March 1, 2002 Prop Life Extension

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by	Kim Santerre
	This article appears in the February 2002 edition of LIGHT PLANE MAINTENANCE and is reprinted here by permission.

There are many things that you as an owner can do to extend the life of a prop, regardless of whether it's made of wood on a Cub or a fancy feathering rig on a Navajo. While inspections can be done by you as part of a normal preflight or extended routine inspection, you are limited on what you can legally change/repair without A&P supervision. The important thing, though, is to spot problems before they get serious, and to optimize the ground and flight environment with good habits.

Ground Procedures

The recommended position for blades is horizontal for two blades and a "Y" shape for three blades. The reasoning is to minimize the harmful effects of water at the base. Professionally made prop covers can be helpful, too. Lastly, the horizontal position reduces two-blade impacts from a plane taxiing by.

One bad habit you may have learned in flight training was pulling a trainer around by the prop. The lighter the plane, combined with a fixed-pitch prop, the less damaging is this procedure. If you continue to use a prop blade as a handle, at least use the root of the blade, so the lever effect of the end of the blade is reduced. Our advice is don't do it.

In flight, operate in the approved rpm range that seems to have the least vibration. Your entire airframe will thank you. Avoid overspend rpm operation, as that is very hard on the prop—particularly a constant-speed model. Be sure to check your tach annually. An error more than two percent calls for replacement according to AC 43-13. Also, AC 20-37D covers metal propeller maintenance, and AC 43-4A has extensive coverage on corrosion detection control and removal. 

I used to park my two-blade prop with the blade vertical to discourage birds from leaving their "calling cards." This is a bad idea. Water will collect around the blade shank (C/S props) and may cause corrosion. Leave the blades horizontal if possible. 

For a three-bladed prop, leave the blades in a "Y," positioned on a line vertical with the ground (i.e. sideways). Not a bad idea to do the same on a fixed-pitch prop, since you reduce the likelihood of a plane taxiing by from clipping your invisible (to him) blade. 

Don't cycle the propeller over 1,700 rpm (or the manufacturer's recommendation), but do get a three-to-four-hundred rpm drop while first exercising the prop during any flight day. This is important for two reasons: 1. To get fresh, warm oil into the prop dome on a regular basis to cut down on sludge accumulation. 2. To purge any air that may have gotten into the prop cylinder while it was sitting since the last run. 

Any air in the propeller system may cause surging, like on takeoff. Prop surging is especially critical in a multi-engine aircraft on take-off, as asymmetrical thrust can really make the pilot's adrenaline flow. 

Don't get carried away by too much of an rpm drop to flush out oil, as it's not especially good for the engine internals to drag the rpm down to nil. 

Ensure that the nose strut has the correct amount of fluid in it and is aired up as high as legally allowed to get the most propeller ground clearance possible. This is particularly important for two-blade props, as some aircraft have very little ground-to-blade clearance. The downside is exposing more strut shaft to flying debris. 

If you know you will be operating in a dirt or gravel strip, check to see if there is a maximum strut inflation value you can use, since this is your only means of gaining a bit more ground clearance-and an inch does matter. I know first-hand, as I have landed at many strips in Alaska, and never had a problem as long as I had my struts at maximum allowable values and exercised due care. 

Be careful that run-ups are not done in an area with loose gravel. If that's impossible, then consider learning to do a run-up while beginning take-off (rpm and mags) if strip length permits. Having another qualified pilot with you is a big advantage, so a second pair of qualified eyes can be looking out for danger. 

This procedure is not recommended for inexperienced pilots or those unfamiliar with the plane and its performance envelope. Usually, gravel strips are not crowded, so run-up alternatives need to be considered. Learn how to do this by practice with an instructor, since it's not normal, and you could end up in the trees from distraction or inadequate runway length. If you are used to this procedure it can be done in a few seconds and minimal visual distraction. 

Risking prop damage is a better choice than risking your life. By the same token, damaging a prop badly during a run-up can lead to prop failure in flight, so consider these elements before dropping in to the nearest gravel strip for fun-and by all means do your homework on the airstrip. There may be safe run up areas not obvious to uninformed visitors. I suspect I'm more sensitive to this type of prop damage since I have spent so much time in the backcountry.

