B737 - retracting LEDs with PTU with Hyd Sys B loss
This might be a question more for an engineer than a pilot. I did not find much information in the FCOM or the QRH.
In the Cockpit Companion, Bill Bulfer states that it would be possible to retract the LEDs if there's fluid in the standpipe of system B reservoir by cycling the Alternate Flaps Master Switch to OFF and back to ARM and toggle the control switch to UP. How does it work actually?
My understanding is that the PTU control valve closes when the alternate flaps control switch is moved to DOWN even momentarily, switching off the alternate system re-opens the PTU control valve and will stay open until the control switch is toggled to DOWN, the UP position will not close the PTU control valve and if there's fluid in the standpipe, the PTU should be able to retract the LEDs.
Is my understanding correct?
This is not something the QRH directs the crew to do in any NNC, but in an exceptional case, it may make it possible to meet a gradient requirement following a loss of hydraulic system B on departure or to make it to an alternate field if things went really really bad.