Propeller Governor Diagnostics
This article will cover things that cause problems with propeller governor systems and how to diagnose them. Often, governor problems don't give any warning until something obvious happens, such as the inability to hold selected RPM, the propeller surges or seeks an RPM, or is airspeed-sensitive. In the case of twins, the problem may manifest itself as the propeller occasionally or often going into uncommanded feather, especially on landing rollout.
There are a number of different components in the system that can cause these symptoms. Let's look at the different causes and troubleshooting procedures, starting with the most common complaint, that being the lack of major RPM control.
In single-engine aircraft, this shows up as the RPM being too high, and the apparent inability of the governor to keep the RPM down. In twin-engine aircraft this shows up as the opposite, with RPM too low, and the governor's apparent inability to bring the RPM up to the set speed.
There are a few things that cause this symptom: 1) Sticky prop -- sticking blades or actuating piston, or hydraulic lock; 2) Bad leak at the case-to-crank transfer point; and 3) Leaking pilot valve in the governor or low-governor oil pressure.
A sticky prop usually happens just after prop overhaul. Sometimes a shop will shim the blade bearings tight to account for wear throughout the TBO life of the prop. If they tighten it up too much, the blades can stick and make the system insensitive to small blade-angle-change commands from the governor.
One key to the problem is whether or not the condition is worsening. Sticky blades will not usually get worse, so it's safe to say that if the problem is worsening, sticky blades aren't your problem.
One interesting problem that can happen at prop overhaul is improper pitch-stop settings. On a single-engine propeller (where oil pressure increases blade angle), a prop may be set up for the naturally aspirated model of the airplane instead of the turbocharged model. For instance, a C-T210 prop may get set up as a C-210, or an A36TC prop may get set up as an A36 prop. The props for a turbocharged aircraft model actually have a higher blade pitch stop setting than the prop of the naturally aspirated aircraft of the same basic model.
The symptom that shows up here is the inability to keep RPM down at a particular density altitude and airspeed. Higher altitude or higher airspeed does nothing but make the symptom worse; i.e., the higher or faster you go, the higher the RPM goes. And when you point the nose down for descent, the RPM really takes off. In essence, it's acting just like a fixed-pitch prop. Once you reach the altitude at which the symptom shows up, any further increase in altitude, airspeed or engine torque causes a corresponding increase in RPM. The prop is against the high pitch stop and can't go any higher.
On twin-engine aircraft (where oil pressure decreases blade angle), it's the low-pitch stop that will occasionally get set improperly. This will show up as the inability to reach proper static-RPM because the blades can't go to a low enough angle to allow the engine to spin up to max. RPM. Once you're rolling on takeoff, however, the RPM comes up, and for the rest of the flight everything is just fine.
A note of caution here: On the single or twin, this particular problem will only happen after prop-hub maintenance of some kind. A similar symptom showing up suddenly or gradually, with no maintenance having been done on the prop or engine, indicates a weakening engine or possible governor-system trouble.
The piston in the prop dome can stick occasionally. It usually happens when a seal gets pinched between the piston and cylinder wall. Sometimes, an engine that has experienced metal contamination will have metal particles wedge between the piston and wall, as well as tear up the seal.
The centrifugal force in the prop dome slings debris right into this area and will eventually cause problems. That's why the prop is one of the components on the list of things that must be disassembled or overhauled after a metal-contamination incident to eliminate debris.
To check the twin for a sticky prop, start the engine and wait for the governor to bring the blades off the feather locks, usually indicated by a slight rise in RPM a few seconds after the oil pressure starts to come off zero. At about 1,000 RPM, bring the prop to feather and immediately shut down the engine. The prop will stop fairly quickly, enabling you to watch it go into feather. There should be a smooth, constant movement to feather that only stops at the feather stop. If the prop requires help to fully feather or is jerky or sticky in its movements, you should get it sent out for repair. To check the single for a sticky prop, see the section below on transfer-system troubleshooting.
A worsening problem is indicative of hydraulic lock or excessive clearance in the oil-transfer system. First, let's look at hydraulic lock.
