Flight simulator

For the flight simulation computer game, see Microsoft Flight Simulator.

Flight simulation is an approximation, or simulation, of flight and various aspects of the flight environment. Flight simulation is used for a variety of reasons, including flight training and aircraft development. Flight simulations have varying degrees of detail and realism that depend largely on the purpose for which they are being used, and may range from simple generic cockpit replicas with minimal software modeling, to actual aircraft cockpits with wide-field visual systems mounted on large six degree of freedom (DOF) motion platforms that feature comprehensive flight and systems models.

History

World War I

Link Trainer

A number of electro-mechanical devices were tried during World War I and thereafter. For example, learning to fire a machine gun requires that the pilot learn to lead targets, so a ground simulator was developed to teach this skill to new pilots.

Post World War I and World War II

One of the best-known early simulation devices was the Link Trainer, produced by Edwin Link in Binghamton, New York USA, which was available starting in 1929. This had a pneumatic motion platform driven by bellows, which provided pitch, roll, and yaw motion cues. A generic replica cockpit was mounted to the motion platform. It was designed for the teaching of instrument flying in a relatively safe and inexpensive environment. While civil aviation was initially relatively uninterested in the Link Trainer, the US Army Air Force purchased four devices in 1934 following a series of fatal accidents that occurred in instrument flight.

Some 10,000 Link Trainers were used during the war to train new pilots of allied nations. They were still in use in several Air Forces into the 1960s and early 1970s.

The Celestial Navigation Trainer of 1941 was a massive structure 13.7 m (45 ft) high and capable of accommodating an entire bomber crew learning how to fly night missions.

In the 1940s, analog computers were used to solve the equations of flight, resulting in the first electronic simulators.

Late 1940s

In 1948, Curtiss-Wright delivered a trainer for the Boeing 377 Stratocruiser to Pan American, which was the first complete simulator owned by an airline. There was no motion or visual system installed on the simulator, but the cockpit was fully functional, and the device provided effective training to flight crews.

Introduction of Visual Systems

A mock-up terrain visual system of the TL39 simulator

The early visual systems used a small physical terrain model. A camera was "flown" over the model terrain, and the resultant image displayed to the pilot. The camera responded to pilot control inputs in order to provide the proper terrain image. Naturally, only limited geographical areas were able to be simulated in this manner, which were usually limited to the immediate vicinity of an airport or, in military devices, typical terrain and sometimes targets.

Improvement in Motion Systems

In 1954, the Link Division of General Precision Inc. (later part of Singer Corporation and now part of L-3 Communications) developed a simulator motion system housing a cockpit within a metal framework, providing 3 degrees of angular displacement in the pitch, roll, and yaw axes. By 1964 improved, compact versions of this system provided increased displacements of up to 10 degrees. By 1969 motion systems that were controlled by hydraulic actuators became available, and six degrees of freedom motion systems were soon put into service. Beginning in 1977, many aircraft simulators began adopting the modern "cab" configuration where non-cockpit hardware, such as the computers running the simulation, are placed on the motion platform along with the cockpit and instructor station, rather than being located off of the simulator platform. In this configuration, equipment is accessible using a wraparound catwalk while the motion system is disabled.

1960s

The use of digital computers for flight simulation began in the 1960s.

Improvement in visual systems

In 1972, Singer-Link developed a collimating lens apparatus, using a curved mirror and beamsplitter, which projected Out of The cockpit Window (OTW) views to the pilot that provided a distant focus point. These collimated visual systems provided an improvement in the realism of pilot perception of distant visual scenes; however, each monitor offered a field of view of only 28 degrees, which required that several channels be installed in order to provide an adequate field of view. These systems were additionally limited in their cross-cockpit viewing capabilities, and the pilot and copilot views would appear somewhat distorted to the other crew member.

