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Was wondering if the pilot would faint, could g-force be enough to maybe break the neck bone with the head and weight of a helmet being pulled downwards. Another relay to this is the effects on the inside of the body, more specifically, the brain. Like a boxer, after taking a few too many knockouts. Is there a limit, or an event horizon to permanent brain damage and bone structure deterioration concerning g-force and pilots in aircraft maneuvers?

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    $\begingroup$ sounds more like a question for biology.SE $\endgroup$ – ratchet freak Aug 18 '15 at 13:38
  • $\begingroup$ I guess i could specify to fighter pilots. I'm pretty sure airliner pilots don't wear or have a helmet standing by in the cockpit. $\endgroup$ – NormLDude Aug 18 '15 at 13:43
  • $\begingroup$ What do you mean by "attempting a prone position"? Fighter pilots are strapped into a sitting position. Yes pilots can be rendered unconscious and can be killed by G-forces. An aircraft with a prone position for the pilot may slightly increase the G that can be withstood by the pilot by reducing the effects of blood pooling in the legs and draining from the brain. But fighter pilots have suits that use pressure to counteract this. $\endgroup$ – RedGrittyBrick Aug 18 '15 at 13:47
  • $\begingroup$ Removed that prone position part. It seemed too long of a question if I were to type the whole brain-damage and bone structure thing. $\endgroup$ – NormLDude Aug 18 '15 at 13:55
  • $\begingroup$ This isn't exactly a flight maneuver, but using an ejection seat can cause compression fractures to the spine or displace vertebrae due to the high number of Gs (~9g). $\endgroup$ – usernumber Aug 18 '15 at 14:39
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Can excessive G-force break or dislocate bones during pilot maneuvers?

Yes.

Even if the pilot remains conscious.

For example see http://medind.nic.in/iab/t00/i2/iabt00i2p1o.pdf which describes compression fractures suffered by pilots.

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    $\begingroup$ That report talks about ejections. I find it difficult to believe that enough G could be exerted to break bones without wrecking the aircraft in any manoeuvring within the envelope. $\endgroup$ – Simon Aug 18 '15 at 14:51
  • $\begingroup$ @Simon: Oops yes, I posted the wrong reference. I'll update the intended reference into the answer. The first case is ejection but some of the others involve high-G maneuvering. $\endgroup$ – RedGrittyBrick Aug 18 '15 at 16:27
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    $\begingroup$ There are no head restraints. During defensive BFM the pilot has to crain his head around to look behind him while also pulling in excess of 7G's. $\endgroup$ – Rhino Driver Aug 19 '15 at 3:43
  • $\begingroup$ Indeed, the G forces generally keep the pilot's head back against the seat - moving it around is quite difficult, and can be painful but it is done under the pilot's control $\endgroup$ – SSumner Aug 19 '15 at 4:23
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No, the moment consciousness is lost, the body will go limp and the hands will release the controls and the aircraft will naturally seek 1g flight. However, several cases exist where pilots have awoke in a supersonic, or near supersonic, dive and sustained massive flail injuries after ejecting.

Its also important to note that simply jumping in the air will exert your body to instantaneous G-forces well above what you'd experience in a fighter aircraft. If such G-forces were capable of causing injury then you'd hardly be able to get out of bed without causing grievous injury.

The danger to high G-forces in the vertical axis is that the blood is pulled from the brain to the lower extremities causing a form of hypoxia commonly referred to as GLOC, or G Induced Loss of Consciousness. For what it's worth, skeletal injuries due to G forces are so rare that they aren't even briefed during training.

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Yes, there have certainly been cases in which fighter pilots break bones while performing high-G turns. Unfortunately, the bone most susceptible to injury is the spine because that is the one of the few structures supporting real weight other than itself.

...pilots of high-performance aircraft frequently sustained cervical spine injuries due to their exposure to high gravitational forces (5), these investigators measured accelerations about the head of an F-16 fighter pilot during simulated air combat maneuvers. With the aid of a spine model, they calculated the forces on the lower cervical spine and noted that the forces were of the same order of magnitude as failure loads of cervical vertebrae and estimations of maximum cervical spine muscle forces.

In one miraculous instance, a USAF pilot was able to land after pulling a 9G turn and breaking his back. I currently cannot find a source for this but I remember listening to my dad's friends (USN) talk about visiting the pilot. I believe he was disabled in a leg or maybe both but he did not fly again.

Less severe maneuvering will not break bones very easily, but they can produce neck injuries.

There are many safety features built into aircraft that can maneuver into such high-G turns, however. The earliest example of this is the G-suit, which pushes blood that pools in the lower half of the body back up to the brain using air pressure lines and sacs. In addition, some aircraft set artificial limits on how tight a pilot can turn to prevent pilot injury and airframe stress (The F-14 could survive much more than 6-7Gs but that limit was set on the controls to reduce wing stress and failure). More recently aircraft have adopted digital systems that will right an aircraft if the pilot fails to respond to an alarm within a certain period of time. If a pilot is in blackout or prolonged greyout, the aircraft ignores pilot input and tries to fly straight and level. This is mainly to prevent unconscious crashes into terrain, but it also prevents pilots from entering tighter and tighter turns that they cannot leave.

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    $\begingroup$ I do this for a living and have yet to hear a single instance of anyone breaking a bone due to G-forces in the jet. If it happens then its so incredibly rare that its probably a statistical outlier. I know people that have developed back problems after several years of flying, but I can't think of a single instance where either the total G's, or the onset rate caused the breakage a bone in the aircraft. There is also no alarm system on any tactical jet. This would greatly hamper the pilots ability to concentrate on flying the aircraft in demanding flight regimes. $\endgroup$ – Rhino Driver Aug 20 '15 at 15:54
  • $\begingroup$ I remember the 9G incident from when I was around 15, so I am recalling that part from distant memory. Sorry, I wasn't clear about the alarm. I am not sure if they have visual cues but I believed auditory alarms were definitely present in military aircraft. Have there not been studies on designing alarms that will not be ignored by pilots, military specifically? However, if you do that for a living, I think you may be a more reliable source. Were you a Phantom Rhino or Hornet Rhino? I always look on with awe when you pilots perform your maneuvers! $\endgroup$ – Kurt Tank Aug 20 '15 at 16:10
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    $\begingroup$ There are auditory alarms, "betty" if you will, but nothing that will automatically correct the aircraft. In fact, there are rare times when the terrain avoidance system actually gets it wrong because we're flying close to the limit. So, especially in those cases, you wouldn't want any automated action to take place. There may have been studies conducted, but the reality is that any extraneous task (like resetting an alarm) that is taking the focus away from an already task saturated pilot is going to reduce mission effectiveness. Wow, you certainly know your stuff, I'm a super hornet guy. $\endgroup$ – Rhino Driver Aug 20 '15 at 16:17
  • $\begingroup$ It looks like I was confused! Thanks for clearing that up for me; I've never had the privilege to ride in such high-performance aircraft much less pilot one for a living. What I was talking about was likely never used for the reasons you explained. $\endgroup$ – Kurt Tank Aug 20 '15 at 16:36

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