For no reason?

Rudder redesign
On Nov. 1, after two years of trying, safety investigators finally found a way to jam the rudder of a Boeing 737 -- a part that is suspected of playing a role in fatal crashes in Colorado Springs and Pittsburgh.

The rudder is a large, vertical flap of metal at the tail that is generally used to steer the plane and correct small deviations from course. But the 737's rudder sometimes seems to jam all the way to one side. That's an emergency setting pilots use only to stay in the air when one engine goes out.

Although it is not been proven that a rudder jam caused the crashes, the Federal Aviation Administration is likely to order a check on rudder controls on all 2,700-odd Boeing 737s in service.

These events are the latest steps in a long effort to unravel mysterious crashes in the world's most common airliner. In March, 1991, a United Airlines Boeing 737 crashed near Colorado Springs, Colo., killing 25.

This was the first major plane crash the National Transportation Safety Board (NTSB) failed to solve. That's not good. Until it figures out the cause of an accident, the Board is in a poor position to recommend mechanical or procedural fixes.

Then, in September, 1994, while flying at 6,000 feet in a clear, quiet sky, a USAir 737 crashed near Pittsburgh. Without warning, the plane began rolling to the left; the pilots never regained control, and 132 people died instantly when the plane hit the earth at 301 miles per hour.

It was a difficult crash to unravel, since the plane was virtually obliterated. As investigators combed through the debris, they could find no obvious reason why this airworthy plane, whose pilots had nearly 8,000 hours experience in 737s, would enter an irreversible roll and dive into oblivion. For more on the Pittsburgh crash, see "The Crash Detectives" in the bibliography.

The NTSB takes all accidents seriously, but when the largest-selling jetliner in history suddenly begins misbehaving, the thirst for answers is particularly urgent.

What happened?
The Pittsburgh investigation began with the flight data recorder, a crash-proof tape recorder carried on every commercial aircraft to record crucial information about speed, direction and orientation. Unfortunately, the USAir 737 carried a basic recorder, which could only track 11 characteristics of the flight. Unlike modern recorders, it did not track the position of the plane's "control surfaces" which the pilot uses to change direction, and which seem to have started the roll that may have caused the crash.

pyr image
Want to meet the control surfaces?

Aileron: the flat rear portion of the upper wing, used to control roll
Elevator: the rear of the horizontal stabilizer, (defined), located on the tail, used to control pitch (angle of the nose)
Rudder: the rear of the vertical stabilizer, also located on the tail; used to control direction and yaw (defined).

Just before the dive started, the flight data recorder showed a brief increase in speed, which may have been due to turbulence. Three seconds later, the nose moved three degrees left, and then abruptly, six degrees more. This kind of yawing motion was probably caused by a movement of the rudder -- one of the many factors that the primitive flight data recorder did not measure. Then the plane started rolling to the left due to simple physics: the leftward yaw accelerated the right wing, increasing lift on the right side and causing a roll to the left.

The roll never stopped. Within eight seconds, it measured 60 degrees, and it continued until the plane was almost upside down -- and heading almost straight at the ground.

How do you know that?

There are 1 2 3 4 5 6 7 8 9 10 11 12 documents. (Glossary | Bibliography)