by Orr Kelly
The F/A-18 was not the first plane to have a HUD. What was different was that, in the Hornet, the HUD became the principal flight instrument, replacing the cluster of vital dials that pilots had always relied on. As a concession to habit, Adam tucked a few of the old familiar dials down in the lower right-hand corner of the cockpit, as a kind of high-tech security blanket. He reasoned that they didn’t cost much, the space they occupied was not very useful and, in the unlikely event that all electrical power failed, they would help get the pilot home. In practice, many pilots routinely use the backup instruments, cross-checking the information supplied by the HUD.
Once Adam had decided on his principal flight instrument, he then turned to the question of what kinds of displays and controls the pilot needed and where they should be placed. Among pilots, there is a running controversy over whether bombing or dogfighting is the more demanding skill. But for the cockpit designer, ground-attack poses the most difficult challenge. Adam started with the problems faced by the green pilot making his first foray into enemy territory in a bombing attack. If he could help that pilot hit his target on the first pass and get home safely, everything else would fall into place.
The first requirement was to help the pilot find his way to and from the target. Adam put the first cathode ray tube, with its television-like screen, at the bottom of the panel, between the pilot’s legs. Then he arranged for five million square miles of jet pilot charts to be reproduced on a sixty-five-foot-long strip of 35-mm film that could be projected on the screen. This gave the pilot instant access to the maps that he would otherwise have to balance on his knees. The map was also cued into the plane’s computers, so that symbols showing navigation check points and even the location and lethal radius of antiaircraft missiles were displayed on the map.
While he was at it, Adam also put into the system 200 color pictures that can be projected onto the screen that normally displays the map. If the pilot sees a ship, he can flip through pictures until he identifies it. If he suffers a malfunction, he can call up a diagram of the system to find the trouble. Pilots consider this a good idea, but they never use it because emergency procedures in the system are not kept up to date.
Adam then added two more screens. The one on the upper right is used primarily for radar contacts and attacks. The one on the left is used primarily for other kinds of sensors, such as infrared; for selecting weapons; and for caution signals.
Each of the screens is surrounded by twenty buttons, which can be used to manipulate the symbols on the screen. By pushing one button, the pilot can see a “menu” of the choices available. Then, by pushing additional buttons, he can, for example, see a diagram of the weapons he is carrying, designate which weapons he wants to use, and select how they are to be delivered. Except for the larger number of buttons and variety of choices, the whole thing works much like an automatic teller machine.
The three displays are identical, so if one malfunctions, the pilot can use either of the others. And the information shown on the cockpit screens can also be flashed onto the HUD so the pilot does not have to focus his eyes on the instrument panel even for a moment.
At the center of the panel is a cluster of buttons that permit the pilot to change radio frequencies—which he may do as often as thirty or forty times an hour. These are the controls that used to be deep down in the cockpit or even behind the pilot. With a little training, most pilots can hit the right button without looking. As soon as he touches a button, a symbol indicating his choice appears on the screen, so if he has made a mistake, he can correct it instantly.
In a dogfight or while diving on a target, the pilot may be pulling so many Gs that lifting a finger to the control panel is difficult, even impossible. Therefore, the controls that a pilot needs to manipulate during those kinds of stressful maneuvers are all placed on either the stick, which the pilot holds with his right hand, or on the throttles, which he holds with his left. This system also has an acronym. It is called HOTAS, which stands for Hands On Throttle And Stick.
In his left hand, he holds the throttles for the two engines. He can move them together or separately. If he wants to go into afterburner, he pushes one or both throttles full forward. Under the index finger of his left hand, there is a pressure-sensitive button that operates like the “mouse” attached to a home computer. With it, the pilot can move a pointer on his radar screen to indicate which target he wants to attack.
In his right hand, the pilot holds the control stick, which is in the conventional position between his knees. The designers of the F-16 replaced the stick with a control lever on the right side of the cockpit. But the McDonnell Douglas designers decided the pilot would be freer to swivel his head and body in a dogfight if the flight control was in the center of the cockpit. Under the pilot’s right thumb on the control stick is a three-position switch that permits him to choose to use his gun or Sidewinder or Sparrow missile. If the plane is flying cross-country or attacking a surface target, and the pilot is threatened by a fighter, moving this control in any direction switches the plane from air-to-ground to air-to-air.
While the ground-attack mission provided the biggest design challenge, Adam knew that a pilot’s survival could depend on how fast he can prepare to defend himself from attack by a hostile fighter.
HOTAS Weapon System Control
Almost all the controls a pilot must manipulate to fly and fight his Hornet are clustered together on his control stick and throttles. The system is called HOTAS, for Hands On Throttle And Stick.
“If you’re in navigation or air-to-ground mode, if someone jumps you, you move that switch in any direction and the whole plane reverts to air-to-air,” Adam explains. “The computer says, ‘This guy wants air-to-air NOW!’ In less than half a second, you have it. By the time you pull your head up and start to maneuver the plane, you’ll be in full air-to-air.”
In the past, a pilot in a dogfight had to estimate the distance to his target and then use his gun sight to calculate how far in front of the other plane to aim so the bullets would arrive at the right moment.
