RPM Surge when pulling the mixture too much

[redbaron1982]

This happened to me twice already, trying to go from ROP to LOP I pulled the mixture too much, to the point when the engine started to quit, and the RPM would surge (for instance I was cruising at 2400 RPM and when did the mixture pull they went up to 2900 RPM). This lasted for less than 1 second as in both cases I pushed the mixture back in and then lean by using the vernier, in a much slower fashion.

My question is, is this normal? So, if the engine quits, for instance, due to fuel starvation, then the propeller would overspeed? Oil pressure should remain fairly stable if the prop is spinning, no matter if it is because of engine own power or air moving through the prop. So why an engine that is quiting makes the prop overspeed?

I've checked in both cases and due to the very short duration of the overspeed, and the actual RPM value, the prop is ok, doesn´t require a special inspection or overhaul.

BTW: The prop is a 3 blades MT.

[M20F]

What engine, what prop, what plane, etc. 

[redbaron1982]

The airplane is in my profile, M20J, the prop is on my post: 3 blades MT. The engine... are there more than one engine on M20J? Lycoming IO 360 A3B6D.

[mcarterak]

12 hours ago, redbaron1982 said:

Are there more than one engine on M20J? Lycoming IO 360 A3B6D.

Yes, IO360-A1B6, -A1B6D, -A3B6, -A3B6D, IO-390-A3A6. Continental IO-550 if you consider a Missile still a J. 

[A64Pilot]

It shouldn’t within reason the prop governor should maintain RPM, of course like everything a change that’s rapid enough will shoot past before it can be controlled.

My guess based on how I could adjust the beta nuts on a turbine prop, that setting min pitch, that your prop whether the min pitch is correct or not is set too flat, so that when power goes away the prop goes flat and becomes driven rather than a driving thing and it’s driven at too high an RPM before the prop governor catches it.

An example of what I’m talking about is a helicopter in autorotation, in normal flight the engine drives the rotor of course, but if engine power goes away the pilot can drop the collective, reducing pitch and the rotor is driven by airflow through it, have to little pitch and the rotor will overspeed.

All unsubstantiated theory, but airflow not the engine must be what’s driving the prop, or you have an engine that when leaned very quickly has a rapid huge power increase, something I find unlikely.

When the prop increases RPM rapidly, you should feel a pretty strong deceleration of the aircraft

[PT20J]

Doesn’t surprise me. The MT prop has lower inertia than a metal prop, so its transient response is probably faster than the governor’s. So, when power is abruptly decreased, the governor commands low pitch. But the airspeed hasn’t changed so the prop is transiently driven overspeed. The governor is a little behind, but quickly catches up. 

[redbaron1982]

33 minutes ago, PT20J said:

Doesn’t surprise me. The MT prop has lower inertia than a metal prop, so its transient response is probably faster than the governor’s. So, when power is abruptly decreased, the governor commands low pitch. But the airspeed hasn’t changed so the prop is transiently driven overspeed. The governor is a little behind, but quickly catches up. 

That's a good point, it could also explain why my RPMs are always fluctuating a bit (like +/- 20RPM).

[Shadrach]

I very much appreciate everything that APS has given to the aviation community over the last three decades. That being said some of their advice has created its own set of “old wives tails”

The severe danger of “The Red Box” and need for an aggressive “Big Mixture Pull” has been over-embraced by many in the flying community. Under most scenarios there is almost no way to hurt an engine with the mixture knob in under 20 secs. There is no need to make abrupt mixture changes when setting cruise power…especially when operating a normally aspirated IO360. The only time when a mixture change might cause an immediate problem is if the engine has already been operating at the margins when the adjustment is made. That is a rare situation. For normal ops just take your time, be gentle…it’s not going to blow up or fry a valve or even shoot your eye out. Smooth and deliberate but not fast works just fine.

[A64Pilot]

39 minutes ago, redbaron1982 said:

That's a good point, it could also explain why my RPMs are always fluctuating a bit (like +/- 20RPM).

That shouldn’t happen either, steady state there isn’t much difference between a light and a heavy prop, it’s accelerations positive or negative that inertia really comes into play. I’m ignoring aerobatics of course. If your speed is wandering plus or minus enough for you to be able to hear and or feel it, somethings wrong and I’d suspect the governor. If you can’t hear or feel a change then maybe it’s your tach, an inexpensive photo tach will tell if it’s really wandering or not.

