Abrasion in Transparent Lens Materials for Exterior Aircraft Lighting

For aircraft manufacturers and operators, LED technology promises to keep planes operating longer with reduced maintenance costs and lower power consumption. As the need to frequently replace burned-out incandescent bulbs is eliminated, the importance of constructing a reliable exterior lighting system capable of maintaining safe levels of light output despite continual exposure to particulate abrasion, thermal, and chemical stresses is crucial for operators to realize the benefits of LED technology.

The ability of pilots to see and be seen in poor weather conditions, especially on and near busy airfields, is critical to ensuring the safety of the public. Exterior aircraft lighting is a critical component that helps provide necessary visibility. Aircraft lighting has followed general conventions used in marine applications to help identify positioning between aircraft, and have several individual lighting systems to help this identification process including position and anti-collision lights and in some cases icing, landing, and taxi lights [1,2,3,4] The light output requirements for anti-collision and position lights are clearly identified in several SAE standards [3,4,5].

In order ensure a long lifetime and reliable performance, the lens covers used in aircraft lighting applications must provide consistent transmission of light, with minimal material degradation after exposure to harsh conditions. In this study, abrasion testing was performed on three different jet aircraft lens materials, a heat strengthened borosilicate glass (Kopp 9000), a hard-coated polycarbonate (Makrolon AR), and an aviation grade acrylic (Plexiglas II UVA), to demonstrate suitability for the extreme aerospace environment. Taber abrasion and high-velocity particulate testing results demonstrated a significant abrasion resistance advantage for glass compared to the plastic. Severe transmission loss was observed in the polycarbonate compared to the glass and far exceeded the transmission losses [theorized or deemed acceptable] by industry sources [6]. Abrasion resistance, the ability of exterior lenses to stand up to these harsh environments, must be a high priority during the lighting fixture design and maintenance processes otherwise the potential cost savings of LED technology cannot be achieved in application.

Lens Materials

Exterior aircraft lighting fixtures use a transparent lens to cover and protect the light source, with the most common lens materials being glass or plastic. Glass lenses are usually made from borosilicate or soda lime silicate compositions and can be provided in annealed or heat strengthened (tempered) states. Plastic lenses are most commonly made of polycarbonate or acrylic, with or without a hard coating applied to improve durability [6]. Table I provides a summary comparison of these three materials.

Table I: Comparative properties of materials tested

Borosilicate glass offers excellent optical performance, durability when exposed to the airstream, and the ability to withstand high operating temperatures. It is resistant against mechanical and chemical abrasion and maintains high levels of light transmission in application. Plastic lenses provide a reduction in lens weight and good impact resistance, but they can be highly sensitive to chemical and mechanical degradation. Intense and prolonged exposure to UV light, such as from solar radiation, has no effect on glass but can cause plastics to become brittle and discolored. Both materials can be molded into a shape or contour [6].

Performance in Application

Light covers or lenses are critical to protecting light sources, like incandescent bulbs or LEDs, from the harsh environment of the airstream, while transmitting as much light as possible, in order to create an efficient and effective system. According to SAE AS8037C, “the light covers or color filters used shall not readily support combustion and shall be constructed so that they will not change shape or permanently change color or shape or suffer any appreciable loss of light transmission during normal use.”[3]

In the harsh, demanding environments experienced by exterior commercial and military jet aircraft lenses, it is necessary to select materials that can provide consistent performance.

Figure 1 shows an example of the light intensity loss of a polycarbonate lens for a wing tip light assembly. SAE guidelines and supporting literature confirm the loss of light output with plastic covers and recommend light fixtures be designed with the presumption of degraded lens output.6 Fixtures designed with plastic lenses must allow for 20-40% loss in transmission, either by compensating with higher wattage incandescent lamps, more LEDs, LEDs of higher intensity, or driving lower intensity LEDs to higher brightness using more electrical power. In addition, SAE has recommended guidelines on anticipating and engineering around LED reduced light output over the service life of the fixture, furthering the need for careful consideration of lighting design.7 All of these compensating design solutions tend to increase the total cost of owning and maintaining a light fixture with plastic lenses to maintain the required light output. Generally speaking, the loss in transmission will negatively affect the required photometric light output of the lens, which could result in a safety hazard as the light then becomes scattered.

