Head-Up Displays (HUD) are an increasingly popular avionics option for all grades of aircraft. The safety benefits HUDs provide for pilots makes them worth the investment.
There are a variety of different HUD options on the market and understanding the technology at hand affords maintenance shops the knowhow to address issues if they arise.
Elbit Systems’ ISTAR division designs, develops and manufactures a diverse range of high-performance HUDs and HUD upgrades for fighter jet cockpits, cargo transporters, commercial commuters and private business jets.
Nick Olivastri, senior director of business development for Elbit Systems, said the company’s HUDs feature advanced technologies offering capabilities such as high-resolution display, enhanced video capabilities and low power consumption. More than 7,500 of Elbit Systems’ HUD systems are operational today on dozens of different platforms
“Elbit Systems has a number of unique HUD products offered as part of a system solution and are readily integrated with cutting-edge sensors such as an Enhanced Vision System (EVS), Synthetic Vision System (SVS) and Combined Vision System (CVS), allowing increased situational awareness and increased safety for the pilot,” he said.
The company recently introduced the SKYLENS wearable HUD for enhanced flight vision system (EFVS) applications by commercial aircraft pilots flying anywhere in the world. Packed in a lightweight device similar to a pair of sunglasses, SKYLENS is suitable for aircraft flight operations in daylight, at night, and in bad weather. Pilots wearing SKYLENS can take off and land in low-visibility conditions and in locations that non EVS-equipped aircraft could not access previously.
Elbit Systems recently introduced a low-profile HUD for next generation fighter jets. Operational features include Wide Field of View design (FOV), Large Head Motion Box (HMB), high image brightness, enhanced symbol quality, superior video image quality and high Mean Time Between Failures (MTBF). The customizable HUD systems are available in either an overhead or panel-mounted configuration, allowing installation in advanced cockpit configurations, in conjunction with a Large Area Display (LAD) using Elbit Systems’ low-prole HUD series.
Most HUD’s have been deemed too expensive, big and heavy for general aviation aircraft. MyGoFlight introduced the SkyDisplay HUD in 2021, which weighs less than 4.5 pounds and projects a multicolored image for better visibility.
Mitch Biggs, director of sales and business development for MyGoFlight/SkyDisplay, said the display information is optimized for the phase of flight operation. SkyDisplay also allows a quick declutter feature and the ability to rotate the combining glass away from the pilot’s field of view if they deem it necessary.
Thermal imaging can be added to SkyDisplay so firefighting pilots can see the hotspots and significantly improve their drop accuracy while still getting all their flight data and record data in one place.
“Essentially, the system is designed to be plug and play and can be installed by any MRO,” Biggs said. “The HUD is designed to be installed by certified avionics installation shops requiring very little training. We use standard connectors, common cabling, and easy-to-read installation guides.”
Excelitas supplies HUD optics and/or combiners for many aircraft including F-22, F-16, Eurofighter Typhoon, Rafale, C-130, A400M, Airbus A350 and A320, Boeing 737, 777 and 787 Dreamliner, and business jets for Bombardier, Dassault and Gulfstream.
Chris Bigwood, vice president, advanced optronics, Excelitas Technologies, said the company offers the capability for holographic combiners used on many of the military platforms and the products are renowned for their image clarity and reliability.
“We have also worked closely with our customers to develop the next generation of HUD, which offers a step change in size and weight whilst maintaining imaging performance,” Bigwood said.
Excelitas is working with customers on waveguide solutions, Bigwood said. A key aspect of these waveguide solutions is the optical light engine, which is critical for maintaining image clarity, brightness and uniformity.
The use of low volume waveguide HUD technology releases valuable display real estate below the HUD, previously occupied by conventional HUD optics allowing integration of large flat screen displays.
Retrofits using waveguide technology HUDs open up opportunities for integration of these flat screen displays into legacy fighter jet cockpits.
“Alternatively, the HUD can be completely replaced by a virtual cockpit solution based on a helmet-mounted display (HMD) such as the one used on F-35. For HUD or HMD avionic displays, full-color solutions are an enhancement that are beginning to be adopted,” Bigwood said.
The Collins HGS-6000 is a digital HUD for commercial airliners that the company called a Head-up Guidance System (HGS). A HGS includes guidance features to enable manually flown CAT III approaches and low-visibility takeoffs (LVTO). This is an alternative to autoland systems that are more costly to install and maintain.
One big trend is the growth of HUDs to include Enhanced Flight Vision System (EFVS) technology. EFVS builds on HUD functionality by adding a camera sensor mounted on the nose of the aircraft. Kowash said these EVS sensors are sophisticated LRUs that contain multiple infrared and visible light cameras, complex image processing, and other functions like sensor window deicing heat. When displayed on the HUD, the EFVS video provides an augmented reality view that allows pilots to “see through” low visibility conditions like fog, snow, rain, smoke, or even just night VFR.
“As these EVS sensors are exposed on the exterior aircraft, they add a new element to HUD maintenance,” he said. “To maintain the best optical performance of these cameras, special care should be taken to avoid scratching EVS windows when cleaning. Additionally, these LRUs can be subject to external damage from hail, bird strikes or lightning strikes.”
Keep HUDs in Service
MyGoFlight works with the MROs, maintenance facilities and avionics for repair and support of the product line, Biggs said.
The SkyDisplay has an embedded watchdog feature checking the functionality of the HUD for the pilot. If the watchdog detects a problem with the video image, then the HUD will display a large Red X for the pilot to alert there is a problem.
