Following a recent increase in laser pen attacks, Ellis Davies looks at the materials and methods protecting aircraft pilots and passengers.
In 2016, 3.77 billion people flew on commercial flights, and aviation analyst Ascend estimates that the total number of aircraft in service was 23,600, including cargo. With the global air transport network known to roughly double in size every 15 years, the safety of an enormous number of people is in the hands of the materials and processes used to build planes.
A number of factors need to be considered during the design and manufacture process. Radiation and lightning protection are some more obvious issues, but BAE Systems, UK, has recently targeted a lesser-known threat. Laser attacks, which involve the use of a laser pen or other device to blind or distract pilots during take-off and landing, increased by 25% at Heathrow Airport, UK, in 2016, leading many pilots to criticise the dropping of a recent government plan to crack down on such incidents.
Reflecting the lasers
Laser attacks cause not only immediate damage to the eyes, but also flash blindness, distraction, additional cognitive load and the blotting out of cockpit instruments.
Currently, the standard intervention is dark goggles or glasses worn by the pilots during take-off and landing. These can be problematic due to their low visible light transition (VLT) and because they can be easily lost. BAE’s solution is more permanent. The company has developed a coating that can be applied to a thin plastic substrate and secured to the windscreen of the aircraft, making the goggles superfluous.
Daniel Black, Research Engineer at BAE, explained how the coating works to Materials World. ‘It is a holographic interference filter, which can be placed onto a variety of materials [...] creating an interference pattern that reflects the incoming light. We programme the film with certain pre-determined wavelengths to reflect light off the interference pattern,’ he said.
The coating has a VLT across the spectrum of around 70%, meaning that the windscreen will remain clear with limited effect on the pilot’s vision. Black said ‘It is slightly darker than looking through a window, but it is hard to find a difference. It's also colour neutral, not tinted, so that it does not affect the pilots’ situational awareness, and allows them to do their job effectively.’ The coating singles out the programmed wavelengths, able to block multiple simultaneously, and reflects them away from pilots with an efficiency of 99.9%, claims BAE. This allows the pilots to see all the wavelengths that are necessary for them to be aware of the aircraft’s surroundings, without the potential exposure to harmful light.
The coating can be updated if a new threat emerges – as it is programmable, the three default wavelengths can be changed as needed. An update involves either replacing the coating entirely or adding a completely separate, new layer.
BAE is continuing lab development of the coating, but does not have a specific timescale for commercial availability as yet. Both military and commercial airlines are being targeted, Black said, ‘We would love to see every commercial airline take on this technology, and we are looking at every avenue.’
As mentioned before,flight altitudes bring pilots and passengers into the reach of harmful radiation in the form of ultraviolet (UV) and cosmic ion radiation (CIR). A high level of exposure to UV, in particular UV-A (longwave UV), is linked to the development of melanoma, as it causes DNA damage. Pilots flying at 30,000 feet for 56.6 minutes receive the same amount of UV-A exposure as a 20-minute tanning bed session, and more if flying over thick cloud or snow, which reflect up to 85% of UV radiation.
A 2015 study titled The Risk of Melanoma in Pilots and Cabin Crew: UV Measurements in Flying Airplanes measured the amount of UV-A at 30,000 feet in Las Vegas, USA, resulting in a measurement of 242 microwatts per square centimetre. The carcinogenic effective dose was calculated using the Skin Cancer Utrecht–Philadelphia human action spectrum, and the dose for a 20-minute tanning session was 2940 millijoules per square metre (MJ/m2) – daily global solar exposure range from 1 to 35 MJ/m2.
Boeing, UK, spoke to Materials World about the methods of protection employed against UV radiation on its planes. ‘Shielding and separation are the primary means to protect against radiation threats. Regarding ultraviolet radiation, all exterior coatings, both decorative and non-decorative, are formulated with UV stabilisers and/or UV absorbers (sunscreen, essentially) to protect both the coating itself and the underlying structure from damage.’ It is in general recommended that crew use sunscreen and periodically have their skin checked, as well as adding UV protection to windscreens.
CIR is a less obvious consideration. It is a form of ionising radiation that comes from outer space, a very small amount of which reaches the Earth. At flight altitudes, passengers and crew members are exposed to higher levels of this cosmic radiation. The World Health Organisation has linked this radiation to cancer in humans, as well as reproductive problems. Figures from The National Council on Radiation Protection and Measurements show that aircrew, based on findings in the USA, have the largest average annual effective dose (3.07mSv) of all radiation-exposed workers. Other estimates of annual aircrew cosmic radiation exposure range from 0.2 to 5mSv per year.
Novel materials, including variants of plastics, have been looked at to safeguard those exposed to high levels of CIR, but this is primarily in the military and space exploration sectors. Cabin crews’ exposure to cosmic radiation has been found to be below the allowed dose limit.
Dangerous weather conditions often ground flights, but in the event of being caught in a lightning storm, an aircraft must be prepared for a potential hit. On 22nd July 2017, a British Airways flight from London, UK, to Chennai, India, was hit by lightning shortly after take-off, but was able to complete its nine-hour flight. The Boeing 787-800 Dreamliner sustained multiple strikes, resulting in between 42 and 46 holes in the aircraft’s exterior. This example shows how modern planes are able to function even after a barrage of strikes.
The plane was saved by a feature used to protect against lightning strike, a lightweight carbon composite covered in a layer of copper. This acts as a Faraday Cage, and protects the interior of the plane from any electrical currents. On the subject of lightning protection, Boeing said, ‘Design features vary with location on the plane as some locations are more susceptible to lightning than others. Specific conductive elements are used in high-threat areas to absorb and disperse lightning energy. Electrical continuity is maintained throughout the aeroplane, and also shielding and separation are effective design measures.’