Figure 1: Plastic lens light intensity loss experienced for a sample wing tip exterior light assembly. Data reproduced from SAE ARP5637 [6]

Experimental Procedure

Two different testing methods were used to assess the abrasion resistance of borosilicate glass and plastics. The Taber Abrasion test method is an industry standard testing protocol used to illustrate abrasion resistance of materials ranging from plastics to glasses to ceramics [8]. A high-velocity particulate impact test designed to simulate erosion effects on aircraft surfaces in flight was used to illustrate the abrasion resistance of glass and polycarbonate in a more application-like environment.

Taber abrasion method

The Taber Abrasion test method was developed and used based on the ASTM D1044 guidelines [8]. A Taber Abrasion Model 174 was used to abrade the samples. The Taber Test experimental procedure followed ASTM D1044 guidelines with the following three exceptions: 1. the vacuum nozzle had an 8mm diameter opening in place of the recommended 11 mm diameter, 2. a spectrophotometer was used to measure transmission in place of a haze, and 3. samples were run up to 2,000 cycles beyond the recommended 100 cycles to help simulate abrasion resistance between materials over a longer time frame.

Figure 2: Schematic of the Taber abrasion test method and testing equipment

Abrasion in this method is simulated by the rub-wear action that is produced by contact of the test specimen against the sliding rotation of the abrasive wheel. As the turntable rotates, the wheels are driven by the sample in opposite directions along a horizontal axis displaced tangentially from the axis of the sample. One abrading wheel rubs the specimen outward toward the periphery and the other, inward toward the center. The wheels traverse a complete circle on the specimen surface, revealing abrasion resistance at all angles relative to the weave or grain of the material [8].

High-velocity particulate abrasion test

A high-velocity particulate abrasion test was performed on September 27, 2017, at the US Air Force Particle Erosion Test Facility at Wright-Patterson AFB, maintained and operated by the University of Dayton Research Institute (UDRI) to serve the international aerospace community. Access is provided for both military and commercial testing. The particle erosion test apparatus, or “dust-rig,” was designed and built in 1983 to simulate erosion effects on aircraft surfaces subjected in flight to dust-laden environments [9]. A schematic of the testing apparatus is provided below.

Figure 3: Schematic of the high-velocity particulate impact abrasion test method and equipment

The test was conducted on six specimens of both glass and hard-coated polycarbonate. Each sample was 4.75” diameter (x3), 3.75” square (x3), and the test setup consisted of five independent variable parameters that define the exposure environment during the test provided in Table II.

Table II: High-velocity particulate impact abrasion test variable parameters

Samples of both glass and hard-coated polycarbonate were exposed to periodic and increasing levels of particulate (sand) at the prescribed impact angle to simulate aircraft lighting lens applications. The erosion and impact of the sand on the samples were controlled by a rostering system to ensure even and consistent abrasion across the defined test sample area.

Results and Discussion

Taber abrasion results

The Taber abrasion results for the borosilicate glass, hard-coated polycarbonate, and acrylic (Plexiglas II UVA) samples are shown in Figure 4. The transmission of these materials is plotted against increasing number of cycles on the test equipment. The samples demonstrated degradation in transmission as the number of cycles increased. The borosilicate glass displayed significantly less transmission loss as compared to the hard-coated plastic samples.

Figure 4: Photopic transmission of glass and polymer samples following increased cycles (exposure) on the Taber Abrasion test

The results illustrate the significant difference in abrasion resistance of the borosilicate glass compared to hard-coated polycarbonate and acrylic. At 1000 cycles, the glass displayed greater than 20% more light transmission than both polycarbonate and acrylic. The glass continues to separate itself from the polycarbonate at even higher cycle counts maintaining over 90% transmission, with the polycarbonate exhibiting only about 65% transmission at 1500 cycles and only 55% transmission at 2000 cycles. In addition, the glass transmission remains significantly higher (approximately 25%) than the acrylic transmission level even as the acrylic transmission levels off after 500 cycles.

High-velocity particulate testing results

The high-velocity particulate test results are shown in Table III. The sample transmission decreases with increasing exposure to simulated atmospheric particulate matter (sand). Intuitively, we can equate an increased amount of sand exposure to more accumulated flight hours. Therefore, the design goal for a lens material is to keep light transmission as high as possible.

