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For its Rotorcraft, Navy is Switching to Aluminum Gearboxes

The Navy is evaluating the potential for replacing select magnesium components used on its fleet of helicopters, because of a recurring maintenance problem. Magnesium is extremely prone to corrosion in saltwater environments.

An aircrewman peers out of an H-60 Seahawk
An aircrewman peers out of an H-60 Seahawk assigned to the "Blackjacks" of Helicopter Sea Combat Squadron (HSC) 21 on June 5, 2008, after delivering a load of ordnance onto the flight deck of the aircraft carrier USS Ronald Reagan. Photo by Joshua Scott, Mass Communication Specialist 3rd Class, U.S. Navy.

The H-1, H-53, and H-60 helicopters have main and tail rotor gearboxes that have been exhibiting severe corrosion problems due to their magnesium construction. Initially, the Naval Air Systems Command (NAVAIR) collaborated with the Army Aviation and Missile Command (AMCOM) to determine if there were any feasible options for mitigating the problem (see related article: "Services Work to Reduce Maintenance Costs and Raise Helicopter Readiness").

According to Bill Nickerson, a materials engineer in the Materials Engineering Division at NAVAIR, this joint service effort was necessary to maximize the use of legacy magnesium parts. During the course of the effort, it was determined that for the long-term, mitigating the problem simply wouldn't be sufficient. "We have a lot of legacy magnesium components out there, and obviously we don't want to replace fully operational assets, so we're going to try to maintain those components for as long as we can to the best of our ability," said Nickerson. "What we found, from a corrosion standpoint, was that the best, state-of-the-art alloys, coatings, and processes that we have to protect and maintain magnesium cannot offer the corrosion performance of aluminum alloys and coatings."

H-60 Seahawk assigned to the Raptors
Ralph Laduke, Aviation Boatswain's Mate Handling 3rd Class, guides an H-60 Seahawk assigned to the "Raptors" of Helicopter Maritime Strike Squadron (HSM) 71 on June 2, 2008. Photo by Elliott J. Fabrizio, Mass Communication Specialist 2nd Class, U.S. Navy.

Legacy data from the H-53D helicopter show that some of the tail gearboxes' center housings have been switched to aluminum components as a result of excessive corrosion maintenance costs. NAVAIR has already acquired performance data for the aluminum gearboxes. Based on maintenance records from the Fleet Readiness Center-East (at Marine Corps Air Station, Cherry Point, North Carolina), the aluminum components on the H-53D model clearly outperformed its magnesium counterpart on the H-53D.

Consequently, NAVAIR began looking for a suitable replacement for the magnesium components that could be easily implemented without altering the performance of the rotorcraft. Aluminum stood out as a legitimate candidate replacement material, despite its inherent drawback of having a significantly higher density, making the strength-to-weight ratio much less attractive than that of magnesium. Weight of course is always a critical factor for rotorcraft.

"The H-53K program has already switched to aluminum components in its design process, reaping a significant estimated cost avoidance over a 20-year-life cycle compared to magnesium designs. If aluminum components can be transitioned to other new programs, such as the H-60 upgrades, the Navy has the potential to realize millions of dollars in cost savings."

What NAVAIR found, at least in one specific example, was that to meet the structural and strength requirements, aluminum only had to have approximately 20 percent of the thickness of the current magnesium component. Consequently, aluminum became a much more feasible candidate, Nickerson said. "We thought—for something like an H-53 main gearbox housing—that there would be a 700- or 800-pound difference to use aluminum on the main gearbox instead of magnesium, and it ended up being an estimated less-than-200-pound difference," said Nickerson. "That's well within the operational capability of the engines, although any weight penalty must be offset against operational payload capabilities for any potential modification."

To further reduce any weight penalty incurred for using a more corrosion-resistant material, NAVAIR is looking into new aluminum casting processes, such as investment and Controlled Solidification Investment Casting (CSIC). The advantage of the new processes is that the density of aluminum can possibly be kept to a minimum while maintaining the structural strength at the required levels. These newer processes are being evaluated to determine scalability for full production and manufacturing.

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Table of Contents

Volume 4, Number 2
Summer 2008

Top Stories

•For its Rotorcraft, Navy is Switching to Aluminum Gearboxes
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• David Erickson Retires as Senior Program Analyst at OSD
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• DoD Corrosion Office Releases Landmark Cost of Corrosion Study
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• John Young Selects Director of DoD Corrosion Policy and Oversight Office
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• Corrosion Office Produces Webcast, Podcasts, and Videos to Advance its Mission
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• What's New on the CorrDefense Web Site?
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