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A heads-up display, or HUD, is any transparent display that presents data without obstructing the user's view. Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other applications.

History Head-up displays were pioneered in fighter jets and military helicopters to minimize information overload by centralizing critical flight data within the pilot's field of vision. In the 1970s, this innovation was introduced to commercial aviation. These early HUDs featured data such as airspeed, altitude, and localizer readings, which had previously been accessible on head-down primary flight displays (PFD).Pope, Stephen. "The Future of Head-Up Display Technology." Aviation International News. Jan. 2006. 12 February 2007

Currently, a typical HUD in a commercial aircraft will display airspeed, altitude, a horizon line, a compass, turn/bank and Slip indicator#Turn Coordinator indicators. These instruments are the minimum required by 14 CFR Part 91.

In 1988, the Oldsmobile Cutlass Supreme became the first production car with a head-up display."Oldsmobiles Pace "the Race"" Oldsmobile Club of America. 2006. 12 February 2007

Types There are two types of HUD. Fixed HUDs require the user to look through a display element attached to the airframe or vehicle chassis. The system determines the image to be presented depending solely on the orientation of the vehicle. Commercial aircraft and automobiles usually incorporate a fixed HUD system. Helmet-mounted or head-mounted HUDs feature a securely-attached display element that moves with the orientation of the user's head. Such systems are often monocular, and are used in the AH-64 Apache and in some versions of the F-16 Fighting Falcon.

Display A typical HUD in civil aircraft contains three primary components: A computer, Overhead Projector Unit (OPU), and a combiner. The computer usually is located with the other avionics equipment and provides the interface between the HUD (i.e. the combiner and OPU) and the aircraft systems to be displayed. Flight data are received from the Inertial guidance system, flight management system, and other flight guidance systems, and processed into a form compatible with the Overhead Projector Unit.This is different between manufactures and also can be dependent on the technology used in the OPU. The OPU takes this data and projects them onto the combiner. The combiner is usually made of glass with a special coating that reflects the monochromatic light from the OPU while allowing all other wavelengths of light to pass through, creating a superimposition image.

Tactical military aircraft usually rely on a projection unit incorporated into the combiner.

Traditionally, the source for the projected image has been a Cathode Ray Tube (CRT), but micro-display imaging technologies are now being introduced. Currently, micro-display technologies that have been demonstrated include liquid crystal displays (LCD), liquid crystal on silicon (LCoS), digital light processing (DMD), and organic light-emitting diode (OLED). HUD systems that project information directly onto the wearer’s retina with a low-powered laser (virtual retinal display) are also in experimentation."Virtual Retinal Display (VRD) Technology." Virtual Retinal Display Technology. Naval Postgraduate School. 13 February 2007 .Lake, Matt. "How It Works: Retinal Displays Add a Second Data Layer." New York Times 26 April 2001. 13 February 2006 . (registration required)

Focus To avoid refocusing of the user’s eyes while reading a HUD, the display is Infinity focus in aircraft, while in automobiles the display is generally focused around the distance to the bumper.

Military Aircraft Applications The use of HUDs was pioneered mainly for use in military aircraft platforms. The main advantage would be to allow the pilot to keep his attention on what was going on around him and not having to look down at his instruments for flight critical information very often.

V/STOL Approaches and Landings During the 1980s, the military was testing the use of head-up displays for the use in aircraft that are capable of vertical take off and landings (VTOL) and short take off and landing (STOL). A head up display (HUD) format was developed at NASA AmesResearch Center to provide pilots of V/STOL aircraft with complete flight guidance and controlinformation for Category-IIIC terminal-area flight operations. These flight operations cover a largespectrum, from STOL operations on land-based runways to VTOL operations on small ships in highseas. The principal features of this display format are the integration of the flightpathand pursuit guidance information into a narrow field of view, easily assimilated by the pilot with asingle glance, and the superposition of vertical and horizontal situation information. The display isa derivative of a successful design developed for conventional transport aircraft.Merrick, Vernon K., Glenn G. Farris, and Andrejs A. Vangas. “A Head Up Display for Applicatoin to V/STOL Aircraft Approach and Landing.” NASA Ames Research Center 1990.

