Last month, when United Airlines Flight 1539 from the Bahamas landed in Newark, N.J., the pilot tried to use the foot pedals to move the rudder, which steers the plane on the ground and keeps it centered as it moves down the runway.
But the pedals on the 737 MAX were stuck, and the rudder wouldn’t move.
The incident didn’t cause a serious problem. The captain was able to keep the airplane near the centerline of the runway, and 30 seconds after touchdown, the pedals began to operate normally.
Still, the incident prompted a National Transportation Safety Board investigation. A preliminary report released Thursday suggests cold temperatures may have affected part of the mechanism that moves the rudder.
The issue also affects the prior 737 model, the 737 NG, which has an identical pedal system for rudder control. Boeing said it is aware of two similar occurrences of stuck rudder pedals on the 737 NG in 2019.
In those earlier incidents, a component of the rudder control mechanism was replaced and both aircraft returned to flying.
In this latest incident, the only occurrence on a 737 MAX, Boeing said Thursday that “the issue was successfully resolved with the replacement of three parts and the airplane returned to service last month.”
John Cox, a pilot and longtime aviation safety expert, said that although the rudder is normally used to steer the plane only on the ground, it may have to be used in flight to correct an abnormal swing to one side caused by engine failure or a heavy crosswind while preparing to land.
In that case, a stuck rudder “could be an unsafe condition,” Cox said.
He said the NTSB report suggests components of the rudder mechanism may need to be inspected more frequently or replaced.
“It definitely bears careful evaluation,” Cox said.
Boeing said in a statement that the company “worked closely with United Airlines to diagnose the rudder response issue.”
“We appreciate the NTSB’s work on this preliminary report and will continue to fully support their investigation,” Boeing added.
How to steer an airplane
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 to the right or left.
It’s not normally used to steer the jet while in the air. That’s done by turning the pilot control column like a steering wheel, which moves flaps on the wings called ailerons to turn.
When a plane is taking off or landing, moving along the ground at a high speed, the pilot uses small deflections of the rudder to keep the jet centered on the runway.
After landing, when taxiing the plane at a low speed, the pilot makes sharper turns off the runway or into a gate not with the rudder but by turning the nose wheel of the aircraft using a steering tiller on a side console.
When the rudder pedals wouldn’t move on the United MAX landing at Newark on Feb. 6, the rudder was stuck in the neutral position and unable to turn the plane left or right.
The captain managed to use the nose wheel tiller to keep the plane near the runway centerline until the pedals unstuck.
NTSB investigators reviewed the flight data and corroborated the pilot’s experience. Despite increasing force inputs to the pedals, the rudder didn’t move.
On Feb. 9, United took the same plane up for a test flight and was able to duplicate the system malfunction in the air. The test flight was discontinued, and the jet landed safely.
Cox said that because of two fatal 737 accidents in the 1990s — 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 — any rudder issue on a 737 will get close attention from the NTSB.
Both those crashes occurred when a system malfunction caused the rudder control power unit to jam and the rudder to fully deflect to one side midflight, a condition known as a rudder hardover.
A rudder that’s temporarily stuck in the neutral position is not such a problem.
“This is not like the hardover conditions that brought down United 585 and USAir 427,” Cox said.
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.
Cox said the NTSB will “need to do a variety of more testing to better understand the failure characteristics” and figure out what action is recommended.
A cold-soak clue
Boeing delivered the MAX that experienced the rudder failure to United just a year earlier. In contrast to the recent spate of news from Boeing, the cause of the incident does not appear to be any quality lapse in production.
Initial testing has already offered a clue to what went wrong in Newark.
The mechanism at the rear of the plane that moves the rudder was removed by the NTSB and tested.
A key component of that system is a servo motor that moves the rudder as commanded, made by Collins Aerospace, a unit of aerospace giant RTX, formerly known as Raytheon.
In late February, the NTSB tested this servo at a Collins facility in Cedar Rapids, Iowa, and discovered that while it operated normally at room temperature, after a one-hour “cold soak,” the movement of the servo’s crank arm that connects to and operates the mechanism was restricted.
This “would prevent the rudder pedals from moving, as observed during flight 1539 and the test flight,” the NTSB report states.
The report does not say how cold the “cold soak” was. And there’s no indication of what the trigger for this might be when it seems to happen so rarely.
There are just three reported cases with about 1,300 MAXs and more than 6,000 of the earlier model 737 NGs currently flying.
The NTSB said it will conduct further examination of the rudder servo system.
Collins declined to comment beyond saying it “is supporting the NTSB in its investigation.”