In 1976, wider angle collimated monitors were introduced, and were called 'WAC windows', standing for 'Wide Angle Collimated windows', and in 1982, the Rediffusion company of Crawley, UK, introduced the Wide-angle Infinity Display Equipment (WIDE) system that used a curved mirror of large horizontal extent to allow distant-focus (collimated) viewing by side-by-side pilots in a seamless display. WIDE-type displays are now utilized in most high level Full Flight Simulators, although helicopter simulators have the option of utilizing real image (non-collimated) display systems.

Use

Flight Training

Interior cockpit of a twinjet flight simulator

Flight simulation is used extensively in the aviation industry for the training of pilots and other flight crew in both civil and military aircraft.

Several different types of devices are utilized in modern flight training. These range from simple Part-Task Trainers (PTTs) that cover one or more aircraft systems to Full Flight Simulators (FFS) with comprehensive aerodynamic and systems modeling. This spectrum encompasses a wide variety of fidelity in both physical cockpit characteristics and quality of software models, as well as various implementations of sensory cues such as sound, motion, and visual systems. The following training device types are in common use:

• Cockpit Procedures Trainer (CPT) - Used to practice basic cockpit procedures, such as emergency checklists, and for cockpit familiarization. Certain aircraft systems may or may not be simulated. The aerodynamic model is usually extremely generic if one is even present at all. CPTs are usually not regulated.
• Aviation Training Device (ATD) - Used for basic training of flight concepts and procedures. A generic flight model representing a "family" of aircraft is installed, and many common flight systems are simulated.
• Basic Instrument Training Device (BITD) - A basic training device primarily focused on generic instrument flight procedures.
• Flight and Navigation Procedures Trainer (FNPT) - Used for generic flight training. A generic, but comprehensive flight model is required, and many systems and environmental effects must be provided.
• Flight Training Device (FTD) - Used for either generic or aircraft specific flight training. Comprehensive flight, systems, and environmental models are required. High level FTDs require visual systems.
• Full Flight Simulator (FFS) - Used for aircraft specific flight training. All relevant systems must be fully simulated, and a comprehensive aerodynamic model is required. All FFS require visual systems.

In many professional flight schools, initial training is conducted partially in the aircraft, and partially in relatively cost-effective training devices such as FNPTs and FTDs. As the student becomes familiar with basic aircraft handling and flight skills, more emphasis is placed on instrument flying, cockpit resource management (CRM), and advanced aircraft systems, and the portion of flight training conducted in these devices increases significantly.

For many commercial pilots, most aircraft orientation and recurrent training is conducted in high level FTDs or FFS.

In comparison to training in an actual aircraft, simulation based training allows for the training of maneuvers or situations that may be impractical (or even dangerous) to perform in the aircraft, while keeping the pilot and instructor in a relatively low-risk environment on the ground. For example, electrical system failures, instrument failures, hydraulic system failures, environmental system failures, and even flight control failures can be simulated without risk to the pilots or an aircraft.

Instructors can also provide students with a higher concentration of training tasks in a given period of time than is usually possible in the aircraft. For example, conducting multiple instrument approaches in the actual aircraft may require significant time spent repositioning the aircraft, while in a simulation, as soon as one approach has been completed, the instructor can immediately preposition the simulated aircraft to an ideal (or less than ideal) location from which to begin the next approach.

Flight simulation also provides an economic advantage over training in an actual aircraft. Once fuel, maintenance, and insurance costs are taken into account, the operating costs of an FSTD are usually substantially lower than the operating costs of the simulated aircraft. For some large transport category airplanes, the operating costs may be several times lower for the FSTD than the actual aircraft.

Engineering Simulation

Engineering flight simulators are used by aerospace manufacturers for such tasks as:

• Development and testing of flight hardware. Simulation (emulation) and stimulation techniques can be used, the latter being where real hardware is fed artificially-generated or real signals (sTimulated) in order to make it work. Such signals can be electrical, RF, sonar, etc., depending on the equipment to be tested.
• Development and testing of flight software. It is much safer to develop critical flight software on simulators or using simulation techniques than it is to develop using actual aircraft in flight.
• Development and testing of aircraft systems. For electrical, hydraulic, and flight control systems, full-size engineering rigs sometimes called 'Iron Birds' are used during the development of the aircraft and its systems.

