Originally Posted by
SLF3
As I understand it the actuator is only used on roll out but if stuck due to icing the rudder would be inoperable in flight.
Is that correct?
Is the rudder required in flight or an optional extra? Asymmetric thrust on engine shutdown / failure?
The actuator is used also during final approach and landing, as well as on rollout. But you are correct that if moisture accumulates inside an actuator with an incorrectly assembled bearing, it can freeze and render the rudder controls inoperable in flight.
Rudder is not typically used in cruise. Rudder is used in phases of flight like final approach and landing, which is why pilots are only likely to discover this particular failure scenario (i.e., jammed rudder controls due to frozen rudder rollout guidance actuator) at the last moment, during a critical phase of flight. There is greater need for rudder in situations like an engine out or crosswind landings.
The NTSB sent a
follow-up letter to the FAA.
Originally Posted by
DaveReidUK
That's an unlikely scenario, given that it's a rollout guidance actuator, though a runway excursion is serious enough.
Dave, your comments have generally been on target in this thread, but I’d like to respectfully point out that this last post of yours evinces a misunderstanding of the scope of the concern. The “smoking hole” scenario is an equal concern. As the NTSB report notes, the rudder rollout guidance actuator remains mechanically connected to the rudder control system even when it’s not active. (In fact, on the United Airlines planes, the rudder rollout guidance actuators had been electrically disconnected but still remained mechanically engaged with the rudder control system.) This means that if moisture freezes in the actuator gearbox, it can jam the whole rudder control system, regardless of whether the rudder rollout guidance actuator is active or not, and regardless of whether the airplane is flying in the air or rolling on the ground. According to the NTSB report, there is concern about the potential for loss of control in flight, during landing, and on rollout—both because the rudder controls can jam but also because Boeing’s procedure for clearing the jam could produce the same result (loss of control) due to a large, sudden, and undesired input and resulting rudder deflection. The scenario could be exacerbated by an engine-out or high crosswind condition.