|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.
March 1, 2002
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.
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.
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.
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 propparticularly 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
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
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.
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.
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.
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.
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.
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.
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
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
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
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.
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
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
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.