Flight test
Flight testing is a branch of aeronautical engineering that develops technologies and equipment required for in-flight evaluation of behaviour of an aircraft or launch vehicles and reusable spacecraft at the atmospheric phase of flight. Instrumentation systems for flight testing are developed using specialized transducers and data acquisition systems. Data is sampled during the flight of an aircraft, or atmospheric testing of spacecraft. This data is validated for accuracy and analyzed to further modify the vehicle design during development, or to validate the design of the vehicle.
The flight test phase accomplishes two major tasks: 1) finding and fixing aircraft design problems and then 2) verifying and documenting the vehicle capabilities when the vehicle design is complete, or to provide a final specification for government certification or customer acceptance. The flight test phase can range from the test of a single new system for an existing vehicle to the complete development and certification of a new aircraft, launch vehicle, or reusable spacecraft. Therefore, the duration of a particular flight test program can vary from a few weeks to years.
Aircraft flight test
Civil aircraft
There are typically two categories of flight test programs – commercial and military. Commercial flight testing is conducted to certify that the aircraft meets all applicable safety and performance requirements of the government certifying agency. In the United States, this is the Federal Aviation Administration ; in Canada, Transport Canada ; in the United Kingdom, the Civil Aviation Authority; and in the European Union, the European Aviation Safety Agency. Since commercial aircraft development is normally funded by the aircraft manufacturer and/or private investors, the certifying agency does not have a stake in the commercial success of the aircraft. These civil agencies are concerned with the aircraft's safety and that the pilot's flight manual accurately reports the aircraft's performance. The market will determine the aircraft's suitability to operators. Normally, the civil certification agency does not get involved in flight testing until the manufacturer has found and fixed any development issues and is ready to seek certification.Military aircraft
Military programs differ from commercial in that the government contracts with the aircraft manufacturer to design and build an aircraft to meet specific mission capabilities. These performance requirements are documented to the manufacturer in the aircraft specification and the details of the flight test program are spelled out in the statement of work. In this case, the government is the customer and has a direct stake in the aircraft's ability to perform the mission. Since the government is funding the program, it is more involved in the aircraft design and testing from early-on. Often military test pilots and engineers are integrated as part of the manufacturer's flight test team, even before first flight. The final phase of the military aircraft flight test is the Operational Test. OT is conducted by a government-only test team with the dictate to certify that the aircraft is suitable and effective to carry out the intended mission.Flight testing of military aircraft is often conducted at military flight test facilities. The US Navy tests aircraft at Naval Air Station Patuxent River and the US Air Force at Edwards Air Force Base. The U.S. Air Force Test Pilot School and the U.S. Naval Test Pilot School are the programs designed to teach military test personnel. In the UK, most military flight testing is conducted by three organizations, the RAF, BAE Systems and QinetiQ. For minor upgrades the testing may be conducted by one of these three organizations in isolation, but major programs are normally conducted by a joint trials team, with all three organizations working together under the umbrella of an integrated project team airspace.
Atmospheric flight testing of launch vehicles and reusable spacecraft
All launch vehicles, as well as a few reusable spacecraft, must necessarily be designed to deal with aerodynamic flight loads while moving through the atmosphere.Many launch vehicles are flight tested, with rather more extensive data collection and analysis on the early orbital launches of a particular launch vehicle design. Reusable spacecraft or reusable booster test programs are much more involved and typically follow the full envelope expansion paradigm of traditional aircraft testing. Previous and current test programs include the early drop tests of the Space Shuttle, the X-24B, SpaceShipTwo, Dream Chaser, Falcon 9 prototypes, OK-GLI, and SpaceX Starship prototypes.
Flight test processes
Flight testing—typically as a class of non-revenue producing flight, although SpaceX has also done extensive flight tests on the post-mission phase of a returning booster flight on revenue launches—can be subject to the latter's statistically demonstrated higher risk of accidents or serious incidents. This is mainly due to the unknowns of a new aircraft or launch vehicle's handling characteristics and lack of established operating procedures, and can be exacerbated if test pilot training or experience of the flight crew is lacking For this reason, flight testing is carefully planned in three phases: preparation; execution; and analysis and reporting.Preparation
For both commercial and military aircraft, as well as launch vehicles, flight test preparation begins well before the test vehicle is ready to fly. Initially what needs to be tested must be defined, from which the Flight Test Engineers prepare the test plan, which is essentially certain maneuvers to be flown. Each single test is known as a Test Point. A full certification/qualification flight test program for a new aircraft will require testing for many aircraft systems and in-flight regimes; each is typically documented in a separate test plan. Altogether, a certification flight test program will consist of approximately 10,000 Test Points.The document used to prepare a single test flight for an aircraft is known as a Test Card. This will consist of a description of the Test Points to be flown. The flight test engineer will try to fly similar Test Points from all test plans on the same flights, where practical. This allows the required data to be acquired in the minimum number of flight hours. The software used to control the flight test process is known as Flight Test Management Software, and supports the Flight Test Engineer in planning the test points to be flown as well as generating the required documentation.
Once the flight test data requirements are established, the aircraft or launch vehicle is instrumented with a data acquisition system, or data acquisition unit and sensors, to record that data for analysis. Typical instrumentation parameters recorded during a flight test for a large aircraft are:
- Atmospheric pressure and temperature;
- Dynamic pressure and temperature, measured at various positions around the fuselage;
- Structural loads in the wings and fuselage, including vibration levels;
- Aircraft attitude, angle of attack, and angle of sideslip;
- Accelerations in all six degrees of freedom, measured with accelerometers at different positions in the aircraft;
- Noise levels ;
- Internal temperature ;
- Aircraft controls deflection ;
- Engine performance parameters.
During the flight, these parameters are then used to compute relevant aircraft performance parameters, such as airspeed, altitude, weight, and center of gravity position.
During selected phases of flight test, especially during early development of a new aircraft, many parameters are transmitted to the ground during the flight and monitored by flight test and test support engineers, or stored for subsequent data analysis. This provides for safety monitoring and allows for both real-time and full-simulation analysis of the data being acquired.
Execution
When the aircraft or launch vehicle is completely assembled and instrumented, many hours of ground testing are conducted. This allows exploring multiple aspects: basic aircraft vehicle operation, flight controls, engine performance, dynamic systems stability evaluation, and provides a first look at structural loads. The vehicle can then proceed with its maiden flight, a major milestone in any aircraft or launch vehicle development program.There are several aspects to a flight test program, among which:
- Handling qualities, which evaluates the aircraft's controllability and response to pilot inputs throughout the range of flight;
- Performance testing evaluates aircraft in relation to its projected abilities, such as speed, range, power available, drag, airflow characteristics, and so forth;
- Aero-elastic/flutter stability, evaluates the dynamic response of the aircraft controls and structure to aerodynamic loads;
- Avionics/systems testing verifies all electronic systems perform as designed;
- Structural loads measure the stresses on the airframe, dynamic components, and controls to verify structural integrity in all flight regimes.
- Weapons delivery, which looks at the pilot's ability to acquire the target using on-board systems and accurately deliver the ordnance on target;
- An evaluation of the separation of the ordnance as it leaves the aircraft to ensure there are no safety issues;
- air-to-air refueling;
- Radar/infrared signature measurement;
- Aircraft carrier operations.
Because the primary goal of a flight test program is to gather accurate engineering data, often on a design that is not fully proven, piloting a flight test aircraft requires a high degree of training and skill. As such, such programs are typically flown by a specially trained test pilot, the data is gathered by a flight test engineer, and often visually displayed to the test pilot and/or flight test engineer using flight test instrumentation.