Prop overspeed question

dji · Post by **dji** » Sun Nov 13, 2005 9:54 am

Hi quick question, yesterday I was in a cessna 206 climbing (probably around 11500 don't remember exactly) at one points I noticed a little vibration so I checked everything and I was back up to 2650 rpm or so.. I was sure I hadn't forgotten to reduce it after take off, so I reduced it again to 2500 and the lever was way farther than normal for that rpm setting. Couple minutes later I was back at 2600something (red line 2700), so I told my guys to prepare. I wasn't very far anyways, but at that point I had to reduce power to keep the RPM so my jumpers went and now I had to keep 10-12' manifold to keep the prop at 2500, I slowly descended and landed, end of story

I'd like and AME's idea on the incident, probable cause and in that situation, is it possible to ask power to the engine or is it just gonna overspeed without giving much thrust.

thanks

Goodwrench · Post by **Goodwrench** » Sun Nov 13, 2005 4:32 pm

A constant speed prop in either a piston or turbine installation is basically a balancing act of the applied forces. You control the speed and power you want by selecting R.P.M. and Manifold Pressure/Torque. The rest is done through the prop. governor (in most cases at least) by applying a variable oil pressure to the prop hub, to change blade angle. Generally, an increase in oil pressure sends the blade angle to fine pitch and decreasing oil pressure toward coarse (feather). From what you wrote, I would suspect the prop. gov. is at fault, unless there is a whole lot of slack in your control system. That should be evident in the system being non responsive, and sloppiness in the lever movement. Better get an A.M.E. on it before something goes real bad!

Billy · Post by **Billy** » Sun Nov 13, 2005 6:17 pm

Goodwrench has covered the items that could be the problem. I would go for the Governor as on a single engine aircraft the prop tends to go fine when the governor fails to become fixed pitch. This gives you some control to land the aircraft, you wouldn't want it to go coarse (to feather) unless your driving a twin. All you will do if you add power with a failed governor is increase RPM as the blade angle will not change to absorb the power to produce more get up and go.

The perfomance with the prop stuck in fine for climb, cruise etc might be in the POH, it might even mention failed governor procedures. Although making all your pax jump out seems a little extreme.

You should have it checked out before you venture into the wide blue yonder again.

chubbee · Post by **chubbee** » Tue Nov 15, 2005 6:54 am

The C206 prop governor is spring loaded to full fine if the control linkage separates.
You can see the prop governor at the front of the left cowl inlet and observe the effect of control inputs.

Turkey · Post by **Turkey** » Mon Jan 02, 2006 10:14 am

It would be interesting to know what the engine's oil pressure was at that time. A loss of engine oil pressure (or a localized blockage) can create a supply problem for the governor, which in turn, is boosting that oil pressure and delivering it to the prop to keep/increase the blade angle (low RPM). A loss of oil pressure will cause the blade angles to change towards fine pitch.

Youngback · Post by **Youngback** » Tue Jan 03, 2006 12:59 pm

What Turkey said.

A loss of oil pressure has different effects depending on whether it is a turbine or a piston aircraft. For a 206, I also suspect you had low oil pressure.

scootermp · Post by **scootermp** » Sun Feb 11, 2007 1:48 pm

Everyone has valid points here, one other thing to check, the easiest, and possilby the most ulikely, would be the cable itself, if any work has been carried out on the engine, someone may have forgotten, to tighten the cable harness bracket, or it may be broken, it's a long shot, but something to check, it's very easy to check. Other than that, gov problem, or oil pressure problem mabee.