View Full Version : Propellor Overspeed


AFFF
17th Feb 2009, 10:27
Been in airport fire and rescue for more years than I care to remember, and come across almost every type of emergency imaginable, however, last year came across a new one on us. We had two calls to the same aircraft (Dash 8 Q400) within a couple of weeks of each other, with a prop overspeed problem. Seem to remember it could be a problem on old piston engined aircraft in the 50's. Hope someone can answer a couple of questions:

1. What is prop overspeed?

2. What causes it?

3. How does it affect the aircraft in flight?

4. What corrective action can the flight crew take?

5. Is prop overspeed the same as a runaway prop?

Thanks in advance,

AFFF.



Checkboard
17th Feb 2009, 12:01
1. What is prop overspeed?

The propellor "twists" in its mounting to "bite" the air harder, and produce more thrust, or twist the other way to slip through the air easier, producing less thrust - for the same engine RPM. Pour on the fuel, and instead of the engine increasing in revs, like in a car, the prop twists (increases pitch). It's the aeroplane equivalent of gears. A prop overspeed is when the pitch is too fine for the power supplied - and the engine/propellor combination exceeds the engine RPM limits. (Like driving down the road in fourth gear, and accidentally selecting second - the engine screams as it over-revs.)

2. What causes it?

Either a governor failure (the piece of equipment which governs propellor pitch), or in hydraulic systems (which use engine oil or hydraulic fluid to drive the pitch changes), a loss of fluid.

3. How does it affect the aircraft in flight?

Depends on how bad it is, and how quickly the pilot reacts to reduce power. Worse case is a catastrophic engine failure. Best case is a minor limit exceedance, requiring an engine inspection before returning to service. There is a (controllable) yaw when it occurs as well, similar to an engine failure.

4. What corrective action can the flight crew take?

Control the yaw, reduce the power.

5. Is prop overspeed the same as a runaway prop?

I would say so.

GotTheTshirt
17th Feb 2009, 14:26
AFFF,
Just to add to checkboard The VP operates from fine (thin bite angle) to coarse (thick bite angle)
With the engine at a given power setting the prop angle will decide the speed and the govenenor selects ( and maintains) this best speed.

When the engine starts ( in MOST cases) the propelleor is fully fine and as the engine speeds up the propellor goes towards coarse the stop the engine overspeeding.
In fact the maximum rpm adjustment is on the propellor not the engine.

During flight the propellor has a natural tendency to go to fine pitch but is held back by the govenor. When a prop overspeeds or runs away it is normally the govenor giving up the fight !
Now with the propellor in fully fine it its like a childs windmill which as you push it through the air it spins so the aircraft flying along the propellor is driven by the airflow. The faster the aircraft the faster the rpm.

On big 4 engine pistons we had a procedure for runaway props which will indicate how serious it is.

1. Reduce power and shut the engine down and cut off the fuel. ( Even in this conditon the forward speed of the aircraft is driving the propellor above the red-line.
2. Continue to try and feather the engine ( again the hi RPM will overpower the feathering pump.
3. Descend and reduce airpeed ( low and slow ). low speed and more dense air should reduce the RPM.
Of course there is a minimum speed that you can go to and in some cases even this is not slow enough and the drag from the propellor disc is very high.


We had a DC 3 overspeed on takeoff which the crew were quick enough to put back on the ground but not before the RPM not only went past the red line but off the clock !!
The engine case, normally air force green, was bright blue from the overheating!!

4.

Old Smokey
17th Feb 2009, 16:07
Just an additional note.

Because of a high degree of commonality of the propeller control mechanisms, a Propeller Control Unit failure leading to an overspeed often also meant the inability to feather the propeller.

Later propellers, such as the RR Dart 528/532 had "High Speed" intermediate stops, much limiting the degree of "fining" of the propeller, and maintaining the aircraft in a controllable situation, even if unable to feather the propeller (The earlier RR Darts did not have this feature).

Regards,

Old Smokey

AFFF
19th Feb 2009, 11:12
Thanks for the replies folks, much appreciated.

AFFF:ok: