Keeping Sharp on Blade Maintenance
Helicopters and other rotorcraft play an important role in society. From fighting fires to executing rescue missions and assisting law enforcement and the military, these aircraft often are put into perilous situations requiring complete operational efficiency. And as such, properly intact and functional rotor blades are a must, as is the maintenance that keeps them that way.
The most common rotor blade damage seen is erosion on the leading edge of the blade, said Heli-One spokesperson Billie-Ann Woo.
“Erosion is caused by the fast flow of air and particles. Damage can vary depending on the environment – sandier environments cause erosion faster. Cracks can also appear in the leading edge. Damage is also found on the tip cap, which can be repaired or replaced. As a result, operators cannot do much to ‘prevent’ operational damage as it is heavily dependent on environmental factors,” Woo said.
Chad Nimrick, Bell’s director of support and service, Americas, agreed, adding that other typical causes of damage are leading edge strike damage from trees, power lines and hangar doors; hard landings and sudden stoppages; and every so-often, the bullet strike on law enforcement or military blades.
Each kind of damage takes a different repair approach. For erosions, this can be as simple as a paint refinish or polishing the leading edges back to contour. Strike damage needs leading edge abrasion strip repairs or replacements, skin patches or even spare repairs. Hard landings require inspections to ensure the blade’s bushings are still within tolerance and not oblong, with blade twist and contour also measured, as well as a full tap inspection to find any voiding in critical areas such as doubles. And for the rare bullet strike, most cases are repaired with a skin patch and core replacement, Nimrick said.
When blades come in for repair at HeliBlade, Inc., Pablo Ejarque, owner and account manager, said each repair process starts out the same.
“We will clean and inspect all blades and give a detailed report to the customer with our recommendations. Then once the customer has approved of the work, we start the repair process,” Ejarque said.
More common than bullet damage, but requiring similar repairs, is hail damage.
“Depending on how bad the damage is, we can fill it with adhesive and fair it out or use a process called a ‘melt down,’” said Ejarque.
Skins patches to repair damaged blade skin vary in sizes from 2 square inches to upwards of 144 square inches and in some cases larger with the approval of Bell Engineering, said Nimrick.
“The leading-edge abrasion strips are replaced if there is significant erosion, pitting or cracking. The original abrasion strip is removed with a replacement being processed IAW Bell’s Bell Process spec (BPS) and installed on the blade,” Nimrick continued.
Blade bushings, Nimrick added, are inspected at incoming inspection processes and, if required, are replaced. The original bushing is pressed out via hydraulic press and a new one installed matching the required specification for each blade type.
“Some typical repair work that is done includes evaluating and repairing erosion and cracking. This could involve sanding, filling, and fairing the blade edge. Poly tape can also be used to patch small cracks. This is done on the titanium abrasion strip, the OEM allows for poly tape to be placed on the strip to patch cracks and mitigate damage until the next inspection or repair interval,” explained Woo.
Tip caps are also repaired even though they are considered a consumable, Woo added.
“Heli-One can rework tip caps and perform major repairs – with the exception of disbonding or major cracks in which case it is sent to CTI or the OEM,” Woo said.
Repairs are most often done in house, although, as Ejarque noted, sometimes field word is required.
“Repairs can be done in the field if needed, but most repairs come to the shop. A blade on average will stay at the shop for three to six weeks depending on the work needed and number of repairs. If there are large repairs, or a more complicated blade like the UH-60, it can definitely take some time,” Ejarque said.
Over the years, blade repair hasn’t change that much, said Ejarque, but what has changed are the blades themselves.
“Composite blades have been more and more prevalent since you have a much longer time life than the standard metal blade,” he said.
Paul Bolton, technical director, Aviation Blade Services (ABS), agreed, adding it used to be that even blades considered a composite still had a metallic structural spar, but with recent advances in composite manufacturing techniques, more and more designs now incorporate composite spars too.
Bolton described the difference between repairing composite and legacy blades.
“The metal blades are comprised of a structural spar with numerous unique bonded pockets to provide the airfoil shape; we find the common areas requiring maintenance to be pocket replacements and leading edge abrasion strip replacement,” Bolton said. “Fortunately, we’ve developed PMA replacement pockets to support these legacy aircraft that still have years of productive service.
"The composite blades are also built around a structural spar but with a nomex honeycomb core and fiberglass skin for the airfoil and strength. Repairs here primarily center around composite skin and honeycomb damage repair as well as lead edge abrasion strip and sheath repair and replacement.”
ABS carries out all composite repairs in the environmentally controlled area of their facility, following complete inspection of the blade, Bolton noted, and generally, there will be a mix of fiber glass skin and honeycomb repairs both requiring different repair techniques — either surface patch or scarf.
“Once the area is prepared and the repair materials installed, ABS uses a combination of custom and industry standard equipment to apply the requisite heat, pressure or vacuum for the controlled curing process. Once cured, the blade is completed and sent for further inspection, final paint and static balance,” he continued.
Bolton said that he sees composite blades as taking a stronger and stronger hold in the industry in the coming years.
“In recent years we have seen a move toward all composite constructions. Traditionally even blades considered as composite still had a metallic structural spar, but with the advances in composite manufacturing techniques the most modern designs now incorporate composite spars,” he said.
