HiCould someone detail the crossbleed start procedure to be used with PIC?CheersTom EvansB767/757/737/777/717/727/737NG/7E7/A320/A319/A321/A330/A340/A380/A318/IL86/IL96/TU154 Senior Captain

Edit: removed non working HTML codeTom,From the Operations Manual:CROSSBLEED STARTThe APU must be shutdown or the APU bleed air switch must be off.Check that the area behind airplane is clear.Advance thrust on operating engine to approximately 70% N2 and accomplish normal ENGINE START PROCEDUREKind regards,Stephan Haaswww.lemont.nl/b767flightdeckhttp://www.pic767flyingclub.com/images/sas544.jpg

Thanks Stephan.

And it works in PIC like a charm...Btw. with APU ON and the BLEED OPEN as well, the situation is of course like normal engine start with one engine already running (APU operated start, that is). I couldn't, however, see any difference in duct pressure whether I then closed the APU bleed or not. What's the point in APU having to be off when doing the crossbleed start ? With APU ON and the APU bleed open, one engine running (ie. normal startup), do the engine and APU both provide for the duct pressure, or is one dominating or automatically shut off of the system, when all bleeds are on, and ISLN swithces open ?Tero

Tom,You might want to abbreviate your sig to:ABELMDCILTU9473Kind regards,Stephan Haaswww.lemont.nl/b767flightdeckhttp://www.pic767flyingclub.com/images/sas544.jpg

Tero,>With APU ON and the APU bleed open, one engine running (ie.>normal startup), do the engine and APU both provide for the>duct pressure, or is one dominating or automatically shut off>of the system, when all bleeds are on, and ISLN swithces open>?I'm afraid this trhead will be going the same way as the one about electrical bus transfer (i.e. way over my head).There seems to be some internal logic behind the scenes. Let me quote some text from the manual (pneumatics)."The APU is protected from engine bleed air pressure by a check valve in the center pneumatic duct and the APU bleed air valve. System logic between engine operation and the APU bleed air valve closes the valve after the first engine is started and opens the valve during the start cycleof the second engine. After the engines are started, the APU bleed air valve cannot be opened unless the center isolation valve is closed or both engine bkeed air valves are closed."Strangely enough the manual doesn't say anything about this in the chapter about engine starting (unless I'm overlooking it). To be honest: I love flying the 767 (the only plane I fly), but at the moment I'm not too much into these sort of details.Kind regards,Stephan Haaswww.lemont.nl/b767flightdeckhttp://www.pic767flyingclub.com/images/sas544.jpg

"What's the point in APU having to be off when doing the crossbleed start ?"It may simply be an extra safeguard, Tero. Not much faith is put in the APU check valve for protecting the APU from reverse bleed air flows from the running main engine. Having bleed air flowing into the APU is not good for the APU compressor. On the 737NG, there is even a "DUAL BLEED" light to remind you to turn off the APU bleed with the engine bleed switched on. The NG has a vaguely similar bleed setup to the 767.Cheers.Ian.

Now I'm confused.Does this mean, that basically everytime I start the engines with only APU bleed (ie. no external lines hooked up), I'm doing a crossbleed start, and I should close the APU bleed before starting the second engine (number one engine, that is)? Surely it isn't so.This is why I don't understand the reasoning behind APU bleed having to be closed when starting the other engine with crossbleed start. Because everytime the engines are normally started with the APU air, the bleed remains open (and APU online) during the entire startup process, for both engines. How does that situation then differ from the crossbleed start ? Some companies do make an issue of closing the APU bleed after both engines are running (along with shutting off the APU of course), some don't. But that's another issue altogether (Finnair for example leaves the APU bleed open all the time).What is the difference between a normal APU-powered engine start process (both engines) and a crossbleed start ? Because I sure don't shut down the APU or close the APU bleed after only one engine running.Tero

