How are the 747-400 rudder pedals and tillers connected to the nose wheel?

Question

The Boeing 747-400 Before Taxi Procedure says (emphasis mine):

[...]

Flight controls .............................................. Check C

[...]

Hold the nose wheel tiller during rudder check to prevent undesired nose wheel movement.

Move the rudder pedals to full travel in both directions and verify:

• freedom of movement
• that the rudder pedals return to center
• correct flight control movement on the EICAS display

The FCOM (14.20.3 Landing Gear - System Description) further says:

Nose Wheel and Body Gear Steering

Nose wheel and body gear steering is powered by hydraulic system 1.

Primary low speed steering control is provided by a nose wheel steering tiller for each pilot. Limited steering control is available through the rudder pedals. The tillers can turn the nose wheels up to 70 degrees in either direction. A pointer on the tiller assembly shows the tiller position relative to the neutral settings. The rudder pedals can be used to turn the nose wheels up to 7 degrees in either direction. Tiller inputs override rudder pedal inputs.

I wonder how the rudder pedals and the tillers are connected. How can the tiller override the pedal input? I assume the tillers would move when the pedals are used, but do the pedals also move when the tiller is rotated?

Answer 1

Both steering methods work via a clutch (12) that engages only when the nose wheel is loaded. So in the air, the nose gear won't turn by either input, whether still extended, or retracted.

The pedals' input to the clutch is a pivot, and is therefore more limited, whereas the tiller's (15) interface is cables.

Now based on my understanding:

1. The pedals move the tiller but not the full range since the cables/clutch/pivot move in that scenario*
2. If the tiller is held in place, the cables that activate the hydraulic power steering actuators cannot move, and the pedals' input will simply pivot freely (clutch slip)
3. The tiller overrides the pedals for the same reason in 2.

For completeness, the main body gear steering starts only when the steering input is big thanks to a cool looking cam (29) that pivots the quadrant (23 around 31).

For double completeness, Terry once told me the linked steering is not on a minority of the -1/200 variants, so for those it was either company option, or disabled.

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* You can watch it here. Note how the bumps on that takeoff bump the tiller as well.

Sources:

• Nose gear steering – Boeing patent (most definitely the 747's judging by the approval date)
• Main body gear steering – Boeing patent (definitely the 747's).

Answer 2

The easy (non-technical) way to understand 74/75/76/77 steering is that the tiller controls the nose-wheel and the pedals control the rudder. And that they are connected by a spring.

So moving the pedals will A. move the rudder, and B. slightly move the tiller which will move the nose-wheel. Moving the tiller will A. move the nose-wheel, and B. slightly move the rudder.