What to Look for on Propellers Before Flying

As vital equipment, propellers must be maintained in tip-top shape at all times. In this day and age, it seems pilots continue to think all they need to do is check roughness on the blades. To do this, they run their hands over the blades. This is dandy, but not fine enough.

It’s necessary to look closely at what you are inspecting. In some cases, you need to use a 10x magnifying glass if you come across something out of the ordinary. Normally, the rule of thumb is depth x 10 width for blending out a nick on the blade or blades. When doing so, do not use a screwdriver to slide over the blades because that may cover up any cracks that may have started and leave the door open for possible blade failure.

The cost of propellers and prop blades has become costly, and service shops don’t like to remove material if they can avoid it. Technicians still have to deal with normal conditions like sand, grass, rain and so forth. Nowadays, some pretty good products are available out there to help prevent certain types of blade erosion. For example, erosion tape seems to work quite well.

The key is proper application. When blade erosion occurs and material needs to be removed, use a body file or rasp to remove only what is necessary. Do this by applying the file or rasp lengthwise, not chordwise, along the blade. When removing any material from the blades, it is very important to maintain the contour of the blades’ leading edge so the air will flow smoothly over it. After removing the damage or erosion, this area must be polished to a smooth scratch-free surface. Once this is done, it is ready to be prepped for a protective coating.

Loose Blades

Quite often, loose blades can create fore and aft movement. This is not uncommon. Any blade movement under 1/16 inch is not enough to set off any alarms, however, it should still be investigated. This movement is normally removed by shimming up the blades. Consult a propeller repair shop if you have any concerns or questions.

Rotational Movement

When internal or, in some cases, external parts are wearing beyond their limits, rotational movement will normally occur. Depending on the type and model of propeller, the maximum angle limits will vary. Again, consult a propeller repair shop.

Grease Leakage

Several things can cause grease leakage. In most cases over-greasing is the reason. Steel hub propellers have a two-piece clamp assembly that contain the bearings and hold the blade onto the main hub assembly. The clamps have two grease fittings to pump grease into the bearings. Between the clamp halves there is a gasket and behind the clamp there is an O-ring. One of the fittings must be removed to prevent pressure inside of the clamp.

If pressure builds up, either the gasket or O-ring will blow out causing grease to escape from the clamp.
This is an easy fix. The clamp must be removed and the gaskets and O-ring must be replaced by a propeller tech. At the same time, the technician will check for broken, worn or damaged parts.

Aluminum hub-type propellers also have grease fittings to lube the internal bearings. This hub is a two-piece assembly bolted together. There are also two grease fittings per blade mounted on the side of the hub. Each blade contained in the hub will have two fittings. For example, a two-blade propeller will have a total of four fittings, while a three-blade propeller will have six fittings.

When pumping grease into the hub, one of the two fittings must be removed to prevent pressurizing the inside of the hub. If pressure builds up, the grease will pass by the main blade seal causing grease to be thrown onto the blades, cowling and wind screen. If this happens, the only way to correct this is to dismantle the propeller.

Once dismantled and cleaned, all seals will be replaced, then the propeller is reassembled and bench tested. Newer model propellers, such as the McCauley, have a red-dyed oil inside of the hub. This oil is used for two reasons: one is lubrication and the other is for means of crack detection.

In most cases, on these types of propellers, leaking oil is caused by a pinched O-ring. Normally, not until the propeller is under a load or not being cycled at a constant speed will it start leaking. When a constant speed propeller is not being cycled properly, the seals dry up and the propeller leaks. If upon inspection, no cracks are apparent, run the propeller for approximately 10 hours and cycle the propeller as much as possible.

If signs of oil are still there, the propeller must be dismantled and seals replaced. If signs of dye are showing up and looking like scratches, this should not be taken lightly and further inspection must be done as a crack may be developing or already be formed.
Vibration

A common problem with propellers is vibration. Many things can bring this on. Steel-hub propellers that have the clamp set-up quite often encounter blade slippage. This is when the blade slightly moves in the clamp, causing a difference in blade angles. If the blades are more than .02 degrees of each other, this will cause vibration.  In order to correct this, the outer clamp bolts must be loosened and angles corrected. The bolts will then be retorqued to the specified torque. A tape mark is installed on the clamp and blade for future reference if this should happen again so that a visual will determine if the angles are off from each other.

Another common problem is tracking of the blades, when the tip height of the blades is not the same. Check this by hanging one blade down and making a reference mark. Carefully rotate the propeller until the next blade reaches that reference point. The difference should not be more than 1/16 inch between each other. If it is greater than this, a qualified technician must correct the track. Fixed-pitch propellers quite often will get too much angle difference between each other. This will be corrected by twisting the blades back to their specified pitch, again within .02 degrees of each other.

Probably the most common cause of vibration is caused by filing the blades to remove any damage. By counting the number of file strokes on each blade and keeping them as close as possible, it should help prevent vibration problems.

A rough running propeller may also lead to many other problems on the aircraft. With these simple steps, you should get many safe and trouble-free hours of flight time and save money in the long run.