 During the period of recorded history, there is little evidence to indicate that man has changed in any major respect. Among living creatures he is not the strongest or the fastest, but rather a unique compromise. He possesses a central nervous system, but not much different from some reptilian antecedent. He has a circulating system with warm blood, but not much better than that of any other mammal. He has a digestive system that gives him a great foraging range, but even this is equal by that of a rat. If we look at man as if he were a machine, we would say that he is made of three interrelated parts. First, he is a physical structure, flexible, light, strong, and functional. Second, he is a biochemical laboratory, complete with the means for converting food into fuel with filtering systems, heat exchangers, and the ability to replace worn-out parts. Third, he is an emotional, psychological computing device of incredible complexity with systems for controlling movement, acquiring and integrating knowledge, and channeling desire. He has systems that can plan, anticipate, and aspire. Modern man's aspirations carry him into new and hostile environments, although technically better prepared he is the same as he's always been. His mechanical, chemical, and psychological abilities are severely limited. When the demands of the situation exceed these limits, man breaks down the result an accident. When we look at man as a physical structure, we see that he has a remarkably strong, light frame, that he is a mechanical system of levers and balances, flexible, resilient. But in spite of his abilities, he has many built-in limits. He can only reach so far. He can train himself to do involved actions requiring coordination and timing, but he cannot perform maneuvers which are too refined. He can only lift. He has a precarious sense of balance. With his eyes closed, he has no sensation of acceleration under the rate of 2 degrees per second per second. When his sense of balance is contradicted by other sensory systems such as by his vision during flight, simple things confuse him. He becomes disoriented. For example, as a pilot changes channels on his radio, he changes hands on the stick and unwittingly pushes it aside. When he straightens up, he is level relative to his plane, but confused. In almost any sensory area, some other animal exceeds him. Man is sensitive to sounds in a very limited range of both frequencies and intensities. Man's noble companion and best friend hears much higher frequencies above 20,000 cycles per second. Sound and mechanical vibration will induce fatigue, reduce working efficiency and produce irritability. Noise over 95 decibels can cause men serious bodily damage and deafness. Preachers surpass man in tactile sensitivity. His sense of touch is severely limited. Shapes are too nearly alike, a limb apart, even when it's important to him. Man protects himself against his environment by designing easily handled mechanical equipment to correct for his limitation. But if it gets cold, his tactile sensitivities are dulled and his hands may become totally useless. As usual, he has a way of coping with the problem. But with gloves, he can hardly rely on touch at all. Man's vision is severely limited. Many birds can see better than he can, can only see so far. The size and shape of objects must be easily identifiable to be interpreted. He has problems with color and low levels of illumination. The world is full of optical illusions. He brings his own emotional attitudes to what he sees. And this may delude him. Medieval man could readily be transposed to the 20th century. Today, man copes with a highly complex environment with physical abilities no better than he has ever had. For although man's civilization has become more complex, his physical limits have remained constant. He could cope with the high velocities in his new environment but not for overreaching his physical limitations. Because he uses equipment in a way that exceeds his real ability, he makes errors of judgment, not only deadly for himself, but for others. Perhaps the most vital physical limit of man is the fact that he operates in time. A standard time sequence starts with the perception of information through his sensory systems. Then he interprets this information, compiles and integrates it with other information and past learning, and makes a decision. Finally, he responds with action. Man is not always aware of this time cycle. For example, when a light stimulus strikes the eye and from there is transmitted to the brain in the form of a nerve impulse, a measurable lapse of time occurs as much as three-tenths of a second. Recognition takes still more time, perhaps a half second or more. Then the evaluation, interpretation and the decision all take more time, about a second. In conclusion, his response takes several more tenths of a second. There's also a time lag as the machine responds to the control and another for the aircraft to deviate from its flight path. If two aircraft were on a collision course at 600 miles per hour at four seconds before the point of collision, these aircraft would be a mile and a third apart. If excessive time were taken in any one of these steps, if the pilot were to look into the cockpit, look outside, and then refocus his eyes on the instrument panel, this action would take about two seconds, two seconds more than he has. A collision would be inevitable. Accidents don't happen. They are caused. The cause can be either failure of the material or failure of the human. Equipment changes, so material failures are different from one piece of equipment to another. Because man does not change, the kinds of errors he commits remain constant. The errors that he will make can be predicted from the errors he has made. It was a beautiful morning and I was scheduled with two other instructors for a pre-brief three-ship student formation training flight. The leader and his wingman got off on schedule. Because of oxygen difficulties, my takeoff was delayed about 15 minutes. Land rendezvous of the left-hand orbit at 18,000 feet in case of separation. After becoming airborne, I climbed to the rendezvous area with 100% power and picked up the leader and his left-hand turn. Now the leadership was in a bank and while cutting him off of the join-up, I saw my rate of closure was excessive. I cut power and increased my angle of bank, fell into trail position. Now thinking I was lagging behind, I went back to 100% and attempted to join on the outside of the turn. Suddenly I realized my high overtake speed on the leader and attempted to break right. Unless I started my turn, my left tip tank must have hit the tail of the lead aircraft. He went into a violent role and started to spin. I saw he couldn't recover and I called him to get out of it. At about 10,000 feet, I saw both the student pilot and instructor eject. So I started checking my aircraft with slow flight characteristics. Well, apparently I was all right and so were they. But moments later the aircraft impacted near the gunnery range. No one fatally injured. Cause of accident? Lack of knowledge. A man unaware of his physical limits as they apply to high-speed aircraft. Can you imagine what his life would be like now if the other two