HMDs

A helmet-mounted display (HMD) is a device used in some modern aircraft, especially combat aircraft. HMDs project information similar to that of head-up displays (HUD) on an aircrew’s visor or reticle, thereby allowing him to obtain situational awareness and/or cue weapons systems to the direction his head is pointing. Applications which allow cuing of weapon systems are referred to as helmet-mounted sight and display (HMSD) or helmet-mounted sights (HMS).

Requirement[edit]

Aviation HMD designs serve these purposes:

• using the head angle as a pointer to direct air-to-air and air-to-ground weapons seekers or other sensors (e.g., radar, FLIR) to a target merely by pointing his head at the target and actuating a switch via HOTAS controls. In close combat prior to HMDs, the pilot had to align the aircraft to shoot at a target. HMDs allow the pilot to simply point his head at a target, designate it to weapon and shoot.

• displaying targeting and aircraft performance information (such as airspeed, altitude, target range, weapon seeker status, "g", etc.) to the pilot while "heads-up", eliminating the need to look inside the flightdeck.

• displaying sensor video for the purpose of:
  • verification that the chosen sensor has been cued to the right target or location without requiring the pilot to look inside the flightdeck
  • viewing outside terrain using sensor video in degraded visual conditions.

HMD systems, combined with High Off-Boresight (HOBS) weapons, results in the ability for aircrew to attack and destroy nearly any target seen by the pilot. These systems allow targets to be designated with minimal aircraft maneuvering, minimizing the time spent in the threat environment, and allowing greater lethality, survivability, and pilot situational awareness.

History[edit]

The first aircraft with simple HMD devices appeared for experimental purpose in the mid-1970s to aid in targeting heat seeking missiles. These rudimentary devices were better described as Helmet-Mounted Sights. Mirage F1AZ of the SAAF (South African Air Force) used a locally developed helmet-mounted sight. This enables the pilot to make bore attacks, without having to maneuver to the optimum firing position. South Africa subsequently emerged as one of the pioneers and leaders in helmet-mounted sight technology. The SAAF was also the first air force to fly the helmet sight operationally. The US Navy's Visual Target Acquisition System (VTAS), made by Honeywell Corporation was a simple mechanical "ring and bead"–style sight fitted to the front of the pilot's helmet that was flown in the 1974–78 ACEVAL/AIMVAL on U.S. F-14 and F-15 fighters

VTAS received praise for its effectiveness in targeting off-boresight missiles, but the U.S. did not pursue fielding it except for integration into late-model Navy F-4 Phantoms equipped with theAIM-9 Sidewinder.^[1] HMDs were also introduced in helicopters during this time.

The first operational jet fighters with HMD (Mirage F1AZ) were fielded by the South African Air Force. After the South African system had been proven in combat, playing a role in downing Soviet aircraft over Angola, the Soviets embarked on a crash program to counter the technology. As a result, the MiG-29 was fielded in 1985 with an HMD and a high off-boresight weapon (R-73), giving them an advantage in close in maneuvering engagements.

Several nations responded with programs to counter the MiG-29/HMD/R-73 (and later Su-27) combination once its effectiveness was known, principally through access to former East German MiG-29s that were operated by the unified German Air Force.

The first successful HMD outside South Africa and the Soviet Union was the Israeli Air Force Elbit DASH series, fielded in conjunction with the Python 4, in the early 1990s. American and European fighter HMDs lagged behind, not becoming widely used until the late 1990s and early 2000s. The U.S.-UK-Germany responded initially with a combined ASRAAM effort. Technical difficulties led to the U.S. abandoning ASRAAM, instead funding development of the AIM-9X and the Joint Helmet-Mounted Cueing System in 1990.