Civil Aircraft Applications 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 IIIc landings.ORDER: 8700.1 APPENDIX: 3 BULLETIN TYPE: Flight Standards Handbook Bulletin for General Aviation (HBGA) BULLETIN NUMBER: HBGA 99-16 BULLETIN TITLE: Category III Authorization for Parts 91 and 125 Operators withHead-Up Guidance Systems (HGS); LOA and Operations EFFECTIVE DATE: 8-31-99 (Document) Falcon 2000 Becomes First Business Jet Certified Category IIIA by JAA and FAA; Aviation Weeks Show News Online September 7, 1998 Design Guidance for a HUD System is contained in Draft Advisory Circular AC 25.1329-1X, "Approval of Flight Guidance Systems" dated 10/12/2004 Studies have shown that the use of a HUD duing landings decreases the lateral deviation from centerline in all landing conditions although the touchdown point along the centerline is not changed. HUD With a Velocity (Flight Path) Vector Reduces Lateral Error During Landing in Restricted Visibility; International Journal of Aviation Psychology, 2007, Vol. 17 No 1, pages 91-108

with a synthetic vision system display.

The image to the right, of a HUD in a NASA Gulfstream GV, shows several different HUD elements, including the combiner in front of the pilot. The green 'glare' in the lower right corner is a result of the backscatter of off-axis light from the OPU, as well as from reflection of the available light in the Cockpit (aviation) off the "only does green" coating on the combiner. Because the combiner has a pronounced vertical and horizontal curve to help focus the image, compensation is applied to the display symbols so they appear flat when projected onto the curved surface. When not in use, most combiners swing up and lock in a stowed position.

The Overhead Projector Unit in the Gulfstream GV image would be directly above the pilots head. In smaller aircraft the design of the OPU can present interesting spacing and placement issues, as room has to be left for the pilot not only when normally seated but during turbulence and when getting in and out of the seat.

Symbology In recent years, several new symbols have been added to the flight deck by HUD designers. A boresight symbol is fixed on the display and shows where the nose of the aircraft is actually pointing. A flight path vector (FPV) symbol shows where the aircraft is actually going, the sum of all energies acting on the aircraft. "Forces in a Climb" NASA Glenn Research Center For example, if the aircraft is Flight dynamics 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 touchdown point on the runway.

An "acceleration indicator" symbol has also been added to HUD designs. This symbol, typically to the left of the FPV symbol, will be above the symbol if the aircraft is accelerating, and below the FPV symbol if decelerating.

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 a 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. Aligning one of these angle lines on the horizon allows the pilot to easily fly a coordinated 30 degree turn while maintaining altitude.

For approach and landing guidance, the flight guidance system can provide head-up visual cues based on navigation aids such as an Instrument Landing System or augmented Global Positioning System such as the WAAS. Typically this is a circle which fits inside the flight path vector symbol. By "flying to" the guidance cue, the pilot flies the aircraft along the correct flight path. Keeping the pilot "in the loop" in this way is an economical alternative to an autolanding system, where the flight guidance system and autopilot system are given the control of the aircraft and the pilot simply provides a monitoring function.

Installation During the installation of a HUD in a commercial aircraft, the combiner is boresighted to the aircraft's centerline so that its displayed data corresponds to reality. For example, the line on the HUD used to depict the horizon must be conformal Note that in this case the word "conformal" has been taken to mean "when an object is projected on the combiner and the actual object is visible, they will be aligned." The displayed horizon line and the actual horizon for example. When Enhanced Vision is used, the display of runway lights must be alighted with the actual runway lights when the real lights become visible. to the actual horizon (at lower altitudes, due the curvature of the earth the display horizon line will be above the visual horizon at higher altitudes.)

