Brain Teasers (page 38)

One day at a busy airport, the passengers on a commercial airliner are seated waiting for the pilot to show up so they can get under way.
The pilot and copilot finally appear in the rear of the plane and begin walking up to the cockpit through the center aisle. Both appear to be blind; the pilot is using a white cane, bumping into passengers right and left as he stumbles down the aisle. The copilot is using a guide dog. Both have their eyes covered with sunglasses.
At first, the passengers do not react thinking that it must be some sort of practical joke. After a few minutes though, the engines start revving, and the airplane begins moving down the runway.
The passengers look at each other with some uneasiness. They start whispering among themselves and look desperately to the stewardesses for reassurance.
Yet, the plane starts accelerating rapidly, and people begin panicking. Some passengers are praying, and as the plane gets closer and closer to the end of the runway, the voices are becoming more and more hysterical.
When the plane has less than twenty feet of runway left, there is a sudden change in the pitch of the shouts as everyone screams at once. At the very last moment, the plane lifts off and is airborne.
Up in the cockpit, the copilot breathes a sigh of relief and tells the pilot: "You know, one of these days the passengers aren't going to scream, and we aren't going to know when to take off!"