Mark,Yes... so it will open the VALVE, and in NORMAL operations, in that situation (with APU-powered startup) I will have the APU on AND APU bleed on, and thus after moving the start selector to GND I will damage the APU (according to what has been said about this kind of start in this thread) ??? This is what I am after. Because to me it seems, that, according to what I've read in this thread, it is NOT OK to keep the APU ON and APU BLEED ON when an engine is running when starting the second one.I'll make it more clear:I have lived under the assumption that it's ok to do this. (this is a very rough example)1. APU running, APU BLEED ON2. packs OFF, all ISLN switches OPEN, ENG BLEEDS ON3. Start number two4. Start number one <--- this is where the question of the x-bleed start restriction of APU having to be off OR the bleed closed comes in, because I change NOTHING before starting number one engine.5. turn OFF APU, close L & R ISLN switches (C in 757), packs AUTOAccording to this, I'm doing the x-bleed start in 4. and the APU should be OFF or the bleed OFF, which I never do. Am I doing this the wrong way ?Tero

Tero,Remember, for a crossbleed start you need to spool up the engine for it to create enough bleed air to start the second engine (I believe 70% N1). This air pressure is enough to damage the APU. At idle power the pressure created by the engine is not enough to do any harm to the APU.Regards,Mark

Mark,This is what I just thought that would provide the explanation. Either that, or I'm doing something terribly wrong (see my edited last message of the rough procedure I use to start engines) ;).Also, with idle, the N2 is around 65%, so the spool up is not much (it needs 70% N2, not N1), which is why I didn't think that was the reason for the restriction. Apparently it is though.Iz ? Ian ?Tero

Tero, the APU bleed valve does not know that you're gonna do a crossbleed start. There is no switch on the thrust lever that tells the APU bleed valve "Hey, stay closed when the start switch is moved to GRD!" If APU bleed is available, the APU bleed valve will open EVERY time you turn the engine start switch to GRD. That is why you need to have the APU off, or the APU bleed switch manually selected off.If you did not do this during your crossbleed starts, then yes, you did it wrong.You need 30 psi duct pressure for engine start, so whatever engine thrust setting achieves that, that's what you need to set. For different engines (757/767/RR/PW/GE), it's different values. But let me assure you, the thrust setting required for that is enough to cause some serious problems during pushback.

Iz,So basically if I start both engines without external air, the other one is always started with a crossbleed start, and in that case the APU should be turned OFF or the APU bleed closed ?I guess it is assumed that the engines are always started with external air, because NOT ONE document (official or PIC-related) mentions this when discussing engine start procedure! Would one think of turning OFF the APU after one engine is running ? Surely not, because turning the APU OFF is part of the after start procedure (turning OFF the APU BLEED is part of the after start procedure in some companies, too). Would one then think of turning the APU bleed OFF after starting one engine ? Not, ... UNLESS there were threads like this ;).So, when ext power/air is not available, and engine start is done with APU, the APU BLEED switch needs to be manually closed before starting the other engine.I think this is pretty significant piece of information, which, I have to admit, I have entirely overlooked! (hence the confusion about x-bleed start all this time)Tero

"So basically if I start both engines without external air, the other one is always started with a crossbleed start, and in that case the APU should be turned OFF or the APU bleed closed ?"It sounds like you're still confused, Tero (and I think Iz and I may have overlooked some of the PRSOV logic in our previous messages). In the above paragraph, are you referring to an APU bleed start?Here's a step by step description of what happens during an APU-assisted start. In this scenario, the APU does not have to be switched off half way through the procedure (as you seem to be saying).Start Switch pulled on Right engine...Right engine bleed valve/s is commanded closed to stop APU bleed air leaking into the engine core.APU bleed air is fed to the Right engine starter (which is downstream (main engine bleed-wise) of the main engine bleed valve known as the PRSOV). At 50% N2, plus x seconds, the Right Bleed valves are commanded open. The PRSOV won't open immediately because of back pressure from the APU. The engine must be supplying sufficient air to open the PRSOV. When the PRSOV is open, a command is sent to the APU bleed valve to close it. This closure (plus the closure of the additional APU check valve) stops air flowing back into the APU from the engine. The APU check valve is mechanical, so will operate irrespective of whether the APU is running or not (safety feature).When the second engine is started, the process begins again. Both L/R engine Bleed Valves are commanded closed and the APU bleed valve is commanded open. At the end of the start process with both engine bleed valves open, the APU bleed valve is once again closed with system logic.Now, if you want to make a crossbleed start, you want to stop the APU participating in the start process and stop the Left (second) Engine start automatically closing the running engine bleed valves (otherwise we will have no bleed air for the crossbleed start). If we close the APU bleed valve manually or shut down the APU (closing the APU bleed valve automatically), this inhibits the circuit which commands the PRSOV (and other engine valves) to close when (any) Start Switch is selected to GND.The APU bleed valve can be closed manually (with the switch), or automatically (when the APU rpms are below 95% (on shutdown).So, basically, irrespective of how many rpms the Right engine is doing (with the APU bleed ON or OFF), the APU should be protected from engine bleed due to the pneumatic system logic and mechanical check valves.Hope this makes sense.Cheers.Ian.