One thing to be aware of is rapid accelerations, like maybe an unplanned go-around could get a light weight prop into surging, could not saying it will.

Been a long time ago but on the search for the perfect prop for an airplane we were developing Hartzell built me a four blade 108” prop. On a Turbine airplane a Hartzell prop has beta nuts, these Beta nuts set min pitch setting and are easily adjustable, with a turbine you can flatten the prop quite a bit and it really causes an airplane to decelerate, sort of like big speed brakes.

Well to make a long story short I found that on that prop when the Beta nuts were set for a nice decel for landing that a rapid pull to idle at a high cruise would cause the prop to run away, then the overspeed governor would catch it with a big bang, overspeed governor operated by adding pitch, but when the prop dropped in RPM the overspeed governor would turn loose of it and then another overspeed, another bang as the overspeed governor kicked in, reducing prop speed stopped that of course but it really scared me

[PT20J]

1 hour ago, redbaron1982 said:

That's a good point, it could also explain why my RPMs are always fluctuating a bit (like +/- 20RPM).

How long since the governor has been overhauled?

[Shadrach]

2 hours ago, redbaron1982 said:

That's a good point, it could also explain why my RPMs are always fluctuating a bit (like +/- 20RPM).

My hartzel fluctuates +/- 20rpm in climb, might do it in cruise too. I’ve never checked. I can’t hear it, it’s only discernible on my optical tach. I would think a fluctuation of under 1% is allowable.

[201Mooniac]

3 hours ago, redbaron1982 said:

That's a good point, it could also explain why my RPMs are always fluctuating a bit (like +/- 20RPM).

that is not something I see on my M20J with the MT prop.  Once set the RPMs are rock solid.  I do notice that I can get to 2730 or so if I apply takeoff power too quickly so I think Skip is correct about the governor being a little slow for the reduce prop inertia.

[redbaron1982]

2 hours ago, PT20J said:

How long since the governor has been overhauled?

The only entry I can find in the log book regarding the governor is from 5/2015 when it was installed new after a prop strike tear down. So it's 8 years and ~1000TT since then.

Looking back at the data from the GI 275 from the previous owner, I see that in 2021 it was already doing this fluctuation:

[PT20J]

36 minutes ago, redbaron1982 said:

The only entry I can find in the log book regarding the governor is from 5/2015 when it was installed new after a prop strike tear down. So it's 8 years and ~1000TT since then.

Looking back at the data from the GI 275 from the previous owner, I see that in 2021 it was already doing this fluctuation:

It shouldn’t hunt like that. I’d check that the prop cable is secure at the governor to rule out vibration, but it’s likely a governor issue. I would discuss it with Dan Long at West Coast Governor Service. 

[PT20J]

On 8/12/2023 at 10:45 AM, Shadrach said:

I very much appreciate everything that APS has given to the aviation community over the last three decades. That being said some of their advice has created its own set of “old wives tails”

The severe danger of “The Red Box” and need for an aggressive “Big Mixture Pull” has been over-embraced by many in the flying community. Under most scenarios there is almost no way to hurt an engine with the mixture knob in under 20 secs. There is no need to make abrupt mixture changes when setting cruise power…especially when operating a normally aspirated IO360. The only time when a mixture change might cause an immediate problem is if the engine has already been operating at the margins when the adjustment is made. That is a rare situation. For normal ops just take your time, be gentle…it’s not going to blow up or fry a valve or even shoot your eye out. Smooth and deliberate but not fast works just fine.

BMP technique seems more useful in turbos as a way from going from ROP high power WOT to a lower LOP power without touching the throttle. In my little IO-360, the only time it would be applicable is at low altitudes, and when I cruise any distance, I’m high enough that mixture is not going to hurt anything.

Some of the APS ideas like BMP and running tanks dry just don’t fit my operational philosophy of smooth flying. I flew a lot of Part 135 for years and we always strived to fly so that the pax couldn’t tell you changed anything. Not saying the APS guys were wrong; it’s just not how I fly.

Skip

[Shadrach]

1 hour ago, PT20J said:

BMP technique seems more useful in turbos as a way from going from ROP high power WOT to a lower LOP power without touching the throttle. In my little IO-360, the only time it would be applicable is at low altitudes, and when I cruise any distance, I’m high enough that mixture is not going to hurt anything.

Some of the APS ideas like BMP and running tanks dry just don’t fit my operational philosophy of smooth flying. I flew a lot of Part 135 for years and we always strived to fly so that the pax couldn’t tell you changed anything. Not saying the APS guys were wrong; it’s just not how I fly.

