cruise power, the prop will change pitch to keep the same RPM.

The smart thing to check is that all the EGTs rise by about 50 degrees. This checks each spark plug for issues.
 
You do the mag check with the blue knob all the way in. You'll definitely get a drop on the mag check.
 
geezer said:
First, the HP drops, and the RPM.
Next, the governor changes the pitch to recover the set RPM.
Now, the RPM is back, but the MP is lower due to the higher RPM flowing more air, even as the HP is lower.

The OP has it right. No change in RPM.

Practical result, the plane starts to descend slowly, as the air speed drops from the lower HP. Looking for the cause, you find the changed MP, and increase throttle to get back the cruise speed that you wanted and flight planned.
Click to expand...
Constant speed = constant speed. Geezer is correct!
 
Salty said:
I’m not convinced the MP will drop. It’s not the same as pulling back the throttle. You aren’t restricting air into the intake. If anything, the MP might go up because the engine isn’t sucking in as much air. But I don’t think it would be enough to move the MP.

tell me why I’m wrong.
Click to expand...
The power would fall a little, running on one mag. I could see the MP not moving much at all. The exhaust has been scavenged as normal by the time intake opens, so no remaining pressure in the cylinders, and at the same RPM the same volume of air is being drawn in, so the MP should stay about the same.
 
flyingron said:
You do the mag check with the blue knob all the way in. You'll definitely get a drop on the mag check.
Click to expand...

Yes, because at that RPM the prop is in full flat (minimum pitch). Any change in power will result in a change in RPM.
 
Tantalum said:
Realistically speaking in practice you would likely have a noticeable roughness as well
Click to expand...

Nope. Ran one for 2 hours and didn't realize it was bad. It passed the mag check on the ground.
 
EdFred said:
Nope. Ran one for 2 hours and didn't realize it was bad. It passed the mag check on the ground.
Click to expand...
what ended up being the issue? My only experience was broken teeth that caused a fairly severe misfire. Previous to that one mag was totally dead, so there was no drop on the one engine - which I guess, in hindsight, otherwise ran smooth on the good one
 
My Cessna 120 has a Beech Roby prop. Mechanically adjustable in flight.

From a practical standpoint, basically have a climb prop AND cruise prop.

It’s kinda cool!
 
The Swift aircraft had an air adjustable propeller, optional.
Springs held it in fine pitch, and thrust kept it from changing.
After reaching suitable altitude, throttle to idle, the thrust went through zero, and centrifugal forces from a pair of weights shifted the blades to cruise.
You had to remember that for a go around, you had cruise still set. It reset at idle on the ground.

From memory, 50 years ago....
 
Early Bonanzas with the E-series motor and electric prop would be classified as controllable pitch. Especially because some of them are poorly maintained and the "Auto" mode of the prop no longer functions properly. :D
 
Tantalum said:
what ended up being the issue? My only experience was broken teeth that caused a fairly severe misfire. Previous to that one mag was totally dead, so there was no drop on the one engine - which I guess, in hindsight, otherwise ran smooth on the good one
Click to expand...

Broken mag spring, but it was only noticeable during the mag check - where the RPM dropped from 1800 to 800. Check was fine in NC, during cruise it still ran smooth, mag check in KY showed the issue. I also had a broken P-lead once (and on this type of mag it did not stay hot, it went cold) and the only reason I knew the mag was out is because the MVP 50 said one mag was out. Still ran perfectly smooth.
 
Not a constant speed or variable (in flight) pitch prop. But a total mag failure (no spark at all) seemed to run just fine until the mag check where the mag drop was 100% (Rotax 912). I assume the failure happened on the previous flight. But, when you assume, you make an ass out of u and me.


(And yes, not an actual "Magneto", it's a capacitive discharge ignition, but...)
 
Thanks for the additional discussion here folks. Lots of stuff to glean from answers to one dumb question. =)
 
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