For Airplanes, It’s One Test After AnotherMay 5, 2015
Aerospace engineers and everyday passengers alike have confidence when getting on an airliner that it’s safe to fly. But few people actually appreciate all the testing that goes into ensuring their airplane can withstand many different conditions and events that can happen during the flight or on the ground.
For example, the next time you’re on an airliner and you go through some light to moderate turbulence, peek outside the window and watch the wings on the airframe flex up and down. Allowing the wings to flex provides a smoother ride for passengers.
Structurally, airplane manufacturers put their new designs through very thorough wing-flex — also known as wing-load — testing. For example, in testing, Boeing flexed the wings of the new 787 Dreamliner “approximately 25 feet.” That’s about 150 percent of the maximum flex that the airframe should ever run into during normal flight conditions. It’s reported that passenger jets can sustain a wing flex of “nearly 90 degrees.” That’s very comforting, even if difficult to imagine.
What’s more comforting is that’s just one of many tests that aircraft designers and those involved in testing the finished product employ. Here at NTS, we can perform them all.
Here’s some more information on some of the various tests that airframes — and the components that go in the airframes — undergo.
Bird Strike Testing
Many of us remember U.S. Airways Flight 1549, which departed LaGuardia Airport in New York City mid-afternoon on January 15, 2009 to be struck by a large flock of geese three minutes into the flight. The plane lost power and Captain Chesley “Sully” Sullenberger guided the plane safely into the Hudson River. Amazingly no one lost their life in the accident, but the event highlights the importance of maintaining a structurally sound and powered airplane, even in the event of a bird strike.
Birds are often heavier than they look. Each of the geese in the Flight 1549 incident reportedly weighed about eight pounds. If you think about it, the force of impact running into an eight-pound bird at about 200 mph is equivalent to having an eight-pound bowling ball hurtled into you at 200 mph. That’s going to do some damage.
Probably the biggest threat from bird strikes is that the birds will get sucked into the aircraft’s jet engines. Jets with engines mounted under the wings are more likely to see birds getting sucked in, as compared with engines mounted near the rear of the fuselage.
NTS offers a bird strike simulator —— sometimes called the “chicken gun” — to test various parts of an airframe or engine. The test includes launching real or simulated birds at airframe parts at speeds up to 400 mph. Various Code of Federal Regulations (CFRs) cover airworthiness standards, and these include damage tolerance from bird strikes of various weights while at cruising speed. There are standardized methods for testing bird impacts, and NTS can help a designer ensure that his or her airframe meets the requirements.
In addition, bird strike analysis suggests that the repair cost to airframes after hitting birds totals over $1.2 billion annually in the United States alone. This highlights the need to prevent bird strikes in addition to ensuring that aircraft can survive them.
Back in high school physics, we learned about the difference between position, velocity and acceleration. Position is where you are at a specific time. Velocity is how fast your position is moving in a particular direction. Acceleration is how fast your velocity is changing.
People sometimes think about speed as acceleration, but really that’s velocity. Again, acceleration is a change in velocity, and that’s what you feel when you are pushed back into your seat when the pilot goes throttle-up on take-off.
Just as you experience the g-forces of acceleration during a flight, so does the airplane. A 60-degree banked coordinated turn puts twice the force of gravity directly perpendicular to the horizontal axis of the aircraft. You’ll likely never experience such a high-banked turn in a commercial airliner, but they are tested for such g-forces and a lot more.
Increased g-forces stress an airframe and can adversely affect many of its structural components. For example, acceleration forces during take-off, landing and during a flight can stress the joints, seals, mounting and other parts of the aircraft. In addition, landing gear, hydraulics and even the seat you sit in each have to endure significant g-forces and remain functional.
For example, imagine your plane being cleared for take-off, and you’re rolling down the runway when the pilot has to abort the take-off. That’s a bit like starting to swing a baseball bat at a pitch and, once the bat is already in motion, deciding to hold back and check your swing. You really have to pull back hard. For an airplane, that braking action puts a lot of g-force on the airframe and especially on the brakes themselves. Brakes are tested under what’s called a “maximum brake energy test” for just this purpose.
