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There are many instances where pilots need to eject out of the cockpit due to emergency situations, and after such an ejection from some modern aircraft, pilots are deemed unfit to fly airplanes for some years. Why is this done? What is the minimum number of years after which a pilot is deemed fit to fly again?

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    $\begingroup$ Depends on the aircraft. Canadian F-18 pilots are allowed to eject once, I believe. After two they are no longer allowed to fly (afaik) - damage to the spine, as noted by @MikeFoxtrot, is the primary reason. $\endgroup$ – J... Jun 2 '15 at 10:20
  • $\begingroup$ thanx @BobJarvis $\endgroup$ – Victor Juliet Jun 2 '15 at 16:47
  • $\begingroup$ Captain Udell ejected supersonically and my understanding is that he was still able to fly again after his ejection. jalopnik.com/5894022/… $\endgroup$ – Rhino Driver Jun 2 '15 at 21:24
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    $\begingroup$ Air force pilots often fly aircraft worth millions, or billions, of dollars. I suspect that the automatic suspension gives them one more reason to stay in the aircraft. $\endgroup$ – Hal Jun 3 '15 at 0:03
  • $\begingroup$ @Hal, I can tell you that the price of the aircraft is not part of the pilot's decision making process when facing a life or death situation. $\endgroup$ – Michael Hall Sep 13 at 23:29
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Ejection Seats are not a free ticket out. They are incredibly violent and rough on your body. This newspaper article has a more chilling quote from an interview:

About one in three will get a spinal facture, due to the force when the seat is ejected - the gravitational force is 14 to 16 times normal gravity and it might be applied at 200G per second. Bruising and abrasions are typical from the shock of the chute opening or the air blast. In the early days, there were cases where pilots would eject into very-high-speed air and it would whip their arms behind and break them, pop their shoulders out; same thing could happen to the legs. Source

Hence, it is very possible to get back pains and a host of other problems as a result of ejecting. Since these type of things are not that easily reversible, you'd rather take the safe path and remove then from the cockpit than put then back in a work environment that known is pretty hard on your body. Aircrew seem to agree:

“It was the most violent thing I’ve ever felt in my life,” says one of the B-1 crew members, whom the Air Force asked me to identify as “Captain IROC.” “I lost a full inch in height,” because his spine absorbed such tremendous G-forces. Source

Modern ejection seats are however increasingly intelligent and will gauge the ejection force applied to the conditions, cutting down the number of serious cases.

This paper discusses the symptoms of four individuals who crashed their jets in mid-air. The medical problems encountered with ejection can be classified as follows:

  • Injuries from the emergency that causes ejection—fire or collision.
  • Canopy jettison: burns from “MDC splatter” and cuts from fragmented plastic. For these reasons, aircrew are always advised to wear their visors down, to protect the face.
  • Firing of ejection gun: spinal injuries.
  • Entering airflow: wind blast may cause lung damage; seat tumbles at variable speed, which may be as high as 180 rpm. (All seats have a drogue parachute or deployable aerodynamic panels to prevent tumbling); flail injuries to extremities.
  • Parachute deployment: snatch injuries.
  • Landing: lower limb injuries.
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    $\begingroup$ For completeness sake, in a single-seat aircraft you're looking at about a 0.4 sec exit time with a parachute deployment starting at between 1-2 secs. That alone is insanity. $\endgroup$ – Rhino Driver Jun 2 '15 at 21:32
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    $\begingroup$ Do you happen to know what fraction of pilots who eject suffer no significant or lasting injuries? $\endgroup$ – Mehrdad Jun 3 '15 at 7:50
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    $\begingroup$ @Mehrdad That sounds like a decent separate question. $\endgroup$ – a CVn Jun 3 '15 at 11:15
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    $\begingroup$ love the "newspaper science" there "200G per second" :) $\endgroup$ – Fattie Dec 15 '15 at 0:03
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    $\begingroup$ @JoeBlow G is a measure of acceleration. G/second is a measure of the rate of change of acceleration. The rate of change of acceleration is sometimes called "jerk." $\endgroup$ – Wayne Conrad Mar 18 '16 at 19:36
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I don't know anything about minimum periods - but that may depend on the air force in question. (Maybe someone else knows.)

Medical complications can arise years after an ejection. A fairly well known case I've heard about is the astronaut Michael Collins, who ejected from an aircraft in the 1950s. More than ten years later he started to notice leg problems, these were traced to bone spurs in neck vertebrae, probably caused by the ejection. Surgery fixed the problem.

(Collins goes into this and many other details in his biography by the way.)

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    $\begingroup$ For those who do not recognize the name: Michael Collins flew again. $\endgroup$ – dotancohen Jun 2 '15 at 10:51
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    $\begingroup$ yes that's right, after having the surgery the problem was (almost completely) fixed and his next spaceflight was... the big one to the moon! $\endgroup$ – Andy Jun 2 '15 at 12:21
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There isn't a limit (in the USAF at least). I seem to remember hearing a story of a SQ/CC at Luke that had ejected twice. I will do my best to find a reg/pub that talks about it.

Additionally, not all ejections cause injuries. I've also known pilots that ejected and were completely fine. A controlled ejection at 200-300 knots is a lot different than an uncontrolled ejection at 500+.

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