How Welding Extends the Life of Aircraft Parts
How Welding Extends the Life of Aircraft Parts
Aircraft repair is a huge business: in 2018, the global 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 and described in the FAA Advisory Circular 43.13-1B. As welding processes and procedures have improved, parts that were once non-repairable assets can now be repaired using welding.
Welding Methods and Materials
Welding is the most prominent form of joining metals in aircraft without the use of fasteners. The four groups of metals that are welded in aircraft are 4130 steel, stainless steel, aluminum, magnesium, and titanium. Each type of metal serves a specific purpose in a particular part of the aircraft, and the types application vary from one aircraft to another. For example, 4130 was a prevalent building material in tube-and-fabric construction (Piper Cub, Pitt’s Special biplane) but is limited mainly to agriculture aircraft for commercial flying. Welding is revolutionizing aircraft maintenance. Repair work on aircraft parts is often performed outside of the original equipment manufacturer’s scope for the part – extending its serviceable life.
Minimizing weight is a paramount consideration in aircraft construction, particularly in helicopters. Magnesium alloy is commonly employed in the construction of transmission cases in helicopters because it is very light and strong. but these cases were never made to be a repairable item. If damaged, they were simply scrapped. However, a new transmission case can easily cost more than $50,000. Companies specializing in aero repairs have pioneered proprietary repairs on transmission cases, gearboxes that pass the FAA’s rigorous muster. These repairs can reduce the cost of dealing with a damaged transmission case by upwards of 90%.
The approved method for repairing magnesium transmission cases cases is by TIG welding which provides the levels of heat necessary to sufficiently join the metal. Oxyacetylene can also be used for this method of joining metals but is not preferred because of its tendency to cause base metal oxidation, distortion, and the loss of elasticity in the welded part.
Benefits of TIG Welding in Aircraft Repair
Gas Tungsten Arc Welding (GTAW), commonly referred to as TIG welding, is an incredibly versatile welding method and can meet most of the demands of aircraft maintenance. Oxy-acetylene welding was the standard process for many, many years because of its relative simplicity and accessibility. Still, TIG welding has surpassed oxy-acetylene as the go-to welding method in recent years, particularly as TIG equipment has become commonplace and come down in price.
In the early years of TIG welding, the most commonly used inert gas was helium and welders would commonly refer to they method as Heliarc and Heli welding. Eventually the use of carbon electrodes gave way to the use of tungsten welding rods. We now have a highly versatile technique that is further increased by the ability to run either alternating current or direct current.
A TIG welder set for DC current, straight polarity is suitable for welding all mild steels, stainless steel, and titanium on aircraft. 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 strong rigid welds that can withstand the the stresses they will have to endure.
Low Carbon Steel
Low carbon steel is also widely used in general aviation in the composition and production of engine mounts for piston engines. These are other areas where welding is the only method of repair instead of purchasing a new unit. 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 the domain of the wealthy, with good reason. Have you ever wondered why airplane owners speak openly about rebuilding their engines rather than replacing them with new engines? It’s because a brand new four-cylinder piston engine easily runs in the $40,000-$50,000 range. A rebuilt zero-time engine for a Cessna 172 is still more than $20,000, and even more for engines with turbochargers or superchargers.
Engine cases on aircraft piston engines are often made out of magnesium alloys, for the same reason that gearboxes and transmission cases are: excellent strength-to-weight ratio. Unlike an automobile engines, which routinely only run at around 10%-15% of maximum output, aircraft engines operate at about 75% output during flight. 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 ability to repair a crack with welding is far less costly.
The cost of replacement parts for more complex aircraft is even greater. The costs of turboprop engine rebuilds (Pratt & Whitney PT-6A, Honeywell TPE331, etc.) are in the hundreds of thousands of dollars. Fixing cracks and building up casting defects and damage using welding is a cornerstone of any cost-effective aircraft repair strategy.
Other Weldable Materials Commonly Used In Aircraft
Aircraft are exciting machines. They are flown through turbulent air at hundreds of knots, blasted with rain, wind gusts, drop out of the sky and bang 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 lightweight, and temperature tolerance are paramount.
Titanium is widely used in turbine engine compressors and stator blades, which are subject to constant high vibration, extreme heat, and foreign object debris (FOD) that routinely include pebbles and rocks, rivet stems, screws, and loose hardware, ice, and birds. Under such harsh conditions, titanium components are easily damaged. TIG welding these damaged components to repair damaged areas saves thousands of dollars and considerable time. Repairs can be performed often on-site, allowing a quick re-installation of the aircraft engine and a subsequent return to flight.
Stainless steel is ubiquitous on aircraft, often used in ducting and baffling, or exhaust pipes, in addition to a slew of other areas. When an exhaust duct on a turboprop cracks (which is not uncommon), it can be easily removed and repaired, saving operators 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 20-30 years old, which takes a toll on metals prone to fatigue. Rather than replace parts, it can be fiscally advantageous to repair whenever possible, and most repairs involve applying a weld to fix damaged or worn metal. Welded repairs continue to keep operators in the black in one of the most expensive transportation markets globally, and also help make it one of the safest.