Technology[edit]

While conceptually simple, implementation of aircraft HMDs is quite complex. There are many variables:^[2]

• precision - the angular error between the line-of-sight and the derived cue. The position of the helmet is what is used to point the missile, it thus must be calibrated and fit securely on the pilot's head. The line between the pilot's eye and the reticle on the visor is known as the line of sight (LOS) between the aircraft and the intended target. The user's eye must stay aligned with the sight – in other words, current HMDs cannot sense where the eye is looking, but can place a "pipper" between the eye and the target.
• latency or slew rate - how much lag there is between the helmet and the cue.
• field of regard - the angular range over which the sight can still produce a suitably accurate measurement.
• weight and balance - total helmet weight and its center of gravity, which are particularly important under high "g" maneuvers. Weight is the largest problem faced by fighter aircraft HMD designers. This is much less a concern for helicopter applications, making elaborate helicopter HMDs common.
• safety and flightdeck compatibility, including ejection seat compatibility.
• optical characteristics – calibration, sharpness, distant focus (or 'Collimation', a technique used to present the images at a distant focus, which improves the readability of images),monocular vs. binocular imagery, eye dominance, and binocular rivalry.
• durability and ability to handle day to day wear and tear.
• cost, including integration and training.
• fit and interfacing the aviator's head to the aircraft – head anthropometry and facial anatomy make helmet fitting a crucial factor in the aviator's ability to interface with the aircraft systems. Misalignment or helmet shift can cause an inaccurate picture.

Head position sensing[edit]

HMD designs must sense the elevation, azimuth and tilt of the pilot's head relative to the airframe with sufficient precision even under high "g" and during rapid head movement. Two basic methods are used in current HMD technology - optical and electromagnetic.^[2]

Optical tracking[edit]

Optical systems employ infrared emitters on the helmet (or flightdeck) infrared detectors in the flightdeck (or helmet), to measure the pilot's head position. The main limitations are restricted fields of regard and sensitivity to sunlight or other heat sources. The MiG-29/AA-11 Archer system uses this technology.^[2] The Cobra HMD as used on both the Eurofighter Typhoon^[3] and the JAS39 Gripen ^[4] both employ the optical helmet tracker developed by Denel Optronics (now part of Zeiss Optronics^[5] ).

Electromagnetic tracking[edit]

Electromagnetic sensing designs use coils (in the helmet) placed in an alternating field (generated in the flightdeck) to produce alternating electrical voltages based on the movement of the helmet in multiple axes. This technique requires precise magnetic mapping of the flightdeck to account for ferrous and conductive materials in the seat, flightdeck sills and canopy to reduce angular errors in the measurement.^[6]

Optics[edit]

Older HMDs typically employ a compact CRT embedded in the helmet, and suitable optics to display symbology on to the pilot's visor or reticle, focused at infinity. Modern HMDs have dispensed with the CRT in favor of micro-displays such as Liquid Crystal on Silicon (LCOS) or Liquid Crystal Display (LCD) along with a LED illuminator to generate the displayed image. Advanced HMDs can also project FLIR or NVG imagery. A recent improvement is the capability to display color symbols and video.

Major systems[edit]

Systems are presented in rough chronological order of initial operating capability.

Integrated Helmet And Display Sight System (IHADSS)[edit]

IHADSS

In 1984, the U.S. Army fielded the AH-64 Apache and with it the Integrated Helmet and Display Sighting System (IHADSS), a new helmet concept in which the role of the helmet was expanded to provided a visually coupled interface between the aviator and the aircraft. The Honeywell M142 IHADSS is fitted with a 40° by 30° field of view, video-with-symbology monocular display. IR emitters allow a slewable IR imaging sensor, mounted on the nose of the aircraft, to be slaved to the aviator’s head movements. The display also enables Nap-of-the-earth night navigation. IHADSS is also used on the Italian Agusta A129 Mangusta.^[7]

ZSh-5 / Shchel-3UM[edit]

The Russian designed Shchel-3UM HMD design is fit to the ZSh-5 series helmet, and is used on the MiG-29 and Su-27 in conjunction with the AA-11 Archer. The HMD/Archer combination gave the MiG-29 and Su-27 a significantly improved close combat capability and quickly became the most widely deployed HMD in the world.^[8][9]

Display and sight helmet (DASH)[edit]

The Elbit Systems DASH III was the first modern Western HMD to achieve operational service. Development of the DASH began during the mid-1980s, when the IAF issued a requirement for F-15 and F-16 aircraft. The first design entered production around 1986, and the current GEN III helmet entered production during the early to mid-1990s. The current production variant is deployed on IDF F-15, and F-16 aircraft. Additionally, it has been certified on the F/A-18 and F-5. The DASH III has been exported and integrated into various legacy aircraft, including the MiG-21.^[10] It also forms the baseline technology for the US JHMCS.^[11]

