The natural flying environment is incredibly tough on aircraft, thanks to the sub-zero temperatures and high winds (over 200 mph in some cases) experienced at altitude plus hazards such as ice and lightning strikes. This is why aircraft OEMs and component manufacturers invest so heavily in environmental testing in the design and prototype phases, before an aircraft is mass-produced for the world market.
Making the grade isn’t easy. “Much of the time, the items being environmentally tested don’t pass due to some unforeseen abnormality,” says Clayton Forbes, director of operations for National Technical Systems’ (NTS) Boxborough-Pittsfield-Tinton Falls test facilities. (NTS has been providing environmental testing services to the aerospace industry for over 50 years.) “In fact, about half of the systems and components that we test at Pittsfield run into initial failures and have to be sent back for redesign.”
Setting Standards
The minimum standards for aerospace environmental testing are set out in “DO-160, Environmental Conditions and Test Procedures for Airborne Equipment.” Published by the RTCA (formerly the Radio Technical Commission for Aeronautics), a U.S.-based standards development organization comprised of industry experts, DO-160 covers altitude, condensation/moisture/liquid penetration, flammability icing, magnetism, lightning, resistance to fungus, sand/dust, shocks/vibration, temperature, voltage spikes, and a host of other vulnerabilities. Basically, if something can affect an aircraft in flight, DO-160 is designed to test for it beforehand.
“The RTCA was formed in 1935 to develop performance standards for aviation,” says Albert Secen, the RTCA’s vice president of Aviation Technology and Standards. “We have been involved in developing performance standards for every major technological innovation in aviation since then. The FAA works with us as we develop standards and uses them as a means of compliance for awarding Technical Standard Orders (TSOs).”
Translating DO-160’s standards into actionable environmental tests is where companies like NTS come in. Serving manufacturers of automobiles, military, medical, and wireless equipment in addition to aerospace, NTS has 28 testing facilities across North America.
“We test specific aircraft components in our sealed environmental test chambers, rather than entire aircraft,” Forbes says. “Depending on the test, the chamber could expose the components to sub-zero cold, ice, and winds; bombard them with electrical signals and magnetism, or vibrate them severely using our Electrodynamic Shaker.”
Lightning Strikes and More
The move towards fly-by-wire and other electronics-based flight control systems (rather than mechanical or hydraulic means) has increased the importance of lightning testing. NTS conducts such tests at its Lightning Center of Excellence in Pittsfield, Mass., in line with standards set out by DO-160 Section 22, MIL-STD 461/464 and others.
“Our direct tests are done using does using high voltage Marx-type impulse generators (up to 2.4 million volts) and high-current generators (more than 200,000 amps) in a high-bay laboratory measuring 40 by 120 feet, suitable for testing smaller aircraft and larger individual components,” says Jeffrey Viel, NTS’ chief engineer EMI/EMC/E3. “These tests simulate the effect of lightning strikes as an aircraft flies through these kinds of environments. We want to see that the equipment maintains normal operations in these circumstances, and isn’t damaged.”
Even when it doesn’t strike aircraft directly, lightning can disrupt aircraft systems by inducing severe voltage spikes. NTS conducts ‘indirect tests’ to assess such disruptions in both electrical and electronic components. They also measure the impact of ‘near strikes’ (cloud-to-ground discharges near the aircraft that generate magnetic and electrical fields), electrostatic discharges (the sudden flow of electricity between two electrically charged objects caused by contact), and static electricity bursts. According to NTS’ web site (www.nts.com), “Static electricity on airplanes can cause shocks that exceed 100,000 volts.”
65 Years of Environmental Engine Tests
GE (General Electric) Aviation has been in the aviation business since 1917, when the company developed a ‘turbosupercharger’ for U.S. military aircraft. Installed on a piston engine, the turbosupercharger used the engine’s exhaust gases to drive an air compressor that boosted power at higher altitudes. To prove that its product actually worked, GE Aviation tested a turbosupercharged 350-horsepower Liberty aircraft engine on the summit of Colorado’s Pike’s Peak, 14,000 feet above sea level.
In 1941, GE was chosen to build the 1-A jet engine based on British designs. Two I-As were subsequently installed on a Bell XP-59A Airacomet aircraft, and flew for the first time in October 1942 at Muroc Dry Lake, California.
