Coping with G-Forces

22 July 2007

Interview with 

Major Todd Dart, United States Air Force


So we've heard about going down to incredible depths, now we're going to find out about going thousands of meters up in the air, with Major Todd Dart, Aerospace Physiologist with the United States Air Force.

Chris -   It sounds pretty trivial, you just get in an aeroplane and fly it.  But what sorts of forces, G - forces will people who are flying very fat planes actually be experiencing?

Todd -   Well, it all depends.  Fighter aircraft for example, pull up to upwards of 9 Gs in the aircraft.  That's what we call 9 positive Gs or at the vector going from the head down to the feet.

Chris -   In real terms, what does that mean, Todd?

Todd -   In real terms it means you have to be able to get that blood, keep that blood, up to your brain, so you don't lose consciousness eventually if you don't have some sort of way of keeping the blood from pooling down.  We have a lot of mechanisms that we have to train the pilots to do, fighter pilots specifically, and also technology that we use to keep the blood flowing to the brain.

Chris -   So are you saying that you tell people not to do movements that would subject them to 9 Gs, because to put that in perspective if your head is feeling 9 Gs the weight of your head is effectively nine times bigger than normal, isn't it?

Todd -   Yes it is.  Unfortunately, in the military environment sometimes you have to pull 9 Gs, so we have to come up with ways to counter that.  We do that by providing training on a human centrifuge, and there are two the air force uses for that.  What we do is we put them on there and we train them how to use the equipment, we put them in G suits - which only gives you about a 1 G increase or so in G tolerance.  The main thing we do is we train them in how to perform what's called an anti-G straining manoeuvre.

Chris -   So when you say G Suit, this suit puts pressure on the body, squeezing say, on the legs and this stops blood from pooling in the legs.  It that the way it's done?

Todd -   Yes, exactly.  Medical doctors will be familiar with the mass suit, which was developed to prevent shock; this was developed from the G-suit, used in the military.  A typical G-suit has an abdominal bladder and bladders in the legs.  Some cover the whole leg, some only cover parts of the leg.  It's squeezes the leg as soon as you hit about 2 Gs.  It helps get that blood up to your brain.

Chris -   So that's the blood going away from the brain, but what about if you do the manoeuvre the other way, does this force blood into your brain, and can that be dangerous?

Todd -   Yes it can, in fact we can't tolerate very much of that, only about -3 Gs, its called negative G.  That's about as much as we can handle before we start getting what's called red-out; you're getting so much blood into the brain that it ends up engorging the eyes, you get some rupturing and it can cause headaches.  It's very uncomfortable and pilots don't like to do those...

Chris -   Not surprisingly!  So how can you compensate for these effects?  What can you say to people, what do people actually do to avoid feeling these effects.

Todd -   Well, to avoid negative Gs pilots will simply roll the aircraft and pull positive Gs, most of the aircraft are not designed to pull negative Gs because it's uncomfortable.  We are evolutionarily adapted to handle 1G pretty well, so we have physiological and anatomical adaptations to help get that blood up to the brain.  Valves in the veins, we also have cardio-vascular reflexes.  We rely on those as well to help give us that tolerance we need to get up there, they help us with positive Gs.  We get pilots in the G-suit, train them in the aircraft and train them how to do the straining manoeuvre.  They will start their strain and before they even get a chance to go up much higher in Gs, they're starting to strain to get that blood up so they can last.  Typically, they'll pull Gs maybe 10-15 seconds.

Helen -   Do you find that there are some people who just can never be trained to go up that far and experience these extreme conditions.  Are some people better than others?

Todd -   Some are, there is a lot of individual variation.  Most people start to lose consciousness if they're relaxed between 5-7 Gs.  Normally, the taller you are the less able you are to handle the Gs and that's because it depends a lot on the heart to brain distance.  The greater the distance, the more distance you have to pump blood up to the brain and that puts more strain so you have to maintain a higher blood pressure to do that.  So people who tend to do a little bit better on average tend to be people who are short.

Chris -   So when you put people on your human centrifuge, can you tell who's going to be okay, and who's going to have problems?

Todd -   We have a general idea, it's not 100% of course.  I've seen people who are very tall, thin marathon runners who are fighter pilots who do great.

Chris -   So Tom Cruise really would be a good fighter pilot because he's nice and short like me?

Todd -   He might be a better fighter pilot because those who tend to be shorter tend to do better.

Chris -   So what about women, because at the moment the military tends to select in favour of men.  Is there an anatomical reason for that or is it just because historically, it's always been blokes who've gone to war and the women haven't so much until recently?

Todd -   Well, at the US Air Force we have female fighter pilots as well, and we found out many years ago in fact that there really is little difference between man and women in terms of G tolerance.  Women do just as well as men do on average.


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