"Now I'm confused.Does this mean, that basically everytime I start the engines with only APU bleed (ie. no external lines hooked up), I'm doing a crossbleed start, and I should close the APU bleed before starting the second engine (number one engine, that is)? Surely it isn't so."Not quite, Tero.When you pull an engine start switch, it automatically closes the PRV and PRSOV (engine bleed valves) on the engines and this allows the APU bleed valve to open (through system logic). At 50% N2, plus a suitable time delay, the engine bleed valves reopen, and the APU bleed valve closes. There is an amazingly complex diagram in the Maintenance Manual showing the valve sequence (very informative). There are a number of mechanical and electric interlocks to prevent air going the wrong way. Sometimes this can go wrong however. Sometimes you will find you cannot start the left engine after starting the right without any problems (You assume there is something wrong with the right engine, but it is actually a valve on the other engine not closing properly which is causing the problem)."How does that situation then differ from the crossbleed start ?"Crossbleed starts are usually for when you don't have APU bleed. In this case, one engine might be started using Ext Air at the gate, the aircraft will push back and the second engine will be started using crossbleed.Hope this makes sense.Cheers.Ian.

Ian,Thanks for your informative answer.The inner workings keep me amazed every time. Sort of independent of the valve positions on the overhead panel the valves do their job, triggered by system logic. Confusing.Kind regards,Stephan Haaswww.lemont.nl/b767flightdeckhttp://www.pic767flyingclub.com/images/sas544.jpg

Ian,Makes a lot of sense :).One additional question.Let's imagine that I do start one engine using ext air, and it's time to start the other. Now, since I don't have APU running, and the external lines are unhooked, I'm doing a crossbleed start on the second engine.Now that the APU is not taking part in the start-up process, what might happen if I had forgotten the APU bleed switch to ON ? I mean, could I damage the APU, if I had the APU bleed ON while doing the x-bleed start ?What I'm after here is this. Since the manual states that APU should be off, or at least the bleed closed when doing the x-bleed start, would the bleed have to be closed ALSO when APU is NOT running, or is there some internal systems logic that closes certain safety valves when APU is offline anyway, so the bleed switch position doesn't matter ?Tero

Hi Tero,In this case the APU bleed valve would close automatically (if the center isolation valve is open)PIC actually simulates this pretty well, just look at the APU bleed button, you'll see it indicate "valve" as the valve opens/closes. Note, the commanded position of the valve is not always the same as what the button is set to!The shortoming of PIC I found is that during an APU start, with one engine running, the APU bleed valve won't open when starting the second engine.Regards,Mark

It's quite simple really. When one or two engines are running and their respective bleed switches are ON (which they always are during normal ops), the APU is automatically 'disconnected' from the bleed air system, the duct is closed.So in normal situations, you don't need to shut down the APU or manually close the APU bleed valve.However, whenever you flick an engine start switch to GRD, the APU bleed valve will open. So if you do this while introducing 30+ psi of air pressure in the bleed ducts during a crossbleed start, you can blow up the APU.So that's why you either need to have the APU bleed air switch to OFF manually, or the APU must be off, during crossbleed start. If the APU is already off or inop, there's no need to manually switch the APU bleed air switch to off.The APU bleed air switch is electrically operated and pneumatically actuated, meaning the open/close command will be commanded electrically, but if there's no pneumatic pressure from the APU side, the switch will remain closed.

I must be either really stupid, or I'm missing something crucial here. But let me re-iterate again.>When one or two engines are running and their respective bleed>switches are ON (which they always are during normal ops), the>APU is automatically 'disconnected' from the bleed air system,>the duct is closed.>So in normal situations, you don't need to shut down the APU>or manually close the APU bleed valve.I'll describe the situation here. We have ONE engine running. Both engine bleed switches ON. APU is running and the APU bleed switch is _selected_ ON as well. In this situation you say that the APU is automatically disconnected from the bleed air. So no harm can be done to APU, irrespective of APU bleed switch position.So why then:>However, whenever you flick an engine start switch to GRD, the>APU bleed valve will open. So if you do this while introducing>30+ psi of air pressure in the bleed ducts during a crossbleed>start, you can blow up the APU.Let's continue on that same situation I described above. Now that I flick the starter to GND, the APU bleed valve opens, yes ? And it will introduce the pressure created by the first engine running into the system ? Above was stated, that if the ENG bleeds are ON, and one engine is running, the APU is disconnected ? Is it or is it not ? Tero

"Above was stated, that if the ENG bleeds are ON, and one engine is running, the APU is disconnected ? Is it or is it not ?"Correct, unless an engine start selector is moved to GND. This will open the APU bleed valve (if other conditions are met)Mark