A man is fighting for his life after apparently surviving stowing away in the undercarriage of a 5,600-mile (9,000km) flight to London. Given extremely low temperatures and a lack of oxygen, how was he able to survive the journey?

Flights from Johannesburg to London take about 11 hours. Boeing 747s tend to cruise at a height of between 30,000 and 40,000ft (9,100m and 12,200m).

The standard ambient temperature at 35,000ft (10,700m) is around -54C. Oxygen levels drop to about 26% of those at sea level. The odds of survival for a stowaway who climbs into the small space above a wheel seem tiny, assuming they're not crushed to death when the wheel draws up or they don't fall out as it lowers ahead of landing.

But a 24-year-old man is currently in a critical condition in a west London hospital, having seemingly endured conditions that should in theory rapidly kill any person. The body of another man was found on a roof a few miles from Heathrow. Police are investigating whether the second man had climbed on to the same plane.

"It's unbelievable that anyone could survive those conditions." says Jack Kreindler, a high-altitude physician at the Centre for Health and Human Performance. "People spend up to three months acclimatising for the top of Everest. Even that's completely different. It's gradual and the clothing is designed for it and they could maybe survive a night at most, and these are people right at the genetic limits - they're not normal.

"If someone were to ask the odds of surviving in a wheel well from Johannesburg to London, there's not one expert who'd give them a chance." Kreindler says that, if it turns out the man survived the flight without the use of an oxygen supply, medics would be very keen to study his genetic make-up.

David Learmount, consulting editor at Flightglobal.com, doubts whether it's even possible. "If someone is to survive as a stowaway then they have to get into an area of the plane that is pressurised and heated," he has said. This is highly unlikely in this case - for obvious reasons there is no way of accessing the pressurised parts of the plane from the wheel well. Learmount thinks the survivor must have brought some oxygen in a container to make the journey remotely achievable.

The South Africa stowaway is not the first wheel well-climber to survive a jet flight. In fact, the rate of survival may be as high as about one in five, according to the US Federal Aviation Authority (FAA). But the lower the altitude the better your chances. At the highest altitudes commercial flights reach, it's hard to explain how anyone could survive.

Is there anything to help would-be stowaways withstand the conditions? Tyre friction on takeoff might mean some heat radiates towards a stowaway, as can warm fluid in hydraulic lines within the compartment.

But these sources fade as the plane ascends, taking the temperature far below anything regarded as survivable. The wheel well on a 747 is protected from the elements during flight by flaps, but these are not pressurised, limiting protection from the cold. There's no obvious way of getting into a space more conducive to life.

Healthy humans are able to regulate body temperatures at 36C to 37.3C in more normal conditions, but, according to the FAA, the limited oxygen supply at high altitude means this is no longer possible.

At this point people might start to become poikilothermic, research sponsored by the FAA says. This means their body temperature becomes more controlled by the external environment. A state "somewhat reminiscent" of hibernation occurs. The body's temperature can fall to 27C or even lower. Unconsciousness and lower heart and breathing rates occur.

The body would pump blood to the brain more efficiently and distribute it only to the automatic nervous system, in an effort to survive. This means less blood getting to the body's extremities. Frostbite is a severe risk, but the most vital organs could be saved, the theory goes.

When the plane nears the ground, the body gradually re-warms, the FAA adds. If the person avoids brain damage or falling out as the wheel lowers before touchdown, they might in theory survive.

Source: BBC