A lot of his friends call Will Johnson a propeller whisperer, a highly-knowledgeable aviator who talks to flyers about protecting their propellers. Johnson acquired his expertise the hard way during 20,000 plus flying hours, most of it in Alaska.
“What all those flying hours add up to is a lot of nicked, damaged, and destroyed props I’ve seen,” Johnson said. “Personally, I have been lucky in not damaging a prop to the point that it couldn’t be overhauled without changing blades. But as a charter and scheduled operator in Alaska I spent a great deal of money over the years paying for unnecessarily expensive overhauls that required blade changes. That led me to trying to educate pilots about how to best take care of an aircraft propeller.”
Despite his extensive Alaska flying, Johnson said it was a long-ago incident in the Lower 48 that focused his attention on propeller damage.
“Good experience comes from bad experiences,” he said. “I learned about rocks on pavement with a new Cessna 172 I bought in 1975. I had flown over to M. Graham Clark Field in Point Lookout, Missouri. When I left, I was careless with my power and in having the aircraft turned into the wind. Back at my home base, Misty Meadows (MO65) at Kirbyville, I discovered a major prop nick. Then I remembered some loose rocks on the pavement I taxied over. From then on, I always kept a sharp lookout for loose rocks on pavement and even tried, to the extent practical, to practice good gravel techniques on pavement.”
Propeller damage occurs even at the best maintained airports, Johnson said.
“And older airports with disintegrating pavement or grass and gravel strips in the Lower 48 pose special dangers for the continued good health of your propeller,” he said. “Plus, the increasing popularity of backcountry flying into rough or relatively unimproved landing spots has added to the danger of propeller damage.”
Johnson regularly flies a Cessna 206 on amphibious floats and a Cessna 207 from Chena Marina (AK28), his home base in Fairbanks.
“Chena Marina is a 4,700-foot gravel airport, so I have to practice what I preach operating from there,” Johnson said.
At the beginning
Protecting your propeller begins with engine startup, Johnson said.
“Use the wind to your advantage on a gravel surface in particular. But it’s better to follow the same precautions on any type of surface,” he said. “Having some wind is better than calm conditions because you can point your plane into the wind for startup. Be aware that a vortex will build under a propeller operating at much above idle rpm. This will suck whatever is under the prop into the blade path unless the plane is pointed into the wind or moving.”
“If parked into the wind, or if the aircraft is moving fast enough regardless of the relative wind direction, then the vortex bends back behind the prop, and rock damage will be avoided,” he explained. “A good wind of approximately 10 knots or more when the aircraft is pointing into it will even allow a run-up over gravel although, if you are uncertain, the prop can be cycled with mags and carb heat checked while rolling.”
A propeller high off the ground, such as exists with a tailwheel or amphibious floatplane, provides no guarantee that rock damage will be avoided, according to Johnson.
“Rocks can rise a couple of feet into the air or more, carried upward by the vortex. Likewise, with an uneven surface or at gross weight or forward CG, some aircraft will have propellers only a few inches above the ground.”
On the Move
Johnson’s formula for avoiding propeller damage continues with movement of the aircraft.
“Always park with the nosewheel or tailwheel facing straight ahead and do not try to start a turn until rolling,” he said. “One brake locked while trying to deflect the nosewheel for a turn when starting to roll is a certain prescription for possible propeller damage, whether a single or a twin. The aircraft will start to roll easier if pointed straight ahead rather than if it is immediately forced into a turn.”
Johnson cautioned, “With most single engine tricycle gear Cessna aircraft, having the nose up with the nose gear strut fully extended locks the nosewheel straight ahead. This is more likely with the aircraft at or near aft CG limits. Trying to make the aircraft turn under these conditions is nearly impossible and if successful, it will take a high power setting with lots of differential braking, while skidding the nosewheel sideways with possible propeller damage.”
He said it is better to let the aircraft move straight ahead and gain momentum, then release up elevator pressure and tap both brakes to dip the nose.
“After that you can immediately use one brake and rudder to initiate a turn,” he said. “Once the nosewheel turns, then up elevator can again be used to help keep the nose higher and the aircraft will maintain its turn.”
He advises that if there is loose gravel on the hard pack or pavement, sweep the area with a broom to move gravel from under the propeller blades and in front for a couple of feet.
“This is especially helpful when operating a turbine aircraft, which has the prop blades in feather during the engine start,” he added.
An additional tip is learning to start the engine without racing to high rpm, even temporarily. “This is possible even with fuel injected aircraft on a hot day, if proper starting technique is used,” Johnson said.
Be especially watchful for even small amounts of loose gravel on pavement, Johnson said.
“That comes from the tires of vehicles or aircraft taxiing onto pavement from gravel,” he said. “Rocks will suck up into the propeller path easier when on pavement than rocks nested together on a gravel surface.”
Johnson advises at the beginning of a turn, try to turn in the direction the wind is coming from and keep the speed up during the turn so that the plane doesn’t stop while pointed downwind.
“All things being equal or in calm wind make turns to the left,” he said. “The aircraft will turn left easier than it will turn right, unless flying a twin with counter-rotating props or a foreign aircraft with a counter clockwise tuning propeller.”
In a calm wind, a short burst of power to start the aircraft rolling helps minimize the danger of a rock-filled vortex forming, Johnson said.
If you’re flying a twin, try not to use differential power so that the rpm on each engine will be lower, Johnson advised.
