Clear Glass
AVIATION WINDSCREENS ARE TAKING A BEATING IN THE HARSH OPERATING ENVIRONMENTS FOUND IN IRAQ AND AFGHANISTAN. HOW BIG IS THE PROBLEM AND ARE THERE SOLUTIONS?
Increases in lifting capacity and the prevalence of extreme environments and unprepared landing zones are placing additional stresses on windshield systems among aviation platforms. Rotary wing platforms offer a particular challenge as their very means of flight exacerbates damage to any glass, the rotors forming an inherent “sand blast” effect. Nearly all the damage that occurs to is done during landing and takeoffs when sand, stones and foreign object debris (FOD) are blown up against the screens, which have limits on the number and size of cracks that have to be replaced, as repair is extremely difficult to achieve.
Iraq and Afghanistan have provided an unusually high source of erosion and abrasion for glass windshields. The DoD is currently undertaking a number of measures working with users and industry in the aerospace transparency community as well as looking to innovative solutions outside this group to reduce the wear and tear on platforms, simply by them being in theater, as well as limit the chance of mission failure by introducing added protection and, in the future, even repair.
USER NEEDS
Sand damage depends on the aircraft platform and is role-dependent, according to Kevin Koloski, maintenance subject matter expert at the 160th Special Operations Aviation Regiment (SOAR). “Our aircraft like the MH-60 or MH-47 land in austere environments where sand and dirt have a major impact on the cooling devices of the aircraft, while our attack aircraft fly in a holding pattern and go back to a hard stand [improved airfield]. There is a difference in maintenance inspections and frequencies determined by what the aircraft mission is and what kind of shape the airframes are in. This is determined by daily inspections.”
The maintenance impact is felt throughout the regiment’s aircraft, according to Johnny Hamer, avionics SME for the 160th. Hamer said, “The aircraft ingest a lot of debris through the cooling fans. A lot of our avionics and cockpit management systems are cooled by an internal fan on the aircraft that pumps in cooling air. Then there are smaller fans on a lot of the components to further cool their internal components. When the aircraft land in an austere environment it ingests all of the dirt and debris into the fans. After a few landings the cooling passages in the components get plugged up and overheating can occur. This causes more upkeep on those aircraft compared to aircraft that take off and land to prepared landing surfaces.” Between them Koloski and Hamer have 23 years active service with the regiment.
Hamer estimates that about 20 percent more maintenance time is required with aircraft that land in an austere environment compared to aircraft that take off and land to the hard stand.
During combat operations windshields get pitted and cracked from landing in rugged landing zones. “We will try to get the windshields replaced if we can bring the aircraft down for a few hours to replace them, but due to mission requirements we might not have the parts available or the time,” said Hamer “To overcome these challenges we will plan to have the windshields available to be able to swap them out during scheduled maintenance or during crew rest.”
PRIME PERSPECTIVES
Rocco DiGenova, project engineer at Sikorsky Executive Transport Helicopter Programs, is the technical focal point for all Sikorsky’s windshield and window issues. He offers a prime contractor’s perspective on the military aerospace transparency issues.
DiGenova identified two damage mechanisms for windshields in sandy environments: abrasion and erosion. Abrasion occurs either when the wipers are turned on or rubbing damage when the windshield is being cleaned by mechanics. Erosion is the results of impacts and impingement. Rotary wing aviation is at an inherent disadvantage in this respect he explained, “When you switch on your rotors you essentially sand blast your windshield and you suffer hits and impact damage.”
Glass is a brittle material, that is, it deforms elastically up until failure. Glass always fails in tension. Glass always fails from pre-existing flaws known as Griffith microcracks. Glass is strengthened by introducing residual compressive stresses that mitigate the effects of the flaws and must be overcome by any applied tensile stress before a failure can result.
Two main ways to strengthen glass are available. Glass is either strengthened by tempering or chemical strengthening. For tempering, the surfaces of the glass are put in compression. The glass is heated to a high temperature and then cooled quickly. Because of the uneven cooling the surfaces of the glass will cool first and then as the center cools, it will leave a residual stress distribution in the pane of glass, and that will put the surfaces in compression and the core in tension. This residual stress distribution balances out and the surface is then less susceptible to starting a failure than if it were left untreated.
Chemical strengthening works a little differently. Glass sheets are embedded in a chemical bath creating an exchange of ions. The smaller ions flow out of the glass into the bath and larger ions flow from the bath into the glass. It is then removed, cooled, and the result is a glass that has its surface in compression and its interior in tension.
Higher strength can be obtained using chemical strengthening; a higher damage tolerance can be achieved because it creates a thicker compressive layer, allowing the glass to take pits and surface damage without causing a failure.
In the desert today Sikorsky has two helicopters, the H-60 Blackhawk aircraft family with laminated glass heated windshield and the H-53 with the main windshield consisting of laminated plastic.
DiGenova outlined the challenges and solutions open to them. “We need two different approaches for two different types of windshield. Some things are good against abrasion, some are good against erosion.”