Enhanced Flight Vision Systems, Head-up display In more advanced systems, such as the Federal Aviation Administration-labeled Enhanced Flight Vision System, DOT Docket FAA-2003-14449-45 Enhanced Flight Vision Systems. 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. In one EVS Enhanced Vision System is an industry accepted term which the FAA decided not to use because "the FAA believes would be confused with the system definition and operational concept found in 91.175(l) and (m) 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 the authorities prior to approval of a HUD based EVS. When the pilot is coming in for a landing and "sees" the runway and runway lights through the EVS display, it is really a good thing when they come out under the clouds and the real world runway is right where the camera said it was as the pilot has a very short period of time to (a) take in the reality of "what is displayed is not what is real" (b) decide that action needs to be taken (c) take action and (d) allow the airplane some time to respond. During the design of such a system, the supplier would perform a safety analysis to determine the consequences of "EFVS Image not aligned with real world at or below decision height." Using regulatory guidance (FAA Advisory Circular 25.1309-1A for example) this would be initially evaluated as a Major hazard There are typically five hazard categoriesCatastrophic - loss of life cannot be prevented;Hazardous - loss of life cannot be prevented except with exceptional skill or intervention;Major - loss of life or injury can be prevented if everything goes as planned;Minor - within the expected abilities and training of crews to handle; andNo Effect - customer satisfaction is not a concern of safety. where if it does occur the design community anticipates that the flight crew will be able to take the appropriate action. The pilot may choose to initiate a missed approach (climb immediately and then figure out what to do because altitude and speed are your friend when trying to deal with "unexpected events") or perhaps to immediately blank the HUD/EVS display (typically there is a thumb switch on the control column for exactly this circumstance) and continue the landing using what can be seen through the window.

While the EVS display can greatly help, the FAA has only "relaxed" operating regulations 14 CFR Part 91.175 change 281 "Takeoff and Landing under IFR" where an aircraft with EVS operating 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, SVS HUD systems are also being designed to utilize a synthetic vision (SVS), which use terrain databases to create a realistic and intuitive view of the outside world. Part 23 Synthetic Vision Approval Approach; FAA Synthetic Vision Workshop, Lowell Foster, February 14, 2006. For additional information see Evaluation of Alternate Concepts for Synthetic Vision Flight Displays with Weather-Penetrating Sensor Image Inserts During Simulated Landing Approaches, NASA/TP-2003-212643 No More Flying Blind, NASA

In SVS image to 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 (-\/-) 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 going across the display with a break at the center, and directly to the left are the 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.

The aircraft in the image is wings level (i.e. the flight path vector symbol is 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 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 could have prevented the crash of American Airlines Flight 965 in 1995.

On the left side of the display is an SVS-unique symbol, which looks like a purple, dimishing sideways ladder, and which continues on the right of the display. The two together 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. The pilot keeps the flight path vector alongside the trajectory symbol and so 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 Required Navigation Performance. 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."A Review of Pathway-In-The Sky Displays, FAA Presentation to 2003 Digital Avionics System Conference Synthetic Vision Workshop, Dick Newman, 15 February 2006

Automotive applications Head-up displays are becoming increasingly available in production cars, and usually offer speedometer, tachometer, and Global Positioning System#Navigation displays. BMW, Nissan, Lexus, Citroën and General Motors currently offer some form of HUD system. Motorcycle helmet HUDs are also commercially available.Mike, Werner. "Test Driving the SportVue Motorcycle HUD." Motorcycles in the Fast Lane. 8 November 2005. 14 February 2007

showing a speed of 47 mph

Add-on HUD systems also exist, projecting the display onto a glass combiner mounted on the windshield. These systems have been marketed to police agencies for use with in-vehicle computers.

Experimental uses HUDs have been proposed or are being experimentally developed for a number of other applications. In the military, a HUD can be used to overlay tactical information such as the output of a laser rangefinder or squadmate locations to infantrymen. A surgical HUD could also display overlaid x-rays or other medical data or imagery onto the surgeon's view of a patient undergoing surgery, allowing the surgical team to "see" structures that are normally invisible. A prototype HUD has also been developed that displays information on the inside of a swimmer's goggles.Clothier, Julie. "Smart Goggles Easy on the Eyes." CNN.Com. 27 June 2005. CNN. 22 February 2007

Further reading

Notes

External links

Commercially Available HUDs

Commercially Available EVS Cameras

A heads-up display, or HUD, is any transparent display that presents data without obstructing the user's view. Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other applications.