The DASH GEN III is a wholly embedded design, where the complete optical and position sensing coil package is built within the helmet (either USAF standard HGU-55/P or the Israeli standard HGU-22/P) using a spherical visor to provide a collimated image to the pilot. A quick-disconnect wire powers the display and carries video drive signals to the helmet's Cathode Ray Tube(CRT). DASH is closely integrated with the aircraft's weapon system, via a MIL-STD-1553B bus.

Joint Helmet-Mounted Cueing System (JHMCS)[edit]

JHMCS

After the U.S. withdrawal from ASRAAM, the U.S. pursued and fielded JHMCS in conjunction with the Raytheon AIM-9X, in November 2003 with the 12th and 19th Fighter Squadrons at Elmendorf AFB, Alaska. The Navy conducted RDT&E on the F/A-18C as lead platform for JHMCS, but fielded it first on the F/A-18 Super Hornet E and F aircraft in 2003. The USAF is also integrating JHMCS into its F-15E and F-16 aircraft.

JHMCS is a derivative of the DASH III and the Kaiser Agile Eye HMDs, and was developed by Vision Systems International (VSI), a joint venture company formed by Rockwell Collins and Elbit (Kaiser Electronics is now owned by Rockwell Collins). Boeing integrated the system into the F/A-18 and began low-rate initial production delivery in fiscal year 2002. JHMCS is employed in the F/A-18A++/C/D/E/F, F-15C/D/E, and F-16 Block 40/50 with a design that is 95% common to all platforms.^[12] This may also be integrated into the system of the F-22.

Unlike the DASH, which is integrated into the helmet itself, JHMCS assemblies attach to modified HGU-55/P, HGU-56/P or HGU-68/P helmets. JHMCS employs a newer, faster digital processing package, but retains the same type of electromagnetic position sensing as the DASH. The CRT package is more capable, but remains limited to monochrome presentation of calligraphic symbology. JHMCS provides support for raster scanned imagery to display FLIR/IRST pictures for night operations and provides collimated symbology and imagery to the pilot. The integration of the night-vision goggles with the JHMCS was a key requirement of the program.

When combined with the AIM-9X, an advanced short-range dogfight weapon that employs a Focal Plane Array seeker and a thrust vectoring tail control package, JHMCS allows effective target designation up to 80 degrees either side of the aircraft's nose. In March 2009, a successfully 'Lock on After Launch' firing of an ASRAAM at a target located behind the wing-line of the ‘shooter’ aircraft, was demonstrated by a Royal Australian Air Force (RAAF) F/A-18 using JHMCS.^[13]

Scorpion Helmet Mounted Cueing System (HMCS)[edit]

Scorpion HMCS mounted on a HGU-55/P helmet with a clear visor

Thales Introduced the Scorpion Helmet-Mounted Cueing System to the military aviation market in 2008. Scorpion was the winner of the Helmet Mounted Integrated Targeting (HMIT) program in 2010. Scorpion has the distinction of being the first color HMD introduced. It was developed for targeting pod, gimbaled sensor or high off-boresight missile cueing mission scenarios. Unlike most HMDs, which require custom helmets, Scorpion was designed to be installed on standard issue HGU-55/P and HGU-68/P helmets and is fully compatible with standard issue U.S. Pilot Flight Equipment without special fitting. It is also fully compatible with the AN/AVS-9 Night Vision Goggles (NVG) and Panoramic Night Vision Goggles (PNVG).

Scorpion uses a novel optical system featuring a light-guide optical element (LOE) which provides a compact color collimated image to the pilot. This allows the display to be positioned between the pilot's eyes and NVGs. The display can be positioned as the pilot wishes. Sophisticated software correction accommodates the display position, providing an accurate image to the pilot. This feature, allows the Scorpion HMCS to be installed onto a pilot's existing helmet with no special fitting. A visor can be deployed in front of the display providing protection during ejection. The visor can be clear, glare, high contrast, gradient, or laser protective. An NVG mount can be installed in place of the visor during flight. Once installed, NVGs can be placed in front of the display, thus allowing the pilot to view both the display symbols as well as the NVG image simultaneously.

