Q&A: What happens to plane stowaways?
A man believed to have been a stowaway has fallen to his death from a British Airways flight from Johannesburg to Heathrow - another man found clinging to the plane is in a serious condition in hospital.
But how could they have got on to the plane and what would conditions have been like during the journey?
What are the risks?
There are serious risks associated with the extreme conditions if someone stows away in the undercarriage of a plane.
These include being crushed when landing gear retracts, hypothermia, frostbite, hearing loss, tinnitus, hypoxia (where the whole or part of the body is deprived of an adequate oxygen supply) and acidosis (the build-up of acid in body fluids which can cause coma or death).
Compartment doors re-open a few thousand feet above ground, which can then cause stowaways to fall to their deaths.
Former head of group security at BAA Norman Shanks has said the threat to anyone other than the stowaway themselves - so to passengers, flight crews and people on the ground - is minimal.
What happens at different altitudes?
At 18,000ft (5,490m), hypoxia sets in which causes weakness, tremors, light-headedness and visual impairment. When the plane has reached 22,000ft (6,710m), the stowaway will be struggling to keep conscious as their blood oxygen level drops.
Above a typical long-haul cruising altitude of 33,000ft (10,065m) - or higher - lungs require artificial pressure to function normally.
What about the temperature?
Temperatures can drop to as low as -63C (-81F), bringing on hypothermia.
Do any survive?
Some have - but they tend to have travelled fairly short distances and been fairly young.
Figures show that 96 people hid under planes during flights around the world between 1947 and 2012 - 23 of them survived. The incidents happened on 85 flights.
How many cases have there been in the UK recently?
Civil Aviation Authority (CAA) records from the last 10 years suggest four stowaways have been found on board aircraft at UK airports in that period.
There were two dead bodies on flights from Africa, one dead body on a flight from Istanbul and one live stowaway on a flight from Austria, the CAA said.
In July 2009, a man's body was found in a plane's undercarriage after a flight from Accra to London Gatwick. In June 2010, a 20-year-old Romanian was found inside a wheel bay after a flight from Vienna landed at Heathrow Airport - he was subsequently arrested. In that case, the jet had flown below 25,000ft due to bad weather.
In August 2012, a man's body was found in the undercarriage bay of a plane at Heathrow after a flight from Cape Town. And in 2013, a stowaway was found dead on a flight from Istanbul. All were in landing gear recess bays, the CAA said.
Incidents where a stowaway falls from an aircraft on its final approach are not reported to the CAA.
When did the last stowaway falls happen in the UK?
Jose Matada, 26, from Mozambique, was found in Portman Avenue, Mortlake, in September 2012. He died of multiple injuries after falling from a plane travelling from Angola.
In 2001, the body of Mohammed Ayaz, 21, from Pakistan, was found in the car park of a branch of Homebase in Richmond. Four years before that, another stowaway had fallen from the undercarriage of a plane on to a gasworks near the store.
Who are the stowaways?
Most tend to be men who are attempting to make their way to Europe or North America from developing countries.
How do they get on the plane?
Airside control areas in some parts of the world do not have the same level of security as they do in the UK, meaning stowaways can get on board if the proper checks and procedures are not carried out.
In the case of the man who died in August 2012 after stowing away on a flight from Cape Town to London, the crew onboard had been told during their journey that a security fence had been breached at the South African airport and that someone had been seen climbing into the undercarriage.
Why do they do it?
It is thought many stowaways are trying to escape persecution in their home country and are fleeing conflict situations, or are trying to find economic prosperity in the West.
What do aviation experts say?
David Learmount, Flight International magazine consulting editor, says of the Richmond incident: "If these two were neither airline nor airport staff and somehow managed to get on a flight then it becomes a serious security issue.
"Just how did they get [on the restricted] airside at the airport? Then there's the question of just where on the plane these two were."
He said the fact that one man survived suggested he may have been in the baggage hold section - as that area would be pressurised and warmer.
Mr Learmount continued: "If a person was in the wheel well of a plane on an 11-hour flight there's really very little chance of surviving.
"You are either going to be frozen to death by temperatures of minus 50C or you are going to die through lack of oxygen with the plane flying at 35,000ft."