In the F/A-18, the plane’s radar measures the distance to the other plane, and the computers do all the calculations with much more precision than the pilot could do in his own mind. The computers even take into account the speed at which a bullet travels, how long it will take to reach the target, and how far the other plane will fly in that time. When the radar is locked on to the other plane and it is within range of the weapon, whether it be the gun or one of the missiles, a strobe light on the HUD flashes SHOOT … SHOOT … SHOOT. If the pilot presses the trigger on the stick, he can’t miss.
As the pilot looks through his HUD, all the information he needs will be right before his eyes while he concentrates on keeping track of the other plane. This is an example of what he might see on his HUD: His target is at 28,000 feet, traveling at .6 Mach. A Sidewinder missile fired at the other plane is ten seconds from a hit. The F/A-18 is at 21,000 feet at 548 knots on a heading of fifteen degrees. The pilot has twenty-four seconds until his first opportunity to shoot at his next priority target.
In addition to the HUD and HOTAS and the other parts of the glass cockpit, the F/A-18 has one more innovative feature that Adam calls the most important development in more than forty years.
In the past, the instruments told a pilot where the nose of his plane was pointing, but it was difficult for him to calculate where he was actually going. If he attacks a target on the ground and then pulls up, for example, his nose will be pointing upward, but the force of gravity may still be pulling him earthward.
In the F/A-18, sensors in the plane feed information to a computer and it calculates where the plane is actually going, not just which direction the nose is pointed. The computer then generates a little blip of light on the HUD (called the “velocity vector”), and it shows the pilot where he is headed.
“If you are coming in for a landing and the velocity vector is off on the grass, I don’t care where you think you’re go
ing, the airplane is going to hit the grass,” Adam says. But it doesn’t just warn when something is going wrong. It also helps the pilot fly more accurately. He adds: “It makes it so easy for the pilot to do something. He puts the velocity vector on the part of the outside world where he wants to be, and he will go there.”
To veteran pilots, all these changes made the cockpit of the F/A-18 a very strange and even threatening environment. Would pilots have to learn to fly all over again?
To help in the design work, and also to help sell this whole new way of doing things to the fleet, the navy set up an Aircrew Systems Advisory Panel, made up of seven pilots representing the East and West Coast navies and both fighter and attack units. The pilots came to St. Louis two or three times a year for seven to ten days to try out the new cockpit arrangement in a simulator.
On one visit, they would concentrate, for example, on navigation problems and leave Adam with a list of suggestions for changes. On the next visit, they would use the improved navigation system on a simulated mission, concentrating this time on attacking a surface target.
“It was heartening to see pilots step out of cockpits built in the 1950s with all round dials, step into that simulator with no round dials, and consistently fly better,” Adam says. When those pilots said, “this is the way to go,” a great deal of the opposition to the new cockpit faded away.
Since then, the F/A-18’s glass cockpit has become the standard for combat aircraft in the Western world. Similar cockpits have been installed in late models of the Air Force F-15 and F-16, the Israeli Lavi, the European Fighter Aircraft, and the Swedish Grippen. The Soviet MiG-29 Fulcrum, a plane similar to the F/A-18, has a HUD and radar display, but the rest of the instrument panel continues to be filled with the old round dials—a throwback to the technology of the 1960s.
The new cockpit would of course be impossible without the compact high-speed computers that were just becoming available when the F/A-18 was designed. In effect, the two mission computers and the sensors built into the plane to monitor its performance take the place of the eyes and ears and fingers of a second crew member.
The two Control Data Corporation mission computers weigh forty-two pounds each, and together they take up about one and a quarter cubic feet of space. Although they are physically identical, the two computers have different software. One is programmed to handle navigation and other tasks involved in flying the plane. The other is dedicated to aiming and shooting the gun, firing missiles, and dropping bombs. Each, however, has a small portion of its memory devoted to the tasks assigned to the other. If one computer goes out, the pilot will still have enough computer power left to fly and fight.
In addition to the two mission computers, there are a dozen other computers connected to sensors, and they constantly feed information into the mission computers. The air data computer, for example, measures the outside air pressure and temperature and helps calculate the plane’s altitude, airspeed, and Mach number.
While much of the work of the computers is involved in gathering information and delivering it to the pilot in a useful way, the mission computers fulfill an equally important function by transmitting the pilot’s instructions to the airplane.
In early planes, and for many years afterward, the pilot guided the plane by moving his control stick and rudder pedals. As he did so, cables running through the fuselage and out through the wings moved the control surfaces. By the time of World War II, that system had been refined so that the control cables activated little electrical motors or hydraulic systems that did the actual work of moving the ailerons and the tail surfaces.
With the fighter planes designed in the late 1960s and early 1970s, a new concept was introduced. Instead of cables, these planes have electrical wires. In this new “fly by wire” system, the movement of the controls by the pilot sends messages through these wires to the wings and tail.
In the F/A-18, the designers took this concept one step further. Instead of sending his instructions directly to the control surfaces, the electrical signals generated when the pilot moves his controls go to the flight control computers. The computers then decide what needs to be done. It is like having two very smart genies with a flock of helper genies responding instantly to the pilot’s every wish.
