B747-400 at AMS in the grass.

[jonw66]

Flyburg
I don't suppose you know what caused the problem with the Nose Wheel Steering Valve Metering Unit do you.
Sorry

[Flying Clog]

Wasn't it a brake problem, or was it a nose wheel steering metering valve issue?

[JammedStab]

It appears that this type of event happens on occasion with Boeing aircraft. You can read about it here.

http://www.bst-tsb.gc.ca/eng/rapport...3/a10q0213.pdf

"A nose-gear steering rate jam occurs when the feedback to the steering system is interrupted, causing the nose gear to continue turning (slewing) at a fixed rate until full travel is reached or until the rate jam is eliminated.

One explanation for a rate jam is trapped debris within the metering valve (slide/sleeve), which prevents it from completely closing. This prevention causes the nosewheel to change angle at a given rate, thereby causing an uncommanded steering input. A metering-valve rate jam is difficult to confirm, since it may not necessarily cause damage to the valve assembly, and the debris causing the jam may be flushed away through the hydraulic system fluid once the jam is cleared, leaving no evidence of a jam."

[harrryw]

Would that not mean that the contaminant remains in the hydraulic fluid as it is a closed system.
Are all the steering hydraulic systems completely flushed and refilled if this is thought to have occurred? If not why not?

[Dannyboy39]

@JammedStab

Correct me if I'm wrong; but isn't there an FAA AD out for that issue?

[Mechta]

Would that not mean that the contaminant remains in the hydraulic fluid as it is a closed system.
Are all the steering hydraulic systems completely flushed and refilled if this is thought to have occurred? If not why not?

A work colleague who had a lot of experience with hydraulics, advised against flushing out hydraulic systems unless absolutely necessary, as the smaller debris in systems that have already had a lot of use often help with sealing(!). Once flushed out, a lot of further maintenance is likely to be required as parts that were sealing no longer do so. I would assume that there are a range of filters in the system to catch the larger debris of the sort that caused the problem in the thread, and one would expect those to be checked and changed at regular service intervals.

Inspection and cleaning procedures here: http://www.faa.gov/regulations_polic...a/ama_ch12.pdf