Scorpion has been deployed on the U.S. A-10C and F-16 Block 30 aircraft. The first squadron to deploy into Afghanistan in early 2013 with the HMIT (Scorpion) system was the 74th Fighter Squadron.

Thales was awarded the US Army Common Helmet Mounted Display (CHMD) program in early 2013. CHMD is part of the Air Warrior program. The Thales CHMD features an upgraded LOE display with a larger field of view than the HMIT version. CHMD is designed to mount to a standard HGU-56/P Rotary Wing helmet.

Aselsan AVCI[edit]

Aselsan of Turkey is working to develop a similar system to the French TopOwl Helmet, called the AVCI Helmet Integrated Cueing System. The system will also be utilized into the T-129Turkish Attack Helicopter.^[14]

TopOwl-F(Topsight/TopNight)[edit]

The French thrust vectoring Matra MBDA MICA missile for its Rafale and late-model Mirage 2000 fighters was accompanied by the Topsight HMD by Sextant Avionique. TopSight provides a 20 degree FoV for the pilot's right eye, and calligraphic symbology generated from target and aircraft parameters. Electromagnetic position sensing is employed. The Topsight helmet uses an integral embedded design, and its contoured shape is designed to provide the pilot with a wholly unobstructed field of view.

TopNight, a Topsight derivative, is designed specifically for adverse weather and night air to ground operations, employing more complex optics to project infrared imagery overlaid with symbology. The most recent version the Topsight has been designated TopOwl-F, and is qualified on the Mirage-2000-5 Mk2 and Mig-29K.

Eurofighter Helmet-Mounted Symbology System[edit]

The Eurofighter Typhoon utilizes the Helmet-Mounted Symbology System (HMSS) developed by BAE Systems and Pilkington Optronics. It is capable of displaying both raster imagery and calligraphic symbology, with provisions for embedded NVGs. As with the DASH helmet, the system employs integrated position sensing to ensure that symbols representing outside-world entities move in line with the pilot's head movements.

Helmet-Mounted Display System[edit]

Helmet-Mounted Display System for the F-35 Lightning II Joint Strike Fighter

Vision Systems International (VSI; the Elbit Systems/Rockwell Collins joint venture) along with Helmet Integrated Systems, Ltd. developed the Helmet-Mounted Display System (HMDS) for the F-35 Joint Strike Fighter aircraft. In addition to standard HMD capabilities offered by other systems, HMDS fully utilizes the advanced avionics architecture of the F-35 and provides the pilot video with imagery in day or night conditions. Consequently, the F-35 is the first tactical fighter jet in 50 years to fly without a HUD.^[15][16] The system has suffered continued technical problems. A BAE Systems Helmet tracker using the optical tracker from Carl Zeiss Optronics South Africa has been selected as a potential replacement.^[17][18]

JedEyes TM[edit]

JedEyes TM is a new system recently introduced by Elbit Systems especially to meet Apache and other rotary wing platform requirements. The system is designed for day, night and brownout flight environments. JedEyes TM has a 70 x 40 degree FOV and 2250x1200 pixels resolution.

Cobra[edit]

Sweden’s JAS 39 Gripen fighter utilizes the Cobra HMD, developed by BAE Systems, Denel Optronics of South Africa, and Saab. It has been exported to the South African Air Force.^[19]

HUDs

HUD mounted in a PZL TS-11 Iskrajet trainer aircraft with a glass plate combiner and a convex collimating lens just below it

OVERVIEW

A typical HUD contains three primary components: a projector unit, a combiner, and a video generation computer.^[2]

The projection unit in a typical HUD is an optical collimator setup: a convex lens or concave mirror with a Cathode Ray Tube, light emitting diode, orliquid crystal display at its focus. This setup (a design that has been around since the invention of the reflector sight in 1900) produces an image where the light is parallel i.e. perceived to be at infinity.