Entertainment

Main article: Amateur flight simulation

Technology

Motion

Large Amplitude Multi-mode Aerospace Research Simulator (LAMARS)

An FFS duplicates all relevant aspects of the aircraft and its environment, including motion. This is typically accomplished by assembling the cockpit, IOS, and visual system on a motion platform. A six degrees of freedom (DOF) motion system is the de-facto (and in some cases, regulatory)standard; however, some legacy devices and low level flight simulators are equipped with three DOF motion systems. Due to the rapid response and significant excursion of modern motion systems, motion equipped training devices are required to provide seat belts as found in the actual aircraft. Since the travel of the motion system is limited, motion systems employ 'acceleration onset cueing', which simulates initial accelerations very accurately, and then returns the motion system to a neutral position at a rate below the pilot's sensory threshold in order to prevent the motion system from reaching its limits of displacement.

Qualification and Approval

Procedure

In order for a Flight Simulation Training Device (FSTD) to be used for flight crew training or checking, it must be evaluated by the local National Aviation Authority (NAA), such as the Federal Aviation Administration (FAA) in the United States. The training device in question is evaluated against a set of regulatory criteria, and a number of both objective and subjective tests are conducted on the device. The results of each test, along with other significant information about the FSTD and its operator, are recorded in a Qualification Test Guide (QTG).

The result of the initial evaluation of the FSTD, called the Master QTG (MQTG), details the baseline performance of the device as accepted by the qualifying authority. A periodic re-evaluation, called a recurrent qualification, is performed regularly, generally in one year intervals (although the interval can be as low as six months for some FAA evaluations and as high as three years for some European evaluations), and the performance of the device is evaluated against the MQTG. Any significant deviations may result in the suspension or revocation of the device's qualification.

The criteria against which an FSTD is evaluated are defined in one of a number of regulatory and/or advisory documents. In the United States and China, FSTD qualification is regulated in 14 CFR Part 60. In most of Europe, as well as several other parts of the world, the relevant regulations are defined in JAR-FSTD A and JAR-FSTD H. The testing requirements vary between the different levels of qualification, but almost all levels require that the FSTD show that it matches the flight characteristics of the aircraft or family of aircraft being simulated.

The main exception to the above process is the evaluation of an ATD by the FAA. Rather than other FSTD, where each device is evaluated on an individual basis, ATDs are evaluated as a model line. When a manufacturer wishes to have a model of ATD approved, a document that contains the specifications of the model line and that proves compliance with the appropriate regulations is submitted to the FAA. If this document, called a Qualification Approval Guide (QAG) is approved, all future devices conforming to the QAG are automatically approved, and individual evaluation is neither required nor available.

Until the publication of Part 60, qualification was called certification, and QTGs were called Approval Test Guides (ATGs). The terms certification and ATG no longer have any regulatory meaning other than for FSTD that remain qualified under FAA AC 120-45 or any other legacy standard.

Levels

The following levels of qualification are currently being granted for both airplane and helicopter FSTD:

FAA

FTD

• Level 4 - Similar to a CPT. This level does not require an aerodynamic model, but accurate systems modeling is required. Helicopter only.
• Level 5 - Aerodynamic programming and systems modeling is required, but it may represent a family of aircraft rather than one specific model.
• Level 6 - Aircraft model specific aerodynamic programming, control feel, and physical cockpit are required.
• Level 7 - Model specific. All applicable aerodynamics, flight controls, and systems must be modeled. A vibration system must be supplied, and this is the first level to require a visual system. Helicopter only.

FFS

• Level A - The lowest level of flight simulator. This is the first level for which a motion system is required. Airplane only.
• Level B - Requires a higher-fidelity aerodynamic model than Level A. The lowest level of helicopter flight simulator.
• Level C - Requires increased response (a lower transport delay or latency) over lower levels. Visual systems requirements are more stringent.
• Level D - The highest level of FSTD qualification available. The first level for which objective evaluation of sounds is required. A number of special motion and visual effects are required.

