Originally Posted by
Yeelep
Because you could have both loops faulted. Under normal operations the fault light will only illuminate with both loops faulted on the same engine, If you have "A" loop fault on eng. 1 and "B" loop fault on eng. 2 it is not MELable. The way to find this out is to isolate the loops during test, hence the manually selected "A" "B" loop switch.
Yes I agree, but my point was that if the system is able to detect a fault automatically, and to isolate automatically a failed loop, you don't need to manually select loop A or loop B because the system does it by itself anyway ! So the only explanation I can come with, is that some failures are not monitored by the system (and thus require a system test performed by the pilot), while some failures are monitored and detected automatically (I assume the loss of gas pressure is one of them).
It would be interesting to know which failures are monitored by the system, and which failures need a system test performed by the pilot. (Not that it is very important, but I like to understand the systems in depth even if it has no "operational" implications. )
Originally Posted by
Capt Quentin McHale
Capt Quentin McHale]A: Yes, exactly that. A loop failure could be any number of things e.g. loss/low gas pressure in loop/s, bad/dirty cannon plug connection etc.
Do you have any reference as to which of those failures actually need a system test to be performed by the pilot ? And which of them are monitored by the system without any pilot input ? Because the FCOM doesn't detail anything, they just talk about a "loop failure" but there are actually different kinds of failures !
Originally Posted by
Capt Quentin McHale
Capt Quentin McHale]As a side note, the Wheel Well loop is salt filled, but that's for another day.
That's indeed interesting that both types of loops are used on the 737. I used to work on an MD80 and I remember they used the resistor-type loops in the engines as well. It is interesting to see that on the 737 the engines use gas-type loops whereas they use a resistor-type loop in the wheel well. Do you know the reason why they use both ?