The combiner is typically an angled flat piece of glass (a beam splitter) located directly in front of the viewer, that redirects the projected image from projector in such a way as to see the field of view and the projected infinity image at the same time. Combiners may have special coatings that reflect the monochromatic light projected onto it from the projector unit while allowing all other wavelengths of light to pass through. In some optical layouts combiners may also have a curved surface to refocus the image from the projector.

The computer provides the interface between the HUD (i.e. the projection unit) and the systems/data to be displayed and generates the imagery and symbology to be displayed by the projection unit .

Types[edit]

Other than fixed mounted HUDs, there are also head-mounted displays (HMDs). Including helmet mounted displays (both abbreviated HMD), forms of HUD that features a display element that moves with the orientation of the user's head.

Many modern fighters (such as the F/A-18, F-16 and Eurofighter) use both a HUD and HMD concurrently. The F-35 Lightning II was designed without a HUD, relying solely on the HMD, making it the first modern military fighter not to have a fixed HUD.

Generations[edit]

HUDs are split into four generations reflecting the technology used to generate the images.

• First Generation—Use a CRT to generate an image on a phosphor screen, having the disadvantage of the phosphor screen coating degrading over time. The majority of HUDs in operation today are of this type.
• Second Generation—Use a solid state light source, for example LED, which is modulated by an LCD screen to display an image. These systems do not fade or require the high voltages of first generation systems. These systems are on commercial aircraft.
• Third Generation—Use optical waveguides to produce images directly in the combiner rather than use a projection system.
• Fourth Generation—Use a scanning laser to display images and even video imagery on a clear transparent medium.

Newer micro-display imaging technologies are being introduced, including liquid crystal display (LCD), liquid crystal on silicon (LCoS), digital micro-mirrors (DMD), and organic light-emitting diode (OLED).

History[edit]

Longitudinal cross-section of a basic reflector sight (1937 German Revi C12/A).

Copilot's HUD of a C-130J

HUDs evolved from the reflector sight, a pre-World War II parallax free optical sight technology for military fighter aircraft.^[3] The first type to add rudimentary information to the reflector sight was the gyro gunsight that projected an air speed and turn rate modified reticle to aid in leading the guns to hit a moving target (deflection aircraft gun aiming). As these sights advanced, more (and more complex) information was added. HUDs soon displayed computed gunnery solutions, using aircraft information such as airspeed and angle of attack, thus greatly increasing the accuracy pilots could achieve in air to air battles. An early example of what would now be termed a head-up display was the Projector System of the British AI Mk VIII air interception radar fitted to some de Havilland Mosquito night fighters, where the radar display was projected onto the aircraft's windscreen along with the artificial horizon, allowing the pilots to perform interceptions without taking their eyes from the windscreen.^[4]

In 1955 the US Navy's Office of Naval Research and Development did some research with a mockup HUD concept unit along with a sidestick controllerin an attempt to ease the pilot's burden flying modern jet aircraft and make the instrumentation less complicated during flight. While their research was never incorporated in any aircraft of that time, the crude HUD mockup they built had all the features of today's modern HUD units.^[5]

HUD technology was next advanced by the Royal Navy in the Buccaneer, the prototype of which first flew on 30 April 1958. The aircraft's design called for an attack sight that would provide navigation and weapon release information for the low level attack mode. There was fierce competition between supporters of the new HUD design and supporters of the old electro-mechanical gunsight, with the HUD being described as a radical, even foolhardy option. The Air Arm branch of the Ministry of Defence sponsored the development of a Strike Sight. The Royal Aircraft Establishment (RAE) designed the equipment, it was built by Cintel, and the system was first integrated in 1958. The Cintel HUD business was taken over by Elliott Flight Automationand the Buccaneer HUD was manufactured and further developed, continuing up to a Mark III version with a total of 375 systems made; it was given a 'fit and forget' title by the Royal Navy and it was still in service nearly 25 years later. BAE Systems thus has a claim to the world's first Head Up Display in operational service.^[6]

In the United Kingdom, it was soon noted that pilots flying with the new gun-sights were becoming better at piloting their aircraft.^{[citation needed]} At this point, the HUD expanded its purpose beyond weapon aiming to general piloting. In the 1960s, French test-pilot Gilbert Klopfstein created the first modern HUD and a standardized system of HUD symbols so that pilots would only have to learn one system and could more easily transition between aircraft. The modern HUD used in instrument flight rules approaches to landing was developed in 1975.^[7] Klopfstein pioneered HUD technology in military fighter jets and helicopters, aiming to centralize critical flight data within the pilot's field of vision. This approach sought to increase the pilot's scan efficiency and reduce "task saturation" and information overload.

