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Boeing 737 � Rudder Actuator Problem

Copied from my wordpress site that was never going to get any footing lol:


The Boeing 737 series plane is known as the most successful and safe jetliners to hit the sky, with an amazing track record dating back to 1967. There has been a total of 8,471 Boeing 737 airplanes constructed, with another 4,244 more confirmed as ordered as of March 2015.

Basically, if you’ve been on at least a few flights in your life, you’ve been on a Boeing 737.

Why i’m writing about this:

  • Does it matter why?

  • Shut up and mind your own business.

The primary users of this jet series are:

  1. Southwest Airlines

  2. Ryanair

  3. United Airlines

  4. American Airlines

Although those four are the most popular operators of the 737, you can see from this Wikipage that this jet is used by pretty much everybody.

Let’s get to the good part now.

Between the years 1991-1999, this model jet has experienced problems with its rudder actuator. This literally causes the jet to at random lock it’s rudder position to the right or left and even cause the reversal of the rudder controls.

As the plane would spiral towards the ground, pilots wouldn’t know in the ten-second average that this crash took that the controls would be reversed and that they are single-handedly driving their own planes into the ground!

There have been two confirmed accidents in the United States involving both a malfunctioning rudder actuator and a Boeing 737. This problematic rudder actuator, has been the only design failure to cause an accident in the 737 series. There have also been other accidents in various countries with this actuator being the cause.

“On March 3rd, 1991, United Airlines Flight 585, a 737-200, crashed in Colorado Springs, CO, killing 25 people.”

“On September 8th, 1994, USAir Flight 427, a 737-300, crashed near Pittsburgh, PA, killing 132 people.”

What is wrong with the rudder actuator?

Under the right conditions, the Power Control Unit’s dual servo valve can jam, which can then cause the actuator to deflect the rudder into opposite positions of a pilot’s input (right = left, left = right). This is caused by thermal shock. The valve itself become cold, while the hydraulic fluid remained hot.

The servo valve, is the component that directs hydraulic fluid.

What makes the situation worse, is that a Boeing 737 does not have enough roll control to counter a full deflection of the rudder at some airspeeds and flap configurations.

This means that after the rudder has been deflected, if the plane is going fast enough, the pilot simply will not be able to re-stabilize the plane until they get control of the actuator again. Something that is only possible if the plane is high-enough from the ground, and the pilot somehow realizes the issue quick enough to direct the rudder into the opposite direction they would normally intend.

Unfortunately, when these incidents occurred the crafts were at an altitude that did not allow enough time for the pilot to do any type of critical thinking (or for the actuator to unjam). They hit the ground at an average of 10 seconds after the problem began. It was only until after the problem happened to a plane high enough in the air, that there was enough time for the actuator to warm up just enough to unjam.

There has never been a case where the pilot was able to realize that they could control the craft by turning the rudder to the opposite direction.

What is most interesting

I can’t help but find it strange that during such a long period of production of this model, that this wouldn’t become a problem until the 90s. Something must have changed. I have yet to do the research myself, but I have my hypotheses.

Perhaps there was a change to the material the servo valve was made of? Or could it be caused by cross jetstreams from other planes? Heck, even global warming technically aligns with this new issue.

All we know for certain, is that the FAA ordered an upgrade of all Boeing 737 rudder control systems by November 12th, 2002. After looking at the document myself, I can see that there were different changes done to different models of 737 rudder control systems. You can take a look at the changes made for yourself, here.

From what I understand, there have been no reported incidents of the rudder actuator malfunctioning on a B-737 aircraft after it has been replaced.

Author: Brent Pantera

Date: October 30th, 2015



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