Table III: The test results - high-velocity particulate abrasion test

The transmission levels start similarly high between the two materials but diverge with increasing exposure. The borosilicate glass samples exhibit consistently higher abrasion resistance to the particles, providing for longer periods of high transmission. Table III demonstrates that glass requires nearly double the amount of sand exposure to drop below 70% transmission compared to the hard-coated polycarbonate. Also, at the exposure level where the transmission loss of polycarbonate has reached approximately 50%, the transmission loss for glass is only at approximately 70%. The glass significantly outperforms the plastic in this testing and correlates well with the results of the Taber abrasion tests.

Another critical result from the high-velocity particulate impact testing is the potential level of severity in the degradation of transmission in the plastic material. SAE guidelines for designing excess light output into a fixture only indicate potential transmission losses of nearly 35% for polycarbonate, as shown in Figure 1 [6]. This testing showed even more significant light loss, nearly 40%, was possible. The additional transmission loss of plastic could be much more significant and may require light fixtures to be designed with an even greater loss taken into consideration in order to meet the prescribed SAE light output levels. Alternatively, plastic lenses will need to be repaired or replaced more frequently before unsafe significant transmission loss occurs to maintain required light output levels, with the related escalation in the total cost of ownership.

Conclusions

The critical function of lenses in exterior aircraft lighting applications is to maintain a consistent transmission of light to ensure the highest level of safety. The Taber abrasion and high-velocity particulate impact testing provide an effective means for comparing current aircraft lens materials to determine suitability. At increasing cycles and exposures, the glass continued to maintain significantly higher levels of transmission than plastic. The tests indicate that the abrasion resistance of glass will significantly outperform that of plastic in exterior jet aircraft lighting applications.

As airframes are pressed into nearly continuous service across varying environments, using materials that can withstand particulate abrasion, thermal, and chemical stresses will reduce the total cost of ownership of an exterior light fixture. When margins of safety, reliability, and maintenance protocols are considered, glass is the superior choice for exterior light fixtures on commercial and military jet aircraft. The testing is clear in showing glass lenses can withstand the harsh operating environment of aerospace for more significant periods of time than their plastic counterparts, helping to provide a consistent, reliable, and safe flying environment.

References:
[1]    Lombardo, D. (2000). Aircraft Systems. 2nd ed. Blacklick, USA:
        McGraw-Hill Professional Publishing
[2]    SAE ARP693D Landing and Taxiing Lights – Design Criteria for Installation
[3]    SAE AS8037C Minimum Performance Standard for Aircraft Position Lights
[4]    SAE AS8017D Minimum Performance Standard for Anticollision
        Light Systems
[5]    SAE AIR1106B Factors Affecting Visibility of Aircraft Navigation
        and Anticollision Lights
[6]    SAE ARP5637 Design and Maintenance Considerations for Aircraft
        Exterior Lighting Plastic Lenses
[7]    SAE ARP6253 LEDs and Aircraft Applications
[8]    STM Standard D1044, 2013, “Standard Test Method for Resistance
        of Transparent Plastics to Surface Abrasion,” ASTM International,
        West Conshohocken, PA, 2013, www.astm.org
[9]    University of Dayton Research Institute, Air Force Research Laboratory
        Materials and Manufacturing: Particle Erosion Test Apparatus:
        Use Policies, Operating Procedures & Specimen Configurations.
        Dayton, OH, 2012, https://www.udri.udayton.edu

The Society of Automotive:
Engineers (SAE) formed appropriate committees to create many of the standards, recommended practices, and aerospace circulars used for guidance in the aerospace industry. These standards and guidelines are necessary to ensure that each aircraft maintain visibility and awareness when in operation, thereby helping to prevent accidents through well-known and consistent visual clues [3, 4, 5].

SAE ARP 5637 statements:
“The most significant shortcoming of plastic lenses is optical performance. When plastic lenses are exposed to the airstream, particulate (ice and rain) and abrasives in the airstream damage the lenses’ exposed surfaces. This is referred to as “lens erosion” damage and it appears as cloudy or opaque areas on the lenses much like sandblasting would produce. The effect of this degradation is reduced light transmission and light being scattered in unwanted directions. The light reduction can be significant.” [6]
“Careful consideration must be given to the total cost of ownership of both types of materials over the life of the aircraft, which is a key consideration for the owners of commercial and military aircraft.” [6]