Aircraft repair is a huge business! In 2018, the worldwide economic impact for global aircraft maintenance, repair, and overhaul (MRO) was over $75 billion (USD), with an estimated compounded annual growth rate of 4.7% through 2025. The commercial air travel industry’s near-term growth outlook is 6.5%, with 20-year traffic growth projected at 4.0%. Welding is a standard maintenance practice in aircraft repair as described in the FAA Advisory Circular 43.13-1B. As welding processes and procedures have improved over the years, some parts that were once non-repairable can now be repaired by welding.
TIG Welder Methods and Materials
Welding is the most prominent form of joining metals in aircraft without the use of fasteners. The five types of metals welded in aircraft are 4130 steel, stainless steel, aluminum, magnesium, and titanium, each used in different areas of the aircraft. For example, 4130 was a prevalent building material in tube-and-fabric construction but is now limited to mainly aircraft used in agricultural applications.
TIG Welding is revolutionizing aircraft maintenance for repairs that fall outside of the original equipment manufacturer’s scope. Magnesium alloy is common in constructing transmission cases in helicopters because it is very strong and light; weight is everything in aircraft (particularly helicopters). Yet transmission cases were never made to be a repairable item. The original intention was to simply scrap damaged cases and replace them. However, a new transmission case can easily cost more than $50,000. Companies specializing in aero repairs have pioneered proprietary repairs on transmission cases and gearboxes that pass the FAA’s rigorous muster. These repairs can save upward of 90% off the bill of a new case or gearbox.
The approved method for magnesium cases is predominantly tungsten inert welding (TIG) for the extreme heat necessary to join the metal. Oxyacetylene can be used for this method of joining metals but it is not the best method due to base metal oxidation, distortion, and the loss of elasticity.
Benefits of TIG Welding in Aircraft Repair
Tungsten inert welding (TIG, also known as Gas Tungsten Arc Welding) is an incredibly versatile welding method and can meet most of the demands of aircraft maintenance. Because of its relative simplicity and accessibility, oxyacetylene welding was the standard process for many, many years. Still, TIG welding has dethroned it as the go-to method in recent years, particularly as TIG equipment has become commonplace and come down in price.
Changing Technologies in TIG Welding
In the early years of TIG welding, the inert gas most commonly used was helium, which gave rise to the early trade names of Heliarc and Heliweld. In time, carbon electrodes made way for the modern tungsten rod. We now find a highly versatile technique that is further enhanced by the ability to run either alternating current or direct current.
A TIG welder set for DC current, straight polarity, is suitable for all mild steels, stainless steel, and titanium typically welded during aircraft maintenance. 4130 is a low-carbon, chromium-molybdenum alloy and is very common in the composition of aircraft structures. The signature standard alloy is used in a tubular form for aircraft fuselages that employ a truss-type of construction. In commercial aviation, agricultural application aircraft (“crop dusters”) use this construction method exclusively because it provides a strong, rigid airframe.
Low Carbon Steel
Low-carbon steel is also widely used in the industry in the production of engine mounts for piston engines. For these parts, welding is the only method of repair available to help operators avoid purchasing new replacement parts. Considering a used engine mount from an aircraft salvage yard runs in the ballpark of $1,000 for a simple Cessna 172, fixing a cracked weld whenever practical is a much more cost-efficient solution.
Historically, aviation has been a hobby for the wealthy, and this is not just talk. If you have ever wondered why airplane owners speak openly about rebuilding their engines rather than replacing them with new engines, the figures will make it clear. Consider that a brand-new four-cylinder piston engine easily runs in the $40,000–$50,000 range. That’s not a misprint. A rebuilt zero-time engine for a Cessna 172 is still more than $20,000, and these are engines without turbochargers or superchargers.
Engine cases on aircraft piston engines are also often made out of magnesium alloys, for the same reason that gearboxes and transmission cases are: excellent strength-to-weight ratio. Unlike an automobile engine, which routinely only runs around 10%–15% of maximum output, aircraft engines operate at about 75% output during the cruise. Accordingly, their crankcases are subject to high stress and are prone to cracking. Also, since the engines are often repeatedly overhauled, the cases may have several thousand hours of use as opposed to the pistons and cylinder jugs, which are replaced at overhaul intervals. A new engine crankcase is predictably quite expensive, so the option to repair a crack by welding is far less costly.
The more complex the aircraft, the higher the cost of replacement parts. Turboprop turbine engine rebuilds (Pratt & Whitney PT-6A, Honeywell TPE331, etc.) often exceed hundreds of thousands of dollars. Fixing cracks and building up casting defects and damage is a cornerstone of cost-effective repair strategies.
Other Weldable Materials Commonly Used in Aircrafts
Aircraft are exciting machines. They are flown through turbulent air at hundreds of knots, blasted with rain and wind, only to just about drop out of the sky and slam onto a hard runway. They are strong, yet they have to be light. Corrosion is a silent killer, and weight is the constant nemesis. Stainless steel and titanium are used in aircraft construction in critical areas where low weight and temperature tolerance are paramount.
Titanium is widely used in turbine engine compressors and stator blades, subject to constant high vibration, extreme heat, and foreign object debris (FOD) that routinely includes pebbles and rocks, rivet stems, screws, loose hardware, ice, and birds. It is a harsh environment where titanium components are easily damaged. TIG welding these damaged components to fill in and blend the damaged areas saves thousands of dollars and considerable time in commission spent waiting on parts. Instead, repairs can be performed often on-site, and the engine reinstalled quickly.
Stainless steel is ubiquitous on aircraft, often used in ducting and baffling throughout the aircraft, exhaust ducts or pipes, and a slew of other areas. When, for example, an exhaust duct on a turboprop crack (not uncommon), it can easily be removed and repaired, again saving a lot of money and a whole lot of time.
Aviation is one of the most heavily regulated industries in the world. The threshold of entry to new designs is so burdensome that old technology prevails. Also, old aircraft are commonplace. Many airliners, particularly cargo aircraft, are 20 to 30 years old, which takes a toll on metals prone to fatigue. Rather than replacing parts, it is fiscally advantageous to repair whenever possible, and most repairs boil down to a weld to fix the metal. Welded repairs continue to keep operators in the black and safety compliant in a transportation market where controlling costs and maintaining safety standards are essential considerations.