Solution
2.4 Introductory Flight Test Concepts
2.4.1 What is a Flight Test?
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The non-military aircraft designation should not be confused with the international aircraft registration prefix code. These codes are unique letters and numbers that precede the aircraft reg-istration number. All civil aircraft in the world must be registered in accordance with regulations established by the International Civil Aviation Organization (ICAO). The aircraft registration pre-fix code for the US is “N”, so that a US registered civil aircraft might have a registration number N1234. In the US, the aircraft registration number is commonly called an N number, because of the prefix code, or a tail number, because the registration number is often displayed near the tail of the aircraft.
k k aircraft instruments and they may be sampled at a higher data rate than achievable with standard
instruments. Flight test equipment or “boxes” that are installed in a vehicle are typically painted orange to distinguish them from other non-flight test items and to make them highly visible. The measured flight test parameters are usually recorded onboard the vehicle and may also be transmit-ted or telemetered to a ground station, where they are recorded or displayed to ground observers.
The data may also be displayed onboard the vehicle to the pilot or flight test engineers.
2.4.1.1 Types of Flight Testing
There are various types of flight testing, with different objectives and approaches. It may be com-monly thought that the first flight of a new type of aircraft is the definition of flight testing, but flight test encompasses much more. Some of the different types of civilian flight testing include experimental, engineering, production, systems, and maintenance flight test. Flight test may also be performed for pure scientific research.
Experimental flight test includes the first flights of a new or prototype aircraft type and the deter-mination or expansion of its flight envelope. It may also include testing of a new aircraft model or of an existing aircraft design that has been significantly modified. Experimental flight test seeks to define the unknown performance, flying qualities, systems operation, or other characteristics of a new aircraft or model.
Engineering flight test is performed on an aircraft within its existing flight envelope. The flight and system characteristics are expected to be the same as already known. This type of flight test may include functional and reliability testing of systems and components. Engineering flight test includes the testing required to certify an aircraft under government regulations. US civilian cer-tification standards are specified by the Federal Aviation Administration for different categories of aerospace vehicles. For fixed-wing airplanes, the most often used standards are the Code of Federal Regulations (CFR) Part 23, “Airworthiness Standards for Normal, Utility, Acrobatic and Commuter Airplanes,” and Part 25, “Airworthiness Standards for Transport Category Airplanes.”
Production flight test is of aircraft being produced by an aircraft manufacturer. Prior to being issued a government airworthiness certificate, production flight test is performed on a newly built aircraft to ensure conformity to the approved or certified design. These flight tests are also per-formed within the defined flight envelope of the existing design.
Systems flight test is conducted to assess the systems onboard the aircraft that have been newly installed, updated, or modified. It is assumed that the aircraft flight characteristics are not affected by the new or changed systems, but this may not always be the case. For example, the installation of a large antenna or sensor on the outer mold line of the aircraft may significantly change the aerody-namics, performance, or flying qualities. In-flight testing of avionics, such as communications and navigation equipment, is a common systems flight test. Flights required for aircraft certification include systems flight tests.
Maintenance flight test or functional check flights (FCFs) are flights that are conducted after maintenance or new installation has been performed on the aircraft. These flights serve to verify the normal performance and flying qualities of the aircraft and the correct operation of its systems.
Types of flight testing that are specific to the US Military include developmental test & evaluation (DT&E) and operational test & evaluation (OT&E). Both DT&E and OT&E can be performed on a new aircraft type or an existing aircraft that has been modified. However, DT&E flight testing precede OT&E, as explained below. Similar to the aircraft certification requirements in the civilian world, there are US Military Specifications (MIL-SPECs) and US Military Standards (MIL-STDs) for the flight characteristics and systems operations of a military aircraft.
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Developmental test & evaluation is performed to quantify the flight dynamics of an aircraft, that is, “how it flies”. Typically, this includes flight test evaluation of the vehicle’s performance and flying qualities. These flight evaluations are usually performed using what is called open loop testing. Here, the pilot provides an input to the vehicle control system and the resulting vehicle output or response is measured, without the pilot disturbing this response. The aircraft systems, such as avionics, autopilots, cockpit displays, radar, and sensors are also usually evaluated in DT&E flight testing.
In Operational test & evaluation flight testing, the vehicle is flown in a manner that duplicates how it will be flown operationally, that is, for its intended use or mission. The test pilot flies the aircraft as the non-test pilot, in its “everyday” use, will fly it. Maintenance of the aircraft is also conducted, as it will be maintained operationally. OT&E testing evaluates the vehicle’s suitability, reliability, and maintainability for its intended use or mission. As an example, if an aircraft is designed for a cargo mission, OT&E flight test involves flying the aircraft along representative flight profiles, with representative cargo payloads, and operating to and from representative types and lengths of runways. The aircraft’s operation may also be tested and evaluated for representative climates. For instance, if the aircraft is going to be normally operated in a hot, humid climate or an artic climate, OT&E flight testing will be performed in these kinds of environments. In contrast to DT&E open-loop flight test, OT&E tends to be closed loop flight testing, where the pilot puts in an input, the aircraft responds, and the pilot stays “in the loop” by applying additional inputs, based on the response.
