NTSB issues urgent safety alert after Boeing 737 rudder failure

The National Transportation Safety Board issued urgent safety recommendations Thursday to Boeing and the Federal Aviation Administration about the risk of a 737 rudder malfunction because of a manufacturing fault in a component.

The FAA said United was the only U.S. airline flying planes with the manufacturing defect in the rudder control system, and that United has already replaced the component on nine 737s, the only jets in its fleet where it was identified as faulty.

However, the NTSB alert may cause the grounding of some 737 MAXs and older model 737NGs flown by foreign air carriers that have not yet replaced the defective part.

The NTSB recommended that the FAA determine whether the faulty components need to be removed from airplanes, and, if so, notify aviation regulators in other countries and encourage them to require removal.

In a statement Thursday, the FAA said it “has been monitoring this situation closely.”

“Tomorrow, we will convene a corrective action review board based upon the NTSB’s interim recommendations and determine next steps,” the FAA added.

The NTSB recommended that Boeing notify flight crews operating 737s with affected components that the rudder control system can jam.

Boeing in a statement said it informed affected 737 operators in August of the potential condition with the rudder and is reviewing the NTSB recommendations.

The component that failed is part of an optional automated landing system that includes layers of redundancy, Boeing added. “We are working with our supplier to develop additional guidance.”

The NTSB also urged Boeing to come up with better instructions for pilots as to how to handle rudder jams.

Boeing’s 737 flight manual tells pilots to “overpower the jammed or restricted system [using] maximum force, including a combined effort of both pilots.”

The NTSB expressed concern that such force applied during landing “could result in a large input to the rudder pedals and a sudden, large, and undesired rudder deflection that could unintentionally cause loss of control or departure from a runway.”

The NTSB asked Boeing to come up with a more appropriate response, “besides applying maximum pedal force” for such situations.

Boeing said it will “ensure flight crews have the appropriate operating procedures.”

An incident in Newark

The rudder is a movable panel on a jet’s vertical tail fin that can deflect to one side so the air pressure on the panel turns the plane’s nose right or left.

The NTSB action arises from the investigation into an incident on Feb. 6 when, as United Airlines Flight 1539 from the Bahamas landed in Newark, N.J., the pilot found the foot pedals that move the rudder were stuck.

The component that failed moves the rudder during precision landings in low visibility conditions to steer the plane upon touchdown and keep it centered as it moves down the runway, a phase of operation known as rollout.

The United incident didn’t cause a serious problem. The captain was able to keep the airplane near the center of the runway using the nose wheel tiller, and 30 seconds after touchdown the rudder pedals began to operate normally.

Boeing had delivered that 737 MAX to United just a year earlier.

In March, Boeing said it was aware of two similar occurrences of stuck rudder pedals on the 737NG in 2019.

In the 1990s, two fatal 737 accidents — United Flight 585, which crashed in Colorado Springs in 1991 killing all 25 people on board, and USAir Flight 427 that crashed near Pittsburgh in 1994, killing all 132 people on board — were caused by a much more serious rudder malfunction.

On those flights, the rudder control power unit jammed and the rudder fully deflected to one side midflight.

A rudder that’s temporarily stuck in the neutral position, as in the United incident in February, is not so dangerous.

Nevertheless, a pilot needs a functioning rudder in case of an engine failure to counteract the turning effect of the asymmetric thrust, or in the case of a crosswind pushing a jet off course when close to the ground.

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NTSB testing identifies the problem

The component that failed — the rollout guidance actuator — has a servo motor that moves the rudder. It’s made by Collins Aerospace, a unit of aerospace giant RTX, formerly known as Raytheon.

In late February, the NTSB tested two of these actuators at a Collins facility in Cedar Rapids, Iowa, and discovered that when operated in very cold conditions “the actuators’ function was significantly compromised.”

Investigators found evidence of moisture inside the actuators.

Collins later determined that a sealed bearing was incorrectly assembled during production of the actuators, “leaving the unsealed side susceptible to moisture that can freeze and limit rudder system movement.” 

The problem is not the design of the component, but its misassembly in the Collins facility, said NTSB spokesperson Peter Knudson.

Collins subsequently notified Boeing that it had delivered more than 353 actuators with this defect to the jet-maker since February 2017. 

Of those, 25 were installed on 737s originally registered to U.S. operators, though 16 of those were subsequently leased to foreign carriers. The remaining nine were installed on United 737s.

RTX in a statement said it is working closely with the NTSB and Boeing and is supporting airlines operating 737s to mitigate the impact.

Neither RTX nor Boeing said how many airplanes globally may still have the faulty actuator.