The Head-up Display is a new flight instrument in the flight deck that projects the information into the pilot's eyes in a way that it appears to be in space ahead of the aircraft.

HUD Concept

The Head-up Display presents flight information to the pilot by projecting an image onto a semitransparent sheet of glass in front of the pilot, called the Combiner. The HUD projector is installed above the pilot's head, and includes an image source that displays the green symbology and EVS video, if selected.

Lenses in the overhead unit are used to project the image onto the HUD Combiner that is precisely located to reflect the image into the pilot's eyes. Because the combiner is designed to only reflect the specific color of light projected by the image source, the pilot is able to see the projected HUD image overlaid on the outside world without any noticeable distortion. The Combiner is also a collimator that makes the projected image appear to come from optical infinity - the same as the outside world - so the pilot does not need to refocus their eyes to see the symbology. The projector is connected to the HGS computer that calculates and draws the symbology on the image source.

Collimated Display

Eliminate pilot's need to refocus from HUD symbology to the outside world.

The Head-up Display (HUD) becomes an invaluable addition to any modern flight deck because the information displayed is collimated. Here's what that means:

The normal human eye can adjust to focus on an image that's near (like a cockpit instrument panel) - or far (like mountains in the distance). For a pilot, it can take over a second to refocus from a distant object to a near object, and frequent refocusing can cause eye strain.

The secret of the HUD collimated display involves using lenses to make a close object appear to be distant. The advantage of displaying a collimated image is that the viewer does not have to refocus to see the outside world. In addition, because the display device appears to be at a distance, the displayed image seems to be very large and, therefore, easier to read.

The Head-up Display uses collimation to improve the way flight information is presented to the pilot. The HUD uses a combiner in front of the pilot's eyes to redirect and collimate light from an image source (a CRT or LCD) toward the pilot. The symbology appears to be more than 200 feet in front of the aircraft, allowing the pilot to easily focus on the flight information and the distant runway at the same time.

With a HUD field of view of 30 degrees, and the collimated, virtual image of the flight information at 200 feet, the data displayed appears to be a transparent billboard over 100 feet tall.

Field-of-View

Compared to typical head-down displays, the Head-up Guidance System can increase Field-Of-View (FOV) width by 80% or more. That's extremely important for keeping the displayed symbology conformal in high-crosswind situations.

How FOV Works

Field-of-view is a characteristic of the Head-up Display (HUD) that determines how useful it is as a flight instrument. It's the "size" of the displayed information as seen by the eye at the normal viewing position, given as an angle in both vertical and horizontal planes. The FOV depends on the size of the display and the distance from the pilot. Because the HUD is a display that presents information on a collimated, virtual display surface, the FOV is more important that the size of the combiner.

For a typical modern flight deck using an ARINC size D Head-Down Display, the display is 8 inches by 8 inches and is located about 24 inches from the normal eye position. This gives a vertical and horizontal FOV of 18.9 degrees.

For a HUD, the combiner is located as far away from the pilot's head as practical (usually between 6 and 12 inches) and sized to meet the FOV requirement. If the combiner is close to the pilot's head, it can be small. If it's farther away, then it will have to be larger.

Today's wider FOV HUDs are able to achieve a 34- and even 36-degrees horizontal FOV.

Generally, the symbology presented on a HUD requires about a 20 degree FOV - similar to that of the HDDs. A wider FOV can be very beneficial in crosswind conditions where there is a large difference between where the nose of the aircraft is pointed versus the actual direction of travel (Flight Path). In this case, the symbology is moved laterally so the symbols still overlay the real world. With a wide FOV HUD, the information remains conformal even in high crosswind situations.

A major push for increased horizontal FOV size is to support the Enhanced Vision System (EVS). This system displays a video image from a camera that is able to "see" through fog or rain. By displaying EVS on a wide FOV image on the HUD, the pilot is able to gain peripheral vision cues that help in flying the aircraft.

Conformality

Important HUD symbology is displayed to the pilot "conformally" - meaning the symbols overlay the real world as seen through the HUD combiner. The flight path symbol is a conformal symbol that shows the pilot where the aircraft is actually going - rather than where the aircraft is pointing (which is represented by the boresight symbol). The boresight symbol shows the pilot the direction the aircraft nose is pointed and the pitch angle of the aircraft compared to the horizon. However, the flight path symbol shows the aircraft track and flight path angle.

Another conformal symbol is the zero pitch line showing whether the pilot is climbing, descending, or in a turn. The zero pitch line includes compass radial/tic marks.

The HGS use of conformal symbology is extremely helpful in all approach situations from Cat III to CAVU. The symbology can be particularly helpful when performing visual approaches to runways not served by an ILS or VASI/PAPI lighting. The pilot laterally aligns the aircraft to the runway by centering it below the displayed flight path symbol. To establish the proper descent path, the pilot maintains level flight until the 3-degree pitch line overlays the touchdown zone. The pilot then lowers the nose until the flight path symbol overlays the 3-degree pitch line (and touchdown zone) and thereby establishes a descent to the runway along a standard 3-degree path. This also allows the pilot to ensure a stabilized 3-degree approach path is flown to the touchdown zone. With the benefit of HGS conformality, this maneuver is easy to perform, even in crosswinds and turbulence.