The effect of this system is visible from the deck of a carrier as an F/A-18 comes in to land. The approach to a carrier is always rocky because the movement of the ship creates a burble of turbulent air in its wake. Even though the pilot of an F/A-18 is making only slight corrections with his controls, the plane is constantly adjusting itself to carry out his instructions. Adm. “Mike” Michaelis recalls the early carrier landing tests on the U.S.S. America:
“I went out to watch the very first landings. The first thing I noticed was how neatly the plane flew through the burble. All those controls are working. The plane is working like hell. The control surfaces do a St. Vitus’s dance. It made me wish I was a kid again.”
The computers are smart enough, in fact, that they can control the takeoff and landing without help from the pilot. When an F/A-18 is launched by the catapult, it is routine for the pilot to keep his hands and feet off the controls. Everything happens so fast that it is safer to let the computers do all the work until the plane is safely airborne. The F/A-18 also has an automatic landing system that will bring the plane right down onto the deck with the pilot riding along as a passenger. When the system was demonstrated for a skeptical navy in January 1983, Hornets made sixty-three perfect landings aboard the U.S.S. Eisenhower. Even when the burble was most severe, the planes strayed from the correct glide path by less than a foot.
Almost everyone was nervous, when the F/A-18 was being designed, about putting so much trust in the plane’s computers. Even one little error in the software might send a plane tumbling out of control. To avoid such errors, McDonnell Douglas set up an elaborate system for testing and proving all the software, including lengthy tests in which simulators ran through every possible flight maneuver to make sure that nothing went wrong. The testing was so intensive, in fact, that it consumed half of the entire time devoted to developing the software. And every time the software is changed, similar tests are run to make sure that no new bugs have found their way into the system.
Although the designers were comfortable with the reliability of their control system, the navy insisted that the plane also needed a backup mechanical control so that, if everything else failed, the pilot could at least return close to the carrier. The backup system helps pilots to feel a little more secure when they are flying out over the ocean hundreds of miles from any landing spot. But the designers are still not sure it is worth the cost. In a technical report to a meeting of avionics experts in 1984, three McDonnell Douglas engineers reported that there had been only two minor software errors detected in flight in five and a half years of flying and added:
“The F/A-18 experience shows that even the simplest backup modes impose major penalties in terms of complexity, weight and cost. … If we in the aircraft industry do our job properly, there should be minimal requirement for backup control system modes, which generally increase cost and complicate the flight control system design.”
With all the testing they did in the simulator, the engineers could not have foreseen the kind of severe test their computerized flight control system would receive in actual service. That test came in the skies over Florida on 10 November 1988.
Lt. Tom Chapin and a squadron mate from VFA-132 took off from Cecil Field for a bombing attack at the nearby Pine Castle range. Following close behind the other pilot, Chapin came in low, popped up to 3,000 feet, rolled over and down, dropped his bomb, and made a hard turn—a jink—to avoid antiaircraft fire.
He was less than 500 feet off the ground, going about 500 knots. He was turning hard, pulling five and a half Gs with his wings nearly perpendicular to the ground. Suddenly, the plane rolled violently to the left.
“I thought I was in somebody’s jet wash,” Chapin says. “The
re was a loss of lift on one wing, as though it had gone through low pressure air. I rolled upside down for an instant. I tried to counter the roll, not knowing why it had happened.” Chapin rolled the plane right side up and then did another jink, pulling about four Gs. But something didn’t feel right. In the F/A-18, the pilot sits high up in his bubble canopy, close to the nose of the plane. To see his wings or tail, he has to swivel around as much as he can in his seat and look back over his shoulder. Chapin craned for a look back at his plane and was startled to see a gap on the left wing where a big chunk of his leading edge flap—later estimated to be twelve to fifteen inches wide and nine feet long—had torn off.
Chapin kept up his speed as he climbed to 10,000 feet—high enough to permit him to eject safely if the plane fell off into a spin. Then he gradually eased back to about 300 knots and began to worry about landing. At Cecil, the traffic controllers cleared the area and sent crash trucks out to the runway. The natural inclination when something goes wrong is to slow down. But, with so much of his wing missing, Chapin wasn’t sure how the plane would handle as he cut back on air speed. He decided to come in hot, landing at 160 knots—185 miles an hour, or about sixty-five miles an hour faster than a normal landing. Despite the high speed and damage to the plane, the landing was uneventful.
When Chapin climbed down from the cockpit and examined his plane, he found that it had not only lost a big chunk of its leading edge flap, but the pieces of metal had battered the vertical and horizontal tails and the fuselage as they tore off.
What caused the damage to the plane was under investigation as this was written. But it was clear to Chapin and other members of his squadron that if he had been in any other type of plane he would have died that day at Pine Castle. They agreed that it was the plane’s computers that had brought him home. When he told the computers he wanted to roll right side up, they instantly compensated for the damage to the plane and used other control surfaces to carry out his commands. And when he came in for a landing the computers got him down safely.