This said, it wasn’t until 1955 that GE Aviation formally opened the Peebles Test Operation facility in Peebles, Ohio. Known as “The Proving Ground,” the Peebles facility initially conducted engine tests using an outdoor platform with performance data being recorded nearby in a former farmhouse.
“GE tested jet engines at Peebles, including the famous J79 fighter jet engine and Liquid Rocket Booster Fuel Cells,” says David Groth, plant leader at Peebles Test Operation. “Over the years, the site grew to encompass 7,000 acres tucked into the thickly forested foothills of the Appalachian Mountains in Adams County, Ohio, roughly 80 miles east of Cincinnati where GE Aviation headquarters is located.”
How times have changed! Peebles’ single test platform has been replaced by 11 large jet engine test cells — including two massive indoor test facilities — as well as engine assembly and repair buildings. “More than 300 people work around the clock at Peebles, where they take new commercial and military development engines through a battery of ground tests, engine certification tests, and final acceptance tests on all production engines before they are delivered to customers,” Groth says. “The Peebles operation also conducts final engine assembly for the GE90, GEnx and GE9X commercial engines, as well as the Passport business jet engine. It is generally regarded as the largest outdoor jet engine test site in the world.”
Since jet engines face all kinds of environmental hazards in flight, GE Aviation has crafted a wide range of environmental tests to check them. “These tests include crosswinds/tailwinds, operability, ingestion/impact testing (hail, hailstone, ice slab, bird, water, dust, sand), failure testing (fan blade release, software failure, oil system failures, fuel systems failures), endurance, performance, vibration and durability testing,” says Groth. “Sand ingestion testing for hot and harsh climates is also conducted at our Avio Aero engine test facility in Pomigliano d’Arco, near Naples, Italy.”
“Simulating the environment, and reproducing the effects of what a jet engine endures, can be challenging,” he adds. “We have developed processes that can be measured and repeated, to assure we meet or exceed the worst conditions Mother Nature can throw at engines in flight. Rather than the engine moving towards environmental conditions, Peebles and GE Aviation test organizations bring the condition to the engine to simulate flight.”
Sometimes it makes sense to enlist Mother Nature’s help in simulating cold weather flying conditions, as is the case with GE Aviation’s Aircraft Engine Testing, Research and Development Centre (TRDC) in Winnipeg, Manitoba. The TRDC is based at the city’s James A. Richardson International Airport and managed by a partnership between GE and StandardAero. Its TRDC’s 122,500 square foot outdoor facility conducts icing, bird strikes, sand/dust ingestion tests on GE jet engines.
“Winnipeg is an extremely favorable location for simulating icing tests as the temperature envelope for the execution of the tests is available for up to eight months of the year,” says Brent Ostermann, StandardAero’s enterprise vice president of Engineering. At the same time, “Being an outdoor facility, we are limited by Mother Nature. The testing must be done under the optimum conditions, which include precipitation, humidity, and temperature.”
The Impact of New Technology
The aerospace industry is undergoing a manufacturing revolution, as OEMs replace aluminum with carbon fibre and other composites. At the same time, onboard avionics are getting smaller as they get smarter and run hotter.
This revolution is affecting environmental testing. “As technology and designs have developed into more advanced materials – particularly in the hot section components in new engine — the testing to validate the durability and reliability of these materials has changed,” says Ostermann. “This may include testing the engine for different durations, cycles, or under more extreme conditions.”
“In my areas of expertise, which include vibration/shock, temperature, explosions, and fire, moving to composites and advanced electronics hasn’t changed our tests that much,” NTS’ Forbes adds. “But the increased reliance on electronics for flight control is making lightning and EMI (electromagnetic interference) testing more important and tightening the allowable tolerances for this equipment.”
Then there’s the impact of smaller, more powerful computer-based avionics. “As onboard computer clock speeds go up due to increased processing power, we are seeing more ‘susceptibilities’ during all forms of testing,” says Kyle McMullen, senior research scientist/ director of the National Institute for Aviation Research’s Environmental Test Lab. “When I say susceptibilities, that’s when the equipment doesn’t perform as optimally as we would. If it’s a display, it may produce artifacts. Or the display may show what we call ‘Red Xs’ where it’s saying, ‘I can’t operate right now’.”
As much as new technology is changing the specifics of environmental testing, the core principles remain the same: To put aircraft equipment and systems through everything that Mother Nature can throw at them and then some. The fact that this is being done in the design and prototype phases is good news for aircraft safety, both for pilots and passengers alike.