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Last Updated: Thursday, 19 July 2007, 14:48 GMT 15:48 UK
Q&A: Sao Paulo plane crash
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Brazilian investigators are trying to establish why a plane landing at Sao Paulo's Congonhas airport overshot the runway and crashed, killing some 200 people.

The crash occurred in wet conditions, on a runway often criticised as being too short.

However, questions are also being asked about how fast the plane was travelling after it touched down on the runway.

Here, Tim Robinson from Aerospace International answers questions on the incident and ensuing investigation.

What do you think will be the main focus of the crash inquiry?

It is still very early to draw conclusions about the causes of this tragedy, but among the factors that will be examined in any safety investigation will be the weather and the condition of the runway - such as how wet it was.

Investigators will also want to establish the condition of the aircraft, especially the brakes and when they were last checked.

Also key will be human factors - the qualifications and experience of the pilots, whether the crew followed procedures properly, and if they were fatigued.

What about the wet conditions and recent work done to the runway?

Wet conditions are particularly hazardous and need even more careful and precise landing by pilots. A 2005 aviation study showed that the risk of an overrun accident on landing was increased by a factor of 10 when the runway was wet or flooded.

Also, the airport at the runway was in the process of being upgraded. Investigators will want to examine this issue carefully to look at whether the unfinished state of the upgrade contributed perhaps to the aircraft aquaplaning out of control.

On 16 July another airliner, a Pantanal ATR 42, also skidded off the runway at Congonhas when its nose wheel collapsed.

However, on that occasion all 21 on board were safe.

Despite being one of the busiest airports in Brazil, Congonhas has one of shortest runways in the country - could this have been a factor?

The airport is nicknamed "aircraft carrier" because of the lack of margin of error in its location. However, the length of a runway should not really be a major issue as airliners have powerful brakes and thrust reversers - the Airbus A320 can land in 1,470m (Congonhas' runway is 1,939m).

But increased weights, higher landing speeds and difficult wet or slippery conditions mean that extra caution and allowance will be needed by the crew.

Congonhas famously sits in a very built up area, will the location of the airport have played a part in this accident? And is there a wider problem with airports, which once had a lot of space, finding that urban sprawl has surrounded them and left them with no room to expand or improve?

Urban planners obviously take into account airports and the safety issues of population growth around them, but in Sao Paulo the population density is extremely high and this is reflected in how close the buildings are to the perimeter - just 150m away.

Most airports have a large overrun zone where, if an aircraft does come off the runway there is adequate space for deceleration without it hitting obstacles.

The vast majority of these overruns are not serious and are classed as minor incidents.

Rio de Janeiro's Santos Dumont airport has an even shorter runway than Congonhas and we understand that pilots need special training to land there. Are there other examples elsewhere of highly tricky runways to land on?

Rio's Santos Dumont has a runway of just 1,323m so pilots are required to undergo extra familiarisation at the airport to ensure that they put the aircraft down precisely at the right speed to stop within the published figures.

Other airports with unusual approaches are London City - where small airliners have to come in at a steeper angle of descent - and Austria's Innsbruck which is situated between mountains.

Perhaps the most difficult of all was Hong Kong's old Kai Tak airport. This involved a low approach over skyscrapers surrounded by hills, then a final visual turn at 1,000ft to land.

Do international aviation rules and regulations specify minimum runway lengths?

Aircraft need more space to take off than to land, so minimum runway requirements are always governed by take-off performance.

For example, in 1960 a Pan Am 707 which mistakenly landed at RAF Northolt in London had to offload fuel and passengers to fly on to Heathrow safely.

Among the factors which are assessed are aircraft type, air temperature, the altitude of an airport (thinner air higher up means less lift so aircraft take longer to get airborne), the take-off weight of the aircraft and obstacle clearances.

What runways are operated are therefore decided by a manufacturer's performance criteria, and thus if a runway is too short to take off from, it is also too short to land on.

In recent years Brazil's airline industry has faced a series of problems, including airlines going bankrupt and air traffic controllers going on strike - is this part of a cumulative effect of an industry in crisis?

If a country's airline safety record drops, the EU has the power to ban its airlines from European skies under a "blacklist" system and the US Federal Aviation Administration (FAA) also has powers to downgrade a country's aviation safety status.

Brazil has not been placed in either category and has a relatively good safety record.

However, last year's Gol mid-air collision which killed 154 people and further air traffic control strikes have perhaps led to a crisis of confidence within the industry.

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