Crash that saved thousands of lives : Boeing 737 “Rudder Hard-over”

Everyday, hundreds of 737’s carry thousands of people across the skies. It’s estimated that a Boeing 737 takes off every two seconds. But what if I say that there was a design fault on all of these 737’s ?

The 737 is one of the famous airplanes in the sky. A twin-engine designed by Boeing it is meant for short haul regional flights designed to carry anywhere from 120 to 170 people on board. The plane is designed to be economically efficient for short haul to medium haul flights. And due to that nature, the aircraft quickly became one of the famous planes in the sky with thousands of them built to date.

Fun fact: Boeing 737 can fly up to 41,000 ft high cruising at speeds of up to 850 kmph

It was all smooth and well until the United Flight 585 crashed on March 3,1991. NTSB which was responsible for the investigation was unable to find conclusive reason for the crash.

On September 8,1994 US Air Flight 427 crashes near Pittusberg. The crash had high resemblance to crash of Flight 585. It was established that the plane quickly pitched to the left and the pilots were unable to recover from the roll. NTSB due to un availability of conclusive data from the flight recorders were unable to determine the cause. But NTSB suggested that the rudder might be to blame on both the accidents.

On June 9th,1996 Eastwind Airlines Flight 517 lost the control of rudder on approach. Unlike the previous occasions, the pilots were able to recover from the un commanded roll and landed the aircraft. This time NTSB was able to closely examine the rudder system.

Fun fact: A Boeing 737 costs approximately $100 million.

On closer examination of the rudder’s PCU a dual servo valve which directs the hydraulic fluid to control the rudder based on the pilots input showed defects under certain scenarios. Testing showed that under certain conditions when pilot applied a particular rudder input, the dual servo could jam and cause the rudder to move in an un commanded direction rather than the pilots input.

PCU on Boeing 737

The issue escaped the light during Boeing’s initial flight tests. NTSB concluded that the rudder PCU defect was to blame in all the three incidents and all the 737 flying could be in danger.

FAA ordered to ground all the aircraft and to replace the old servo valves with new ones. New training protocols were introduced to train pilots on scenarios like uncontrolled movements of flight surfaces.

Today thousands fly daily on the 737. Though not asked, the crash of flight 427 and flight 585 helped to save thousands of lives.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.