There’s a variety of acceleration tests that are conducted on airframes and their components, including the use of machinery that spins components in a centrifuge, generating a force up to 50,000 times the force of gravity. Equipment is often tested by applying alternating forces or combinations of linear and rotational forces. NTS can perform these and other tests that verify that an aircraft frame or components will meet and exceed the standards for acceleration testing.
Aircraft climb to very high altitudes, known as “flight levels.” For example, flight level 210 (FL210) is 21,000 feet. Modern aircraft operate at flight levels between 300 and 400, and the air pressure at these altitudes is very low relative to the air pressure at sea level.
Airframes are pressurized to provide oxygen to the crew and passengers, but this means that there is the potential for an “explosive decompression,” should the airframe fail. The pressure inside the cabin then rapidly equalizes to the pressure outside, and everything inside that isn’t bolted down or strapped in will be blown out from the high winds caused by the pressure equalization.
NTS provides altitude testing for explosive decompression and other facets of altitude performance. We can test airframes going from sea level to 100,000 feet, with various simulated atmospheric conditions. That includes pressure, temperature and humidity. We even have space simulator facilities if you want your equipment to go higher than 100,000 feet.
EMC and EMI stand for electromagnetic compatibility and electromagnetic interference, respectively. There are thousands of electrical components in today’s jetliners. Each of these components must be resilient to electrostatic discharge, irregular voltage, magnetic fields and other unpredictable changes. Testing for EMC and EMI has to be done in variable temperatures and humidity because events are often dependent on these environmental conditions.
High intensity radiated fields (HIRF) testing is related to EMC/EMI testing. It tests electronic components to high electromagnetism. Newer composite materials offer less electromagnetic shielding. Metallic enclosures often reduce radiated emission and improve immunity to the effects of magnetic radiation. But metal is heavier than composites, making an airplane more costly to fly.
NTS operates all the equipment necessary for EMC/EMI and HIRF testing. Our facilities and technicians are state-of-the art, and we can assuredly meet virtually any EMC testing requirement you may have.
If you keep shaking something, it’s likely to fall apart eventually, even if it’s an airplane. Airplanes have to undergo rigorous vibration stress testing to make certain they won’t fall apart in flight.
One of the main sources of vibration in an airframe is turbulence, and that places random vibrations forces on the aircraft. But airplanes must meet standards for both random and sine-wave vibration. That goes not only for the airframe, but the engines and other components.
Perhaps the most concerning type of vibration is called “flutter.” Flutter is when “vibrations occurring in an aircraft match the natural frequency of the structure.” Should this happen, the vibrations can amplify, building on each other. This is similar to what’s occurred in some bridges that have failed. Airplanes undergo various tests for flutter to help ensure that such oscillations can be dampened and will not cause the loss of the aircraft.
Testing facilities typically employ hydraulic and elctrodynamic levers (or shakers) to impart vibrating forces to aircraft structures and components. At NTS, our test facilities can generate up to 70,000 force pounds with tandem shakers and exceeding a force of 200 g’s. (This is denoted as 200 GRMS, in which GRMS stands for the root mean square of the g-forces.) It’s critical to know where and how to measure vibration testing on different components, and at NTS we have all the expertise needed to do it right.
Extreme Environmental Condition Testing
Airplanes operate in all sorts of environmental conditions. They have to start on the ground in high or low humidity or temperature, windy conditions, rain and facing possible icing. They have to fly through a wide range of atmospheric pressure, temperature, and weather conditions — including violent thunderstorm and dust storms. They even have to withstand UV and solar radiation.
There are environmental condition tests that all approved jetliners go through. For example, hail strike testing is employed to determine the structural soundness of the main fuselage and wings, as well as the windshield, antennae and radar domes.
