Originally Posted by
wiedehopf
Maybe they also want to avoid the possibility of "managing" the fluid level in the hopes of landing with all hydraulics.
If the quantity is almost zero when you land you could have a HYD C failure on short final.
This in turn means you either need to go around or land without having applied the checklists for landing with HYD C inoperative.
Basically Boeing probably prefer a failed hydraulic system over an unreliable one.
I can definitely see the merit in that. For the manufacturer it is almost always better to plan for the least amount of in the air trouble shooting and go for the standard procedure, so low level, unknown leak, high temperature any other thing seems wrong, just use the checklist to switch it off and the procedure will tell you what is/isn't going to work, without any last minute surprises.
Sometimes that won't work, on some of the A319 (now "fixed" with an OEB) with a green or yellow pump overheat, the procedure was to switch the pump off, and have the system pressurized via the PTU of the opposite side system. Sometimes the overheat would be due to low quantity, and this would lead to the PTU running continuously, and overheating the engine pump on the opposite side. You would then have to switch that pump off too, and would end up with dual failure. Now we have to check if the overheat is due to low quantity, and if so, switch off the affected engine pump and the PTU.
If there was any system that would reduce the risk during the landing phase (not just for A319, but in general like anti-skid, nosewheel steering, diferential braking aso) that I could get back by switching the system back on while on the approach, I would definitely plan for it not to work, but I could see myself switching it back on.