Any of you guys able to offer an explanation for the 'barking' being reported from the cabin crew taxiing out to the holding point for t/o? Hydraulic system page checked, all normal and PTU shown to be not running.
If memory serves, the PTU indications are based off the pump and system low pressure switches,rather than a position pot on the PTU control valve, but the hydromechanical bit opens the valve when you have 500psi differential pressure (as noted above). If you get the chance, grab an AMM and have a look at the indication sections.
So a suggestion to what
could be occuring: during flap/slat extension and controls checks, and low engine RPM, if you drop (for example) you could (theoetically) get the presure dropping low enough for the PTU to pop in without the system dropping low enough to trigger the indications.