Composite Rotor Blade Inspection and Repair
Helicopter composite rotor blades offer an exceptional challenge to those wishing to make repairs because of the critical role that the rotor plays in flight. Most technicians today choose not to make any type of repairs to the rotor, in fear that an incorrect repair might result in disaster. While these fears are understandable, taking the time to know composites and learn basic preventive maintenance and minor repairs can actually result in the prevention of catastrophic failures.
Practice makes perfect
Prior to attempting any types of repairs on any composite component, it is imperative that two things be done. First, there must be a thorough understanding of the type of material that is going to be repaired or inspected. And second, the technicians skill level when working with the material should be developed by practicing the repair on practice parts. It is highly advisable to attend a composite school where practical hands-on experience can be gained and repair technique, that can contribute to an airworthy repair, can be learned.
What's a composite?
A composite is a combination of two materials: a mass of fibers, the principal load-carrying material, and a matrix (commonly epoxy), which bonds the fibers together and gives them lateral support. The combination of the two leads to a very high strength-to-weight ratio material. Today's advanced composites are also referred to as "fiber-reinforced plastics."
Know the materials
Fiberglass is one of today's most commonly used composite materials. Fiberglass is actually made of molten silica and, for the purpose of aviation, is divided into two categories: Electrical grade, which is referred to as Type "E," and structural grade, which is referred to as Type "S." Fiberglass has been used in aviation since the 1950's. Kevlar® is a lighter (about 40 percent), more durable material than fiberglass. It is an organic aromatic polyamide that is derived from nylon. One of its more distinguishable features is its yellow color. Kevlar 49 is the only grade that is approved for use on aircraft structures. It has superior tensile strength and toughness but is inferior to carbon in compressive strength. Carbon also referred to as graphite, is strong but rather brittle. It is black in color, stiffer than titanium, and 40 percent lighter than aluminum. Boron, according to FlightSafety, is made by depositing boron gas vapor onto a thin filament of tungsten or carbon. The resulting fibers are approximately 0.004 inch in diameter, have excellent compressive strength and stiffness, and are very hard. Boron is black in color and highly toxic.
Ceramic fibers such as Nextel®, made by 3M™, are used in high temperature applications of around 2,200 degrees F and its fibers are white.
Matrix refers to the resin system that is use to bond the reinforcing material together. Epoxy is the most common matrix material used today. Core Material is the central material that is bonded between two surface skins. The core material, usually foam or a honeycomb structure, provides a rigid light weight component that supports the two external skins. According to FlightSafety, Honeycomb has a greater strength-to-weight ratio, but foam is more durable and if damaged, has a memory and will return to about 80 percent of its original strength.
Many sophisticated methods of inspection are available, but the most valuable and economical inspection is still done with no special tools. The eyes, ears, and hands can give you valuable indications that there are defects in the rotor. It is important, however, to refer to manufacturers recommendations for proper inspection techniques.
"I like to get a mechanic in the habit of touching and feeling along the trailing edge of the rotor blade for damage all the way to the tip." says Dana Kerrick of IAC Ltd. Co., in Haslet, TX. "Many people don't realize how critical a nick on the trailing edge of a blade can be. It is extremely important to repair it right away." Kerrick explains that the condition of the trailing edge can ultimately determine the life of the blade. A small nick can set up stresses that can propagate to the leading edge and cause blade failures.
Kerrick suggests polishing out any small nicks as soon as they are discovered to relieve stress in that area. IAC Ltd. Co. is a rotor blade overhaul facility actively engaged in the repair of Bell, Agusta, Sikorsky, Eurocopter, MD Helicopter and Schweizer main and tail rotor blades. Kerrick explains that while many technicians are reluctant to remove the rotor from the helicopter for a good thorough inspection, it is important to place the rotor in a position where the entire blade can be inspected and where there is suitable lighting. According to FlightSafety, a good visual inspection with the aid of a magnifying glass should be the first step in inspecting a blade. An acoustic impact (coin tap) should follow to verify visual indications of damage. The coin tap test is conducted with a special light weight (1-ounce) hammer, coin, or other object (depending on the manufacturers recommendation).
By simply tapping on suspected areas of the blade, a change in the sound can indicate delamination, bond separation, or other damage. Becoming proficient at coin tapping takes practice. Different materials, complex curves, and varying thickness in materials can effect the sound of the tap, thus, the importance of knowing the material, construction and composition of the blade that you are inspecting. Other methods of inspection that may be spelled out by the manufacturer include hardness tests, moisture tests, ultrasonics, X-ray, and thermography, to name a few. "If technicians have questions regarding damage," Kerrick says, "call us, send a drawing, or e-mail a photo. We'll give a reasonable assessment of the damage."
"Practically all damage," says Kerrick, "is due to one of four things: corrosion, erosion, trees, or strikes." Of these four, corrosion and erosion are the only two that can be addressed by the technician. To prevent corrosion, Kerrick recommends a daily washing of the blades with whatever solution is recommended by the manufacturer. "You can't use just any soap or cleaner," Kerrick says. "There are certain cleaners that are approved and others that are forbidden. Some solvents or cleaners may contain chemicals that will damage the blade."
Kerrick adds, "Fiberglass rotor blades should not be waxed. Many technicians wax blades assuming that the blades will be protected, but the wax can imbed itself in the composite, and prevent paint or adhesives from sticking. This can make repairs more difficult and possibly more expensive."
"Erosion," explains Kerrick, "is an ever present problem and should be addressed frequently. Flying in rain, or in harsh environments, such as near the ocean or in dusty areas, can quickly erode the blade to the point of putting it out of service."
