Flight simulators are devices in which air crews can train without the use of actual aircraft.  In their most sophisticated form, they simulate an aircraft's instrumentation, vehicular motion and sounds, gravitational forces, cockpit environment, radar and electrooptical sensor displays, and out-of-the-window views.  Stated simply, flight simulators are computer driven, electro-mechanical systems which provide an economical, safe and effective alternative to in-flight training of pilots.

Less than a decade after the Wright Brothers first flight, early aviation enthusiasts recognized the importance of learning to fly in an environment safer than the plane itself.  Ground based trainers appeared as early as World War I.  However, it was 1929 before a young aviator named Ed Link designed the first practical simulated aircraft trainer.  Mr. Link combined his knowledge of automation of musical instruments (using pneumatic controls) and his knowledge of flying, to design a device which simulated aircraft instrument flight.

His invention was not readily accepted by the aviation world when first introduced.  He was, however, successful in selling it as a coin operated amusement device.  Later, he set up a flight training school.  For eighty dollars, he guaranteed to teach someone to fly.  The school included training in his simulator, with only two flights in an actual aircraft.  It really took World War II to get Ed Link's invention accepted.  It became the key to solve the requirement to train thousands of pilots to fly over a short span.  As a result, the Link Trainer, or "Blue Box" as it was often referred went into full production.

Early ground based trainers were used principally as instrument trainers.  In spite of their widespread use over many years, instrument-flying trainers have never been received enthusiastically by the whole of the piloting fraternity.  The illusion of flight is only successful if the pilot can relate to the flight situation and divorce himself from the idea of sitting in a box, performing a stylized, though difficult task.  Acceptance of the simulator as an aircraft is greatly assisted by a visual display, whereby the pilot is made aware of the position, orientation, rotation, and translation of his aircraft in space.

The visual flight simulation (display) is that part of the simulation which presents scenes for the pilot to view out-of-the-window of the mock cockpit.  The visual display presents the flight trainee with scenes representative of those that would have been seen if the actual mission being trained for was flown.  On takeoff for example, the pilot is made aware of the increasing speed and height of the aircraft over the ground from the visual display  an impression that is then reinforced by the readings of his simulated flight instruments.

Visual simulation has been without a doubt the most elusive phase in the development of flight simulators for aircraft training.  When one ponders the requirement, however, it is not hard to understand that simulating the world as seen by the human eye is much more difficult than duplicating a set of flight equations, controls, dials, and switches.

Out-of-the-window visual simulation is a formidable challenge because of the fantastic performance capabilities of the human eye.  The human visual system provides the user with most of his sensory input: the peripheral retina answers the "where" question and the central fovea area the "what" of pattern vision.  The eyes are our most important sense organs for gaining information about the world around us.  It is estimated that over 90% of the information that we receive during our normal daily activities come through the eyes, and certainly that much or more when an aviator is involved in flight tasks.  Even with the tremendous advancements of the most sophisticated visual systems, computer generated image systems lack the processing and storage capacity to provide anything approaching real world detail.

The real world is infinitely variable; scenes produced by the simulator's computer image generator, however, must be realistic enough only to foster development of the student's skills.  Nevertheless, if even one aspect of the imagery falls below the threshold of acceptability, the training value of the device is diminished, if not lost.

The importance and usefulness of out-of-the-window visual simulation displays can be best judged by the recent large increase in the use of this equipment in air-carrier trainer simulators.  The military has followed suit in that the visual system for flight simulators have become the major portion of an already large simulator budget, especially since the feasibility and flexibility of computer generated image systems have been demonstrated.

Equipment for out-of-the-window visual simulation was largely developed in the 1960's by aircraft manufacturers who used the simulators for engineering and test purposes.  By the end of that decade, the equipment had proved sufficiently useful and dependable that it was being adopted for training.  At the end of the 1970's, over 300 visual simulation systems were in use by the world's air carriers.  The relatively late and rapid utilization of these visual cues in training simulators, compared with inside-the-cockpit cues and with motion cues, may be ascribed to several factors.  One of these factors is the recent rapid development in the techniques and hardware of computers and television which have greatly increased the quality and reliability and have decreased the cost of the visual simulation equipment.  Another is the success of early users of visual simulation equipment in decreasing the cost of pilot training by substituting simulation for training time in the actual aircraft.

Visual simulation for air carrier use had largely been restricted to a forward-looking view of the airstrip and its surroundings during takeoff, approach, and landing.  The conventionally used 48 degree horizontal by 36 degree vertical field-of-view has proved satisfactory for training pilots for a straight-in approach of an aircraft; regulatory agencies (FAA) have certified systems of this type as capable of substituting for a large portion of aircraft training time.  As system capabilities expand, the requirements have also expanded.  Decreases in cost per channel of information have resulted in the desire for increases in the field-of-view by employing multiple displays.  That is, the same geographic region which is viewed through the front "window" of the simulator will moments later appear moving past the side windows.  Today's commercial aircraft simulator typically requires three to four channels to present views for the front and adjacent side windows.  A common commercial configuration is a three channel, four window system, where the pilot and copilot are presented with identical forward views.

Military training use has lagged behind commercial systems because of the more varied visual simulation requirements associated with military missions.  The tasks performed by aviators in the modern battlefield environment are many and diverse.  They range from takeoffs and landings at airports with all the normal aids, to operations out of hastily prepared landing strips in unfamiliar terrain; from air-to-air combat to nap-of-the-earth missions; from in-flight refueling to air-to-ground weapons delivery. These military tasks impose severe requirements on the out-of-the-window visual simulation system and make the hardware solutions much more difficult.  But it was the realization of the value gained from such simulation which has pushed visual technology to todays standards.  Modern combat aircraft simulators, with their sophisticated visual systems, provide a means by which flight crew members can experience manuevers or situations that in reality would be either too dangerous to risk in training or are such that they would only be encountered in actual combat.  The full capability of the flight simulator and its visual data bases can be exploited with the aid of additional parameters.  Instructors may schedule weather effects such as fog, snow, rain, lightning and clouds; they can trigger threat effects including planes, helicopters, or tanks with their associated bullets, missiles, and rockets. 

Rapidly advancing simulator technology now permits full scale, dynamic, high fidelity reproductions of virtually all sensory elements of flying, including: the motion and sounds of the aircraft, the scenes observed through the cockpit windows and the manner in which controls and instruments respond.  With a high degree of realism, pilots can achieve proficiency by practicing take-offs, climbing to and flying at altitudes, approaches and landings under normal, peak workload and emergency conditions  all with no danger to the pilot, instructor, or "aircraft".

The simulator system that is mainly responsible for this state of affairs is the visual system.  Although aviators have a variety of electronic aids to assist them, many tasks still require the human visual system and the intimate knowledge that system gives the aviator of his external environment.