History Head-up displays were pioneered in fighter jets and military helicopters to minimize information overload by centralizing critical flight data within the pilot's field of vision. In the 1970s, this innovation was introduced to commercial aviation. These early HUDs featured data such as airspeed, altitude, and localizer readings, which had previously been accessible on head-down primary flight displays (PFD).Pope, Stephen. "The Future of Head-Up Display Technology." Aviation International News. Jan. 2006. 12 February 2007

Currently, a typical HUD in a commercial aircraft will display airspeed, altitude, a horizon line, a compass, turn/bank and Slip indicator#Turn Coordinator indicators. These instruments are the minimum required by 14 CFR Part 91.

In 1988, the Oldsmobile Cutlass Supreme became the first production car with a head-up display."Oldsmobiles Pace "the Race"" Oldsmobile Club of America. 2006. 12 February 2007

Types There are two types of HUD. Fixed HUDs require the user to look through a display element attached to the airframe or vehicle chassis. The system determines the image to be presented depending solely on the orientation of the vehicle. Commercial aircraft and automobiles usually incorporate a fixed HUD system. Helmet-mounted or head-mounted HUDs feature a securely-attached display element that moves with the orientation of the user's head. Such systems are often monocular, and are used in the AH-64 Apache and in some versions of the F-16 Fighting Falcon.

Display A typical HUD in civil aircraft contains three primary components: A computer, Overhead Projector Unit (OPU), and a combiner. The computer usually is located with the other avionics equipment and provides the interface between the HUD (i.e. the combiner and OPU) and the aircraft systems to be displayed. Flight data are received from the Inertial guidance system, flight management system, and other flight guidance systems, and processed into a form compatible with the Overhead Projector Unit.This is different between manufactures and also can be dependent on the technology used in the OPU. The OPU takes this data and projects them onto the combiner. The combiner is usually made of glass with a special coating that reflects the monochromatic light from the OPU while allowing all other wavelengths of light to pass through, creating a superimposition image.

Tactical military aircraft usually rely on a projection unit incorporated into the combiner.

Traditionally, the source for the projected image has been a Cathode Ray Tube (CRT), but micro-display imaging technologies are now being introduced. Currently, micro-display technologies that have been demonstrated include liquid crystal displays (LCD), liquid crystal on silicon (LCoS), digital light processing (DMD), and organic light-emitting diode (OLED). HUD systems that project information directly onto the wearer’s retina with a low-powered laser (virtual retinal display) are also in experimentation."Virtual Retinal Display (VRD) Technology." Virtual Retinal Display Technology. Naval Postgraduate School. 13 February 2007 .Lake, Matt. "How It Works: Retinal Displays Add a Second Data Layer." New York Times 26 April 2001. 13 February 2006 . (registration required)

Focus To avoid refocusing of the user’s eyes while reading a HUD, the display is Infinity focus in aircraft, while in automobiles the display is generally focused around the distance to the bumper.

Military Aircraft Applications The use of HUDs was pioneered mainly for use in military aircraft platforms. The main advantage would be to allow the pilot to keep his attention on what was going on around him and not having to look down at his instruments for flight critical information very often.

V/STOL Approaches and Landings During the 1980s, the military was testing the use of head-up displays for the use in aircraft that are capable of vertical take off and landings (VTOL) and short take off and landing (STOL). A head up display (HUD) format was developed at NASA AmesResearch Center to provide pilots of V/STOL aircraft with complete flight guidance and controlinformation for Category-IIIC terminal-area flight operations. These flight operations cover a largespectrum, from STOL operations on land-based runways to VTOL operations on small ships in highseas. The principal features of this display format are the integration of the flightpathand pursuit guidance information into a narrow field of view, easily assimilated by the pilot with asingle glance, and the superposition of vertical and horizontal situation information. The display isa derivative of a successful design developed for conventional transport aircraft.Merrick, Vernon K., Glenn G. Farris, and Andrejs A. Vangas. “A Head Up Display for Applicatoin to V/STOL Aircraft Approach and Landing.” NASA Ames Research Center 1990.