Ian,Now this is what I was looking for.You see, I understood the quote from the AOM the wrong way. I thought that the requirements for APU to be shut off OR the APU bleed be closed were ONLY because of protecting the APU. Your explanation basically tells that you cannot do a crossbleed start UNLESS the requirements are met. I did not know that with APU running and its bleed open the ENG bleeds will automatically close when starting (GND). I thought that the x-bleed start can be achieved with APU running and the bleed open, and that they should be turned off and closed only as a safety measure.So basically, my APU-assisted starts have in fact been correct, and I have not damaged the APU :). Finally, to summarize (say yes if you agree :)):1. APU assisted start (I skip the obvious parts)- APU running, APU bleed OPEN- L and R ENG bleeds OPEN=> Normal startup: first right, then left. The APU and APU BLEED can be ON during the whole startup process of both engines. The system closes L/R ENG bleeds when starter activated (GND), and x-bleed start is not possible2. X-bleed start- APU running, APU bleed OPEN- L and R ENG bleeds OPEN=> Normal startup for the right engine. Then manually CLOSE APU BLEED. => APU is no longer taking part in the process, and thus when starter is again activated for the left engine, the ENG BLEEDS _remain_ open and will make the X-bleed start possible.=> Start the left engine3. EXT-air start ?- APU running OR shutdown, depends if EXT pwr connected or not- APU BLEED CLOSED (but APU can still be used to power up the a/c)- L and R ENG bleeds OPEN=> Normal startup, similar to APU assisted start, BUT:=> additional question regarding EXT-air start (bear with me heh): - With EXT air assisted start, what will happen during the starting of SECOND engine, now that the APU bleed is CLOSED and the number one engine already running (=resembles x-bleed start!). Can the EXT air still be connected to the system while starting up the other engine, ie. is the air assisting the number two engine start now the SUM from air from ENG ONE BLEED & EXT AIR ?Tero

Hi, Tero.Sounds like you almost have it :-)1. OK"2. X-bleed start- APU running, APU bleed OPEN- L and R ENG bleeds OPEN=> Normal startup for the right engine. Then manually CLOSE APU BLEED. => APU is no longer taking part in the process, and thus when starter is again activated for the left engine, the ENG BLEEDS _remain_ open and will make the X-bleed start possible.=> Start the left engine"My engineering (training) manuals indicate that there is a requirement for the person in the cockpit to close the BLEED VALVE on the engine which is to be cross-bled started (in this case, the Left one).... presumably to stop air going into the engine core(?). If the air goes into the core, there may be less air available for the starter."3......With EXT air assisted start, what will happen during the starting of SECOND engine, now that the APU bleed is CLOSED and the number one engine already running (=resembles x-bleed start!). Can the EXT air still be connected to the system while starting up the other engine, ie. is the air assisting the number two engine start now the SUM from air from ENG ONE BLEED & EXT AIR ?"Good question :-) Not really sure about this one. To be on the safe side (you don't want to damage/upset the EXT AIR ground cart with the engines at high rpms... or damage/upset the engine with the EXT AIR cart), I would either disconnect EXT AIR before crossbleed starting the second engine... or close the Right ENG BLEED AIR air switch and use only EXT AIR to start the second (Left) Engine. As with the normal crossbleed start, I would also switch off the Left Engine Bleed to stop the air going through that engine core). The aircraft EXT AIR connections have reverse flow check valves, but it may be best to play it safe and not rely on them to stop the engine air going back into the start cart. It's difficult to know what will happen as I don't know how much pressure a typical start cart produces.Note that these procedures are for the CF6 Engine. Other engine procedures may vary (especially RB211's as the Start Valve is on the upstream side of the PRSOV. In this case, it will be necessary to keep the ENG BLEED switch ON (engine valve armed to open) on the engine to be started for all kinds of starter-assisted starts. This is true for the 747-400 with RB211's also). Also note that I have no official pilot procedures for these kind of starts. It may be best to ask Iz, HPSOV or other real pilots about this. If one of your start methods is not covered in their books, then it is probably not done.Another thing to note is that the logic which closes the engine bleed valves when the Start Switch is positioned to GND is also affected by the airplane AIR/GROUND status. If the airplane is in the air, the engine bleed valves do not close during start (Even if the APU was running and its bleed switch was ON). I can only guess at why this is the case.Hope this makes sense.Cheers.Ian.P.S. I'm not sure how close the 757 resembles the 767 when it comes to start logic (in case you were wondering) ;-)

Ian,You made perfect sense. A very educational thread.I have a pile of those books myself, so I will try to look for the procedures there. Hopefully one of "our own" pilots here can chime in before that though, and fill in the details. And yes, I was wondering about the 757. :)cheersTero

Tero and others,I think the logic about WHEN to do a crossbleed start is also missing here.Normally, you ALWAYS start up with APU air, unless the APU or APU bleed system is inop. If that is the case, you need an external air cart. This cart can start up both engines. The fact of the matter is that these carts are REALLY noisy and sometimes have a hard time achieving the required air pressure. Engine start takes a long time and you can even notice the strain on the air cart by listening to the pitch of the noise. It is possible to connect two air carts to the 757 but those are usually not available, more hassle, more expensive and more noise.That is the reason why a crossbleed start is done. A crossbleed start is easy and quick, sometimes even quicker than an APU assisted start.So, bottom line: When the APU is working, you NEVER do a crossbleed start. You could, if you wanted to, but in that case, you should shut down the APU or the APU bleed valve.You also NEVER do a crossbleed start with the external air cart connected. This is done either at the gate before the pushback, or on the taxi track, after pushback (because most pushback trucks cannot handle the increased thrust during the pushback).

Iz,Thanks for bringing that point up as well.I think we pretty much covered the topic for now :)Tero