Normally, jet engines push air push air out the back of the engine, resulting in forwarding thrust, but commercial aircraft have thrust reverser that can change the direction of the ejected air, sending it forward.

When might these thrust reversers be useful in practice?

• If the ejected air is deeded forward then thrust force is backward This might tics desirable to slow the plane down
• If the ejected air is deeded downward, the thrust force is up Jets can take oft vertically without needing a runway this way.
• If the ejected at is directed forward then thrust force is backward This might be desirable to accelerate the plane.
• If the ejected air is directed forward then thrust force is backward This might be desirable to conserve the speed of the plane