Civil Aircraft Applications 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 IIIc landings.ORDER: 8700.1 APPENDIX: 3 BULLETIN TYPE: Flight Standards Handbook Bulletin for General Aviation (HBGA) BULLETIN NUMBER: HBGA 99-16 BULLETIN TITLE: Category III Authorization for Parts 91 and 125 Operators withHead-Up Guidance Systems (HGS); LOA and Operations EFFECTIVE DATE: 8-31-99 (Document) Falcon 2000 Becomes First Business Jet Certified Category IIIA by JAA and FAA; Aviation Weeks Show News Online September 7, 1998 Design Guidance for a HUD System is contained in Draft Advisory Circular AC 25.1329-1X, "Approval of Flight Guidance Systems" dated 10/12/2004 Studies have shown that the use of a HUD duing landings decreases the lateral deviation from centerline in all landing conditions although the touchdown point along the centerline is not changed. HUD With a Velocity (Flight Path) Vector Reduces Lateral Error During Landing in Restricted Visibility; International Journal of Aviation Psychology, 2007, Vol. 17 No 1, pages 91-108

with a synthetic vision system display.

The image to the right, of a HUD in a NASA Gulfstream GV, shows several different HUD elements, including the combiner in front of the pilot. The green 'glare' in the lower right corner is a result of the backscatter of off-axis light from the OPU, as well as from reflection of the available light in the Cockpit (aviation) off the "only does green" coating on the combiner. Because the combiner has a pronounced vertical and horizontal curve to help focus the image, compensation is applied to the display symbols so they appear flat when projected onto the curved surface. When not in use, most combiners swing up and lock in a stowed position.

The Overhead Projector Unit in the Gulfstream GV image would be directly above the pilots head. In smaller aircraft the design of the OPU can present interesting spacing and placement issues, as room has to be left for the pilot not only when normally seated but during turbulence and when getting in and out of the seat.

Symbology In recent years, several new symbols have been added to the flight deck by HUD designers. A boresight symbol is fixed on the display and shows where the nose of the aircraft is actually pointing. A flight path vector (FPV) symbol shows where the aircraft is actually going, the sum of all energies acting on the aircraft. "Forces in a Climb" NASA Glenn Research Center For example, if the aircraft is Flight dynamics 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 touchdown point on the runway.

An "acceleration indicator" symbol has also been added to HUD designs. This symbol, typically to the left of the FPV symbol, will be above the symbol if the aircraft is accelerating, and below the FPV symbol if decelerating.

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 a 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. Aligning one of these angle lines on the horizon allows the pilot to easily fly a coordinated 30 degree turn while maintaining altitude.

For approach and landing guidance, the flight guidance system can provide head-up visual cues based on navigation aids such as an Instrument Landing System or augmented Global Positioning System such as the WAAS. Typically this is a circle which fits inside the flight path vector symbol. By "flying to" the guidance cue, the pilot flies the aircraft along the correct flight path. Keeping the pilot "in the loop" in this way is an economical alternative to an autolanding system, where the flight guidance system and autopilot system are given the control of the aircraft and the pilot simply provides a monitoring function.

Installation During the installation of a HUD in a commercial aircraft, the combiner is boresighted to the aircraft's centerline so that its displayed data corresponds to reality. For example, the line on the HUD used to depict the horizon must be conformal Note that in this case the word "conformal" has been taken to mean "when an object is projected on the combiner and the actual object is visible, they will be aligned." The displayed horizon line and the actual horizon for example. When Enhanced Vision is used, the display of runway lights must be alighted with the actual runway lights when the real lights become visible. to the actual horizon (at lower altitudes, due the curvature of the earth the display horizon line will be above the visual horizon at higher altitudes.)