Besides the windshield, the primary material used by for monolithic windows is stretched acrylic. More could be done to strengthen this type of windows, but it is less expensive to replace than harden.
For the plastic windshields, laminated plastic is the basic configuration for the H-53 fleet. A hard-coated version was developed in a U.S. Navy-funded Sikorsky program undertaken with several windshield manufacturers, beginning in 2002, specifically for sandy environments.
DiGenova explained. “We were looking for ways to make windshields more survivable in sandy environments. We worked with them to develop a hard polysiloxane coating, which puts a very thin glass-like coating over the plastic.” He explained that this provides good results against abrasion and some degree of success with erosion. This is now available for H-53 windshields the U.S. forces in Iraq and Afghanistan. DiGenova estimates this coating would at least double the life of a windshield.
Prior to this Sikorsky funded a study into glass to increase impact resistance, looking at innovative ways to solve the problem. DiGenova said, “One was combination glass, combining both thermal tempering and chemical strengthening where we would end up with a high compressive layer on the surface from chemical strengthening plus the increased depth of the compressive layer that was more representative of the thermal tempering process. It had some promise but it never came to fruition.”
While Sikorsky’s work was geared toward improving performance on plastic windshields, the U.S. Army has taken the lead on protecting the H-60 windshield. DiGenova explained that while glass is good in avoiding abrasion damage, it is susceptible to impact damage that creates many fine points on the windshields. The Army has developed a concept for putting a sacrificial liner on a glass windshield, known as advanced screen saving aviation layered tear-away (ASALT) using a Mylar material similar but not identical to that used by NASCAR racing cars. This is now being used on the H-60 family. This work been led by the Army’s Redstone Arsenal in Alabama and Defense Supply Center working with Pro-Tint Inc and United Protective Technologies. The Army found that in Iraq windshields on the Blackhawk had to be replaced every nine months, down from two years under normal conditions.
DiGenova explained, “They couldn’t use the right-off-the-shelf NASCAR configuration because our requirements were more demanding in the area of optics. Its one thing to drive a race car, it’s another thing to be flying a helicopter and so our optical quality had to be greater. Because of that they couldn’t use multiple layers of Mylar like some racing cars do use. The Army instead uses a single ply of Mylar that they attach down to the windshield, and that becomes a sacrificial layer. That takes the damage, and when the optics in that become degraded it can be peeled off and a replacement liner put on. You can’t, however, replace the liner in the field because it is important to have a very clean environment so you don’t embed other particles.”
DiGenova said that it would not be possible to change the geometry of the windshield to avoid sand damage from rotor downwash but practically this was only an option at a design phase of the platform.
Temperature extremes associated with deserts are not an issue by themselves in regards to windshields explained DiGenova, as they do not exceed existing design requirements. Temperature however could be factor if the windshield does have damage to aid the propagation of failure.
PPG Industries Inc produces all the windshields for the H-60 fleet and has done so from the beginning. Other competitors in a narrow field include Sierracin, acquired by PPG in 2006, and GKN, formerly Pilkington.
Repair is difficult with replacement often being the only alternative. DiGenova said, “The answer to that is to try and polish scratches, which is a successful technique on plastics, but it is much harder to polish with glass using polish material. With deep scratches, they are not polishable; you could do more damage.”
Repair rather than replacement is very much being actively sought. DiGenova continued, “One thing we are going to evaluate is the use [of] different waxes and materials to ‘repair’ a pitted surface. You are not really repairing the surface, but what you are doing is filling the pits and holes, and by doing that you can recover some of the degradation you have lost in your optical quality. There isn’t a specific project within Sikorsky at the moment although I am pushing for that.” It is, however, something that is being looked at in the aerospace transparency community to look for and identify suitable materials. Outside the company DiGenova points to the American Society for Testing and Material subcommittee on Aerospace Transparent Materials and Enclosures on which he sits, which is looking at short- and medium-term solutions for this issue.
FLIR Systems is also in the military transparency business, its day and night vision solutions experience similar exposure to sand erosion to windshields and require at least as good optical performance to offer distance reach in all circumstances.
“We use various types of optics in our gimbals; the IR cameras, TV cameras and lasers and various other sensors all have to be protected," said a FLAIR representative. In our gimbals the windows are both aerodynamically shaped and designed to protect the optics inside from sand and dust. Depending on the windows they can be an optical glass or for the infrared world we prefer to use germanium because it is a hard material that resists abrasion and damage. We then put a permanent layered diamond-like coating (DLC) on the front that is both anti-reflective to improve optical characteristics and also very tough.” The actual recipes for the DLCs are proprietary and a tightly guarded combination of materials.
Like all straightforward tasks, ensuring the crew and pilots and particular can observe their surroundings poses a number of technological problems. Materials used to ensure transparency are relatively fragile compared to other materials in the airframe and defy easy maintenance and repair to damage caused by sand, which is a permanent and inescapable environmental factor in today’s ongoing operations. ♦