Definition: The process by which an experimental aircraft is proved to be safe, functional, and economically viable to produce.
Significance: Testing allows airplane engineers and designers to ensure that their design visions are practical and safe before an aircraft is put into commercial production. The procedure involves both real and computer-simulated tests of all elements of the craft, both individually and systematically.
Testing aircraft is often thought of as only encompassing test flights, more often than not conjuring a picture of a brave pilot climbing into the latest supersonic or hypersonic craft and driving it to the edge of space or beyond. This image has some validity, but aircraft testing also includes less glamorous processes involving computer simulations, laboratory tests, and other types of trials intended to assess how the aircraft will perform when it finally runs a test flight. However, such flights are of the utmost importance to any aircraft research program, which is the basis for the future of aviation.
Military flight tests are usually run at locations such as Edwards Air Force Base, California; the China Lake Naval Weapons Test Center, California; or the Patuxent River Naval Air Warfare Center, Maryland. Commercial tests are performed at sites such as Yuma, Arizona, and Moses Lake, Washington. These test flights are the extension of laboratory tests and computer simulations. Advances in technology must be tested in order to be put to use, and yesterday’s cutting edge soon becomes run-of-the mill. For instance, after World War II, Douglas Aircraft tested its bright red experimental Skystreak model D-558, which reached speeds of Mach .80 to Mach .90. In 2001, approximately one billion airline passengers sat comfortably on airliners traveling at roughly the same speed.
Testing Oversight
There is much more to flight testing than trying out new concepts. Commercial and military aircraft are delivered daily to their ultimate users by Boeing, Lockheed, Airbus, and other manufacturers. When a B-777, an A340, or an F-16 comes off the production line, it is first turned over to production test personnel, whose function is to test the components of the new aircraft to ensure that it is ready for the ultimate customer. The customer, American Airlines or the U.S. Air Force, for example, then conducts a production acceptance flight to determine whether the aircraft meets specifications and is ready for delivery and payment. The military has its own personnel that perform these acceptance tests, and the Civil Aeronautics Authority (CAA)—the forerunner of the Federal Aviation Administration (FAA)—did likewise.
During the 1950′s and 1960′s, FAA inspectors were on the scene, personally checking the output of the manufacturer’s engineers and technicians. Beginning in the 1970′s, however, the FAA became increasingly dependent upon manufacturers to perform both standard testing and FAA inspections themselves, as “designated representatives of the FAA.” With the increasing use of computer simulation in place of hard testing, the FAA has neither the personnel nor the training to oversee the manufacturer. The Boeing B-777 is a prime example in which the manufacturer performed all the testing oversight and approval on the extensively computer-designed aircraft.
There is also an ever-widening gap between the requirements of commercial and military aircraft performance envelopes. The modern subsonic commercial airline transport is designed to cruise at about Mach .85 at altitudes of 28,000 to 40,000 feet. The passengers are afforded living-room comfort, complete with pressurization holding the cabin at a comfortable maximum altitude of 8,000 feet, in-flight entertainment, meals on longer flight legs, cool drinks of their own choice, and an atmosphere of relaxation and safety. Although some modern commercial transport aircraft have found their way into military service as aerial refueling/cargo transports, medical evacuation airplanes, and VIP carriers, the requirements of the modern military have diverged into very specialized equipment. The military is interested in high speed, maneuverability, rugged survivability, deadly weapon delivery systems, and the ability to escape enemy radar detection. Although the armed forces are interested in economy of operations, cost is a secondary consideration compared to military performance. As a result, advances in engine performance have historically taken place in military development; in fact, the core of every engine model currently used on commercial aircraft had its beginning as a military engine. The testing requirements for new engines can run into billions of dollars, and if the military requirements diverge too far from commercial interests, there will be a significant demand for the establishment of a government-sponsored commercial jet development center.
The FAA, which is part of the Department of Transportation, is responsible for licensing all general aviation, business aviation, and commercial airline aircraft, a task that consumes a significant part of its $11 billion yearly budget. Aside from the few homebuilt experimental aircraft that fly in fairly quiet traffic areas, the FAA’s mission is to certify that any new airplane will safely carry passengers and is airworthy. A new model is taken to the edge of its performance envelope by testing it under the most unusual and exacting conditions, conditions that might not occur in real life during many hundreds of thousands of flight hours.
