听力原文： Before a new plane goes into service, every part of it is tested again and again. But there are two tests that are more important than ail the others. In the first test, a modern airliner must fly at very high altitudes. Air must be pushed into the airliner so that the passengers can breathe. The metal structure of the plane has to be very strong for this reason. When the airliner is filled with air, the air presses against the skin of the plane inside. The pressure on a small window, for instance, is like a huge, giant foot that is trying to get out. If a small part of the plane were to crack, the plane would explode in the sky. It is lowered into a huge tank of water to test the stmctrue of the plane. Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is in the air. Finally, there is an explosion. Engineers can discover which part of the plane has cracked. In the second test, the test pilot must find out exactly what happens when all the engines are shut off at once. He takes the plane up very high. Then he shuts the engines off, the plane begins to fall like a stone. It is the pilot's job to find out how he can control the plane again. These two tests are examples of how planes are made safe before they ever carry passengers. What's the main topic of this passage?