Enhanced Flight Vision Systems, Head-up display In more advanced systems, such as the Federal Aviation Administration-labeled Enhanced Flight Vision System, DOT Docket FAA-2003-14449-45 Enhanced Flight Vision Systems. 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. In one EVS Enhanced Vision System is an industry accepted term which the FAA decided not to use because "the FAA believes would be confused with the system definition and operational concept found in 91.175(l) and (m) 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 the authorities prior to approval of a HUD based EVS. When the pilot is coming in for a landing and "sees" the runway and runway lights through the EVS display, it is really a good thing when they come out under the clouds and the real world runway is right where the camera said it was as the pilot has a very short period of time to (a) take in the reality of "what is displayed is not what is real" (b) decide that action needs to be taken (c) take action and (d) allow the airplane some time to respond. During the design of such a system, the supplier would perform a safety analysis to determine the consequences of "EFVS Image not aligned with real world at or below decision height." Using regulatory guidance (FAA Advisory Circular 25.1309-1A for example) this would be initially evaluated as a Major hazard There are typically five hazard categoriesCatastrophic - loss of life cannot be prevented;Hazardous - loss of life cannot be prevented except with exceptional skill or intervention;Major - loss of life or injury can be prevented if everything goes as planned;Minor - within the expected abilities and training of crews to handle; andNo Effect - customer satisfaction is not a concern of safety. where if it does occur the design community anticipates that the flight crew will be able to take the appropriate action. The pilot may choose to initiate a missed approach (climb immediately and then figure out what to do because altitude and speed are your friend when trying to deal with "unexpected events") or perhaps to immediately blank the HUD/EVS display (typically there is a thumb switch on the control column for exactly this circumstance) and continue the landing using what can be seen through the window.

While the EVS display can greatly help, the FAA has only "relaxed" operating regulations 14 CFR Part 91.175 change 281 "Takeoff and Landing under IFR" where an aircraft with EVS operating 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, SVS HUD systems are also being designed to utilize a synthetic vision (SVS), which use terrain databases to create a realistic and intuitive view of the outside world. Part 23 Synthetic Vision Approval Approach; FAA Synthetic Vision Workshop, Lowell Foster, February 14, 2006. For additional information see Evaluation of Alternate Concepts for Synthetic Vision Flight Displays with Weather-Penetrating Sensor Image Inserts During Simulated Landing Approaches, NASA/TP-2003-212643 No More Flying Blind, NASA

In SVS image to 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 (-\/-) 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 going across the display with a break at the center, and directly to the left are the 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.

The aircraft in the image is wings level (i.e. the flight path vector symbol is 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 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 could have prevented the crash of American Airlines Flight 965 in 1995.

On the left side of the display is an SVS-unique symbol, which looks like a purple, dimishing sideways ladder, and which continues on the right of the display. The two together 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. The pilot keeps the flight path vector alongside the trajectory symbol and so 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 Required Navigation Performance. 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."A Review of Pathway-In-The Sky Displays, FAA Presentation to 2003 Digital Avionics System Conference Synthetic Vision Workshop, Dick Newman, 15 February 2006

Automotive applications Head-up displays are becoming increasingly available in production cars, and usually offer speedometer, tachometer, and Global Positioning System#Navigation displays. BMW, Nissan, Lexus, Citroën and General Motors currently offer some form of HUD system. Motorcycle helmet HUDs are also commercially available.Mike, Werner. "Test Driving the SportVue Motorcycle HUD." Motorcycles in the Fast Lane. 8 November 2005. 14 February 2007

showing a speed of 47 mph

Add-on HUD systems also exist, projecting the display onto a glass combiner mounted on the windshield. These systems have been marketed to police agencies for use with in-vehicle computers.

Experimental uses HUDs have been proposed or are being experimentally developed for a number of other applications. In the military, a HUD can be used to overlay tactical information such as the output of a laser rangefinder or squadmate locations to infantrymen. A surgical HUD could also display overlaid x-rays or other medical data or imagery onto the surgeon's view of a patient undergoing surgery, allowing the surgical team to "see" structures that are normally invisible. A prototype HUD has also been developed that displays information on the inside of a swimmer's goggles.Clothier, Julie. "Smart Goggles Easy on the Eyes." CNN.Com. 27 June 2005. CNN. 22 February 2007

Further reading

Notes

External links

Commercially Available HUDs

Commercially Available EVS Cameras



SportVue ® Heads Up Display for Sports
Heads Up Display for Sports, Motorcycles, Racing, Skydiving

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Heads Up Display



 
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