Materials Testing
Flight testing, which is the ultimate appraisal of the finished vehicle, differs from the many preliminary tests, computer simulations, and other explorations that contribute to the success of the complete airplane. Laboratory testing of the airframe starts off with material science, and engineers must make decisions considering questions of the material’s strength, cost, weight, and ease of manufacture, maintenance, and repair. The first aircraft frames, constructed prior to World War I, were composed of fine-grain spruce or similar wood covered with fabric stitched over the skeleton. Steel was used for the engine mounts, some fuselage structures, guy wires and braces, landing gear or skids, and various attachment fittings. Aluminum was still in short supply, very expensive, and had some serious fabrication drawbacks. Aircraft through World War I and up to the mid-1920′s continued to utilize these materials, and they allowed new aircraft models to be constructed very rapidly. Fabric skins slowly became replaced by plywood, and aluminum crept into use in castings and other high-strength-to-weight parts. The wide use of aluminum in the 1930′s allowed the rapid expansion of the airline industry with the Douglas DC family of aircraft. World War II combat requirements saw the development of new high-strength aluminum, then called 75S, and this material was immediately applied to the commercial airline business after the war in aircraft such as the Douglas DC-6 and the Lockheed Constellation. By 1950, titanium was becoming available, but the cost of $25 per pound was about the same cost as sterling silver. The continued search for new and better materials opened the door to nonmetallic materials such as fiberglass, carbon fiber/epoxy, and other synthetics such as Nylon, Dacron, Kevlar, Mylar, Spectra, and Technora. Aircraft material science both creates designs that demand the invention of new fabrics and utilizes new technology to create new designs.
Once it is possible and desirable to start the fabrication of parts, these components are tested either through computer simulation or on actual test fixtures. Testing is done to determine ultimate strength, fatigue resistance, crack propagation, notch sensitivity, and wear resistance. Once the individual parts are assembled into minor or major sub-assemblies they are again run through tests or simulations in a structural test laboratory to prove their suitability for incorporation into the finished airframe. Computer-controlled hydraulic jacks and fixtures, or sometimes just sand bags such as the Wrights used, torture the structures until they turn to scrap material.
In other laboratories, the major systems of the finished aircraft are also “wrung out.” As an example, the DC-8, Douglas Aircraft’s first jetliner, developed in the late 1950′s, had the complete pneumatic, pressurization, and air-conditioning systems laid out in a ground test laboratory. Giant compressors and heaters simulated the “bleed air” from the aircraft’s four jet engines, which was ducted through the simulated wings and fuselage to test the anti-and deicing system, and to run the cabin pressurization superchargers and the refrigeration compressors. The pressurized and air-conditioned air was then fed to a huge steel tank simulating the cabin volume. This pneumatic test program proved that a water separator was needed on the DC-8 air-conditioning system. On an early production DC-8 that lacked such a separator, a jet of cold water came out of the air-conditioning system, hitting Donald W. Douglas, Sr., the chairman, on the head. This same early jetliner had an “iron horse” fixture to test the control system layout for the flaps, ailerons, rudders, and elevators; an engine test stand to develop noise suppressors and thrust reverses; a fuselage panel test rig to test explosive decompression resistance of the DC-8′s rip-stopper fuselage skin; a landing gear drop fixture to test impact resistance; a hydraulic laboratory to test the Skydrol-based system; a heavy, concrete bomb shelter and noise generator to design cabin noise insulation; a power generation laboratory to check out the aircraft’s alternators; a complete layout of the radio rack and flight deck instrumentation; a microwave test facility to test the aircraft antennas; a full-scale mockup of the interior so that interior designers could simulate the look and feel of the passenger cabin (this facility was also used to examine the toilet and galley layout); and finally, a toilet test rig.
Many of these test programs can now be simulated with computers in three-dimensional models, which prevent the interesting arrangements found on the DC-8, where hydraulic, pneumatic, and electrical lines were planned to occupy the same space at the same time. The use of fiber optics and fly-by-wire (completely computerized flight control) will not only save weight, but also free space that in the past was taken up with heavy cables and giant wire harnesses. Testing will be different, but it will be testing nonetheless.