Damage

Some bent prop blades can be straightened or cut down (both sides obviously), but this must be done by a certified prop shop. Do not make any effort to straighten the blade for shipping to the prop shop, as this will ruin any chances of salvaging the bent blade.

Prop nicks and scratches or corrosion spotted anywhere on the prop should be taken care of ASAP. A huge chunk doesn't need to be taken out of the blade for failure to occur. Time and lots of flex of the blades takes place in flight, (or any time rpm is run up) so minor damage can be made much worse by letting it go. 

Corrosion never sleeps, so it will only progress if you wait, leading to stress risers, which can lead to blade failure. This holds true whether the prop is constant-speed or fixed-pitch. 

Some people take a file along in an airborne toolbox for just such contingencies. If the prop needs immediate attention, and you are a licensed A&P or can work directly under the supervision of one, use the 1 to 10 rule of thumb for dressing out prop dings. 

For every measure of depth to the bottom of the ding it should be dressed out 10 times that. So a ding that is 1/16 inch deep should be dressed out 5/16 inch on either side of the damaged area for a total of 10/ 16 or 5/8 inch. Some say you should count the number of filing strokes you take off each blade so that you can take an equal number off the opposite blade. 

If you have any deep nicks or the prop hasn't had any inspection in years, it's often better to send the prop to a prop shop to have the nicks dressed out and other anomalies corrected. This costs in the neighborhood of $350 for a fixed-pitch prop, and can restore an engine smoothness and a bit of speed if the blades were out of specification. 

The asymmetrical loads put on an engine after blade failure are so great that the engine can shake completely out of the mount and depart the airplane before you can reach up and kill the engine. There is over 20 tons of pull spinning out there, trying to rip the prop apart. 

There is no provision in FAR Part 43 allowing unauthorized personnel to dress their own prop, so use good judgment and carefully evaluate your knowledge and circumstances before you dress your own prop as an unlicensed person. It's definitely not legal according to the FARs. 

AC 43.13-1B. Chapter 8, Sections four and five (Acceptable Methods Techniques, and Practices Aircraft Inspection and Repair) shows the right and wrong ways to dress out dings, as well as extensive inspection and evaluation criteria. Such drastic things as shortening damaged blades, or using bushings in elongated boltholes can sometimes repair apparently disqualifying damage. 

This manual includes graphs and formulas on allowable damage repairs (but the specific prop maker's guidance should be considered the definitive source for proper and acceptable repairs). AC 43-13, September 1998 edition, an invaluable reference for any owner. 

Buy the commercial version ($20 from Jeppesen.com) as it's 1/3 the cost of the same government printing office version, which comes in a loose-leaf format. Go figure.

Maintenance Issues

This horrible blade was on an actively flying Cherokee — yikes. This is the result of the owner "fixing it up." The leading edge was nearly blunt. This prop is dangerous, and badly in need of professional repair.

When removing a propeller always leave one nut or bolt on at least four or five threads until you have the prop most of the way off. Many props are stubborn and hard to get off. They will stick and bind and then let go all at once, when you least expect it. 

The nut or bolt that you left on loosely may prevent you from dropping your umpteen thousand-dollar investment on the floor or falling on your posterior (which may cost even more to fix than a damaged prop). 

Oftentimes a propeller will be damaged in the field by improper maintenance. For example, some props have two grease fittings, one on either side of each blade shank. 

You are supposed to remove one of the grease fittings and pump grease in until it flows freely out the other fitting hole. You are not supposed to try to force grease into each fitting without removing the other one. 

This may damage to the seals. It's also very important to use only grease called for in the maintenance manual. The wrong grease may react improperly with the seals or the grease that is already in the hub. 

Regular oil changes are very important to help cut down on the amount of sludge that forms in the crankshaft oil passageway and in the prop cylinder itself (constant-speed props). Also, whenever there is metal contamination in an engine the prop and the prop governor should also be inspected or overhauled to ensure no damage has been done there. Metal particles that run through the engine also go to the prop and governor. This is often overlooked. 

