US statistics taken from between 2000 and 2009 show that car drivers or passengers suffer 7.3 deaths per billion miles. In contrast, air travel comes in at just 0.07 deaths per billion passenger miles. Of course, statistics cannot cure someone's fear of flying, but they do prove that it is one of the safest ways to get from A to B. This safety is thanks to the testing that every single part of the aircraft has to go through before it is allowed to undertake its first flight. While passengers get to enjoy the comfort of air conditioning, the engines have to be able to cope with temperatures as low as -55 °C during cruise conditions. Temperature tests are important to the airlines because they want to make sure that their planes can operate in as many environments as possible. Each time they have to ground an aircraft because of bad weather, they lose money. Another thing to be considered by the airlines is the migrating birds. They can travel at altitudes of over 8000m and threaten the cruise phase of the flight, although the greatest risk is during take-off, when the craft is at its heaviest and the engines are close to maximum power.