# g-force passout question



## garuda-one (26 May 2010)

New guy here,

So i was watching this show called Jetstream and it is about new CF-18 recruits doing their training at CFB Cold Lake. This one pilot on the show fails the g-force test with the centrifuge and is grounded, preventing him from ever flying a CF-18. I was wondering, how is it that some people can withstand g-forces and still stay conscious while other people blackout from the same amount of g-forces. Is it something to do with your fitness? your blood pressure? I am hoping to be a CF-18 pilot myself and i will be applying for ROTP next year as soon as i am finished my last year of college.


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## PuckChaser (26 May 2010)

I've watched the same show, I believe they explain the fitness level required to continuously perform the breathing technique is pretty high. Like in most things, some humans are just better at doing it than others.


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## DexOlesa (26 May 2010)

From what I understand it comes down to a little bit of everything. Impossible to look at a person and say "Yup he'll definitely pass"


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## Zoomie (26 May 2010)

I am sure that Max will weigh in soon - as he is most probably the only person on these means that has experienced this..

AGSM - Anti-Gravity Straining Maneuver is not something to be practiced outside of supervised events or under actual +1G.  It involves clenching and releasing muscles in your lower extremities (to force the blood out and to stop it pooling in your feet).  It also involves forcing your internal organs and muscles (lungs, diaphragm) to assist in forcing O2 enriched blood from your core into your brain.  This is all in an attempt to stave off G-LOC (Gravity induced loss of consciousness).   

That being said - physical fitness is extremely important.

FWIW - that guy who GLOC'd took the course again (when the cameras were off) and passed the course.


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## Jorkapp (26 May 2010)

It's a blend of physiological factors. I am neither a doctor nor a fighter pilot, but what I can tell you is:

A blackout (in this situation) is caused by a lack of oxygen to the brain (hypoxia). In the centrifuge, the forces acting on your body force blood to move more towards your lower extremities, thus cutting off blood-flow (and with it, the oxygen that's supposed to go to the brain). A stronger heart can pump blood more effectively under such stress, so yes, physical fitness does play a factor.

Fighter pilots do get the luxury of g-suits, but those don't completely negate the effects of g-forces.

I'm sure if any pilots are hanging around, they can probably go more in-depth than my simple understandings.


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## Eye In The Sky (26 May 2010)

You mean someone like...ohhhh I don't know..Zoomie who posted before you?

 8)





You ain't felt Gs until you've flown the Gonzo...


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## SeanNewman (26 May 2010)

Guy Incognito said:
			
		

> A blackout (in this situation) is caused by a lack of oxygen to the brain (hypoxia)...



Completely correct but this is not simply a fighter pilot phenomenon.  Happens all the time due to non-G-related issues when pilots of small airplanes or helos try to push altitude far past 10,000 AGL.  Of course in those cases the hypoxia is caused by less oxygen *pressure* (edit) in the air as opposed to blood being pooled in the feet.

Edit - As pointed out below by the experts, these are both hypoxia but more detail is given.


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## Eye In The Sky (26 May 2010)

Petamocto said:
			
		

> Completely correct but this is not simply a fighter pilot phenomenon.  Happens all the time due to non-G-related issues when pilots of small airplanes or helos try to push altitude far past 10,000 AGL.  Of course in those cases the hypoxia is caused by less oxygen in the air  as opposed to blood being pooled in the feet, but essentially the same things is happening and will result in the same thing (loss of consciousness until the factors change or a crash).



Umm...actually the % of oxygen is the same...the partial pressure is lower at altitude.

 8)

From the AMT bible:

One of the most important hazards to man during flight is the reduction of the partial pressure of oxygen (PO2) produced by ascent to altitude. A decrease in the PO2 reduces the ability for oxygen to pass from the lungs into the bloodstream. Hypoxia is defined as a state of oxygen deficiency in the blood, cells and tissues sufficient to cause an impairment of function. PO2 at altitude is calculated using Dalton’s law as follows:

Dalton's Law
Dalton's Law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of each gas in that mixture. The application of this law to aviation tells one that even though the percentage of oxygen in the atmosphere is constant (21%), its partial pressure (PO2) will decrease as atmospheric pressure decreases with ascent to altitude.


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## 2010newbie (26 May 2010)

I incorrectly assumed that human centrifuge testing was performed during the effects of High G's on the body training in AMT. So is it only Fast Jet candidates that undergo the centrifuge?


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## Fishbone Jones (26 May 2010)

Another thread nearly ruined. Some just have to realize eventually, that a LITTLE knowledge is a dangerous thing. Especially when they try to espouse it like a pro and only end up confusing people, then it has to be rebutted and corrected.. Eventually, maybe eventually............

Until Max or another pro gets here, and asks a mod to unlock it so the PROPER info can be posted. It will be locked.

Milnet.ca Staff


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## Loachman (26 May 2010)

GLOC and Hypoxia are caused by two different mechanisms, and the symptoms and onset are also different.

