Supersonic Hearts

20 June 2017

Interview with

Gaz Kennedy, Nick Green, Royal Air Force Centre of Aviation Medicine

Fighter pilots shoot across the sky at supersonic speeds and this means that they’re subjected to extreme G-forces such as when they need to turn in a tight circle or rapidly change direction. So how do they handle it? Chris Smith found two people to help him out...

Gaz - Squadron Leader Gaz Kennedy; I work for the Royal Air Force Centre of Aviation Medicine and I’m the officer commanding the aviation medicine flight.

Nick - I’m Wing Commander Nick Green; I’m a defense consultant in aviation medicine. I joined the Air Force in 1990 and I also work at the Royal Air Force Centre of Aviation Medicine.

Chris - Gaz; you’re going to actually dress me up as a pilot. I don;t look like Tom Cruise - not half as good looking as him - but you’re going to give me some of the same gear…

Gaz - Oh I don’t know. We’re starting with a flying suit.

Chris - This looks a bit like a boiler suit that you’re putting me into. It’s quite lightweight material.

Gaz - It’s Nomex, so it’s flame resistant. It’s what Formula One drivers wear.

Chris - I look like I’m going to do the decorating in a bit. Zip that up.

Gaz - The next thing is some angi-G trousers. These will give you protection against pulling G. Basically, they’re a pair of inflatable trousers that connect to the aircraft engine bleed system.

Chris - You're putting a sort of belt round my waist. This is already very heavy Gaz.

Gaz - Yeah, but you get used to it.

Chris - So there’s a belt going round my waist and I look a bit like a cowboy actually with those sort of leathers they used to wear down the outside of their…

Gaz - Chaps.

Chris - Chaps yeah. Absolutely. There’s a hole in the front of it where my knee goes on each side. That’s to enable me to bend my legs presumably?

Gaz - That’s right. On the thigh there are two white plastic rectangles - they’re for writing information down on.

Chris - Now you’re just reaching between my legs and they zip up on the inside of each leg.

Gaz - That’s right. They’re a tight fit - normally, you’d have these put on and they’d be specifically your size and then some safety equipment fitters would tighten up some strings at the back of the trousers and pull them really so they’ve very tight fitting.

Chris - So it’s almost like a victorian corset? There’s a tube emerging at waist height out of these trousers - what’s that?

Gaz - That tubes connect to the aircraft bleed air system that comes from the engine. There’s a spring loaded system there and every time you pull more than about 3G, that opens a valve air comes in the trousers, inflates the trousers, and that stops the blood from pooling your legs to prevent you from going unconscious.

Chris - It’s amazing that you’re actually connected to the jet engine. What are you giving me now?

Gaz - This is a life jacket which we wear every time we go flying, because as well as giving you support over the water, it also contains all the survival aids as well, such as a beacon to locate you, flares, heliograph mirror, first aid kits, spare water. So quite a lot of stuff for carrying around with you.

Chris - I’m now fit to fly?

Gaz - Almost. There a helmet to go on yet.

Chris - Okay. Got the helmet and some gloves, of course. I can't possibly take to the air without my gloves. Just get the helmet on…

Gaz - The oxygen mask on. You won’t be able to speak now but still be able to breath. And you’ve got a visor as well that you can put down that we fly with all the time in case we get a bird strike through the front of the aircraft, and it also gives you protection against the sun as well.

Chris - Nick; why am I wearing all this?

Nick - Exposure to high G-force runs a risk for pilots of loss of consciousness, which we call G-LOC, or G induced loss of consciousness and that’s really because of the effect that the G-force has on your blood. Under increased G, the weight of your blood is effectively increased, and as it gets heavier it has a couple of effects. One of them is to cause the blood to pool in the lower limbs, and the other is to reduce the amount of blood pressure at head level. Basically, anything above heart level we see a reduction in blood pressure, and anything below heart level we see an increase in blood pressure due to the increased hydrostatic gradient caused by the G-force.

