Greetings all,Just wondering if one of the many knowledgeable ones out there could tell me what the technical reason is for closing the isolation valves after engine start-up (other than the fact that it is on the checklist). My speculation is that it is to maintain two isolated systems, so that a malfunction in one will not affect the other. but if someone could verify this it would be much appreciated!Best regards :-wave.Chris BrunnerCYVR

Yep, spot on the money!Its to split the left and right systems so that they run independant of each other, so no single failure can knock out the entire system (unless the body cross-over duct ruptures....).You need to systems un-isolated on the ground so the packs can get air from the APU!

Hi Chris!Thank you for the reply. Your help is much appreciated!Regards :-wave,Chris BrunnerCYVR

Hang on, are the isolation valves only opened during takeoff? I thought that in flight the valves were closed as well (ie the left and right systems are NOT issolated in flight)Have I got that right?

This is quite simple actually - you have 3 ducts to pressurize: the center duct (used for bulk and aft cargo heat, ADP and center hyd res pump), plus the right and left duct (left for left wing AI and left hyd res pump, right for right wing AI, right hyd res pump and fwd cargo heat).the connection between the ducts is as follows:1. center duct may pressurize the right or left ducts if isolation valves are open.2. any of the side ducts can pressurize the center duct through a one way connection bypassing the isln valves.3. the left duct can pressurize the right duct if right isln valve remains open (through the center duct actually, though you open both isln valves normally).you can get a good schematic of this system by looking at the pneumatics controls panel.last thing to know: There are four air sources possible: left and right engines, APU and external air.Now in order to pressurize the three ducts, you position the isln valves according to the air source you use, like this:1. APU on, engines off: APU is connected to the center duct. in order to pressurize all ducts you must position both isln valves on to allow flow from the center duct to the left and right duct.2. APU on, engines on: the engines will pressurize all ducts, thus the apu valve commands itself closed. now this is when you command the isln valves closed: otherwise the air supplied to the center duct from the side ducts will flow back to the side ducts with no need, losing pressure in the center duct.3. APU on, one engine on - one engine is enough for pressurizing the ducts (i.e. before starting the second engine), so the apu valve commands itself closed. however you still need the isln valves open to allow the working engine pressurize the other engine starter (and duct). BTW when starting the other engine, apu valve will also open to add pressure, until engine is started.4. last - the two external air sources are connected to the left duct. in order to pressurize all ducts, isln valves must be open to allow flow from the left duct to the right duct and not only to the center duct.Whoa that was a long answer... anyway hope it helps.Bye,Ilan

Hi Ilan,Thanks for the great info! It definitely does help. I absolutely love highly technical answers, and I, for one, can never know too much!Thanks again!!Chris BrunnerCYVR

hey Chris,if really so you should go check out this address:http://www.smartcockpit.com/b767.html

Hi Denis,Thanks a lot for the link. I've actually been to this site before, and it's exceptional! Lot's of excellent reading there.Regards :-wave,Chris Brunner