There are steps that can be taken to reduce erosion and significantly reduce operating costs. Exposed areas, where paint has been worn off the blade should be touched up with paint immediately. It is much cheaper to wear out paint, than to wear the actual blade. It is not necessary or recommended to repaint the entire blade because a re-paint can cause serious balance problems with certain blade models. Kerrick also suggests sealing joints where leading edge erosion strips meet. If this joint is neglected, the original sealant will wear away. Moisture will then enter into the joint and propagate from that area, causing the erosion strip to separate from the rotor.
Kerrick offers that manufacturers sometimes provide kits to perform this repair and that it can be done in a couple of hours. "Technicians don't have to perform these tasks," says Kerrick, "but they'll realize a much greater blade life if they do.
There are actually many repairs that a technician can comfortably perform in the shop, but the principal factor that limits the repairs is the applicable maintenance manual. Procedures are clearly called out that specify repair limits, techniques, and methods.
It is important, however, to remember that if a repair cannot be accomplished in accordance with the existing maintenance manual, it may still be repairable by an approved overhaul shop with expanded engineering approval. Minor damage such as nicks, scratches, and minor delamination can usually be repaired if this damage is in a non-critical area.
There are usually no special tools that are required, but to accomplish professional looking repairs, it may be necessary to purchase special equipment such as vacuum and heating devices. Kerrick says, "With the recent development of portable repair tooling and technology, we are now doing increasing numbers of repairs in the field at customer request. Obviously, the main advantage is the savings in time and freight costs."
Practical Thermocouple Troubleshooting
By Peter G. Tanis
Athermocouple is a simple device. Two dissimilar metals are joined together and the junction produces a small voltage when heated. The voltage produced is determined by the make-up of the metals in the junction — not the quality of the junction. They either work or they do not work. They will not give an inaccurate reading in most cases.
If you check thermocouples with an ohmmeter, they will show resistance or open. An "open" will not work at all. If there is continuity, the thermocouple will give the correct output because it is a characteristic of the material that the thermocouple is made of. The lead wire is also made of the same wire that the thermocouple is made from.
Some Cylinder Head Temperature (CHT) systems are operated by the voltage output of the thermocouple alone. An example of this type of system is the Alcor® 2 1/4-in. gauge used on some aftermarket cylinder head temperature gauges. These self-powered units must have a certain total resistance of the thermocouple and lead wire together. This resistance is marked on the back of the gauge (it's usually 2, 4, or 8 ohms). The resistance of the total system is a function of the diameter of the lead wire. Lead wire is rated in a number of ohms per foot. For this reason, never shorten the wiring harness on one of these systems.
Another type of system is the amplified gauge. In this type of system, the power for the operation comes from the aircraft electrical system. The thermocouple voltage output is used only to provide a reference voltage at very low current. On this type of system, the lead length is not important — they can be shortened or lengthened. Examples of this type of gauge are: Insight, JPI, and Electronics International.
Thermocouples are made in many "types" and each type has a different voltage output. Because of this, the wire type in the lead, instrument, and thermocouple must be matched. Common aircraft types are Type J (Iron-Constantan) and type K (Cromel-Alumel®). Thermocouple systems may also require grounded or non-grounded thermocouple junctions. Electronics in some equipment is not designed to handle a grounded thermocouple and so they insulate the thermocouple. It's also easier from a manufacturing standpoint to make a grounded thermocouple because it doesn't require the extra step of insulating it.
Thermocouples have the lead color marked. Type J wires are red and white (yellow and black on old military units). Type K wires are red and yellow. The negative lead is always red. Harness and thermocouple leads should always connect to the same color wire.
Things that can make a thermocouple read incorrectly:
Below, in Figure 1, is a thermocouple system with the junction at 'A.' The leads pass through a baffle hole at 'B.' 'C' and 'D' are lead connectors and 'E' is the instrument.
The instrument reads the voltage output of the closest junction of the wires. If the wire insulation chafes through at 'B,' then the temperature indicated will be the temperature at 'B' and not that at 'A.' If a cylinder thermocouple probe has the lead so badly abused that the insulation is damaged, then it will read the temperature where the wires touch and not the probe tip. This can be avoided by not bending the wires sharply where they exit the probe and by securing the leads properly.
If there is an "open" in any of the wires, such as a bad crimp in any of the connectors at 'C' or 'D,' then the system will not read at all. Again, handling the wires roughly when installing the CHT probes or improper securing can result in broken wires.
In inexpensive self-powered systems, the connectors at 'C' and 'D' are made from the same material as the wire they are used on. However, in most aircraft systems, the connectors are common electrical terminals. Each place different metals join, such as in these connectors, another thermocouple is formed, which puts out voltage. These terminals are in series with the sensing thermocouple and add to or subtract from the output. It is fortunate that these put out a negative voltage on one lead and a positive voltage on the other. The net result is no change in the reading of the sensing thermocouple 'A.' An inaccurate reading can occur if one connector is hotter than the other. This can occur when one of the junctions at 'C' or 'D' is close to an exhaust stack and the other one is somewhat removed. They must both be in the same temperature environment.
If a thread "adapter" thermocouple, as shown in Figure 2, is placed under an OEM resistive CHT sender and the OEM sender is loose or not properly grounded, voltage may be placed on the thermocouple lead, resulting in problems with both systems.
The OEM CHT sender is a resistive unit that changes resistance with temperature. The gauge reads the current flow to ground, which changes as the resistance changes. The center terminal of the sender has a positive voltage on it. If the body is not grounded to the engine properly (and the engine grounded) the cockpit gauge will not work properly and the thermocouple that you've placed under it may be in the path back to ground. Be sure the sender is grounded to the engine and the engine ground has continuity to the airframe.