HUD evolved from the reflectors sight, a pre-WWII parallax-free optic sight technology for military fighter planes.
During the early 1940s the TRE, in charge of UK radar research, found that RAF night fighter pilots were having difficulty reacting to the verbal instructions of the radar operators as they approached their targets because the radar operators could not see them in the dark sky.
They experimented with adding a second radar display for each pilot, but found they couldn’t look up from the lit screen and find the target. In 1942, they successfully combined the image from radar tubes with a projection from their existing GGS Mk.
II Gyrosights onto a flat surface of the windscreen, then in the gunsight itself.
A key upgrade included moving from the original “reflector” sights to the new “head-up displays”. A HUD system displays information about the current location of the plane, the speed of the plane, the direction of the wind, and the position of the target.
The HUD provides pilots with vital information about the plane and the weapon systems.
How Do They Work?
The windshield acts as a screen that allows us to see data projected onto them. A projector built into the dashboard sends the image onto the windshield.
The image then goes through the projection device and reflects several mirrors, magnifying and inverting the image so that the information comes out correctly.
There are many types of windshields available, including those with a combiner. These are used to create a uniform image no matter what shape the windshield is.
What Do You See?
HUD is used in cars and planes. With HUD a pilot or driver can see a variety of information. Since 1988 the Corvette has been equipped with a HUD system. It displays performance data like engine speeds, g-forces, gears, and oil temperatures.
On certain top end cars, the HUD offers night vision – showing images taken from behind your grille onto your windshield may save you from running into animals. On the lower end of the spectrum, companies aren’t just calling it merely a heads-up display.
Rather, it’s AI – arrows and pathways overlaid on roads ahead, providing foolproof navigational guidance, and the ability to see how fast that truck is going in front of you, and if there’s anything behind it, you might miss.
The HUDs used on aircraft provide information about altitude, airspeed, heading, vertical speed, and ground speed. They also display the current position of the aircraft relative to the runway threshold or glide path. They can be used to guide the pilot during take-off and landing.
When flying at night or in bad weather, pilots rely heavily on the HUD to help them land safely. During take off and landing, the pilot uses the instrument panel to monitor the airplane’s performance.
There are many factors that play into the design of a HUD:
The field of View-also known as “FOV” – is the angle(s), horizontally and vertically, subtended at the pilot’s eye, at which the display symbology appears in relation to the outside view.
A narrow FOV means the view (of a runway, for example) through a combiner might include little extra information beyond the perimeter of the runway environment; while a wider FOV allows a “broader” view.
For aviation applications, a wide FOV is beneficial because an aircraft approaching the runway could have the runway in view even if the aircraft was pointed far away from the runway threshold.
With a narrow FOV, the airport runway would be off the edge of the combiner, outside the HUD’s view.
Human eyes are separated, so each eye receives a different view. The HUD image is visible by one or both eyes depending on technical and budget constraints in the design process.
Current expectations are that both eyes receive the same image, in effect a binocular field of view.
A collimated HUD displays the projected image at or near infinity to give pilots a clear image without needing to refocus. This feature allows pilots to see the outside world while using the HUD.
Eyeboxes are devices used to project images onto a user’s retinas. These images are projected onto the retina, allowing the user to see them. Luminance and contrast displays have adjustment settings to account for ambient light.
Scaling is used by the flight computer to show the pilot what he should see when flying. Objects such as runways or obstacles are shown at negative angles. This makes them easier to spot. The scale is also used by the HUD to show pilots exactly how far away things are.
The correct seat position is crucial to get full use of HUD. The HUD is generally in a set position. Therefore, it is necessary to alter the seat position accordingly.
There is a test feature that can be used to create reference symbols at the bottom and top of the display. When the symbols are visible, the seat height is correct.
The longitudinal position exists when the information at the outer edges can be viewed without significant movement of the head. Rudder pedals should be adjusted to the seat height.
Screen brightness should be set according to the display technology used. Symbols should be easy to read, but not too bright. Objects outside of the aircraft should not be obscured by the display.
To accommodate personal preference, HUDs will often have both a brightness control and an enhanced vision brightness control.
The contrast should be adjusted using the enhanced vision brightness control such that the symbology can be seen over the enhanced vision picture.
The overall brightness should then be adjusted so the HUD information can be read without obscuring external references or other displays. There is often an auto-brightness mode that will adjust the symbology relative to changing ambient light.
However, many pilots choose to manually adjust the brightness as the light changes.
A HUD can enhance situational awareness during takeoff and landing phases of flight and can help pilots avoid CFITS. A new report shows that using a HUD could prevent 33 per cent of all jet crashes and 29 per cent of major partial plane crashes.
This is because the HUD provides pilots with information about the plane’s condition.
A head-up display enhances the pilot’s ability to see what is going on around him. He can still use instruments but he doesn’t have to look away from the cockpit. It helps pilots land safely at night.
When flying, pilots must be aware of the plane’s speed, altitude, and direction. This information helps them know how much fuel they need to get back to an airport safely. With this knowledge, they can make adjustments to their flight plan and avoid dangerous situations.
To achieve the benefits of HUD it must be used as intended and flight crews need to be properly trained, practised and proficient with it.
A comprehensive list of training items related to the head-up display is available in the IFALPA position paper “Head-up Display (HUD) and vision systems”.
Modern technology has certainly created features that make complex jobs much easier to navigate and it’s fascinating living in a modern and ever-advancing world.
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