I understand the principle that the 'brake source' caption illuminates when pressure in the right hydraulic system is low. I've noted (only recently) that it illuminates during descent. I presume that this is because HYD pressure is low with the engine at idle. Is that correct? It doesn't sound right as clearly there are other systems requiring HYD pressure in flight which would also suffer. The caption only lights during descent and is gone by the time the approach starts and it's not actually a failure. Failures are enabled, though.Is the brake system powered by the engine pumps only and the other stuff from the electric HYD pumps. Can anyone point me to a schematic? One for Ian R perhaps?AndyEGTR(Standard Disclaimer Applies - Forum Searched Before Question Asked)

"I've noted (only recently) that it illuminates during descent. I presume that this is because HYD pressure is low with the engine at idle. Is that correct? "It doesn't sound normal, Andy (I may have to check this out). Have you made any changes to PIC which might cause this (e.g. POSKY merge)?"Is the brake system powered by the engine pumps only and the other stuff from the electric HYD pumps.In most cases, all the (demand/engine) pumps in a particular (L, R or C) hydraulic system share the load for that particular system. You can't get one particular pump to feed only brakes or flight controls under normal circumstances. However, when hydraulic pressure becomes critically low, the hydraulic systems are designed so that certain airplane systems will work whilst others won't (i.e. some systems are able to run on lower pressure than others, so continue working, whereas others will stop). At the moment, I can't recall how far up the priority list the brakes are (this is the sort of stuff an Engine/Airframe guy would be more familiar with, rather than an Avionics guy like me ;-) If you don't get any responses to this one on the PIC forum, you may find someone like that at the PRRuNe site).Also, when things start getting really bad (with the engines below idle), the RAT, of course, pops out. This is designed to feed only certain airplane flight controls. In this case, a system of non-return valves in the hydraulic plumbing prevents the RAT hydraulic pressure reaching non-critical systems. I thought there was a schematic in the PIC manuals showing this, but I may have been mistaken (I just had a quick look, but couldn't find anything. There are some pics in the Abnormal Checklists however, which may help you)."Can anyone point me to a schematic? One for Ian R perhaps?"I'd probably get into trouble if I posted too many official Boeing diagrams, Andy, but I'll see what I can come up with in the next few days ;-)Cheers.Ian.

Here's a diagram I found in my files which may help... not sure where it came from...http://www.ozemail.com.au/~iriddell/767/Hy...lockDiagram.gifThe firm lines show the normal hydraulic feed to the systems. The dashed lines show the abnormal feed. E.g. during RAT operation. BTW, it looks like I overlooked the "reserve" (emergency) braking and steering system: If the hydraulic fluid level (not pressure) in the Center system falls below 48%, valves automatically isolate certain sections of the hydraulic system, allowing the remaining fluid and the #1 Center Elec Hydraulic pump to power only the brakes and steering.Cheers.Ian.

Thanks so much, Ian. However, as you suspected, I'm none the wiser! To answer your specific question, it isn't the vanilla PIC. I'm using the panel for a 757 but it's not a conventional merge. What I did was to simply copy all of the panel and sound from the PIC directory, and use the exact .air and aircraft.cfg files from PIC, only changing the bit's at the top of the .cfg file so that it looks like a 757 to MSFS. As far as the panel is concerned, it's PIC, but it's wearing a 757 coat. It performs and operates exactly as a 767, including 767 weights, but looks like a 757. The only reason I do it is so that I can maintain the fiction (in my somewhat warped mind) that it's a 757, but I don't have to suffer the unbelieveable performance degredation that is present in other attempts to use PIC with 757 models. The net effect should be that it is, to all intents and purposes, PIC, and it fits into 757-shaped spaces.Also, I don't believe it's breaking any copyright issues because I'm still using it on the same PC as I've installed PIC and I can only use one at a time.But it's interesting that you mention it in the context of this brake source issue. I'll try a couple of flight using the original and see if I can duplicate the snag.Thanks again for the info,AndyEGTR

Reading Ians answers in this forum is teaching me more than ground school did!But to answer in a slightly simplified way once again.Normal brakes are powered by the right hydraulic system. In the event of low pressure in the right system, alternate brakes are powered by the centre system. This transfer is automatic and not annunciated. So anytime you have a low right system pressure, you assume the brakes are operating UNLESS you have a BRAKE SOURCE light and EICAS message. In this case the brakes are not recieving hydraulic pressure from anywhere, and the reserve brakes must be used. This is accomplished by pushing the reserve brakes and steering switch located on the captains panel.This isolates a portion of the centre systems, and the Centre 1 primary pump for use by brakes and steering only.Your brake source light on descent is VERY strange, and would be cause for some alarm, as it implies a failure of normal AND alternate brake systems. OR a failure of the centre and right hydraulic systems. I'd say, as Ian suggested, its a problem associated with the software merge.