While we are making clear distinctions between different types of flight testing, in reality, there is often overlap between these various kinds of testing. For instance, in performing Certification flight testing for a new engine installation in an aircraft, DT&E performance and flying qualities flight testing may need to be completed, in addition to OT&E flight testing, to determine how well the aircraft can still perform the desired mission.
2.4.1.2 Who Does Flight Testing?
Flight testing and flight research are performed by a wide variety of entities, including large and small commercial aerospace companies, the military, and the government. The various organi-zations may work together as a team in performing the flight testing or research. Flight testing may even be performed by individuals, as when the builder of an experimental, homebuilt aircraft performs the testing of his or her newly constructed aircraft. Often, organizations have their own, dedicated flight test groups or departments, staffed by pilots, engineers, managers, technicians, and other support personnel who are specifically trained to perform flight operations and testing. The organizations may also have non-test aircraft to support flight testing and to be flown for aircrew proficiency.
Government agencies often act in an oversight or regulatory role, ensuring that the vehicle being tested ultimately meets the government regulations or standards. Some of the major government aviation regulatory authorities in the world that oversee flight testing include the Federal Avia-tion AdministraAvia-tion (FAA) in the United States, the Civil AviaAvia-tion Authority (CAA) in the United Kingdom, and the European Aviation Safety Agency (EASA) in Europe.
The US military has several major flight test facilities or complexes in the USA. These include the US Air Force flight test centers at Edwards, California, and Eglin, Florida and the US Navy facil-ities at Patuxent River, Maryland and China Lake, California. The test locations are large, remote areas that are conducive to flight testing of high-speed military aircraft and weapons systems.
Non-military companies and government organizations also use these test ranges by arrangement
k k with the military. NASA operates the Armstrong Flight Research Center as a tenant on Edwards
Air Force Base, California.
Where does one learn to do flight testing? Early in aviation, there was little formal flight training, much less, any formal flight test training. Flight testing was often done by trial-and-error, hopefully with the result that the flight tester survived their flight to learn from their mistakes. Eventually, it was recognized that formal training in flight test would be beneficial, and several formal schools and training programs were established. Today, many aerospace companies train their flight testers
“in-house”, developing their flight test skills through a mix of formal academic training and prac-tical experience, under the tutelage of veteran flight test personnel. There are also several formal flight test training facilities and test pilot schools worldwide, where student test pilots and student flight test engineers attend a formal, typically year-long curriculum of academics and flight train-ing. These include military, industry, and civilian operated training facilities. The major flight test schools and training institutions include:
• Divisão de Formação em Ensaios em Voo, Brazilian Air Force Test Pilot School, São José dos Campos, Brazil
• Empire Test Pilot School, Boscombe Down, Wiltshire, England
• Aircraft and Systems Testing Establishment (ASTE), Indian Air Force Test Pilot School, Ban-galore, India
• International Test Pilot School (civilian), Woodford, England, and London, Ontario, Canada
• L’Ecole du Personnel Navigant d’Essais et de Reception (EPNER), French Test Pilot School, Istres, France
• National Test Pilot School (civilian), Mojave, California, USA
• Russian Ministry of Aviation Industry Test Pilot School, Zhukovsky, Moscow Olast, Russia
• US Air Force Test Pilot School, Edwards, California, USA
• US Naval Test Pilot School, Patuxent River, Maryland, USA.
Typically, flight test training includes the basic areas of performance, flying qualities, systems testing, and test management. All of these subject areas draw heavily upon the technical aspects of aerospace engineering. The management of flight test projects is also typically covered. Aca-demic theory is usually taught in a classroom environment, which is then coupled with practical application of the theory in flight. The flight exercises are typically flown in a variety of aircraft, to give the students exposure to a wide range of different aircraft. Student test pilots and student flight test engineers often work as a team in preparing for, flying, and analyzing test flights. In addition to flying real aircraft, flight simulators are also used, due to their unique capabilities and cost effectiveness.
2.4.1.3 The X-Planes
Flight test has had a rich history in the USA, starting with the first test flights of the Wright Flyer I by the Wright brothers. As the flight testing of aerospace vehicles became more formalized, a special designation was created for the flight test vehicles that were the first of their kind or unique in other ways. Starting with the Bell X-1,5 the “X-plane” designation has become synonymous with US experimental aerospace vehicles that expand the frontiers of air and space.
Many of the early X-planes were experimental, rocket-powered aircraft, expanding the airspeed and altitude boundaries of high-speed flight. However, the X-plane designation includes a wide
5The Bell X-1 was initially designated the XS-1, the “S” signifying “supersonic”. The “S” was deleted from the designation early in the project.
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range of aerospace vehicles, such as low-speed propeller-driven airplanes, unmanned aerial vehi-cles, vertical takeoff and landing vehivehi-cles, unmanned missile test beds, space access vehivehi-cles, and prototypes of advanced aircraft, missiles, and spacecraft. Despite their differences, the flight test of X-planes share the common goal of advancing the research and technology boundaries of aerospace engineering. The US X-planes, from the Bell X-1 to the present, are listed in Table 2.11. We refer back to many of the significant accomplishments of the various X-planes throughout the text.