men had died in that crash? Here at the Air Force Safety Center, accident reports come in from every area. Flight, ground, missile and nuclear. They've been coming in from the day in 1942 when General Hap Arnold learned he was losing more aircraft in accidents than in combat. These men analyze them and try to develop a set of criteria to help keep you safe. They say, you must be aware of and accept the physical limits of man. And a man can only reach so far. We were in the process of transferring liquid nitrogen when it happened. We were both experienced men. We came into the missile program from the beginning. We had performed this operation many times. We knew the danger, all the statistics, how many seconds it takes for nitrogen to incapacitate. We always used the buddy system. My buddy was a lot bigger than I was. Well, I kind of followed his lead. We're supposed to use the gauge to tell us when the tank is full. But instead, my buddy was always using the valve to double check. And he'd left it open. When he saw the valve bending, he decided to shut it off quick. It's only to hold his hand while he reached over into the pit to shut off the valve. Just a little out of reach. It was just a little out of reach. The unwillingness to accept a simple obvious physical limit ended in sudden death. His hand went limp. I couldn't hold it. We're supposed to use safety equipment. We hadn't used any. That's a pretty deep hole, about 170 feet. I guess I was one of the first men assigned to work in the silo. But first, I like these. We've got an elevator that goes up and down the length of a big bird. Whenever the elevator moves, this bell rings and it goes on ringing until the elevator stops. It makes quite a racket, but it keeps you on your toes and no one has ever got caught on it. We have a public address system down there, too. It's a little hard to understand because it echoes through the hole and there's a lot of background noise. Whenever they run a test or there's an emergency, you get a couple of blasts on this klaxon. I remember the first time I heard it. I was to jump three feet, but you get used to it and it gives you a comfortable feeling that if anything goes wrong, you're going to hear about it. After working down there for several months, I hardly paid any attention to the noise. One day they were testing the doors. I remember that I was walking through the tunnel at the time. They sounded the klaxon and I could tell from the other man's face that he was hearing it loud and clear. But I couldn't hear it at all. The flight surgeon told me I was going deaf. I needed treatment. But he said that I wasn't as bad off as some of the men who had been working in the generator room. Three of them, he said, were in bad shape and they had more protection. When this operation first began, three highly trained men had to be pulled off duty. At that time the situation in this room demanded too much from man's highly limited hearing apparatus. Sometimes the limitation is built in, man. And other times it is built into his machine. I was on a local transition flight. The mission? Climb the aircraft out at Mach 2 to 35,000 feet. You'd never think something like your sense of touch could be so vital until something like this happens to you. And then you'd never forget. I'd made a normal take-off and afterburner climb to 35,000 feet. Leveled off and continued an afterburner for about 30 seconds. When I changed to full military power, I noticed the exhaust nozzles failed to change from the wide open position. I then tried the emergency nozzle handle, but it didn't actuate. I attempted to relight my afterburner, but was unsuccessful. Military power would not maintain my altitude. So I set up an emergency landing pattern. In order to increase my glide, I delayed lowering my landing gear until approaching the runway. The aircraft was sinking rapidly in the flare. I reached for the emergency gear handle and pulled the drag chute handle instead. The drag chute pulled the aircraft to the ground and he landed four feet short on the overrun and skidded 6,000 feet down the runway. Case of design, which did not anticipate man's limited ability to discriminate in an emergency. Two similarly shaped handles activated by the same motion by the same hand. The result, momentary confusion and a damaged aircraft. Emergency here. Crane operator was highly qualified. He operated a great variety of cranes. This was one of the easy jobs. The non-com directing the operation pointed to the wires looming overhead. The crane operator sized up the situation. From where he sat, there weren't any problems. No sweat. But the crane operator's vision failed it. He had misjudged the distance by a hair. When the crane touched the wire, the men holding the equipment were instantly electrocuted. A man's vision is severely limited, not only in depth, but in his peripheral vision, what he can see out of the corner of his eyes. It was a cold day and the men were dressed for the weather. As the aircraft turned, the tail walker misjudged his position and disappeared behind the huge wheel so he could no longer be seen by the crew supervisor. Before anyone knew it or could do anything about it, he slipped under the wheel and was crushed to death. I used to think seeing was believing. Sure, I'd heard all about optical illusions, railroad tracks meet and all that. But when you're up here all alone, it's mighty hard not to believe what you see. I'd completed a routine instrument training flight. And with the exception of the weather being a little worse than brief, everything was normal. I made one practice penetration at low approach and then called a tower. The tower gave me runway 35 because the normal active had two other aircraft on it. My pattern and approach looked fine, though the sky was white and the ground was white. From my position, the snow looked perfectly level. And then suddenly I hit something. The nose gear collapsed and the speed break separated. That teabird bounced on the runway with a nose gear folded. The aircraft skidded to a stop on the nose and main gear and the pilot was not injured. The picture of the runway threshold told the story. There it was four feet of snow. There wasn't any way in the world to see it from above. Man's sensory systems are fallible, but the fact that he must operate in time is perhaps the most severe physical limit of all. At any point in this time cycle, during perception, during the decision, or during the response, the process can break down. It was raining very hard and I could feel this guy coming up behind me and he was in a hurry. I decided I'd rather be behind him than in front of him, so I slowed down. I saw a car coming in the distance. Before I knew it, this maniac was trying to pass. I slowed down even more. Then I saw him lose control of his car, go off the road and hit a tree broadside. Killed him instantly. A closer investigation of this accident revealed that his tires were bald, no tread, and on a wet road. Here was a man who didn't know his own physical limits or those of his machine. Whether you're coming in for a landing, about to launch a missile, or merely driving home after a day of hard work, be aware of and accept your physical limits. After all, you're only a man.