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Tuesday, 10 September 2013 08:31

Prop Talk 101

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September 2013

If you’ve never thought that your propeller will need an overhaul, think again.

Without a doubt they’re the hardest working, most underappreciated part of your airplane. I’m of course talking about your propeller.

Most of us just think of a propeller as a chunk of metal spinning around on the front of our airplane. How wrong we are.

Your propeller is one of the most highly stressed components on your airplane. During normal operation it has to withstand 10 to 20 tons of centrifugal force, which is trying to pull the blades right out of the hub. And that’s not including the other stresses put on the blades as they strain to literally pull the entire weight of your aircraft through a sea of air.

Even on the best of days, a propeller is routinely subjected to bending, flexing and vibration. And it does all this with little or no maintenance from you. In fact, beyond your cursory glance during preflight, your propeller never gets a second look, let alone any consideration for a routine overhaul. And that can be a big mistake.

“People think that thing sitting on the front of the engine—which disappears when you start the engine—never needs looked at, or is indestructible,” stated Chris Bell, field service engineer for McCauley Propeller Systems. “So indestructible, that all too many pilots use it as a makeshift tow bar.”

“Using the prop instead of a tow bar can actually pull the blades out of track,” he said. “Obviously, this is something we do not recommend." But pilots do this and other things that can inadvertently damage their propellers. Then on top of it all, many owners neglect any proactive maintenance and ignore propeller overhaul recommendations. With that lack of respect in mind, it’s no wonder that some owners never even consider a proactive propeller overhaul.

“I’d say that first and foremost, the most common misconception among aircraft owners is whether or not overhauls are even necessary,” explained Gary Chafin, the director of OEM sales at Hartzell Propeller Inc. “Most owners say that their propeller is working just fine—so why overhaul it?”

Good question. But most of these same owners would never purposely fly their engine beyond its TBO, even if it is running as smooth as silk. Pilots just don’t want to run the risk of an engine failure. So why do they routinely run the risk of propeller problems?

“Propellers don’t fail very often,” Chafin said. “But when they do, it’s usually catastrophic. When an engine fails, it rarely causes a pilot to lose control of the airplane. But when a propeller throws a blade or the hub breaks, the imbalance will be so great it will literally pull the engine right off the mount and that most often renders the airplane uncontrollable.”

Chafin theorizes that the reason most pilots don’t appreciate the severity of a prop failure is that it is rarely ever listed as the cause of an accident. “Propeller failures are rare and when they do occur they don’t often generate much media attention,” he said. “So pilots just aren’t attuned to the problem. They’ll just keep flying on their prop and never give it a second thought until it starts to leak grease or oil; then it’s into the shop.”

Of course by then, what the owner is left with is usually little more than scrap metal. Fatigue, wear and corrosion have rendered all of the mechanical components of the propeller’s system useless. So what should the owner have done?

“The typical TBO recommendation on a Hartzell constant-speed propeller is divided into two parts: a six-year calendar limit and/or 2,400 hours of operation—whichever comes first. (Yes, there’s a TBO recommendation for fixed-pitch props, but we’ll get to that later.)

“Hartzell has Service Letter 61 posted on its website that spells out all of our recommended intervals for propeller servicing and overhaul,” Chafin said. (See Resources for the web address. —Ed.)

“We consider these to be requirements, but technically they are only recommendations, because owners operating under Part 91 of the FARs are not required to do any propeller maintenance.” That’s probably why a lot of us don’t do it.

 

Your propeller can tell you a lot
The fact is the majority of General Aviation aircraft owners wait until their propeller is damaged or begins to show signs of problems before they take them in for maintenance. So, what kind of hints will your prop give you?

“The main thing is grease or oil leaks from around the hub,” Chafin said. “Often they are signs that the seals have finally given way, or even [warning of] a hub that’s getting ready to fail. If you see grease or oil streaks on the propeller blade, have a qualified mechanic look at it right away.”

Another telltale indication of problems is any sudden onset of vibration. “That’s a real cause for concern,” he added. “If it’s sudden and seems to increase with power, land as soon as possible and have it looked at.”

Of course, if you spot any dings or dents in the propeller, you need to have those dressed out as soon as possible. Why?

Chafin explained that propeller blades have nodes and the stresses at these nodes are extremely high and generally increase as power is applied or during operation in restricted power ranges. If you get a ding at one of these points and leave it, it creates a stress riser that can initiate a crack in the blade. And depending on the location of the crack and the overall health of the blade, these cracks can progress quite rapidly.