JAA

FNPT

• Level I
• Level II
• Level III
• MCC - Not a true "level" of qualification, but an add-on that allows any level of FNPT to be used for multi crew cooperation training.

FTD

• Level 1
• Level 2
• Level 3 - Helicopter only.

FFS

• Level A
• Level B
• Level C
• Level D

Credits

The training or checking credits allowed of an FSTD are based on the level of qualification and the operator's training curriculum. For some experienced pilots, Level D FFS may be used for Zero Flight Time (ZFT) conversions from one type of aircraft to another. In ZFT conversions, no aircraft flight time is required, and the pilot first flies the aircraft (under the supervision of a Training Captain) on a revenue flight.

Manufacturers

Civil Full Flight Simulators include FlightSafety International (FSI), Frasca International, Inc., Rockwell Collins, Opinicus in the USA, Indra Sistemas in Spain, CAE Inc. and Mechtronix in Canada, Sim Industries in the Netherlands, Havelsan in Turkey and Thales Group in France and the UK, the UK site being the ex-Rediffusion simulator factory at Crawley, near Gatwick airport.There are currently about 1200 Full Flight Simulators in operation worldwide , of which about 550 are in the USA, 75 in the UK, 60 in China (PRC), 50 each in Germany and Japan, and 40 in France. Of these, some 450 were made by CAE, mainly in their Montreal factory, about 380 by Thales and its predecessors Rediffusion, (Singer) Link-Miles, and Thomson CSF, and about 280 by Flight Safety International. L-3 Communications operates a facility in Arlington, Texas which manufactures flight simulators for the military; the division (Link Simulation and Training) traces its legacy back to Link's original invention.

Flight simulators are also extensively used for research in various aerospace subjects, particularly in flight dynamics and man-machine interaction (MMI). Both regular and purpose-built research simulators are employed. They range from the simplest ones, which resemble video games, to very specific and extremely expensive designs such as LAMARS, installed at Wright-Patterson Air Force Base, Ohio. This was built by Northrop for the Air Force Research Laboratory (AFRL) and features a large scale five degrees of freedom motion system to a unique design and a 360 degree dome-mounted visual system.

The TL39 3-DoF motion simulator with IOS at MAI University

Instructor operating stations

Most simulators have Instructor Operating Stations (IOS). At the IOS, an instructor can quickly create any normal and abnormal condition in the simulated aircraft or in the simulated external environment. This can range from engine fires, malfunctioning landing gear, electrical faults, storms, downbursts, lightning, oncoming aircraft, slippery runways, navigational system failures and countless other problems which the crew need to be familiar with and act upon.

Many simulators allow the instructor to control the simulator from the cockpit, either from a console behind the pilot's seats, or, in some simulators, from the co-pilot's seat on sorties where a co-pilot is not being trained. Some simulators are equipped with PDA-like devices in which the instructor can fly in the co-pilot seat and control the events of the simulation, while not interfering with the lesson.

In the past full motion flight simulators had been limited to multi-million dollar hydraulic devices used at large training centers such as those provided by FlightSafety International, CAE, Alteon (a Boeing company) and at the training centers of the larger airlines. Recent advances in electric motion platforms have led to their use in Full Flight Simulators at these and other training centers and also permitted full motion simulation to be provided economically for much smaller aircraft including single-engine piston aircraft at training centers such as Flight Level Aviation.

Flight simulators are an essential element in individual pilot as well as flight crew training. They save time, money and lives. The cost of operating even an expensive Level D Full Flight Simulator is many times less than if the training was to be on the aircraft itself and a cost ratio of some 1:40 has been reported for Level D simulator training compared to the cost of training in a real Boeing 747 aircraft.

Modern high-end flight simulators

High-end commercial and military flight simulators incorporate motion bases or platforms to provide cues of real motion. These are important to complement the visual cues (see below) and are vital when visual cues are poor such as at night or in reduced visibility or, in cloud, non-existent. The majority of simulators with motion platforms use variants of the six cylinder Stewart platform to generate motion cues. These platforms are also known as Hexapods. Stewart used an interlinked array of six hydraulic cylinders to provide accelerations in all six degrees of freedom. Motion bases using modern Stewart based hexapod platforms can provide about +/- 35 degrees of the three rotations pitch, roll and yaw, and about one metre of the three linear movements heave, sway and surge.

