Titan Abrasive Systems’ president, Brandon Acker looks at how abrasive blasting can play huge role in bringing aging aircraft back to life.
The effect of Covid-19 on the fiscal health of the airline industry is readily apparent in the numbers. The staggering losses incurred are a major catalyst behind the decision of the major airlines to refurbish their current fleet, rather than purchase new aircraft.
The process of refurbishing older aircraft, which falls under the larger category of MRO, has been going on for decades. With Covid-19 severely hampering the revenue sources and eroding profit margins, more airlines began to turn to MRO to maintain fleet efficiency.
There are a number of methodologies employed in the refurbishment of an aircraft; perhaps the most critical is abrasive blasting. Through abrasive blasting, an extremely wide range of parts can be rendered virtually new. Abrasive blasting modifies surfaces of parts or structures in a variety of ways depending on the media type and blasting parameters.
For aircraft refurbishment, the abrasive blasting process is generally executed for two purposes: 1) Cleaning and abrading a surface, which not only removes all dirt, grime, oils, etc. from the surface, but also ‘roughs up’ the surface to provide improved adhesion for a new surface finish, whether it’s paint, powder coating, or polyurethane. The fuselage provides a prime illustration of this type of application. 2) Cleaning and deburring a surface without producing any kind of ‘profile’ or removing any materials from the surface of the item. This would be the process used for parts in jet turbines, for example, where the removal of even the most minute amount of metal could affect engine performance – and safety.
Simply put, softer media at lower pressure can gently remove coatings, while very hard abrasives projected at high pressures can provide texture to surfaces that require finishing.
On a commercial aircraft, there is almost no part or system that cannot be refurbished. However, it’s the engine components that are most critical. The abrasive blasting performed on these valuable items may involve simply knocking off some dirt or an entire overhaul (though a complete overhaul is not performed nearly as often as a refurbishment).
Handle with care
Within the engines, the most expensive parts are the turbine blades, bearings, and engine control systems. These must be handled with kid gloves during refurbishment, not only to avoid huge fiscal losses but to ensure the overall safety of the engine itself. Often, these parts are reconditioned through a process called micro-abrasive blasting. It’s used for extremely localised tasks and can be carried out by hand or by machine.
Micro-blasting involves combining air and an abrasive into a highly focused stream. The ‘laser’ focus of the micro-blasting is ideally suited for removing the coating from recessed and protected areas of the engine blades as well as inside small air passages. The precision that micro-blasting is known for is a product of the small nozzles and fine, sharp abrasives, such as aluminium oxide. If more difficult coatings are involved, silicon carbide can be used as an alternate medium.
Vacuum, centrifugal, soda, steel grit, bristle, dry ice, shot peening, and bead blasting are other abrasive blasting procedures. Abrasive particles can be organic, metal, silicate, plastic or stone. The most widely used include glass beads, aluminium oxide, plastic, steel shot, silicon carbide, coal slag, even corn cobs. The question is, what are the optimal types of media used to refurbish aircraft? And how does one know which media to use on each part?
First off, the reason so many media exist is because they each excel at performing different tasks. The performance of each material is determined by a number of key abrasive properties of the medium: shape, size, hardness and density. Normally, with anything made out of steel, you would use a material to create a profile so it can hold the coating. Conversely, engines parts don’t need a rough finish to them; as mentioned earlier, the desired result is a smooth finish where no material has actually been removed from the part. Planes are mostly all aluminium, so they’re generally blasted with some type of plastic. It will clean aluminium without making it rough or removing material.
All shapes and sizes
Abrasive blasting is performed by companies who specialise in this procedure, rather than the airlines themselves (although some airlines actually own companies or have separate divisions that perform this work). Depending on the size of the part, the procedure – regardless of which process or medium is used – is performed in either a blast room or a blast cabinet. The difference in the two as their names imply is simply size: both have the same basic components, namely, a blasting system, dust collectors, and a media recovery system.
A blast room, which can be massive, would be used to handle the fuselage, landing, gear, and other large items. The larger blast rooms are usually built to custom measurements; the size is determined by a number of factors, including the size and weight of the largest piece that will blasted. A blast room is an area you can literally walk into and perform the work inside the structure.
Many jet engine components, conversely, would be able to fit nicely into a blast cabinet. With the blast cabinet, the work is performed from outside using protective gloves.
How the media is applied is a critical part of the blasting process as well. On a website called Aerossurance, there is a report entitled, “Aggressive Grit Blasting Maintenance Leads to Engine Fire and IFSD.” Apparently, during the maintenance of a particular aircraft, the plasma spray coating on the blade roots was being removed and replaced. The surface of the blade was grit blasted with aluminium oxide prior to the application of the plasma coating.
The problem was the operator performing the procedure had held the grit blasting gun about 2 inches from the surface instead of the specified 4 to 6 inches. According to a post-incident audit, the decreased distance, as well as “incomplete masking of the blade root neck that allowed the no grit blast area being exposed during the overhaul of the blade,” resulted in “blasting media being embedded in the blade root from which a fatigue crack developed. The fatigue crack propagated until separation of the blade occurred that resulted in a complete loss of engine power.” Though the plane landed safely, and no one was hurt, the damage was significant – not to mention the frayed nerves of the pilot and crew.
Performed correctly, and using the right media, an aircraft in need of refurbishment can be given new life through the use of abrasive blasting. With the cost of new aircraft being prohibitive due to the fiscal fallout from Covid-19, every airline should give refurbishment – and abrasive blasting – serious consideration.