B747 Hyd Qty Low 1
 

In the HYD QTY LOW 1 non-normal checklist for the B747, it gives us two possible choices. If the HYD PRESS SYS 4 is not shown, procedure states for the flight to continue normal operation. If HYD PRESS SYS 4 is shown, then there will be some procedures to follow.

As for the HYD QTY LOW 4 non-normal checklist, the QRH does not provide the given step : If the HYD PRESS SYS 1 is shown.......

What is the reason for that? Does it have to due with the service bulletin : Potential Sequential Loss of Multiple Hydraulic Systems?

I am a junior first office and would very much appreciate any help. Thanks in advance.

As far as I know that's exactly right, it has to do with the possible sequential loss of systems 4, 1 and 2, due to common tubing on the brake lines.

Thus if you get HYD QTY LOW 1 without any problems on system 4 first, your problem is with system 1 only. If instead you get a HYD QTY LOW 4 followed by HYD QTY LOW 1, you may well be in the multiple sequential loss scenario; the checklist then directs you to depressurize system 1 (to conserve fluid) until required for gear and flap extension.

MD