Prop Life Extension

Vitamin E won't do the job here but, as Kim Santerre explained in Light Plane Maintenance, there are simple things you can do that can have salutary effects on your prop's life-span. Here's what you can do to keep it spinning like a top.

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There are many things that you as an owner cando to extend the life of a prop, regardless of whether it’s made of wood on aCub or a fancy feathering rig on a Navajo. While inspections can be done by youas part of a normal preflight or extended routine inspection, you are limited onwhat you can legally change/repair without A&P supervision.The important thing, though, is to spot problems before they get serious, and tooptimize 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 traineraround by the prop. The lighter the plane, combined with a fixed-pitch prop, theless damaging is this procedure. If you continue to use a prop blade as ahandle, at least use the root of the blade, so the lever effect of the end ofthe blade is reduced. Our advice is don’t do it.

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

I used to park my two-blade prop with the blade vertical to discourage birdsfrom leaving their “calling cards.” This is a bad idea. Water willcollect around the blade shank (C/S props) and may cause corrosion. Leave theblades horizontal if possible.

For a three-bladed prop, leave the blades in a “Y,” positioned on aline vertical with the ground (i.e. sideways). Not a bad idea to do the same ona fixed-pitch prop, since you reduce the likelihood of a plane taxiing by fromclipping 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 propduring any flight day. This is important for two reasons: 1. To get fresh, warmoil 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 wassitting since the last run.

Any air in the propeller system may cause surging, like on takeoff. Prop surgingis especially critical in a multi-engine aircraft on take-off, as asymmetricalthrust 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 notespecially 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 upas high as legally allowed to get the most propeller ground clearance possible.This is particularly important for two-blade props, as some aircraft have verylittle ground-to-blade clearance. The downside is exposing more strut shaft toflying debris.

If you know you will be operating in a dirt or gravel strip, check to see ifthere is a maximum strut inflation value you can use, since this is your onlymeans of gaining a bit more ground clearance-and an inch does matter. I knowfirst-hand, as I have landed at many strips in Alaska, and never had a problemas 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’simpossible, then consider learning to do a run-up while beginning take-off (rpmand mags) if strip length permits. Having another qualified pilot with you is abig advantage, so a second pair of qualified eyes can be looking out for danger.

This procedure is not recommended for inexperienced pilots or those unfamiliarwith the plane and its performance envelope. Usually, gravel strips are notcrowded, so run-up alternatives need to be considered. Learn how to do this bypractice with an instructor, since it’s not normal, and you could end up in thetrees from distraction or inadequate runway length. If you are used to thisprocedure 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 sametoken, 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 forfun-and by all means do your homework on the airstrip. There may be safe run upareas not obvious to uninformed visitors. I suspect I’m more sensitive to thistype 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 betaken care of ASAP. A huge chunk doesn’t need to be taken out of the blade forfailure to occur. Time and lots of flex of the blades takes place in flight, (orany 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 stressrisers, which can lead to blade failure. This holds true whether the prop isconstant-speed or fixed-pitch.

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

For every measure of depth to the bottom of the ding it should be dressed out 10times that. So a ding that is 1/16 inch deep should be dressed out 5/16 inch oneither side of the damaged area for a total of 10/ 16 or 5/8 inch. Some say youshould count the number of filing strokes you take off each blade so that youcan 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’soften better to send the prop to a prop shop to have the nicks dressed out andother anomalies corrected. This costs in the neighborhood of $350 for afixed-pitch prop, and can restore an engine smoothness and a bit of speed if theblades were out of specification.

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

There is no provision in FAR Part 43 allowing unauthorized personnel to dresstheir own prop, so use good judgment and carefully evaluate your knowledge andcircumstances before you dress your own prop as an unlicensed person. It’sdefinitely 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 todress out dings, as well as extensive inspection and evaluation criteria. Suchdrastic things as shortening damaged blades, or using bushings in elongatedboltholes can sometimes repair apparently disqualifying damage.

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

Buy the commercial version ($20 from Jeppesen.com) asit’s 1/3 the cost of the same government printing office version, which comes ina 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 fivethreads until you have the prop most of the way off. Many props are stubborn andhard to get off. They will stick and bind and then let go all at once, when youleast expect it.

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

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

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

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

Regular oil changes are very important to help cut down on the amount of sludgethat forms in the crankshaft oil passageway and in the prop cylinder itself(constant-speed props). Also, whenever there is metal contamination in an enginethe prop and the prop governor should also be inspected or overhauled to ensureno damage has been done there. Metal particles that run through the engine alsogo to the prop and governor. This is often overlooked.

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

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

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

Wooden Props

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

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

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

Buying Tips

When buying a used airplane it can be a real heartbreak to have to spendadditional 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 andinspection before you buy the airplane. However, there are a few things you cando 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 normaluse will most likely have had some dings honed out, but if the leading edge hasa 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 theleading edge. Deep-dished areas that have been honed out are not always readilyvisible with the black paint that is frequently on the blade backside. A darkhangar and a flashlight beam shining down the leading edge and the back of theblade may enhance areas that couldn’t be seen in daylight.

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

Checking blade tracking isn’t particularly difficult, and it’s best to do itwith one set of spark plugs removed (you will probably have removed the plugsanyway for an acceptance compression check). Just use an appropriately sizedblock of wood that it is just about touching the tip of the blade, and the blockfirmly held in place so it won’t move; or use masking tape so you will notice ifthe wood has moved during the check.Now carefully rotate the next blade into position on the block and note thedistance between the block and prop tip. There shouldn’t be much more than a1/16-inch difference. See graphic example.

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

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

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

In Short

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

It’s clear that doing frequent checks and simple things like proper bladepositioning while parked can catch problems before they become major or slowdown the process of corrosion. By all means avoid using your prop as a towinghandle.

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

mccauley.textron.comsensenich.comhartzellprop.com
(800) 621-7767(800) 791-7767(800) 942-7767

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