What scientists haven’t solved and hot-shot pilots won’t talk about.

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One of life's humbling moments comes for the fighter pilot racking the simulator around at 30,000 feet with the bad guys closing in. Even though he has a thousand dogfighting flight hours and is attached to the ground like a slab of granite, he  feels airsick. Why does it hit him in the simulator but not in his airplane?

Every time he takes to the sky, the pilot takes in hundreds of physical cues, some very subtle, that help orient him within the cockpit. Even the best simulators can’t reproduce something as seemingly inconsequential as the odor of hydraulic fluid. For a veteran military pilot, such omissions might cause disorientation, which in turn can bring on waves of nausea, sweats, irregular breathing, vision disturbances, and other symptoms of motion sickness.

Ron Kruk, a motion sickness expert for CAE, a Canadian company that manufactures simulators, says, “A simulator can’t replicate the real world completely, like giving the acceleration of a real aircraft. Parts of one’s nervous system are fooled by that and other parts are not. So you get sensory mismatches. Simulator sickness is a special case of motion sickness. Maneuvers that get you sick in a simulator might not get you sick in the aircraft.”

Scientists don’t fully understand why a pilot can feel fine in a simulator one day but turn green the next, but they do know that disorientation often originates in the inner ear, where tiny hairs, or otoliths, can indicate you’re stationary when your eyes say you’re not. So even though the pilot feels his feet on the floor and his inner ear says everything is straight and level, the simulator’s electronic scene tells his eyeballs that he’s in a steep bank or, worse, a roll.

To get reoriented, a pilot might have to step back for a moment and sit quietly. But, as Captain Angus Rupert, a flight surgeon at the Naval Aviation Aeromedical Research Laboratory in Pensacola, Florida, points out, “These are people who do not tend to say ‘Hey, I have a minor problem here.’ They don’t even say anything when they have a major problem.”

The military itself is unwilling to say it has a “problem” with motion sickness. The official response from Colonel James Little, who directs the motion sickness training program for the Air Force: They weed out the most sensitive recruits early on so it rarely becomes an issue. But the military and NASA currently fund the bulk of motion sickness studies, including investigations into simulator sickness, so they’re obviously looking for answers.

People like Kruk contend that airline and military transport simulators have reached a stage of fidelity that eases adaptation. But even the less susceptible pilots can face discomforting electronic aberrations, like jitter, which is analogous to the flickering of a TV screen. More disconcerting is latency, in which too many milliseconds pass between the time the pilot moves the stick or ailerons and the time the scene responds. Another problem is off-axis viewing, in which the student flying is fine and the instructor to the side is uncomfortable. High fidelity or not, simulator software and hardware have to be in tune.

Dennis McBride, a physician and former executive director of the Institute of Simulation and Training in Orlando, Florida, believes that superhigh fidelity in the newest simulators actually causes the trouble. The higher the fidelity, the more likely experienced pilots in particular will run into problems. No simulator can offer a perfect reproduction of all the forces at work in a modern jet in flight.

McBride recalls one dilemma the Navy faced with the A-12 stealth attack airplane before the program was canceled. Its concept of operations, he says, called for intense shipboard simulator rehearsal just before a mission. Since adaptation to minute changes in cues is a factor in motion sickness, McBride says, a pilot could adjust to the simulator and then get disoriented in the real airplane, where his performance could deteriorate.

Exactly how much the disconnect between a simulator run and a real flight can affect performance is unclear. Several researchers believe not enough work is being done on this crucial issue. While no one could cite an instance of a pilot crashing a jet or even getting in a car accident after an extended stay in the simulator seat, there’s plenty of evidence to suggest such a mishap is possible.

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