On Boeing 737’s ‘defective’ rudder system | Explained

The story so far: Certain variants of the Boeing 737 (fitted with a ‘rollout guidance actuator’) are under the scanner over an issue that concerns the potential for the occurrence of a jammed or restricted rudder control system. The United States National Transportation Safety Board (NTSB) has put out safety alerts to the manufacturer, Boeing, and the Federal Aviation Administration (FAA). Flowing from this, India’s Directorate General of Civil Aviation has issued directives to Boeing 737 operators in India. The issue has arisen after an NTSB investigation into an incident on February 6, 2024, involving a United Airlines Boeing 737-8 MAX flight, UA 1539, from Nassau in the Bahamas to Newark, U.S. In 2019, two similar events had occurred with foreign airlines.

What was the issue?

During the landing rollout phase of flight 1539, the rudder pedals were found to be “stuck” in the neutral position. Despite the use of pedal force of about 42 pounds by the crew there was no movement which resulted in the captain having had to use the nosewheel steering tiller to control the aircraft’s direction. The crew then found the copilot to be experiencing the same problem.

The crew applied more pedal force (flight data recorded it to be about 59 pounds) which resolved the issue. The problem was reported to the airline’s maintenance department. There was no damage to the aircraft which was removed from service for checks.

What did the flight data show?

Examination of the Flight Data Recorder (FDR) data aligned with the pilot’s statements of a malfunction. The data also showed increasing force inputs to the rudder pedals ‘even as the rudder surface position remained near its neutral position’. But with significant pedal force by the crew for the second time after landing, the rudder pedals and rudder began to function normally.

On February 9, 2024, United Airlines did a test flight with the incident aircraft wherein there was a recurrence of the malfunction. After landing, the NTSB was notified. An investigation began which included United, the FAA, Boeing and Collins Aerospace, the manufacturer of the rudder component in question, called the rollout guidance actuator. A preliminary report which detailed the circumstances of the event was published on March 12.

The FDR data were analysed at the NTSB laboratory in Washington DC. In addition, there was an inspection of the rudder control system but no obvious malfunctions were detected. But still, certain parts of the rudder system components were removed for detailed inspections. After this, United conducted a second test flight where the rudder control system operated normally.

How does the rudder issue come into the picture?

As a part of the aircraft tail, the rudder plays a crucial role in aircraft control around the yaw axis (one of the axis of aircraft motion) and is vital in coordinated turning, ground directional control, crosswind landings and even in the event of engine failure.

As a Boeing 737 pilot explains, the rudder pedals on the 737 which move the rudder in different ways are connected through cables to what is called a torque tube mechanism. This in turn is connected to a Power Control Unit or PCU; there is a standby unit as well. All this is in what is called a closed loop mechanism, so the position of the pedals corresponds to the position of the rudder. While in flight, the PCU also helps the flight crew coordinate turns with the roll from the ailerons (on the aircraft wings) which help in banking to turn.

Some 737s have a separate system to move the rudder and this is where the component made by Collins Aerospace comes into the picture. The United Airlines 737 was fitted with a Collins Aerospace SVO-730 rudder rollout guidance actuator. This is a component that is linked to another aspect of aircraft operations. In conjunction with the aircraft autopilot, the actuator plays a role in a certain type of instrument landing system (ILS) approach called CAT III, more specifically in CAT IIIB operations. In normal aircraft operations, ILS approaches are also linked to weather and visibility. In India, ‘CAT III landing’ is a term that comes into focus in airports and flight operations in northern India especially during the winter season when fog/bad weather affects visibility.

There is another aspect of aircraft operations in CAT III and when the aircraft’s autoland function is used. Here the aircraft will basically land by itself using the autopilot systems. In CAT III, the crew inputs occur during the landing phase where, after the autopilot is disconnected, there is need for directional control. In CAT IIIB, the autopilot remains connected. And the rudder rollout guidance actuator, which is an additional rudder component feature, enables the autopilot to work directly with the rudder through the landing phase. Out of the thousands of 737s that have been manufactured, only a small number of 737s have been equipped to carry out CAT IIIB autolands.

In the course of the investigation, the NTSB conducted ‘cold chamber’ tests on the actuator. The component was found to jam, with significant force (which were beyond the design limits) having to be applied to get parts moving. There was also evidence of moisture and corrosion. In its technical review, Collins determined that a seal in the actuator, in a clutch bearing, had not been assembled correctly during the manufacturing process. In August 2024, Collins notified Boeing that 353 rudder rollout guidance actuators it had delivered to Boeing from February 2017 were affected. Reports are that this could impact about 40 airlines across the world.

What has Boeing said?

In an email to The Hindu, Boeing has said that it has reminded operators of the proper actions that flight crew should take were they to encounter rudder restriction. This includes following the checklist for jammed or restricted controls during flight and using various tools to maintain ‘centerline’ on the ground. It added that it has also shared technical guidance from the supplier.

Boeing also said, “In March and August, we informed affected 737 operators of a potential condition with the rudder rollout guidance actuator, which is part of an optional autoland system. The optional system allows landing during limited visibility CAT IIIB autoland conditions. The actuator is the part of the system that assists pilots to maintain centerline steering. Redundancies within the 737 design allow continued safe operations while the potential condition is being addressed.”

About the defence variant of the Boeing 737 or the P-8 (which the Indian Navy operates), Boeing said, “We are working to determine the full scope of this issue as it relates to the P-8 Poseidon maritime surveillance aircraft based on the 737 NG and are in contact with our customers. Redundancies within the 737 design allow continued safe operations of the P-8 fleet.”

What has the DGCA said?

The DGCA has asked Indian operators of the 737 to simulate scenarios that involve a jammed or restricted rudder control system during pilot recurrent training and instrument rating/pilot proficiency checks. It has also asked airlines to discontinue CAT IIIB operations that include practice or actual autoland until further notice.

Of the three main 737 operators in India, Air India Express has said that five of its aircraft are under inspection while SpiceJet said none of its aircraft is affected. Akasa Air did not respond to queries, but an aviation source said that around 14 of its 737s might have to be checked.