I experienced greyout almost to the point of blackout in Moose Jaw while pulling 6.5 G in a Tutor in 1981 (not intentionally). During both of these stages, the brain is still functioning. Colour vision is lost first (greyout), followed by complete vision loss (blackout). I also seem to remember losing peripheral vision. Unconsciousness will follow, if high G is sustained long enough or increases enough. Onset is generally quite quick.

Hypoxia is the result of reduction of atmospheric oxygen, as cabin altitude increases or supplied oxygen dimishes. Onset is usually slower than GLOC, unless it is due to sudden depressurization at high altitudes, or a sudden problem with oxygen supply while breathing through a mask etcetera. A helicopter crew climbing through 10000 feet will not suddenly become hypoxic and pass out; that is simply a legal limit imposed on flight without supplemental oxygen. One may feel a little light headed at that point, or maybe no symptoms at all. As cabin altitude increases, a variety of other symptoms will show up. These vary between individuals, which is why we do (or at least did) high-altitude chamber runs before Moose Jaw, so that everyone would recognize their symptoms. It is similar to being drunk. It is insidious; it creeps up on one, and one's judgement is affected.

As has been stated, a G-suit will increase G tolerance a little, by tightening on the lower legs and forcing blood back up to the head. In those days, 1 to 2 additional G units could be tolerated; better suits today may offer more, but it has been many years since I have worn one. The straining manoeuvre should, if done properly, add about the same amount of extra tolerance. Reducing the height difference between heart and brain can also help, which is why an F16 pilot occupies a more reclined seat.


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## Loachman (26 May 2010)

Unlocked by request.

And now, from the Wilds of Cold Lake, I bring you...

Supersonic Max!!!!!!




Thanks, Max - we wuz waiting for you.


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## SupersonicMax (26 May 2010)

Here's a few pieces of info on G-LOC.

G-LOC is indeed a form of cerebral hypoxia.  Under Gs, the blood, which carries the oxygen, is forced out of the brain. The first symptom will be the loss of color vision.  Everything becomes grey.  If you keep pushing it, so to speak, you will experience tunnel vision, where you lose your peripherals.  If you push it further, you will black out.  At that point, you are still concious, but cannot see a thing.  The last thing that will happen to you is a loss of conciousness.  In that case, you have no control over anything.

There are a few factors that influence your G Tolerance.  First, and the most important one in my mind, is your natural vertical distance between your heart and your head.  The shorter that distance is, the less pressure is required to carry the blood to the brain.  Shorter people will tolerate more Gs, in general.  One way aircraft designer capitalized on that is by reclining the ejection seat, minimizing the vertical distance between the heart and the brain.  The F-16 has a 30 degrees recline on the seat, diminishing that distance by 20%.

The second factor is your blood pressure.  The higher it is, the more Gs it will take for your blood to be forced out of your brain.  There are ways to increase your blood pressure as you are pulling Gs.  The first one is with a G-Suit.  By compressing the lower legs, the thighs and the abdomen, you increase your overall blood pressure.  The second one is by doing a G-Straining manoever.  By compressing every muscle you can, you will increase your blood pressure.  Holding your breath and "pushign it" against your chest will help as well.  So, being in a good shape can help.  Having a greater muscle mass will help you increase you blood pressure as you are straining your muscles.  However, a long distance runner with a resting heart beat of 42 and an extremely low blood pressure will not tolerate Gs better than a smoker, fat food eating individual with a blood pressure up the roof.

The last factor I can think of is your VO2 max.  The more oxygen your body can process and utilize, the better.  Now, remember when I said that a marathon runner with a very low blood pressure will not do good?  Well, it's probably true, however his VO2 max is probably very good.  In the end, you want to achieve a balance. 

Now, onto the CF training.  It is only given to pilots that are selected to fly jets.  Once you are finished Phase IIA in Moose Jaw and are selected to go jets, you'll be sent on a 3-day paid vacation trip to Toronto to complete the training.  The half of the first day is ground school.  You learn all the theory and practice the G-Straining on a chair.  In the PM, you do your Centrifuge run.  There are 4 profiles, the first one being a "warm up".  It's a gradual onset (no g-pants) with no straining until you start feeling the symptoms.  The second run is a rapid onset (no g-pants) to 5Gs for 20 seconds.  The third run (and probably the most failed one) is a rapid onset to 6Gs (no G-Pants) for 15 seconds.  The last one is a rapid onset to 7.5Gs for 10 seconds and down to 5G for 30 seconds (with G-Pants).  

The Gs in the centrifuge are very different than what you feel in the jet.  I don't know why, but that's the way it is.  I personally thought that 6Gs in the centrifuge was much, much more agressive than 6Gs in a Hornet.

The second day is for anyone that failed to redo the runs.  

As far as the individual in Jetstream that didn't pass the centrifuge training, he still flies the Hornet.  He eventually passed the centrifuge training.

Hope this helps,


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## Fishbone Jones (27 May 2010)

Thank you Max, for the professional, correct 'from the horse's mouth' answer to the OP's question.

Milnet.ca Staff


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## SeanNewman (27 May 2010)

Original post corrected to reflect "less oxygen *pressure*" instead of just "less oxygen".

I have no problem atmitting when someone knows more than I do.


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