What is a HUD?


Whether through the grainy footage from actual combat or the Hollywood hype of ‘Top Gun’, most have seen the tumbling gun-sights and symbology as a fighter rolls about the horizon in the midst of a dogfight. To the pilot, these numbers, arrows and symbols are critical flight information projected into the field of view by a ‘Head Up Display’. Today, the HUD is no longer limited to the military domain and is used to assist civilian pilots around the world every day.

Beyond the Cross-Hairs.

From the time of the first aerial combat in World War One, aircraft designers were in constant search of a better targeting system than simply placing cross-hairs on the target and squeezing the trigger. They realised that the numerous variables of manoeuvring aircraft in the heat of battle impacted directly upon the fundamental ability to shoot straight in what was a three-dimensional rifle range. Equally they appreciated that the first generation of fighting biplanes were just as likely to flick into a spin as tightening turns pushed them beyond their limits. A manoeuvre that often proved fatal in these early days.

And so, through two world wars, targeting systems evolved. World War Two saw the first gun-sights projecting light onto glass in front of the pilot and utilised a gyroscopic mechanism to compensate for such variables as ‘lead’, or the distance to aim ahead of a moving aircraft. The pilot would lay a ‘sighting ring’ on the target and rotate a knob on the throttle until the ‘ring’ lay over the wingspan of the enemy aircraft. The system then computed the range and ‘lead’ on the target, relieving the pilot from the guess-work of how much ‘lead’ was required. By the 1950s, radar ranging rather than a ‘sighting ring’ calculated the range and the advancements continued to the modern day.

However, the thrust of all the information presented to the pilot related to targeting. There was no information relating to aircraft performance such as speed or altitude and so manoeuvring the aeroplane into a firing position still called for an element of seat of the pants flying or a glance down to the instruments. This took the pilot’s focus away from the fight outside and the ground below, both of which had lethal consequences.

To provide vital flight data, ultimately, the early reflector gun-sights evolved into the high-tech HUDs our military aircraft carry today. Aside from targeting and weapons information, the pilot has a ready reference to attitude, speed, altitude, angle of attack, rate of climb or descent, G-loading and much more. Critically, this information is provided in the pilot’s field of view outside the aeroplane, where the fight is taking place. However, in flying an aircraft there doesn’t always need to be armed conflict to warrant the pilot’s eyes to be looking outside and so the HUD was destined to find a new vital role in civil aviation.

While the first fully fledged military HUD entered service in the 1960s, it would be a number of years until the technology was applied on any scale in the civil realm. In 1985 the first HUD was certified to CAT III standard, fitted to an Alaskan Airlines Boeing 727 and in 1998, the first HUD was fitted as standard equipment on the Boeing 737NG BBJ. Over this time, “Flight Dynamics” emerged as the leading light in commercial HUD technology and was ultimately absorbed into the Rockwell Collins group. Today, thousands of Rockwell Collins-Flight Dynamics Heads Up Guidance Systems (HGS) are in commercial airliners and the number is growing steadily.

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Another Hostile Environment.

All instrument-rated pilots have been there; particularly on dark, rainy nights. That ‘twilight zone’ between flying on instruments and visually acquiring the runway at the bottom of an instrument approach. The eyes hunting at a rapid rate between the ILS indication and the blurred runway lighting that seemingly morphs out of the darkness ahead. The eyes dart in and out of the aeroplane as the brain endeavours to process the flashes of information. For some, there may be a tendency to ‘duck under’ the glide-slope on becoming visual, while others may tense a little in the forearms and subconsciously pitch the nose up, ultimately landing further into the runway. Either way, the difficult transition to visual flight has not taken place smoothly as cold hard instrument data within the cockpit has been traded in on the real world environment beyond the windscreen.

Such a situation is made for the application of the Head Up Display as it offers the ability to overlay that cold hard instrument data on the real world picture. And it can do it the whole way down the approach path, so any ‘transition’ at the bottom of the approach is minimal. Yet environmental conditions don’t have to be poor for the HUD to be of service, although undoubtedly the system comes into its own in the low visibility environment.

In a daily sense, the HUD can provide the cues for rotation on take-off and flaring on landing, giving extra protection against potential tail-strikes. It can display all primary flight information as well as flight director guidance and sensitive ‘speed error tapes’ which make flying the correct airspeed a very precise and instantaneous exercise. Yet beyond normal operations, guidance is also provided for less frequent occurrences such as windshear events, unusual attitude recovery and TCAS resolutions amongst others. And all of this information is overlaid on the view outside the flight deck.

The benefits are numerous. In addition to increasing the amount of ‘heads out’ time, it reduces the repeated eyeline transition from inside to outside. The flight information is provided continuously to the pilot, rather than interpreted from instruments and then applied to visual flight-path. There is no need for a ‘last look’ at the airspeed crossing over the airfield fence; it’s right there in the field of view to touch-down. Furthermore, critical events outside such as runway incursions or birds crossing the approach path are more likely to be detected immediately as the pilot is looking outside. Most importantly, combined with the reduced work load, HUDs have impacted positively on the operation of civilian aircraft.

But how does it work?

CLICK HERE to read part two of 'What is a HUD?'

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