How Does a HUD Work?
As far as pilots are concerned, the head-up display may as well consist of the HUD combiner glass and little else, because that is about all they see. Obviously, head-up displays in modern business airplanes are sophisticated systems comprising a host of components, and while it might not be crucial to know what each one does, a brief explanation can help pilots understand how that seemingly magic piece of glass hanging in front of them derives its glowing green symbology.
The most important piece of any HUD is the central computer, which receives data from a variety of aircraft sensors and generates the display symbology. Typical HUD computers consist of a number of processor cards that are capable of interfacing with multiple avionics sources to create the HUD symbology that the pilot sees. The computer is also responsible for monitoring the integrity and performance of the system, vital duties when pilots are executing low-visibility approaches.
The next component of the HUD is the overhead unit, which houses the CRT or LCD projector and contains a relay lens that beams the HUD symbology onto the combiner. The combiner is a piece of nearly clear glass in front of the pilot’s face that reflects the projected image into the eyes. The system is optimized so that the projector sends a specific green-hued image, and the semi-reflective combiner glass reflects only that specific green color. This way, when the pilot looks through the glass there is no color distortion of the outside real-world view.
The HUD’s lens system makes the image on the combiner glass appear as though it is being focused at optical infinity, producing what HUD developers refer to as a “collomated” image. The major benefit of focusing the HUD image at infinity is that the pilot does not have to continuously refocus his vision between the outside world and the HUD symbology. Rather, both appear to be at the same focal length.
When the pilot looks through the HUD combiner glass, the symbology appears as though it is hovering several hundred feet out in front of the aircraft. One way to think about the HUD image is as a clear billboard, about 200 feet wide by 100 feet tall, that is locked to the horizon. More important than the horizon line, however, is the flight-path symbol, which continuously shows the velocity vector of the airplane, regardless of which direction it is pointing. When the aircraft is in a crab, for example, with its nose up and the path coming down toward the runway on an approach, the flight-path symbol shows where the airplane is going, regardless of where it is pointed. If the pilot maneuvers the airplane so that the flight-path symbol is sitting on the runway touchdown zone, it doesn’t matter which way the nose is pointed; the airplane will touch down on the desired spot.
The small flight-path symbol looks like the backside of a fat little airplane with stubby wings and tail. It informs the pilot of the precise vector of the aircraft’s center of gravity and is calculated in the HUD computer based on input from inertial sensors. Additional symbology provides a graphical view of the thrust of the airplane by portraying lateral acceleration and velocity. Used together, the symbology allows the pilot to know whether the airplane is on speed, whether it is accelerating or decelerating, and that it is on the proper path.