What is a HUD? Part 2.

Aviation Blog.
                                           Image: Boeing.com

The technology employed in modern aircraft continues to evolve at an incredible rate. For the pilot, this is particularly evident in the cockpit. Aircraft, both civil and military, recreational and commercial, are employing increasingly advanced flight deck systems. Here we look at the Heads Up Display, or the 'HUD'.

Part One of 'What is a HUD?' can be read HERE at The Pilot's Blog.


The Magic Box.

At the heart of the 'Heads Up Guidance System', or HGS, are five core components. Firstly, the HGS computer situated beneath the floor in the Electrical and Electronics Compartment receives the relevant data from the aeroplane before converting it into standard symbology. The computer then relays information to the Overhead Unit (OHU) above the pilot which contains a Cathode Ray Tube (CRT) and the optics required to project the symbology to the lowered glass panel in front of the pilot. This holographic glass panel, or ‘Combiner’ reflects the light from the OHU and blends the projected symbology with the pilot’s view outside.

Supporting these three components, a centrally located HGS Control Panel facilitates the selection of different modes as well as the setting of glidepath angles, runway lengths and elevation. Finally, an HGS Annunciator Panel provides an indication of the systems status and warning annunciations should errors arise.

The pilot’s head position is also critical in the operation of the HUD. To maintain a wide field of vision and be able to sight all of the projected symbology, the pilot must have their head positioned in the ‘eye box’. Achieved by correct set position, it is an invisible geometric box which permits minimal head movement without the loss of any projected symbology. So that the pilot doesn’t have to change focus from the outside environment back into the HUD symbology, the combiner is ‘collimated’, or focussed to infinity. In practice, the pilot looks ‘through’ the HUD rather than setting a gaze upon the symbols.


The symbology found across HUDs is standard and much of it is simply replicating the format of conventional flight displays. Pitch and roll scales, heading bugs, horizon bars, compass roses, flight mode annunciations, speed tapes, altimetry tapes and so on can all be found in HUD symbology. The old ‘wings’ on the artificial horizon are still there representing the aeroplane, although they are now referred to as the Aircraft Reference Symbol, or ‘Boresight’.

Additionally, a winged circle ‘Flight Path’ symbol represents the actual flight path vector of the aeroplane and consequently, wherever the Flight Path Symbol is laid, that is where the aeroplane is going. This is very useful in situations such as approaches to runways without a vertical path guidance as laying the Flight Path symbol on the runway aim point will guide the aeroplane on a standard 3 degree flight path. Furthermore, certain manoeuvres such as flaring provide their own cues represented in a circular fashion. As such, flying that phase of flight simply calls for overlaying the circle of the Flight Path symbol on the circle that is guiding the manoeuvre; keeping the ‘ball in the hoop’ if you like.

There is so much information available, that one must guard against having their attention ‘captured’ by the symbology in a type of technological target fascination. To offset this, the system offers a variety of modes that provide the most appropriate information, guidance and symbology for that phase of flight. Whether it is established in the cruise, or rolling out in minimum visibility following an autoland, there is a mode applicable to the situation, combating any potential clutter and confusion.

Looking Ahead.

The growth of HUDs continues and the  Boeing 787 Dreamliner features the system as standard for both pilots, replacing the previous situation where the HGS is usually only on the Captain’s side of the flight deck. Beyond the simple duplication of the system, there is ongoing research into the best means of projection and resolution for the human eye in varying light conditions.

In military circles, the HUD is rapidly being overtaken with helmet-mounted displays, or HMDs. These do not come with the limitation of an ‘eye box’ and provide the information regardless of where the pilot is looking. The next leap for the civilian application of the HUD utilises the new technology of synthetic vision. This innovation will see a 3D image of terrain, obstacles, runways and the like in various shades of green projected onto the HUD combiner. Yet another step forward in enhancing the pilot’s situational awareness.

For now, the HUD is becoming the standard rather than an option as its benefits are increasingly appreciated in both the commercial and safety domains. What began merely as a means of improved targeting has resulted in one of the most significant changes to flight deck design in the modern age and it is a path of advancement that is set to continue. To see what comes next, ‘Heads Up’!


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