Chris - So as the Gs make the blood heavier and effectively fling the blood into my legs, there’s less blood coming back up to my heart, my heart’s got less to pump out into my head and it’s facing a bigger upward struggle to get it into my brain, so my brain basically suffers a lack of oxygen for a while?

Nick - Exactly. And it’s all about the delivery of oxygen to the brain. So we’re not interested in the blood pressure by itself, what we’re really interested in is the flow of blood carrying that vital oxygen to keep you conscious. Around about 4 seconds after the oxygen delivery is stopped you lose consciousness.

Chris - Gaz, you’ve flown some of the most powerful aircraft that the RAF has. Say you’re going to pull a very high load of G, what do you do personally to make sure that you don’t pass out?

Gaz - If you’re aware of it you can perform an anti-G straining maneuver, which is basically muscle tensing just as the Gs about to come on in your legs, buttocks, abdomen. You also take a deep breath, you then hold the breath for between 3 and 4 seconds, and then you exhale and inhale as rapidly as you can within about a second. You want to do probably 2 or 3 of those and then you’re at a level where you can then judge greyout, which is a slight loss of vision and you can relax slightly until you start to greyout and you can retense to get rid of the greyout - the loss of vision.

Chris - Nick; why does that work - the muscle tensing in the legs and then the deep breaths sequence?

Nick - The problem is all about not enough blood pressure and all of these maneuvers are designed to increase blood pressure. The leg tensing has two effects: it acts on the arterial system and squeezes the arteries, particularly the smaller vessels (the arterioles), which increased the peripheral resistance, and it’s actually the peripheral resistance that's the main determinant of your circulation’s blood pressure. So, if you can squeeze those vessels, make them narrow, pressure goes up and that’s what we want. The other convenient benefit of squeezing your legs is blood tends to pool in the veins - they’re floppy elastic vessels, and if you squeeze your muscles hard you can empty that blood out of the veins back up into the chest and then it can be pumped upwards to the heart and to the brain.

Chris - What is Gaz talking about when he mentions greyout?

Nick - Greyout is an interesting phenomenon useful for pilots because it gives them warning that there is a loss of consciousness impending. The reasons that you get greyout is through a failure of blood supply to the retina. The globe of the eye is actually pressurised just to keep the eye in the right shape, but what that means is you need an extra 10 or 20 mm of mercury of blood pressure to perfuse the retina rather than perfuse the brain. In practical terms, if your blood pressure’s falling, your eye stops working before your brain does.

Chris - The first time you were doing all this, Gaz, did you just take to it like a duck to water? Are there people to whom this comes naturally or have you had to learn this and train yourself to become resistant to these effects?

Gaz - There are some people whose anatomy makes them more resistant to G, so short, squat people. I’m not suggesting that I’m molded in that manner. But tall, thin people usually have more difficulty pulling G than shorter, squatter people.

Chris - Have you ever passed out?

Gaz - No. But I’ve flown with quite a few people who have, but it’s nothing to do with my bad flying. That's why we do the training and why it's in a training environment. Because they could go off and do them by themselves next time so at least we’ve discovered that nice and early and realise that they have to then work on their anti-G strain maneuver.

Chris - When people pass out like this Nick, how long do they stay unconscious for ?

Nick - In a study done by the Americans when they went back and looked at their video records from centrifuge training they found it was around about 10 seconds on average, although that did include the time to stop the centrifuge. So in the aircraft it may be a little bit quicker than that you would wake up. But the problem with G-LOC is once you’ve woken up, there’s a period of around about 30 seconds where you’re dazed and confused. You can’t necessarily operate the aircraft. One pilot told me when he was waking up, he looked ahead of him and saw a load of alarm clocks and thought gosh, why did I set all those alarm clocks in front of me? And then suddenly realised it was actually the aircraft instruments and he was in an aircraft travelling at 500 miles and hour, and needed to do something about it. So the confusion is a really important, and yet dangerous, part of the experience.

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