Machine gun fire and rocket propelled grenades are known enemy threats against the nation's Army helicopter force.
But, so, too, is a relatively passive culprit that causes damage over time -- the minute fragments of salt that remains on a helicopter after flying near or over seawater.
Salty residue left on the nation's helicopters can cause extensive corrosion, with the chlorides found in seawater coating their metal exteriors, and finding their way into helicopter magnesium transmission houses and gear boxes. The Aviation and Missile Command spends about $1.6 billion a year to mitigate corrosion on helicopter components and materials. And as budget constraints are requiring that the Army maintain its existing helicopter fleet for several more years, issues with seawater corrosion are sure to rise.
"Years ago, we pinpointed a need for an automated and portable helicopter rinse system," said Steve Carr, program manager for the Aviation and Missile Command's Corrosion Program and an employee of the Aviation and Missile Research Development and Engineering Center.
"The Army has a requirement to rinse aircraft that have operated low-level flights over seawater or in areas where there is lots of seawater. Helicopters are very sensitive to saltwater. There are lots of places where we operate close to seawater -- Savannah, Ga., Corpus Christi, Texas, and Hawaii, for a few -- and we've never really had a good way to rinse those aircraft. Rinsing on a frequent basis will prevent corrosion or at least extend the life of a helicopter, even one that spends nine months working on a beach."
Helicopters flying over seawater should be rinsed daily. Yet, that's a tough requirement to uphold when there's a whole battalion worth of helicopters -- anywhere from 12 to 24 -- operating in a seawater environment at the same time.
"There's no way to rinse those aircraft in a timely manner. Soldiers have used (garden) hoses and fire truck hoses or anything they can find sometimes. But even using a fire truck to rinse them requires a good length of time. It can be as much as 30 days before an aircraft gets that required rinse," Carr said. "Even when you get some help from nature with rain, it just doesn't come often enough."
Although large military aircraft -- such as the C-130s, F-113s and C-5s -- do have rinse facilities, the Army does not have a rinse facility for rotary wing aircraft.
"They are not designed for rotary wing," Carr said. "The pressure and the direction of the spray is not good for the blades of a helicopter."
In 1998-2000, studies were conducted to determine the amount of damage seawater can do to the Army's helicopters. Since then, the Army fleet has grown, operations have expanded throughout the world and the requirement for rinse standards has become more significant. In 2008, the Aviation and Missile Command, with the help of the Aviation and Missile Research Development and Engineering Center, conducted a Clean Water Rinse System Demonstration and Validation Project, with the findings submitted to the Office of the Secretary of Defense in 2010.
"We're trying to help the Aviation and Missile Command sustain its assets. That's the mission we have," Carr said. "That's always been a big focus, but it's even a bigger focus as we go through the drawdown with the Army. The drawdown makes it even more important to sustain our fleet."
The rinse system that was demonstrated uses a relocatable aluminum pad or a fixed concrete pad that is 200 feet by 290 feet wide. It is automated and uses different configurations of ground and over-the-rotor spray nozzles to fit the different types of aircraft. One demonstration at Wheeler Army Airfield in Hawaii involved the CH-47 Chinook, UH-60 Black Hawk and the OH-58 Kiowa while another demonstration at Hunter Army Airfield in Savannah, Ga., involved the AH-64 Apache and Special Operations aircraft.
"We were able to successfully demonstrate a system for rinsing aircraft using off-the-shelf technology that had never before been used to rinse aircraft," Carr said.
"The CH-47s needed the most nozzles because they had to reach over the rotor blades in both the front and the back of the aircraft. The Kiowa required a lot less nozzles and a shorter rinse time."
The rinse system also captured used water and recycled it through a filtration system so that it could be reused. That feature is important as many seawater locations are limited in the amount of clean water that is available, Carr said.
During the demonstration phase, water was reused to show the effectiveness of the filtration systems. Engineers also checked helicopter fluids to ensure that the rinses did not cause water to get into the aircraft's hydraulic system or into its oil reservoirs.
"We wanted to make sure the rinse system was not damaging the aircraft," Carr said.
"We wanted to make sure it rinsed well and that we did not get water intrusion in the oil and hydraulics."
With the demonstration successful and the report complete, the Army Corps of Engineers is now completing the standard design for a helicopter rinse system that will become the Army standard.
The next step will be to get Army funding for four permanent rinse facilities -- one each at Wheeler Army Airfield, Hunter Army Airfield, Camp Humphreys, Korea, and Corpus Christi Army Depot, Texas, and for re-locatable rinse facilities for theater outposts. There is also a possibility the Navy will purchase rinse facilities for some of its installations and depots.
The Army Standard Rinse System Design can be used to provide the rinse facilities for all military helicopters and small fixed wing aircraft. The cost is $2 million-$4 million per rinse facility. But compared to the price tag of a $25 million helicopter, the investment would be worth it, Carr said.
"From 2000 to November 2012, it has taken us 12 years to get a standard," he said.
"Now we have an Army Corrosion Executive and a formal corrosion prevention program. In the last few years, we've added an Army strategic plan for preventing corrosion. There's lots of emphasis on trying to address corrosion, so I'm optimistic that we will get funding for these corrosion rinse facilities."