Use of HUDs then expanded beyond military aircraft. In the 1970s, the HUD was introduced to commercial aviation, and in 1988, the Oldsmobile Cutlass Supreme became the first production car with a head-up display.

Until a few years ago, the Embraer 190, Saab 2000, Boeing 727, Boeing 737-300, 400, 500 and Boeing 737 New Generation Aircraft (737-600,700,800, and 900 series) were the only commercial passenger aircraft available with HUDs. However, the technology is becoming more common with aircraft such as the Canadair RJ, Airbus A318 and several business jets featuring the displays. HUDs have become standard equipment on the Boeing 787.^[8] Furthermore, the Airbus A320, A330, A340 and A380 families are currently undergoing the certification process for a HUD.^[9] HUDs are also added to the Space Shuttle orbiter.

Design factors[edit]

There are several factors that interplay in the design of a HUD:

• Field of View – also "FOV", indicates the angle(s), vertically as well as horizontally, subtended at the pilot's eye, that the combiner displays symbology in relation to the outside view. A narrow FOV means that the view (of a runway, for example) through the combiner might include little additional information beyond the perimeters of the runway environment; whereas a wide FOV would allow a 'broader' view. For aviation applications, the major benefit of a wide FOV is that an aircraft approaching the runway in a crosswind might still have the runway in view through the combiner, even though the aircraft is pointed well away from the runway threshold; where a narrow FOV the runway would be 'off the edge' of the combiner, out of the HUD's view. Because the human eyes are separated, each eye receives a different image. The HUD image is viewable by one or both eyes, depending on technical and budget limitations in the design process. Modern expectations are that both eyes view the same image, in other words a "binocular Field of View (FOV)".
• Collimation – The projected image is collimated which makes the light rays parallel. Because the light rays are parallel the lens of the human eye focusses on infinity to get a clear image. Collimated images on the HUD combiner are perceived as existing at or near optical infinity. This means that the pilot's eyes do not need to refocus to view the outside world and the HUD display...the image appears to be "out there", overlaying the outside world.
• Eyebox – The optical collimator produces a cylinder of parallel light so the display can only be viewed while the viewer's eyes are somewhere within that cylinder, a three-dimensional area called the head motion box or eyebox. Modern HUD eyeboxes are usually about 5 lateral by 3 vertical by 6 longitudinal inches. This allows the viewer some freedom of head movement but movement too far up/down left/right will cause the display to vanish off the edge of the collimator and movement too far back will cause it to crop off around the edge (vignette). The pilot is able to view the entire display as long as one of the eyes is inside the eyebox.^[10]
• Luminance/contrast – Displays have adjustments in luminance and contrast to account for ambient lighting, which can vary widely (e.g., from the glare of bright clouds to moonless night approaches to minimally lit fields).
• Boresight – Aircraft HUD components are very accurately aligned with the aircraft's three axes – a process called boresighting – so that displayed data conforms to reality typically with an accuracy of ±7.0 milliradians. In this case the word "conform" means, "when an object is projected on the combiner and the actual object is visible, they will be aligned". This allows the display to show the pilot exactly where the artificial horizon is, as well as the aircraft's projected path with great accuracy. When Enhanced Vision is used, for example, the display of runway lights are aligned with the actual runway lights when the real lights become visible. Boresighting is done during the aircraft's building process and can also be performed in the field on many aircraft.^[7]
• Scaling – The displayed image (flight path, pitch and yaw scaling, etc.), are scaled to present to the pilot a picture that overlays the outside world in an exact 1:1 relationship. For example, objects (such as a runway threshold) that are 3 degrees below the horizon as viewed from the cockpit must appear at the −3 degree index on the HUD display.
• Compatibility – HUD components are designed to be compatible with other avionics, displays, etc.