Hydraulic lock is pretty unusual. It will act similarly to improper low-pitch-stop settings. On single-engine aircraft, the prop will not bring the engine RPM down as low as it should. The engine will also speed up with an increase in airspeed or an increase in manifold pressure (engine torque). On a twin-engine prop, it's just the opposite: The engine won't go to correct static RPM any more. On the twin this could also indicate a bad transfer system, so you'd better troubleshoot for both.
The oil-filled McCauley propeller is the most likely to fall prey to hydraulic lock because it is designed to trap the red-dyed oil in the hub and retain it there. The other McCauley models are vented through a small hole in one of the hub-mount dowels, making them less likely to lock up. If oil is coming out that dowel hole, you have the same leak by the prop piston seals that could lock up a Hartzell.
Hartzell hubs have no vent and, therefore, can be susceptible to hydraulic lock, however unusual it may be. To check for this condition in a Hartzell, set the hub with a grease zerk pointing down and remove the zerk. If oil comes out, you're getting oil past the piston (single) or piston-rod seal (twin) in the hub. The oil may also blow out the blade-shank seals (grease seals) and make a mess of the cowl and nacelle. On the Hartzell twin props, if the front-side piston seal is going bad, the air charge in the dome usually leaks out into the oil cavity and is vented back to the engine during prop cycling.
A continuously low prop dome will cause other problems discussed later. A proper piston/dome reseal will cure this problem in either make propeller.
The oil-transfer system is the only other cause of this RPM control problem. This includes the governor and oil-transfer components. The governor may have a bad internal leak in the pump, pilot valve or pressure-relief valve. Any of these will cause the governor to lose some or all control. There is no direct troubleshooting for this unless you have a governor bench handy. The problem must be found by process of elimination. Since the governor has to be pulled off to send in, first check out the transfer system.
Oil is transferred in from the prop governor to the prop through one of the below-mentioned methods. If there is excessive leakage in this transfer system, the prop won't be able to hit high-pitch/low-RPM on the single, or low-pitch/high-RPM on the twin. The twin may even go into an uncommanded feather. This is because too much oil is leaking from the excessive clearance of the transfer system.
This problem can come from several sources. On the system with bearing transfer, either a standard bearing on an undersized crank or bearing galling causes the clearance to increase and thereby leak oil. On the transfer-collar system, the transfer collar will be too large for the crankshaft, either because of galling or improper fit. On the transfer-tube type, the tube-end clearance will be too large for the same two reasons as excessive transfer-collar clearance.
To check for this problem, the prop governor must be removed. Use shop air and an air gun with the rubber tip inserted into the prop transfer hole in the governor pad. Put full shop pressure on this transfer hole until all the oil is blown out of the main bearing (no gurgling is heard). When you start this you should notice the prop blades go to immediate high-pitch setting on singles or against the low-pitch stop on most twin-engine governors.
If you want to kill two birds, you can just pull the prop governor and do this check, seeing if the blades go to high pitch, checking for hydraulic lock, sticky blades, and blowing all the oil out of the bearing at the same time. You'll have to do this two or three times so that the oil will be removed from the prop dome. Caution: Do not pull the rubber tip out of the hole until all the pressure inside the prop dome is released. If you do, you will be covered with a huge spray of oil. Also note it should only take about 30 to 40 psi for the prop to go to its pitch stop.
Now hook up a compression tester to the shop air hose and the rubber-tipped air gun to the compression tester instead of a cylinder fitting. Set the compression-tester regulator to 80 psi, as you would for cylinders. With the rubber tip held up in free air, activate the air gun. Make sure that the source gauge reads 80 psi and that the cylinder-side gauge reads zero, with all air escaping from the air gun. Now, put the air-gun rubber tip back in the oil-transfer hole and fully activate it. The only air leakage should be a small amount from the transfer point.
