"We have eliminated the need for a head-up display. Instead, the helmet connects the pilot to the airplane," says Jon Beesley, F-35 chief test pilot. "We've taken pieces that are essential for combat operations, such as targeting information, crucial flight measurements, and night vision capability, and merged them into the helmet to give the pilots more complete situational awareness."
The helmet-mounted display system, or HMDS, displays head-steerable symbology, meaning the pilot's line of sight dictates the content that appears on the visor. As soon as, or even before, a pilot sees another aircraft in the distance, the system projects a marker on the visor to locate, identify, and track the aircraft. If the designated aircraft is determined to be hostile, the pilot can use the targeting info to cue weapons—without looking down at the cockpit displays and while pulling g's.
"No matter where pilots look, they have all the flight information right in front of their eyes," says Dave Perkins, lead engineer of HMDS integration. "The helmet displays airspeed, altitude, rate of climb, and the aiming information for all the weapons. The helmet even displays all the information needed should something go wrong. For example, it provides an alert and directs the pilot's attention if there's something nasty coming from the nose of the airplane while the pilot is looking somewhere else."
These HMD capabilities apply to F-35 air-to-ground missions as well. Pilots can mark new target locations by simply looking, pointing, and clicking a designator on the sidestick as they fly by. Then the aircraft's datalink system allows pilots to send the marked precise target coordinates quickly and easily to bombers and other combat aircraft in the same mission.
The F-35 helmet provides pilots a unique ability to see through their aircraft. Even though the helmet doesn't come equipped with x-ray vision, the HMD correlates images from a set of cameras, called the distributed aperture system, mounted on the outer surfaces of the jet. These cameras provide a constant 360-degree view of the aircraft's surroundings. When a pilot looks down, the image of what is below the aircraft shows up on the HMD. This feature is helpful not only in combat, but also during carrier and vertical night landings with the Navy and Marine variants, respectively.
Through a night vision camera built into the front of the helmet, the F-35 HMD visor can also display flight and targeting information on top of night vision images. "No helmet provided the combination of night vision and symbology at the same time until now," explains Beesley. "With legacy systems, pilots have to choose between the two capabilities." This combination is a huge advantage for F-35 pilots because all night vision devices limit peripheral vision. The symbols help pilots interpret more of their environment than night vision capability alone.
For the display to correlate with what direction the pilot is looking, a magnetic field in the cockpit senses the direction the helmet is pointing. A transmitter on the seat emits the field while a receiver on the helmet reads the magnetic flux as it moves in that field. "Most HMD systems require pilots to go through an alignment process before each flight," explains Beesley. "They may have to realign the system several times during a flight because the systems can drift. This magnetic tracking system makes all the corrections itself, so that we pilots never have to adjust the alignment."
Additionally, the night vision camera and a day camera right next to it ensure that the images and symbology correctly represent the direction the pilot is looking. "The helmet cameras look out at all times, take a picture of the outside scene, and relate that image to other images from the fixed camera on the glareshield to make sure the line of sight is correct," says Perkins. "If the two images are even a little bit off, the system self-corrects."
Flight display systems have evolved from the head-down displays of every fighter through Vietnam to the head-up system now used in the F-16 and many other modern fighters. However, no flight display system has achieved functioning HMD capability to the level of the F-35 helmet. In the 1980s, engineers developed the Falcon Eye system for F-16s—a project for which Beesley flew test flights. Falcon Eye was also head-steerable, with targeting and flight information symbology projected on the helmet display. However, imagery was analog with high latency, or lag times, in information transfer.
Current F-16 pilots fly with digital imagery on the Joint Helmet-Mounted Cueing System, or JHMCS, but this system provides only targeting information and not the head-up display information pilots use for flying. "Providing that content was never a design goal for JHMCS," notes Beesley. "If we tried to put all the F-35 information we need on the JHMCS, the potential latency would be too great for it to be effective."
Advances in computer technology have significantly reduced latency of the content displayed on the F-35 HMD. The HMDS currently exists in two forms. The first is a binocular system with optics that display images and symbology so that the right optic shines to the left eye and the left optic shines to the right eye. This bifurcated helmet is characterized by sharp, distinct edges down the center and around the perimeter of the visor where curved surfaces come together. While the bifurcations allow for a larger field of view, they are visible as lines to the pilot and have proved to be problematic. "The bifurcated helmet was rejected as the operational answer even though we built a half dozen so we could still fly with the HMD capability," explains Beesley.
The Generation II helmet that evolved from the bifurcated system is the HMDS that F-35 pilots around the world will fly with. It is still a binocular system with essentially the same shell as its predecessor, but instead of the bifurcated visor, the Gen II HMD has a more traditional, rounded visor that can also be flipped up and down. The visor is part of the display system on the bifurcated helmet, so pilots have to lift the whole optic system and visor off instead of just flipping the visor up to access their faces. The visor improvements make the Gen II more in line with every other current service helmet.