Aircraft[edit]

On aircraft avionics systems, HUDs typically operate from dual independent redundant computer systems. They receive input directly from the sensors (pitot-static, gyroscopic, navigation, etc.) aboard the aircraft and perform their own computations rather than receiving previously computed data from the flight computers. On other aircraft (the Boeing 787, for example) the HUD guidance computation for Low Visibility Take-off (LVTO) and low visibility approach comes from the same flight guidance computer that drives the autopilot. Computers are integrated with the aircraft's systems and allow connectivity onto several different data buses such as the ARINC 429, ARINC 629, and MIL-STD-1553.^[7]

Displayed data[edit]

Typical aircraft HUDs display airspeed, altitude, a horizon line, heading, turn/bank and slip/skid indicators. These instruments are the minimum required by 14 CFR Part 91.^[11]

Other symbols and data are also available in some HUDs:

• boresight or waterline symbol—is fixed on the display and shows where the nose of the aircraft is actually pointing.
• flight path vector (FPV) or velocity vector symbol—shows where the aircraft is actually going, the sum of all forces acting on the aircraft.^[12] For example, if the aircraft is pitched up but is losing energy, then the FPV symbol will be below the horizon even though the boresight symbol is above the horizon. During approach and landing, a pilot can fly the approach by keeping the FPV symbol at the desired descent angle and touchdown point on the runway.
• acceleration indicator or energy cue—typically to the left of the FPV symbol, it is above it if the aircraft is accelerating, and below the FPV symbol if decelerating.
• angle of attack indicator—shows the wing's angle relative to the airflow, often displayed as "α".
• navigation data and symbols—for approaches and landings, the flight guidance systems can provide visual cues based on navigation aids such as an Instrument Landing System or augmented Global Positioning System such as the Wide Area Augmentation System. Typically this is a circle which fits inside the flight path vector symbol. Pilots can fly along the correct flight path by "flying to" the guidance cue.

Since being introduced on HUDs, both the FPV and acceleration symbols are becoming standard on head-down displays (HDD). The actual form of the FPV symbol on an HDD is not standardized but is usually a simple aircraft drawing, such as a circle with two short angled lines, (180 ± 30 degrees) and "wings" on the ends of the descending line. Keeping the FPV on the horizon allows the pilot to fly level turns in various angles of bank.

Military aircraft specific applications[edit]

FA-18 HUD while engaged in a mock dogfight

In addition to the generic information described above, military applications include weapons system and sensor data such as:

• target designation (TD) indicator—places a cue over an air or ground target (which is typically derived from radar or inertial navigation systemdata).
• V_c—closing velocity with target.
• Range—to target, waypoint, etc.
• Launch Acceptability Region (LAR)—displays when an air-to-air or air-to-ground weapon can be successfully launched to reach a specified target.
• weapon seeker or sensor line of sight—shows where a seeker or sensor is pointing.
• weapon status—includes type and number of weapons selected, available, arming, etc.

VTOL/STOL approaches and landings[edit]

During the 1980s, the military tested the use of HUDs in vertical take off and landings (VTOL) and short take off and landing (STOL) aircraft. A HUD format was developed at NASA Ames Research Center to provide pilots of V/STOL aircraft with complete flight guidance and control information for Category III C terminal-area flight operations. This includes a large variety of flight operations, from STOL flights on land-based runways to VTOL operations on aircraft carriers. The principal features of this display format are the integration of the flightpath and pursuit guidance information into a narrow field of view, easily assimilated by the pilot with a single glance, and the superposition of vertical and horizontal situation information. The display is a derivative of a successful design developed for conventional transport aircraft.^[13]

Civil aircraft specific applications[edit]

The cockpit of NASA's Gulfstream GV with a synthetic vision system display. The HUD combiner is in front of the pilot (with a projector mounted above it). This combiner uses a curved surface to focus the image.