Frame Testing
Once the major systems and components are tested, there is the final “proving-up” of the aircraft. Sometimes a sacrificial aircraft structure is constructed, a complete structural aircraft without electronics, hydraulics, pneumatics, and other such items. This aircraft is placed in a giant fixture surrounded by a steel framework and computer-controlled hydraulic jacks that bend and twist the airframe to simulate flight loads up to what is called limit load. Limit load is the load that the designers expect the airframe never to exceed in even the most violent maneuvers. Once the airframe has passed the limit load test, it is repaired, if necessary, and then undergoes a fatigue life test program. The airframe is cycled. One airliner cycle includes pushback from the gate at maximum takeoff weight; taxi along a bumpy taxiway to takeoff position; the takeoff run itself; the pressure cycle, in which the fuselage is pressurized to 8 pounds per square inch; flight loads due to turbulence and normal flying; the depressurization ofthe fuselage on descent; the landing load, sometimes known as an organized “crash” on the runway; and the taxi back to the gate. Military aircraft have similar cycles, especially in the Navy where the plane is catapulted from the deck and arrested by a wire upon landing. Experience tells the designers how many cycles per day, week, or year the aircraft can expect to undergo. Fatigue testing simulates these cycles by taking the airframe through the equivalent of twenty-five to thirty years on a commercial jetliner. Military aircraft life is usually shorter since the planes do not fly every day except in combat. However, both military and civilian jets are being used over longer periods, far beyond their original design specifications. As a result, service life extension programs (SLEPs) have been initiated on numerous aircraft models. These aircraft are twisted, bent, and tortured in the test rigs until something breaks. The broken parts are either replaced or repaired and the torture continues, until airframes designed for twenty thousand to thirty thousand hours are SLEPed to sixty thousand to eighty thousand hours. Finally, when the engineers have tortured the airframe to its most extended life, it is taken to destruction by loading up the hydraulic jacks until catastrophic failure of a major component, such as the wing spars, occurs. The designers now know how much safety they have in their calculations—110 percent, 120 percent, and so on. The percent over 100 is how much load it took over maximum design load to create a catastrophic failure, and this is the margin of safety.
Flight Testing
The finished aircraft is now on the flight ramp ready for its first flight. The major components have been tested, the systems have been tested, and the airframe has been tested, but there are a few things yet to be done. During the construction of the first example of a new craft, extensive instrumentation is routed throughout the aircraft to measure parameters from every conceivable point of view. Stress and strain on the structure, pressures, temperatures, voltages, and frequencies are to be measured and recorded both on the craft and remotely. Safety equipment and systems have been installed, for as the old saw goes, “I told the Wrights, and I’m telling you, it’ll never get off the ground.” Preliminary tests are run before the plane takes off for the first time: low-speed taxi tests, braking tests, and high-speed taxi tests, in which the nose wheel is lifted off the ground. Finally, the first flight is undertaken, often with much media fanfare.
Once the first flight is accomplished, testers slowly expand the envelope by flying ever higher and faster, thereby testing all the systems of the aircraft. Back on the ground, or rather very near it, rejected take off tests are performed, in which testers accelerate the aircraft to flight speed, chop the power, and slam on the brakes to see if it can stop in the required field length. Burning tires usually accompany this test, since on the rejected takeoff test the engine thrust reversers cannot be used. Another exciting ground test on commercial aircraft is the evacuation test. The number of passengers that will be allowed on a commercial aircraft is determined by the FAA requirement that all passengers must be evacuated within ninety seconds with 50 percent of the doors or escape windows blocked. Furthermore the test passengers, usually manufacturer’s employees and relatives, must be a cross section of the traveling public, so there must be children as well as senior citizens in the mix. They are seated in the aircraft’s seats with seat belts fastened and an alarm is sounded. They have not been told which exits will not work, and all must go down the typical airline exit slide. The only unrealistic aspect of this test is that all test passengers know the test is coming, all know their jobs are dependent upon good test results, and they have not been frightened out of their wits by an actual emergency on the aircraft and know that the threat of real fire is not present. The final elements to prepare for commercial flight include training the airline crews, both in the cockpit and in the cabin, readying the ground handling equipment and ground crew, positioning spares, training the maintenance workers, and actually flying the routes the aircraft will fly when full of passengers.
