PropOverspeed? How can you tell?

[dueguard1]

Has anyone here ever experienced a Prop Overspeed condition?
What are some of the indicators if this happens?
Has my Govenor failed?
Should I land immediately?

I was reading that since the govenor is such an efficient peice of equipment it is hard to tell this condition has happened?

 

[Kream926]

ummm

i had this happen on a duchess once.......

not sure if it was the tach or what but the left engine was givin me about 3500 rpms

 

[PositionandHold]

Main indicator is an overspeed condition. I.E. something like what Kream926 described. You'll probably hear it, too. Procedure, in the Arrow at least, is prop full decrease, then set at any available RPM. IIRC, that's the procedure in most pistons.

Oh. Yes. And land ASAP. If you have a failed governor and you can't set it at one RPM setting, good chance you're not going to be able to set at any RPM. You've got to secure the engine and land.

 

[timeoff]

I had it happen in a 182 a few weeks ago. Started the t/o roll....full power...check the engine guages and I see the rpm up around 2900. The prop governor check was fine and showed correct indications during the run-up. I believe this was some sort of static prop governor setting as opposed to a legit overspeed/failure.

And yes, the sound is definitely noticable. That was my first indication.

 

[avbug]

I've had propellers run away in piston and turopropeller airplanes. On the piston side, I've had it happen on small single engine horizontally opposed installtions with two blade constant speed props, and large radial three and four blade ham standard hydromatic props.

How can you tell you're in overspeed? Look at your tachometer. Is it indicating faster than your upper limits? You're overspeeding. Is speed increasing above where you set it, or above were you saw it last and you haven't touched the prop control? It's overspeeding.

In a nutshell, an overspeed is caused by a governor malfunction in which the governor allows the propeller to go to the fine pitch/high RPM position. Your propeller has just become a fixed pitch propeller at the low pitch stops. It's going to go as fast as either the engine will drive it, or the slipstream will drive it...which ever imparts more energy to the prop. An overspeed may also be caused by other problems, such as a failure of a low pitch stop, or a runaway engine, or excess airspeed.

If your propeller is running away, engine speed increasing, and you can't bring it back, you need to get it under control, and your only choice may be to shut it down. However, you can try other things first. Reducing power, right to idle, if necessary, may fix your problem. If you reduce power and your RPM falls within limits, and you can keep it there at your current airspeed, then you can use that amount of power...whatever it is, for the rest of the trip in level flight. Any more at that airspeed is going to overspeed the prop.

The secret is slowing down. Pull the power back, get to a slower airspeed where the slipstream isn't driving the propeller, and then start applyng power again. Let the engine impart torque to the propeller and the propeller convert that to thrust, rather than letting it absorb slipstream energy as the slipstream drives it. This is most important in a multi engine airplane, where a propeller that's overspeeding and aborbing energy will be similar to one going into beta or reverse...it's not doing ou any good. It's windmilling out there, if you've got the power back and it's still trying overspeed; the slipstream is driving it, and not your engine. It's aborbing energy from the slipstream, creating a lot of drag on that side, and needs to be either feathered or slowed down.

Many moons ago we lost a C-119. The airplane went into overspeed, and the crew pulled the power back. The aircraft began descending with the reduced power, but it also slowed down, and as it did, the RPM came back under control. The crew applied power now that the airspeed was under control, the aircraft climbed, the propeller ran away again, and they reduced power. Over the course of about 45 minutes, this cycle continued, grdually losing altitude, until it crashed on the high desert floor.

If the crew would have just kept it slow and used engine power and a slower airspeed, they'd have been fine. Slow down, don't let the slipstream drive the propeller, and let the propeller take your engine power and give you something meaningful in return instead of working against you.

Watch your tach, follow your AFM proceedures.

 

[Metro752]

great read thanks

 

[svcta]

A prop overspeed is no small affair from a Mx standpoint. I saw a chart published by Hartzell once that gave the percent in overspeed(beyond engine redline) and time in that condition VS. what needed to be done to the propeller assembly afterword. I was shocked at how small an overspeed would (in the eyes of the manufacturer) render the prop scrap metal.

I've been trying to find the chart and can't locate it, but it's out there in internet land...it was published either by Hartzell or Lycoming, I can't remember which site it came from.

 

[avbug]

Overspeed doesn't necessarily mean the propeller exceeds a limit, goes too fast, or even reaches the low pitch stops. Overspeed is a normal condition which prompts action by the propeller governor...the governor senses the propeller in overspeed above that speed to which it is set, and increases propeller pitch under normal operation.

From a pilot perspective, overspeed is occuring any time the RPM increasesand stays increased above a predetermined setting. If the pilot has set 2400 RPM, for example, and the propeller RPM has increased to 2500 RPM and is steadily creeping up, you have a governor failure, the propeller is overspeeding, but it hasn't reached it's stop limits, and it hasn't exceeded it's published limits.

Speed limits are generally not metalurgical limits, but aerodynamic limitations.

That said, the forces acting on a propeller disc, and upon each propeller blade, are enormous. The blades cone and flex a lot during flight; if you realized just how much those blades flex and move, you might be a little nervous during your next flight.

Another problem of speed ranges that might occur outside the published ranges is something that's not given you in the AFM or even the maintenance manual...because it's occuring outside the published range, and that's harmonics. This can destroy a propeller very quickly to the point of failure, or fatigue the propeller or engine such that it appears normal, but will develop problems or fail at a later time.