Stewart platform

These limited angular and linear movements (or "throws") do not inhibit the realism of motion cueing imparted to the simulator crew. This is because the human sensors of body motion are more sensitive to acceleration rather than steady-state movement and a six cylinder platform can produce such initial accelerations in all six DoF. The body motion sensors include the vestibular (inner ear, semicircular canals and otoliths), muscle-and joint sensors, and sensors of whole body movements. Furthermore, because acceleration precedes displacement, the human brain senses motion cues before the visual cues that follow. These human motion sensors have low-motion thresholds below which no motion is sensed and this is important to the way that simulator motion platforms are programmed (and also explains why instruments are needed for safe cloud flying). In the real world, after conditioning to the particular environment (in this case aircraft motions), the brain is subconsciously used to receiving a motion cue before noticing the associated change in the visual scene. If motion cues are not present to back up the visual, some disorientation can result ("simulator sickness") due to the cue-mismatch compared to the real world.

In a motion-based simulator, after the initial acceleration, the platform movement is backed off so that the physical limits of the cylinders are not exceeded and the cylinders are then re-set to the neutral position ready for the next acceleration cue. The backing-off from the initial acceleration is carried out automatically through the simulator computer and is called the "washout phase". Carefully-designed "washout algorithms" are used to ensure that washout and the later re-set to about neutral is carried out below the human motion thresholds mentioned above and so is not sensed by the simulator crew, who just sense the initial acceleration. This process is called "acceleration-onset cueing" and fortunately matches the way the sensors of body motion work. This is why aircraft manoeuvre at, say, 300 knots, can be effectively simulated in a replica cabin that itself does not move except in a controlled way through its motion platform. These are the techniques that are used in civil Level D flight simulators and their military counterparts.

The NASA Ames Research Center in "Silicon Valley" south of San Francisco operates the Vertical Motion Simulator. This has a very large-throw motion system with 60 feet (+/- 30 ft) of vertical movement (heave). The heave system supports a horizontal beam on which are mounted rails of length 40 feet, allowing lateral movement of a simulator cab of +/- 20 feet. A conventional 6-degree of freedom hexapod platform is mounted on the 40 ft beam, and an interchangeable cabin is mounted on the hexapod platform. This design permits quick switching of different aircraft cabins. Simulations have ranged from blimps, commercial and military aircraft to the Space Shuttle. In the case of the Space Shuttle, the large Vertical Motion Simulator was used to investigate a longitudinal pilot-induced oscillation (PIO) that occurred on an early Shuttle flight just before landing. After identification of the problem on the VMS, it was used to try different longitudinal control algorithms and recommend the best for use in the Shuttle programme. After this exercise, no similar Shuttle PIO has occurred. The ability to simulate realistic motion cues was considered important in reproducing the PIO and attempts on a non-motion simulator were not successful (a similar pattern exists in simulating the roll-upset accidents to a number of early Boeing 737 aircraft, where a motion-based simulator is needed to replicate the conditions).

AMST Systemtechnik (Austria) and TNO Human Factors (the Netherlands) have developed the Desdemona flight simulation system for the Netherlands-based research organisation TNO. This large scale simulator provides unlimited rotation via a gimballed cockpit. The gimbal sub-system is supported by a framework which adds vertical motion. Furthermore, this framework is mounted on a large rotating platform with an adjustable radius. The Desdemona simulator is designed to provide sustainable g-force simulation with unlimited rotational freedom.

See also

• Flight training

References

[AeroFiles] www.aero-files.com/shop addons shop for Microsoft Flight Simulator series and x plane.

External links

• Black Magic and Gremlins: Analog Flight Simulations at NASA's Flight Research Center by Gene L. Waltman
• The Art of Flight Simulation (Aersopace MEng Thesis on Flight Simulation)