The use of head-up displays allows commercial aircraft substantial flexibility in their operations. Systems have been approved which allow reduced-visibility takeoffs, and landings, as well as full Category III A landings and roll-outs.^[14][15][16] Studies have shown that the use of a HUD during landings decreases the lateral deviation from centerline in all landing conditions, although the touchdown point along the centerline is not changed.^[17]

Enhanced flight vision systems[edit]

In more advanced systems, such as the FAA-labeled Enhanced Flight Vision System,^[18] a real-world visual image can be overlaid onto the combiner. Typically an infrared camera (either single or multi-band) is installed in the nose of the aircraft to display a conformed image to the pilot. EVS Enhanced Vision System is an industry accepted term which the FAA decided not to use because "the FAA believes [it] could be confused with the system definition and operational concept found in 91.175(l) and (m)"^[18] In one EVS installation, the camera is actually installed at the top of the vertical stabilizer rather than "as close as practical to the pilots eye position". When used with a HUD however, the camera must be mounted as close as possible to the pilots eye point as the image is expected to "overlay" the real world as the pilot looks through the combiner.

"Registration," or the accurate overlay of the EVS image with the real world image, is one feature closely examined by authorities prior to approval of a HUD based EVS. This is because of the importance of the HUD matching the real world.

While the EVS display can greatly help, the FAA has only relaxed operating regulations^[19] so an aircraft with EVS can perform a CATEGORY I approach to CATEGORY II minimums. In all other cases the flight crew must comply with all "unaided" visual restrictions. (For example if the runway visibility is restricted because of fog, even though EVS may provide a clear visual image it is not appropriate (or actually legal) to maneuver the aircraft using only the EVS below 100' agl.)

Synthetic vision systems[edit]

A synthetic vision system display

HUD systems are also being designed to display a synthetic vision system (SVS) graphic image, which uses high precision navigation, attitude, altitude and terrain databases to create realistic and intuitive views of the outside world.^[20][21][22]

In the SVS head down image shown on the right, immediately visible indicators include the airspeed tape on the left, altitude tape on the right, and turn/bank/slip/skid displays at the top center. The boresight symbol (-v-) is in the center and directly below that is the flight path vector symbol (the circle with short wings and a vertical stabilizer). The horizon line is visible running across the display with a break at the center, and directly to the left are numbers at ±10 degrees with a short line at ±5 degrees (the +5 degree line is easier to see) which, along with the horizon line, show the pitch of the aircraft. Unlike this color depiction of SVS on a head down primary flight display, the SVS displayed on a HUD is monochrome – that is, typically, in shades of green.

The image indicates a wings level aircraft (i.e. the flight path vector symbol is flat relative to the horizon line and there is zero roll on the turn/bank indicator). Airspeed is 140 knots, altitude is 9450 feet, heading is 343 degrees (the number below the turn/bank indicator). Close inspection of the image shows a small purple circle which is displaced from the Flight Path Vector slightly to the lower right. This is the guidance cue coming from the Flight Guidance System. When stabilized on the approach, this purple symbol should be centered within the FPV.

The terrain is entirely computer generated from a high resolution terrain database.

In some systems, the SVS will calculate the aircraft's current flight path, or possible flight path (based on an aircraft performance model, the aircraft's current energy, and surrounding terrain) and then turn any obstructions red to alert the flight crew. Such a system might have helped the pilots of American Airlines Flight 965 prevent the fatal accident in 1995.^{[citation needed]}

On the left side of the display is an SVS-unique symbol, with the appearance of a purple, dimishing sideways ladder, and which continues on the right of the display. The two lines define a "tunnel in the sky". This symbol defines the desired trajectory of the aircraft in three dimensions. For example, if the pilot had selected an airport to the left, then this symbol would curve off to the left and down. If the pilot keeps the flight path vector alongside the trajectory symbol, the craft will fly the optimum path. This path would be based on information stored in the Flight Management System's data base and would show the FAA-approved approach for that airport.

The tunnel in the sky can also greatly assist the pilot when more precise four-dimensional flying is required, such as the decreased vertical or horizontal clearance requirements of RNP. Under such conditions the pilot is given a graphical depiction of where the aircraft should be and where it should be going rather than the pilot having to mentally integrate altitude, airspeed, heading, energy and longitude and latitude to correctly fly the aircraft.