Besides controlling blade angle on C/S props, the prop governor is also an oil pump. A typical governor runs at crankshaft speed and puts out upwards of 290 lbs of pressure to operate the piston in the prop hub that changes the blade angle. Normal engine oil pressure is around 50 pounds at cruise rpm. 

The prop governor only has a metal screen that traps larger contaminants, but can gunk up with debris and result in erratic operation, so have this checked and cleaned periodically. Removing, reinstalling and rigging the prop control is essentially all that can be done legally in the field by an A&P or repair facility. 

Governors should only be serviced by a repair station that has the specialized knowledge and tools to do the work and test the repair before re-installation.

Wooden Props

These babies are beautiful, and some are a work of art, but they need care and attention if they are going to last. They need frequent inspection to assure the exterior finish is intact and environmental effects on the wood have not caused delaminating, cracks, warping or glue failure. 

One thing especially needing attention is checking the proper torque on the prop hub, since moisture, especially in wet weather, can get into the hub through the boltholes. When the wood swells, the bolts prevent normal expansion, and some wood fibers are crushed. When it dries the prop hub bolts become loose. 

Proper moisture preventative such as asphalt varnish on the hub bore and boltholes will help prevent this problem. New prop installations should have the torque checked at 25 hours, and every 50 thereafter, or as prescribed by the airframe maker or prop service bulletins.

Buying Tips

When buying a used airplane it can be a real heartbreak to have to spend additional money for propeller repairs or replacement that you didn't expect. It's also normally impractical to send the prop to a prop shop for tear-down and inspection before you buy the airplane. However, there are a few things you can do to lessen the risks. 

Check the blades for prop nicks and the leading edge for general straightness. You can do this by sighting down the leading edge. Any prop that has seen normal use will most likely have had some dings honed out, but if the leading edge has a multitude of hills and valleys it may be a candidate for overhaul. 

Check the backside of the prop too. It is subject to nicks just as much as the leading edge. Deep-dished areas that have been honed out are not always readily visible with the black paint that is frequently on the blade backside. A dark hangar and a flashlight beam shining down the leading edge and the back of the blade may enhance areas that couldn't be seen in daylight. 

A rough prop back can actually reduce prop efficiency by disrupting airflow, or be indicative of corrosion. Roughness in the metal may be pitting, which will contributes to stress risers and promote prop failure or shorten overhaul life much more so than a glassy-smooth finish. Corrosion anywhere on a prop warrants further checking by a qualified person. 

Checking blade tracking isn't particularly difficult, and it's best to do it with one set of spark plugs removed (you will probably have removed the plugs anyway for an acceptance compression check). Just use an appropriately sized block of wood that it is just about touching the tip of the blade, and the block firmly held in place so it won't move; or use masking tape so you will notice if the wood has moved during the check. Now carefully rotate the next blade into position on the block and note the distance between the block and prop tip. There shouldn't be much more than a 1/16-inch difference. See graphic example. 

Get in touch with your neighborhood prop shop or contact the manufacturer and find out what the minimum prop length is and minimum blade width at one or two blade stations for your particular propeller aircraft application. Most prop makers also have web sites that have varying degrees of information, including service bulletins, care and maintenance procedures. 

You may find during inspection that you have either an un-airworthy prop or one that may not make it past an overhaul-or that the prop is an unapproved one for the plane. A good IA will spot this, but they still get past inspections all the time. We see this on older planes with some frequency. Just because a prop will mount doesn't mean it's either approved, or safe for that application. 

At each overhaul, surface metal is removed from the prop to relieve stress concentrations, so even a fixed-pitch prop can undergo only so many overhauls or re-pitches not to exceed a certain cumulative inch value (normally 8 total inches).

In Short

These steps can vary from no brainers to a lot of trouble, but on most airplanes the propeller is one of the major cost components. It just depends on how much expense you want to risk you want to incur when buying a new plane, or how much extra life you want from your prop. A great deal of prop life extension depends on simple habits. 

It's clear that doing frequent checks and simple things like proper blade positioning while parked can catch problems before they become major or slow down the process of corrosion. By all means avoid using your prop as a towing handle. 

Fixing dings as they occur can lead to significant increases in potential prop life as well. Lastly, visit the Web site of your prop maker, as you may find a wealth of information previously very difficult to get. You will be pleasantly surprised.