NTS has a hail strike simulator that can fire clear ice, stones or pieces of aluminum at speeds in excess of 700 mph. Using lasers, the impact results can be measured reliably, even when testing in conditions well below freezing.
Icing and freezing rain pose additional threats to airplanes in flight. When ice accumulates on the leading edge of a lifting surface, it can break the laminar flow of air over that surface. This reduces the lifting capacity of the surface. In addition, ice accumulation adds weight to the plane, and the weight may not be distributed symmetrically. NTS has extensive capabilities for testing equipment in icing conditions.
Rain and wind can also erode the structural soundness of an airframe. At NTS, we have a testing environment that can blow rain at up to 90 mph. Items can be tested at temperatures ranging to 50 degrees Fahrenheit above the temperature of the rain.
Airplane pilots try to avoid thunderstorms, but lightning strikes are still a routine part of every commercial jet’s experience. In fact, the Federal Aviation Administration (FAA) estimates that every airliner in the U.S. is hit by lightning once per year on average.
Normally, when an airplane is struck by lightning, the electrical discharge passes through the airframe and continues back out to the air where it may then go to the ground. A single bolt of lightning may send a current of 200,000 amps through the airplane. It’s easy to appreciate that having such a huge electrical discharge on the surface of the airframe could have pretty bad consequences if those consequences weren’t anticipated.
Airplanes are designed to ensure that fuel tanks, fuel lines and anything else combustible are not positioned where a spark from a lightning strike could ignite them. But as today’s aircraft rely on fly-by-wire systems, those systems also have to be able to withstand the electromagnetic effects of a lightning strike.
Engineers appreciate that flowing electricity creates a magnetic field. So when lightning strikes an airplane, a magnetic field is created around the airframe. In turn, change in magnetic flux then induces electrical current, which is known as Faraday’s Law of Induction. These new lightning-induced currents may be incompatible with an aircraft’s components, and components must be tested to ensure they can withstand such possibilities.
NTS is able to simulate lightning strikes in a test chamber, including single stroke strikes, multiple stroke strikes and multiple burst sequences. NTS generators meet standards from Boeing and Airbus, and we can help ensure your components will be able to meet legal requirements for lightning-strike safety.
DO-160 testing is a standard maintained by the Radio Technical Commission for Aeronautics (RTCA) for testing avionics hardware. It has a 40-year history and applies to equipment in helicopters, general aviation aircraft and commercial airplanes.
DO-160 is revised as new information and testing become available. The most current version is DO-160G, which was approved in December 2010. DO-160 applies to the United States, and there’s an identical set of standards for Europe, known as EUROCAE ED-14. (EUROCAE is the non-profit European Organisation for Civil Aviation Equipment.)
DO-160 testing covers a plethora of different conditions. For example, avionics must be tested not only under the obvious conditions of different temperatures and pressures, but also for condensation, shock, impact, sand, dust and even resistance to fungus, salt and flammability. DO-160 testing covers basically everything you can imagine that might cause an avionics component to experience a problem or fail.
NTS offers the complete range of DO-160 testing, not only for aircraft avionics, but even for satellites, too.
Call Us: We’re the Experts
NTS was founded in 1961. We’re one of the largest and oldest commercial test laboratory networks in the United States. We work from start to finish, from engineering design to implementation and supply management.
You’ll find that at NTS, our test, inspection and certification services meet a lengthy list of international standards across various industries. Every quote we give is absolutely free.
When you have aircraft components or an airframe to test, keep in mind that in addition to all the testing mentioned here, we also test materials for fatigue, shock and pyro shock. We test materials for their ability to withstand a drop, and we can handle hazardous materials and lithium batteries, too. We even can do testing for tin whiskers, which can develop when tin is used as a finish for electronic components. The tin develops “conductive whiskers” that can make unwanted and dangerous conduit electrical paths.
If you have any questions about aviation testing, contact us today. We have relationships with all the important safety-related agencies, including agencies in the United States and internationally. For information about corporate events and industry news, sign up for our free electronic newsletter.