There are two main things that make aircraft engineering difficult, the need to make every component as reliable as possible and the need to build everything as light as possible. The fact that an airplane is up in the air and cannot stop if anything goes wrong, makes it perhaps a matter of life or death that its performance is absolutely dependable. Given a certain power of engine, and consequently a certain fuel consumption, there is a practical limit to the total weight of an aircraft that can be made to fly. Out of that weight as much as possible is wanted for fuel, radio navigational instruments, passenger seats, or freight room, and of course, the passengers or freight themselves. So the structure of the aircraft has to be as small and light as safety and efficiency will allow.' The designer must calculate the normal load that each part will bear. This specialist is called the 'stress man'. He takes account of any unusual stress that may be put on the part as a precaution against errors in manufacture, accidental damage, etc. The stress man's calculations go to the designer of the part, and he must make it as strong as the stress man says is necessary. One or two samples are always tested to prove that they are as strong as the designer intended. Each separate part is tested, then a whole assembly—for example, a complete wing, and finally the whole aeroplane. When a new type of aeroplane is being made, normally only one of the first three made will be flown. Two will be destroyed on the ground in structural tests. The third one will be tested in the air. When a plane has passed all the tests it can get a government certificate of airworthiness, without which it is illegal to fly, except for test flying. Making the working parts reliable is as difficult as making the structure strong enough. The flying controls, the electrical equipment, the fire precautions, etc. must not only be light in weight, but must work both at high altitudes where the temperature may be below freezing point and in the hot air of an airfield in the tropics. To solve all these problems the aircraft industry has a large number of research workers, with elaborate laboratories and test houses, and new materials to give the best strength in relation to weight are constantly being tested. The two main requirements of aircraft design are______.