The primary flight display (PFD) combines the instruments that display the information most vital to the aircraft into a single space. Unlike older instruments, a PFD provides information in electronic form. The advent of this technology has allowed for greater attention to flight information while increasing the ease of use and amount of space available for other instrumentation.
There are many purveyors of this technology, and the primary flight display made by each vary to some degree. The use of PFDs is not limited to one particular aircraft, as both airplanes and helicopters can employ them. The differences between PFDs become even more apparent when the different missions and needs of each aircraft are considered.
While the primary flight display does not provide the pilot with all information about the physical condition of the aircraft itself, it does show the aircraft's flight conditions. Every PFD shows the aircraft's airspeed, altitude, and attitude. Altitude refers to the aircraft's distance above sea level. Attitude is the orientation of the aircraft, it's heading, bank, and pitch. If a PFD malfunctions, the pilot has to fly the aircraft using backup mechanical gauges.
Though designs may differ, the general layout is usually similar. The center of the primary flight display typically contains the attitude indicator. This instrument indicates the aircraft's position in space through the display of an artificial horizon. This means that the indicator shows the pitch, which is the angle of the aircraft above or below the horizon, and roll, the tilting of the aircraft from side to side along a central axis running from nose to the rear.
On the sides of the attitude indicator are two more gauges for airspeed and altitude. These are most commonly shown vertically next to the artificial horizon. Airspeed is represented in units of knots, or nautical miles per hour (1.151 miles or 1.852 kilometers/hour). Units above mean sea level (AMSL), which is the average height of the surface of the world's seas, are used to describe the aircraft's altitude. Another indicator that might be on a primary flight display is a compass rose.
Some similarities are apparent when comparing a heads up display (HUD) and a primary flight display. A fighter plane may utilize an HUD, which is placed directly ahead of the pilot in his or her field of vision. This allows the pilot to concentrate more on the task at hand and spend less time looking down at gauges. A HUD, however, must only display the minimum amount of information necessary for the pilot to maintain proper control. A PFD is capable of greater customization and of displaying much more information than a HUD and would most likely also be present in any modern aircraft equipped with a heads up display.