He also strongly recommends that you make sure your tachometer is calibrated to avoid inadvertent operation in restricted power ranges.

“It’s always surprising to us just how many pilots say that they had some kind of warning of a problem before they had catastrophic propeller failure,” Chafin said. “If they had known the signs they probably could have avoided the problem altogether.”

 

Overhaulin’

Sooner or later every propeller is going to need service at some level, but there are some things you can do in the meantime to extend the inevitable.

“The best preventive maintenance for a propeller starts with periodic cleaning of the leading edges of the blades and lubrication as recommended by the propeller manufacturer,” stated Rocky Mountain Propellers’ general manager, Dave Hampel. “Also, follow the manufacturer’s overhaul recommendations.”

Hampel urged owners to stop saying, “I operate Part 91 and don’t have to overhaul my propeller.

“You don’t have to change your oil, or replace bald tires, or clean spark plugs either. They are all ‘factory recommendations,’” he continued. But we all fly by them because they are all good, safe ways to operate our aircraft. Okay, so you’re going to follow the propeller OEM’s guidelines and overhaul your prop. We all pretty well know what goes on during an engine overhaul, but what does your constant-speed propeller go through when its overhaul time comes?

To help give you an idea, Hampel provided the following outline of this top prop shop’s overhaul processes. (Rocky Mountain Propellers also has a video of the process on YouTube. See https://www.piperflyer.org/rockypropvideo for more information. —Ed.)

When it arrives, the propeller is inspected and a document is prepared that will accompany it through the overhaul process. All applicable ADs, current specifications and manufacturer Service Bulletins are researched for incorporation during the overhaul process.

Next, the unit is disassembled and cleaned. All parts are thoroughly inspected and those requiring replacement or rework are noted in the documentation. Many specialized tools and fixtures are required for the proper disassembly and reassembly of propeller components.

 

Hubs.

Nonferrous hubs and components are media blasted to strip away paint and anodized coatings. Then the parts are inspected for cracks using a liquid dye penetrant inspection procedure. This will detect flaws that are below the surface of the material. Magnetic particle inspection is used to detect flaws in the steel parts.

Additionally, components that are subjected to wear are dimensionally inspected to manufacturer’s specifications.

After passing all inspection steps, aluminum parts are anodized and painted, while steel parts are cadmium plated for maximum protection against corrosion.

 

Blades.

Blades get media blasted to remove paint, then etched to remove anodized coatings. Then they are dye penetrant inspected for cracks. Blades that pass the crack inspection move on to have their airfoils precision hand ground to remove all corrosion, scratches and surface flaws.

“This is where the art happens,” Hampel said. “Our technicians use a variety of hand tools to return the propeller leading edge and airfoil to its original shape.”

Next, each of the propeller blades is balanced through a multi-step process to perfectly match each other. Then the blades are anodized and painted for long-term corrosion protection.

The last step for the blades is a final dimensional inspection to ensure they are within the manufacturer’s specifications. “We use an advanced digital unit specially developed to do propeller inspections,” he said. “It provides extremely accurate and consistent information. We are one of the very few shops in the country that have one on-site.”

 

Reassembly.

Once the components are completed, it’s time for reassembly following the original manufacturer’s overhaul manual. After completion, the assembly is checked for proper operation and leaks.

The reassembled propeller is checked for static balance. If necessary, weights are placed on the hub areas of each “light” blade socket. These weights should be considered part of the basic hub assembly and should not be moved during subsequent dynamic balancing of the engine.

 

Fixed-pitch props need love, too

Up until now all of our talk has been about the needs of constant-speed propellers, so you owners who have fixed-pitch props probably thought you are off the overhaul hook.

Well, think again. Your simple fixed-pitch propeller needs just as much care as its complex cousin.

“Unfortunately, the propeller is one of the most neglected pieces of equipment on the airplane even though it’s one of the most important,” explained Bell. “If something happens to the propeller, everyone is in for a bad day.”

Bell explained that like their constant-speed cousins, fixed-pitch propellers do have TBO requirements, which in the case of the McCauley is 1,500 or 2,000 hours (depending on specific type) or 72 months—whichever comes first. Bell also added that the propeller mounting bolts torque should be checked at annual.

Details on McCauley propeller inspections and maintenance can be found on McCauley’s website. (See Resources for more information.)

 

Fixed-pitch overhaulin’

Okay, so you want to play by the rules. So how does a fixed-pitch propeller get overhauled?

