Before a new airliner goes into service, every part of it is tested again and again. But there are two tests that are more important than all the others. One of them is very strange and the other is very dangerous. The first of these is called the “tank test”. A modern airliner must fly at very high altitudes. Air must be pumped into the place so that the passengers can breathe. The metal structure of the plane has to be very strong for this reason. When the plane is filled with air, or “pressured”, the air presses against the skin of the plane inside. The pressure on a small window, for example, is like a huge foot that is trying to get out. If a small part of the plane were to crack, the plane would explode in the sky. This is what happened to the first Comets. In order to test the structure of the plane, it is lowered into a huge tank of water. Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is inthe air. Finally, there is an explosion. Engineers can discover which part of the plane has cracked. This part is made stronger. The most dangerous test happens when the new plane is going through test flight in the air. The test pilot must find exactly what happens whenall 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 get control of the plane again. These two tests are examples of how planes are made safe before they ever carry passengers.

The purpose of the “tank test” is to find out（）.