Skip

Good practice indeed. I try to catch the tanks at the first hint of a fuel pressure drop. I find the system will fluctuate quite a bit before the fan stops turning. That isn’t to say that I haven’t missed it before.

I don’t have much experience with Aircraft turbos. I’m guessing there’s some advantage to not touching the throttle while leaning to LOP. Most of the reading I’ve done about operating big radios suggest that they lean first and then add MP back with a throttle.

i’ve done a lot of experimenting IO360 at low altitude, high power LOP settings.  Even under those conditions you’d be hard-pressed to heat it in under 20 seconds except for on the hottest days.

[hubcap]

My understanding of the danger of the “red box” has more to do with turbocharged engines at high power settings like Skip mentioned, and the parameters for the red box require higher than 400 degree CHT’s for the internal cylinder pressure to be at its highest.

[Fly Boomer]

On 8/12/2023 at 12:45 PM, Shadrach said:

The severe danger of “The Red Box” and need for an aggressive “Big Mixture Pull” has been over-embraced by many in the flying community. Under most scenarios there is almost no way to hurt an engine with the mixture knob in under 20 secs.

I suspect the advice was given so that some pilots wouldn't make an all-day affair out of backing the mixture off in tiny increments and waiting for temperatures to stabilize before the next tiny increment.  I don't know about 20 seconds, but I think it's possible to damage some engines at some power settings with ill-informed mixture settings in 60 seconds.  Part of that advice was to pull it back swiftly enough that it feels like someone stepped on the brakes -- more of a seat-of-the-pants change than remaining studiously focused on the temperatures.

[Fly Boomer]

22 hours ago, PT20J said:

Some of the APS ideas like BMP and running tanks dry just don’t fit my operational philosophy of smooth flying. I flew a lot of Part 135 for years and we always strived to fly so that the pax couldn’t tell you changed anything. Not saying the APS guys were wrong; it’s just not how I fly.

There are some things I would do when solo that I would not do with non-pilot passengers on board.

[Parker_Woodruff]

This happens because the prop has low inertia and accelerates very quickly.  Adding power or RPM should be done slower with the MT propeller.

Prop governor needs the pressure turned all the way up with this prop and it will lose pressure over time.

[Shadrach]

3 hours ago, Fly Boomer said:

There are some things I would do when solo that I would not do with non-pilot passengers on board.

This you???

 

[Fly Boomer]

5 hours ago, Shadrach said:

This you???

Yikes!  No thank you.

Let me change my original statement to “There are some perfectly safe things I might do solo, that I wouldn’t want to risk giving a passenger a bad experience.”  Could be as simple as rate of descent.  I can probably tolerate a faster descent than some passengers.  First time I flew with my Mother, I was doing great until she made the first turn.  I was not okay with the bank.  I might tighten my belt and bounce along in some light chop but, in deference to a passenger, I would try to get out of the bumps.

[Pinecone]

Most of the development of the LOP movement in GA was done with NA and turbo normallized engines.

The original work was with supercharged and turbo supercharged big radials.

[quote]I suspect the advice was given so that some pilots wouldn't make an all-day affair out of backing the mixture off in tiny increments and waiting for temperatures to stabilize before the next tiny increment.[/quote]

This is a big part of it.  A smooth pull over a few seconds to a knonw LOP fuel flow.   And the BMP for NA was done in the climb out at full power.  It was instead of pulling the power back to 25" as was common then.

Don't dawdle, but pull slow enought to not seriously overshoot.

[A64Pilot]

17 hours ago, Shadrach said:

This you???

 

Notice how much quicker she went to the left and how much difference in pitch was required.

It rolls of course better going with engine torque than against it. But be careful if anyone tries this, on a slick airplane like a Mooney airspeed can easily get out of control if you let the nose drop too much.

I’ve got neighbors that will teach you aerobatics in a Zlin if anyone had the itch, better than trying it yourself in a Mooney.

I’m old and don’t have the stomach for it anymore myself

[Fly Boomer]

8 minutes ago, A64Pilot said:

I’m old and don’t have the stomach for it anymore

A CFII (who also happened to be a friend) wanted me to put my head down to get the hood on and get it adjusted.  While I was doing that, he would roll the airplane a couple of times, then jank into some weird attitude and immediately ask for an unusual attitude recovery.  Not my cup of tea either.