“In most cases, simply cycling the power will solve the problem. No troubleshooting is really required,” Biggs said. “There are brightness and contrast buttons to adjust during bright and low light situations. The HUD simply projects existing flight data and sensor information onto the combiner.”
HUDs have a long life span since the basic required functionality has remained very consistent over the years. Aging electronics tend to have reduced MTBF, and a failure of mission critical equipment such as the HUD would cause the mission to be aborted.
“We specialize in HUD upgrade programs, allowing its customers the benefit of increased MTBF, Diminishing Manufacturing Sources (DMS) and prolonged operational life, Olivastri said. “Additionally with modern electronics calibration and diagnostic routines become more automatic making maintenance easier, all these factors reducing life cycle costs.”
Due to the visual nature of the system, Kowash said the pilot will quickly see if there is maintenance required. Until then, maintenance generally does not need to be concerned. Many of the models are part of the MSG-3 process with no preventive maintenance determined.
“Many operators include the HUD in their Minimum Equipment List as a system eligible for deferred maintenance,” he said. “The system does not cause AOG unless it is operationally needed for low-visibility operations. And it is optional equipment on many airframes.”
Kowash said there’s a large amount of built-in test (BIT) that is useful for troubleshooting. Information on BIT for Collins HUDs can be found in the maintenance manuals published by Collins Aerospace.
The system is typically maintained “on condition,” with no preset inspection intervals. Some systems need a desiccant inspection of the Overhead Unit. Some operators have a Category III landing approval specification that requires interval testing.
The vast majority of inside the box repairs are done at a Collins Service Center, Kowash said. In the field, Collins has an extensive global network of customer support engineers that help our customers with troubleshooting faults or other maintenance issues.
“Collins offers flight line maintenance training for maintenance personnel that will be operating, maintaining, and troubleshooting our HUD systems, Kowash said. “These courses are about 2 days and can be offered either at customer sites or at Collins facilities.”
Elbit Systems provides Intermediate Level Maintenance and Depot Level Maintenance for HUDs at its maintenance facilities. Repair of the optics assemblies requires a clean environment and the circuit cards use automated test equipment (ATE) to repair. The complete HUD requires very sophisticated test and alignment equipment.
“The Head Up Display is a very precise optical/electrical piece of equipment that requires very specific processes, procedures and equipment for proper alignment and calibration to achieve optimum accuracy, Olivastri said. “We support the end user with hands on initial training and refresher training. We assist with in-country troubleshooting and problem solving to ensure this essential piece of equipment does not cause their aircraft to be grounded.”
Elbit Systems HUDs must be certified by the applicable civil authorities to the highest design assurance level since they are utilized as the pilots’ Primary Flight Display. This means that a failure of the HUD would ultimately cancel the mission or ground the aircraft.
An improperly calibrated HUD could result in multiple issues such as blurry or shifted imagery, display brightness while display refresh issues could cause a noticeable blinking of the image.
For the optics, chips and scratches could go unnoticed. A chip or scratch within the field of view (FOV) may seem like very minor issue but could result in a profound effect on the pilot’s ability to read the display
“All these calibration and optics issues may go unnoticed for the untrained maintainer but could result in critical information unreadable by the pilot resulting in an unsafe operating environment,” Olivastri said.
Bigwood said Excelitas supports all of its HUDs throughout the lifetime of the aircraft. HUDs are sent back to its facility for inspection and test. Excelitas determines if repair is viable or whether the HUD is beyond economical repair (BER). In most instances, the company can maintain/repair the HUD and re-life the product getting it back to the user.
“As an OEM, we can re-manufacture failed or damaged elements, or in some cases, re-work components to restore their as-new condition,” he said. “All repaired HUDs are then fully tested prior to return to the platform integrator.”
If there is a suspected fault in the avionics head up display system, Bigwood said a team of specialists exchange the line replaceable unit (LRU) to maintain operation of the aircraft. The suspect LRU will then go to the airbase or airline maintenance back shop for further diagnosis. Using the repair procedures created with Excelitas expertise, the HUD is diagnosed and either repaired by fitting Excelitas supplied replacement sub-assemblies or returned to Excelitas is the OEM where the LRU can refurbished.
Keep an Eye on Combiner Damager
Bigwood said the most common issues are damage to the combiner, where if used as a step for entry and exit to the aircraft, they can get damaged. Sometimes on commercial airliners, the combiner is used as a handle for the pilot to haul themselves out of their seat. This can cause wear in the combiner mechanism.
If a combiner is scratched, Bigwood said it needs to be returned for repair or replacement, otherwise it can obscure symbology. Similarly, if the combiner mechanism is damaged, it needs to be returned to the OEM for strip down and repair.
“Combiner damage is visible by inspection and mechanism operation is simple to assess by exercising the stow and deploy operation,” he said. “The HUD projector assembly is a sealed and purged unit, which has a maintenance schedule. If there is a reduction in in image brightness of the HUD, a re-purge may be the solution or a replacement of the display source itself might be necessary.”
If you wear a ring, Kowash said to be careful working around the Combiner to avoid chipping the glass edges. When deploying the Combiner, let the arm snap into the detent for repeatable positioning.
“One unique procedure some of our systems may require when swapping LRUs is electronic alignment,” he said. “To accurately provide precision flight information to pilots, HUDs and EFVS sensors must be aligned to the outside world with tight tolerances. When replacing LRUs, it may sometimes be required to confirm or recalibrate this alignment using electronic adjustments. Collins provides detailed instructions for these procedures.”