Set the compression tester to 80 psi and read the cylinder-side gauge. On a Lycoming, the pressure held by the main bearing should be between four and 20 psi. The low figure here is a little hard to see sometimes, especially with some gauges starting at 10 or so. One reason a Lycoming transfer system can go so low is the engine operating oil pressure. They run between 60 and 90 psi, which aids in lessening the oil leakage from the transfer region in the center of the front main bearing.
A Continental should be no less than 40 for the engines with transfer collars. For the Continental engines with the transfer in the main bearing, the cylinder-side gauge should be no lower than 18 psi. If the cylinder-side gauge falls out of these specs (on the low side), then the transfer system is bad and the engine must be disassembled. If this is not the problem, the governor should be sent off for a thorough bench check.
There are a few problems caused by the governor that we should talk about next.
The flyweight toes can sometimes wear a flat spot in the on-speed condition position, causing the governor to lose sensitivity and do some RPM "seeking."
A worn pilot-valve bearing can also cause the governor to "seek" or maybe even surge. A badly worn pilot valve will leak too much oil and act like a bad engine-transfer system, as can a bad governor oil pump or its associated relief valve. The transfer system check will narrow this to the governor or the engine.
A few factors to also consider are the peculiarities of some models of props. The Hartzell used on the Aztec and some twin Comanches relies on just the air-charge in the prop, the ATM (aerodynamic twisting moment) and the internal spring to counteract or balance the governor pressure. There are no counterweights. If the dome air charge is low, the RPM will be airspeed sensitive; that is, the RPM will increase with airspeed and decrease with airspeed. This is true of most Hartzells but especially of this particular model because of the lack of counterweights.
Hartzell also changed the air pressure in the domes of most of their other piston propellers (see Hartzell Service Bulletins 111D, 112, 114D, and 115D). These Service Bulletins add a feather-assist spring in the air-charge side of the dome and lower the air-charge pressure accordingly. In these props the dome pressure is now down around 40 psi, whereas before it was around 70. If you put 70 psi in a prop that has had this Service Bulletin accomplished, you may have trouble with props going into feather at low idle-speed and hot oil. Like a leaky transfer system, this will generally show up on landing rollout.
Sometimes prop-sync systems on twins will give some similar symptoms. If the system is mechanical and doesn't park the governor control in the right spot, you may not be able to get maximum RPM (parks too low) or feather (parks the lever too high). Be sure the system parks in the center position.
Some older Cessna systems, with the driver motor on the governor control arm, can continue to run and cause the RPM to fluctuate constantly. The cause is the park micro switch doesn't activate and the motor runs continuously. Pulling the prop-sync circuit breaker will stop this fluctuation if it's the problem.
Check control rigging before tearing into the system too deeply. The governor arm should hit its high-end stop and the arm should not be excessively loose on the speeder screw shaft. The control cable mounting should be solid and mounted to the engine, not the airframe. Sometimes a piece of baffling can move with air loads and interfere with the governor control arm (especially Continentals, due to the forward low-governor position). Engine movement, due to torque, thrust or worn engine mounts, can also cause control-cable movement when the control-cable mounting is loose.
There isn't much that will affect the governor control system, but the things that do can cause expensive repairs if you don't watch it. Keep the oil changed regularly -- as we always recommend. Oil contaminated with moisture will corrode the aluminum parts in the propeller and cause prop piston-seal leakage.
High oil temps (consistently at or near redline) can cause transfer-collar-equipped Continentals to wear the collar more than normal, causing eventual loss of RPM control.
A great help in case of oil contamination from some sort of debris is the governor gasket with screens in the "smile" (intake port of the governor). The screen can catch particles that would otherwise cause scoring or galling in the governor and transfer system. They can save you from replacing a lot of governor parts in case of metal contamination. The transfer collar in the Continental is especially vulnerable to this problem when it occurs. The clearance on the collar/crankshaft is less than two-thousandths of an inch and the tolerances are measured in 10-thousandths of an inch. Galling or scoring in this system can easily result in a crankshaft needing to be reground or even failing inspection altogether, meaning you'll be buying another one.
This is a huge expense best avoided with a little care. Just about every system in an engine benefits tremendously from clean oil.