Heads-up!

Beyond the Cross Hairs

From the time of the first aerial combat in World War 1, 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 2 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 guesswork 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, which could have 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 infor-mation, 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, well over 3000 Flight Dynamics Head Up Guidance Systems (HGS) are in commercial airliners and the number is growing steadily. Chosen for the 737NG range, the Flight Dynamics HGS 4000 was the first to make its mark in Australia when Qantas opted to utilise the new technology in all of its Boeing 737-800s. The commercial HUD had arrived down under.

Another Hostile Environment

All instrument rated pilots have been there, particularly on a dark, rainy night – that ‘twilight zone’ between flying on instruments and visually acquiring the runway at the bottom of an instrument approach.

The eyes hunt 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 glideslope 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 takeoff and flaring on landing, giving extra protection against potential tailstrikes. 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 flightdeck.

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 flightpath. 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 touchdown.

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 workload, HUDs have impacted positively on the safe operation of civilian aircraft.

The Magic Box

At the heart of the HGS system 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, a HGS annunciator panel provides an indication of the systems statusand 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 his/her head positioned in the ‘eye box’. Achieved by correct seat 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 flightpath symbol represents the actual flightpath vector of the aeroplane and consequently, wherever the flightpath 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 flightpath symbol on the runway aim point will guide the aeroplane along a preset flightpath, normally 3 degrees.

Furthermore, certain manoeuvres such as the landing flare provide their own cues represented with circular symbology.

Flying that phase of flight simply calls for overlaying the circle of the flightpath symbol on the circle that is seemingly rising from the runway and guiding the manoeuvre.

There is so much information available, that one must guard against having your 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 new Boeing 787 will feature the system as standard for both pilots, replacing the current situation where the HGS is usually only found on the captain’s side of the flightdeck. 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. This will be 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 flightdeck design in the modern age and it is a path of advancement that is set to continue.

To see what comes next, ‘Heads Up’!