“High power on one engine while the other engine is at idle will cause rocks to be drawn into the engine with the high rpm. Twins offer good learning opportunities since the props are visible over gravel and the vortexes that carry the rocks into the blade path are also visible, as there is usually dust mixed in with the gravel.”
After startup with the aircraft facing into the wind complete all takeoff checks possible before moving the aircraft, Johnson said.
“Only move the aircraft when it is possible to keep moving without stops unless into the wind all the way through to getting airborne. When turning onto the end of the runway for takeoff, keep the speed up and do not allow the aircraft to stop or be excessively slow while pointing downwind.”
Johnson noted that while turning around at the end of a gravel runway differential braking at the end may cause the inboard wheel to dig in.
“If this happens, let up on the brake while still moving so that the wheel can climb out of the hole dug by the wheel, then brake again to continue the turn,” he said. “This seems almost like pumping the inboard brake while turning. The runway manager may not be happy with your holes in the surface, but your propeller is more important. And on a high-wing plane, the wheel can be observed and between sight and feel it is quite clear when the wheel is digging a hole.”
If you become stuck, do not attempt to power out.
“Get help and either push the plane out onto a firm surface or get a vehicle to tow the plane out of the soft area,” he said. “While this seems inconvenient, do not feel pressured to get out quickly. A little time getting unstuck can save thousands of dollars in damage to the aircraft, especially to the propeller.”
Propeller Nicks
Despite all the precautions, there is always the possibility of propeller nicks.
“At every opportunity, inspect the propeller for rock nicks and have them addressed by a mechanic as soon as possible,” Johnson said. “Nicks are stress risers and can result in crack initiation, which can rapidly proceed to blade failure and loss of the aircraft in a worst-case scenario.”
“Compressed metal under the rock strike can wedge the metal below the nick apart and often rock fragments and other foreign material embedded in the nick can initiate stress corrosion,” he added. “Propeller blades, most of them being metal, exhibit notch sensitivity.”
“A rare condition can occur in the fall wherein light snow on top of loose gravel can result in rocks sticking to the tires,” Johnson continued. “As takeoff speed increases, these rocks can fly off the tires and into the propeller blade path, causing severe damage.”
“Take every precaution with an aircraft taxiing in front of you,” Johnson said. “If the aircraft slows down over gravel, keep your distance. And let the aircraft complete the takeoff before moving your aircraft. Also be wary of innocent-looking water puddles on the ground or tarmac. Use good gravel techniques over puddles of water as water can cause severe blade erosion under the right conditions.”
“Students do not always get informed advice about gravel operations,” Johnson continued. “Most flight instructors are not familiar with gravel operations and often simply leave the impression with students that the only thing needed is to go slow over gravel with low rpm. This can be exactly the wrong thing to do and can prolong the time in a bad situation with your aircraft pointing downwind stopped or at low speed, resulting in more time to pick up rocks and adding even more prop damage.”
“And finally, I believe most pilots will learn and improve on their own if they look at the prop after each flight and think back to what they might have done wrong if they find damage.”
I have a Tennessee laminated prop on my experimental aircraft. During a forced off airport landing the prop, which was stopped, contacted a power line. The end of one blade was scuffed. The laminate did not separate. Can the prop be safely used after removing about 2 sq. Ins of scuff mark. It seems that Tennessee are out of business, no telephone no.
I’d be interested to hear some advice and technique suggestions from the author & seaplane pilots on minimizing water damage. There’s not much we can do about flying thru rain, but I’ve personally witnessed the water vortex that can be pulled into the blades taxiing, and resulting water damage from it, as well as the spray of rough water landings. The props edge can start to feel like sand paper.
I had a prop guard on a former float plane, but noticed even it started to show a tattering of its edges toward the tip, and IAW instructions, the prop guard could be trimmed back. But then it seemed you’d have the tips unprotected and potential to develop a bit of a step in your prop with wear over time where the prop guards protection ceased at the edge of the plastic.
The Prop Guard instructions that I have lists that a 2 inch wide section to be cut away and replaced with new tape from the excess in the kit.
Although the ‘accelerator’ liquid in the kit looks to have a short shelf life.
DONT RUNUP WITH PROP OVER CRACK OR SEAM IN PAVEMENT.
I attended a five week McCauley Propeller Repairman’s Course while I worked for Cessna decades ago.
We we were taught to remove the minimum material from the blades using a special stone similar to a grill cleaning stone made up of aluminum oxide.
Then finish with sand paper. Blend the edges out to eliminate sharp areas or points. Use AC 43-13.
On composite blades the prop manual calls out the limits and type of repair filler materials to use for nicks.
“Most flight instructors are not familiar with gravel operations”… Most CFI’s have never landed or departed on a gravel strip, just soft field or grass strips. Like most things in aviation, experience is the best teacher. I’ve written many of those things down in a little green booklet I’ve kept, to pass on to my students. My next entry might be to just avoid gravel strips if at all possible to prolong prop life.
I don’t operate off gravel, but the older paved taxiways and runways I do use had ‘sand blasted’ all the paint off the prop leading edges.
I recently installed an overhauled prop and ask the shop how I could protect the paint on the prop.
They recommended ‘Prop Guard’, an STC’d tape, that covers the leading edges. So far it has worked.
There are a few dings, scratches and gouges in the tape, but it has protected the prop…