The prime contractor on the F-35 HMD is Vision Systems International, or VSI, of San Jose, California. VSI is a joint venture between Elbit, an Israeli company responsible for the helmet's operating system, and Rockwell Collins, an American company responsible for building the helmet itself. Lockheed Martin produces the software and imagery that the HMD displays. The F-35 Joint Program Office in Crystal City, Virginia, is responsible for defining requirements, implementing capabilities, and testing the functionality.
The Gen II helmet is made of carbon fiber and Kevlar. It was carefully designed to distribute weight without altering the pilot's center of balance. If the majority of the helmet's weight is too far forward, high g's may force the pilot's head in the direction of the turn. To keep head movement under the pilot's control, designers brought the optics to the top of the forehead of the Gen II helmet.
Every F-35 pilot will wear the same Gen II HMD. The lens of the visor is made of strong polycarbonate for impact protection. Pilots will also be able to wear one of two outer visors: a darker, sun-screening visor for a bright day or one with protection against laser eye damage.
The HMD connects two ways to the F-35 cockpit to provide power, symbols, images, oxygen, and communication to the pilot. The first connection carries the power, flight systems, and sensors to and from the HMD through a cable that runs from the middle of the back of the helmet and down the pilot's flight suit before plugging into the right side of the seat. The second connection carries the oxygen hose and the communication link to the pilot interface connector that sits to the left of the seat. Both connections automatically disconnect if the pilot ejects.
The HMD runs the same gamut of checks that all F-35 parts must run—and pass: temperature, altitude, vibration, electronics, alignment, and general specification tests. Engineers also conduct detailed safety research to ensure that the helmet will protect the pilot's head. For example, the helmet is fitted to a fully outfitted mannequin, put into a wind blast facility, and exposed to 600-knot wind blasts like those after an ejection. It is put on a simulated head equipped with sensors and dropped from eight feet onto a steel block to test impact safety. It is put through a simulated ejection to confirm it sufficiently protects the pilot. Before the HMD is cleared for operation, it must emerge from the wind blast, impact, and ejection tests unscathed.
The communications system is also carefully examined to ensure that friendly forces hear everything and enemy forces hear nothing. Noise-reduction ear cups are checked for functionality. The communications system is also tested for noise and electronic interference: an F-35 pilot must be able to understand all incoming communications, and friendly forces on the ground and in the air need to understand the F-35 pilot as well. At the same time, the electronics for the communications system is tested for surveillance susceptibility to ensure that conversations cannot be picked up by enemy forces.
Each of some 8,000 F-35 pilots worldwide will be custom-fitted to his or her own helmet. Fit is vital to both safety and symbology because the functionality of the HMD relies on precise positioning of the optics. The optics, in turn, project the symbols onto the visor directly in front of the pilot's eyes, creating two green spots. "If the helmet shifts when pulling g's, the green spots can rotate out causing the pilot to lose all the symbols," Beesley explains. "If the green dots aren't in front of the eyes, the pilot doesn't see anything from the HMD. It's called 'losing the exit pupil.' That's why the fit must be exact and tightly positioned."
The fitting process of the HMDS is as high-tech as its capabilities. First, the pilot puts on an elastic cap so his or her head can be scanned by a laser to create a computer profile. The three-dimensional image created from the scan determines whether the pilot needs a small, medium, or large shell. The image relays to a milling machine that scoops foam from the inside layer of the shell so that the pilot's head fits snugly inside.
Next, technicians position the optics using data from the head scan. Measurements of distance between the pilot's eyes, between the eyes and the visor (exit pupil distance), and from the eyes to the top of the head are all taken into account. The exit pupil distance determines how much the display can move around and still allow the pilot to see the image. The measurement from the top of the head is used to position the optics to get the optimum image size.
To fit the visor, the pilot puts on the oxygen mask so that the visor can be hand-trimmed around it. "We have to hand-trim the visor because we don't want it touching anything apart from its own fixings," says Perkins. "Otherwise it will distort the image the pilot sees." The gap between the visor and the oxygen mask should be small to allow a little bit of movement while pulling g's. However, it also needs to fit closely enough to protect the pilot's eyes from debris during ejections. "We are looking at a method to profile the face to quickly trim the visor to fit," Perkins adds, "but right now the fit is done by hand."
To test the fitting process, a group of four F-35 test pilots went to the United Kingdom and flew RAF Hawk T.Mk. 1A trainers with the displays in front of their eyes (not with the whole system). They flew at nine g's to determine whether they would lose their exit pupils. "The results were very good," Beesley says. "The pilots all had genuine success with keeping the display in the right position. We learned that the basic helmet capability and fit system is on the right track."
As the Gen II system progresses alongside the F-35, more and more pilots will learn to fly with advanced HMD capability. Beesley, a veteran pilot who has flown with HMD prototypes for more than twenty years, foresees no adaptation problems for new pilots, whether they're fresh out of flight school or they're transitioning from other fighters.
"At first we keep things easy because the helmet is new," he says of the training process. "Pilots train some with the HMD in the simulator, but it falls short of what it's like to really fly. One of the best testimonies I've heard is that, after they've flown for a while, pilots forget they're flying with the symbols on their heads."