Airplane propellers have been around, well, since the first powered flight. In the early days they broke at an alarming rate, a function of being carved from wood, a porous and fibrous structural tissue.
Today aluminum and structural composite blades are the norm and repairing them when they get dinged has become commonplace. Damage can result from stones and other objects being swept into the prop wash and from impact with external objects or the ground.
With aluminum blades, repair consists of removing material and using metalworking tools – files, sandpaper, grinding discs, etc. – to re-establish a smooth finish for the airfoil. Repairing a structural composite blade is quite different, because instead of just removing material, composite repairs replace material lost to gouges and other damage.
Infinite Fatigue Life
First, some background on their makeup: Hartzell structural composite blades are composed of a metal blade-shank retention section. A low-density foam core is molded onto this section, and it supports built-up layers of composite laminate. Beyond their obvious weight advantage over aluminum blades, structural composite blades have additional advantages.
These advantages are a longer service life and the ability to maintain a more optimum airfoil shape over the service life of the blade, resulting in an infinite fatigue life. This is possible because most damage to composite blades can be repaired, and they can be returned to service without adversely affecting the airfoil shape.
A composite blade is far more robust than an aluminum blade from an erosion and impact point of view. The stainless-steel leading edge of a composite blade is three to four times harder than the aluminum blade’s leading edge.
It can withstand a lot more impact. A FOD strike that may not hurt the composite blade’s leading edge may damage an aluminum blade. A composite blade may continue to fly to the next service date while a similar size object hitting an aluminum blade might ruin its airworthiness.
With aluminum repairs, the material is ground down to remove all the pits and with repeated repairs the blade gets thinner and thinner and must be scrapped after two or three overhauls. A composite blade can be repeatedly repaired to factory-new shape and aerodynamics.
While the composite materials that form propellers are incredibly durable, they are not entirely immune to operational damage. Propeller manufacturers have created and published protocols to help airplane operators and maintainers determine when and how to repair composite blades.
Hartzell Propeller’s Five Step Evaluation Process
1) Stop Flying
This is the single most important step you can take. Even a small crack or gouge can escalate quickly under the intense forces of flight. The cost of a propeller repair is far less than the cost of a wrecked aircraft. Inspect your prop before and after every flight. If you see damage, do not fly again until you have it professionally inspected or repaired.
2) Assess the Damage
Gouges on the outboard region of a blade and damage to the tip or trailing edge may be minor enough to repair yourself. Hartzell recently published a series where Kevin Ryan, a Hartzell technical representative, demonstrates these repairs in several Hartzell Propeller videos at HartzellProp.com/Composite-Propeller-Repair. If the damage is more severe, it is best to contact a Hartzell Recommended Service Facility.
3) Identify the Material
If you choose to make the repair yourself, the first step is identifying whether your blade is made from Kevlar or carbon fiber, which look similar. Even if you think you know, it is a good idea to verify this using the blade model number. This is located on a sticker on the cylinder of the propeller. This sticker may be located on the blade of older composite props. If you cannot find the sticker, the blade model number will also be on the inspection sheet that came with your prop.
4) Read the Manual
The Hartzell Propeller Composite Propeller Blade Field Maintenance and Minor Repair Manual is the ultimate guide to composite propeller repair. It is more commonly identified as Manual 170. Within its pages, you can determine whether you are capable of making your own repairs or should trust them to the manufacturer. The manual contains step-by-step instructions on each minor repair and is a vital resource for any repair and maintenance. Download it at HartzellProp.com/manual170.
5) Contact Your Nearest RSF
If your propeller has sustained more than minor damage, contact a Hartzell Recommended Service Facility. This worldwide network of propeller repair stations meets rigorous standards, including regular on-site quality systems and process audits, technicians who attend required training updates at Hartzell and approved tools and equipment.
The Coin Tap Test
Best practices include getting factory training and advice, along with making sure you understand overhaul requirements, inspection differences, repair criteria, and repair limits. One of the primary tests is a “coin tap” audible check of the blade body and the blade erosion shield to identify any discontinuities.
For the coin tap, you create a grid on the blade and use a round metal disc to tap the blade, listening for changes in the tone. You look for any de-bonding in the blade body and in the laminates. If you find issues, then you have to evaluate them from a size, depth, and location perspective. In some specific instances, an ultrasonic test may be needed, but that is very limited.
Dean Ward is the director of aftermarket parts and service for Hartzell Propeller Inc. Through his 30+ years with Hartzell Propeller, Ward has gained wide ranging experience that uniquely qualifies him to lead the Aftermarket Parts and Service Team. His team is responsible for aftermarket sales and customer service, forecasting, export compliance, shipping and the Hartzell Service Center.