According to Bell, it’s a rather simple process that includes stripping the paint and inspecting for damage. “The blade angles, thickness, chord and face alignment are measured and adjusted,” he said. “The prop is then dye penetrant inspected. Lastly, a surface treatment such as alodine or anodize is applied and the prop is painted.”

Bell stopped here and strongly cautioned against polishing any McCauley propeller. “McCauley does not approve any fixed-pitch [prop] to be polished,” he stated. “All of our fixed-pitches were certified with surface protection.

“If the owner wants to have a polished propeller, he will have to get a field approval or STC from the FAA,” he explained.

And a freshly overhauled prop doesn’t just look new; it flies like new, too. In fact, I had one technician tell me that pilots are amazed just how much better their airplane performs with a newly reconditioned propeller on it.

“Many of them say it’s like they got a bigger engine on their airplane. They just haven’t noticed that over time their propeller has gotten out of its optimum shape. After it’s overhauled, it delivers the performance the way it’s designed to,” he said.


Overhaul or replace?

While a propeller overhaul is probably the solution of choice for most owners, Chafin said that major propeller repairs are often the time when an owner will decide to upgrade their legacy prop to a new-generation design.

“Overhauling a propeller is very expensive, especially if you have to replace the blades and a bunch of the internal components,” he said. “When you add it all up, suddenly the delta between an overhaul and a new propeller becomes pretty small.”

And, of course, there’s always that allure of putting a cool-looking propeller on the front of your beloved airplane. Ramp appeal can be a very strong motivator.

“There are a number of options available for both aluminum and composite propellers today,” Chafin said. “For example, with [Hartzell’s] aluminum propellers we now offer blended-technology airfoil design. People see them on the ramp as the new swept or scimitar design. Using new computer-aided design and machining technologies we can literally optimize the design of the entire airfoil from the hub to the tip.”

Speaking of optimizing the design, Chafin also explained that companies like Hartzell are now creating purpose-designed propellers for specific aircraft types.

“Using our proprietary propeller design codes, we can design and manufacturer propellers that will get every last bit of efficiency out of a specific aircraft,” he said. “That’s what we call our Top Prop Program. We actually get the STC for the aircraft and supply the owner with the new prop, spinner and paperwork to make the change easy.”

 

Picking a prop shop

As you can see, overhauling a propeller is a sophisticated and demanding process that requires a combination of specialized training and equipment. It’s not something that any maintenance facility can, or will, attempt.

“Many pilots don’t realize it, but the FARs spell out clearly what types of propeller maintenance can be done by a licensed mechanic and which need to go to an approved propeller shop,” Chafin said.

“And in my opinion, that’s one of the biggest mistakes owners make—when they decide to go to have their propeller overhauled, they all too often choose a shop based on price. A low price may attract a lot of business, but you may not be getting the quality of overhaul you need.”

 

Chafin’s suggestion to owners: “If it’s a Hartzell propeller, finding a factory-authorized shop is as easy as visiting our website. There you’ll find a complete list of shops that we know have the equipment, documentation and training to perform quality propeller overhauls.”

“If there isn’t a Hartzell authorized shop in your area, I suggest you talk to other owners to find out who they have used, then visit the shop yourself and ask questions,” he added.

“Just like you’d do with your engine when shopping for someone to do your overhaul. Get information, ask questions—and don’t let price be your only guideline.”

 

Dale Smith has been an aviation journalist for 30 years. When he’s not writing aviation articles, Smith does commission aircraft illustrations specializing in seaplanes and flying boats. Smith has been a licensed pilot since 1974 and has flown 35 different types of General Aviation, business and World War II vintage aircraft. Send questions or comments to This email address is being protected from spambots. You need JavaScript enabled to view it..

Resources

Hartzell Propeller Inc.

hartzellprop.com

Hartzell Service Letter 61

hartzellprop.com//wp-content/uploads/2012/12/SL061Y-R03-W.pdf

Hartzell Top Prop program

hartzellprop.com/products/top-prop/

McCauley Propeller Services

mccauley.textron.com

McCauley propeller inspections and maintenance information mccauley.textron.com/MPC26OWrev2.pdf

Rocky Mountain Propellers Inc.

rockyprop.com

Rocky Mountain Propellers’ video tour and overhaul process youtube.com/watch?v=S0PyFPOvnYY

Other PFA Supporters for Propellers, Spinners

Ameritech Industries, Inc.

ameritech-aviation.com

Flight Resource, LLC

flight-resource.com

Sensenich Propellers

sensenich.com

TCB Composites

tcbcomposite.com


Last modified on Thursday, 06 November 2014 20:30
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