You are right, the engines are very heavy. This means that the wings have to be very strong to support them, and engineers spend a lot of time making sure that the wings are strong enough. You might also notice that when the plane is on the ground, the wings will sag a little from the weight of the engines. When the plane starts moving in preparation for takeoff, the wings move upward a little and don't sag as much. If you check out the answer to the previous question about how airplanes fly, you should be able to figure out why the wings stop sagging when the plane starts moving. Good Luck!