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Crossbleed engine start

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Hi Folks,Can someone explain in details the procedure for a crossbleed engine start on the PMDG 737-600/700.On boeings 767 and 777 series the procedure is:APU shut down or APU bleed air switch off, advance thrust on operating engine to 70% of N2 then normal engine start procedure.Does this procedure applies to the 737-600/700?Can it be done with APU on and APU bleed on?which of APU or engine provides for duct pressure?DISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michael

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>Hi Folks,>>Can someone explain in details the procedure for a crossbleed>engine start on the PMDG 737-600/700.>>On boeings 767 and 777 series the procedure is:>>APU shut down or APU bleed air switch off, advance thrust on>operating engine to 70% of N2 then normal engine start>procedure.Hi Michael,If it's just an APU air snag, then you can use the APU for AC power. Anyway, you must get one engine started the normal way, either with ground power and air or with the APU. With one engine running, close the APU bleed valve, bleeds open on both engines and Isolation valve open. Turn the Air Con packs off. Advance the running engine to 70% N1, about 78-80% N2 and carry out a normal start.>>Does this procedure applies to the 737-600/700?The PMDG 737-600/700 can do this.>>Can it be done with APU on and APU bleed on?APU bleed should be turned off during the crossbleed start.>>which of APU or engine provides for duct pressure?The engine already running provides the duct pressure.Cheers,JohnBoeing 727/737 & Lockheed C-130 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

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Hi John,Thanks a bunch. This is exactly the kind of answer I was looking for.DISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michael

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Michael...Further info....The DUAL BLEED light is a reminder not to advance your engines above idle with the APU bleed still on. The APU doesn't like bleed air being forced back into it :-)John.... Do you know why the bleed valve on the engine being started has to be open? (the start process closes it automatically if it is open)Thanks.Cheers.Ian.

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Hi,Now we are getting to the bottom of it.Does the pneumatic logic in the PMDG 737 series reproduce adequately the real the system logic? ie. is the APU fully protected from engine bleed?Furthermore, is it correct to say that the APU should not be active (play a role) in te start phase and that the second engine should not close automatically the bleed valves? If not so then there should not be any bleed air for the crossbleed start. Or am I missing something here?DISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michael

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"Does the pneumatic logic in the PMDG 737 series reproduce adequately the real the system logic? ie. is the APU fully protected from engine bleed?"Not sure I understand what you are trying to say, Michael....Are you asking if the real aircraft is fully protected... or PMDG's NG (unrealistically so)?I'm not completely sure how good the safeguards on the real NG are, regarding dual bleed sources. There is a "check" valve in the APU bleed duct (a simple mechanical device which closes if the air from the engines is greater than the air from the APU). There is an electronic control system for the APU bleed output, but I don't know how it reacts with engine-supplied bleed pressure.... or whether it can close the main APU Bleed Valve completely. I'd guess not, that is probably why you have to close the APU bleed valve manually when making a crossbleed start on the NG. Note: The engine will most likely still start if you don't close the APU bleed valve, it's just not a good thing to do...i.e. have two bleed systems designed to work solo, feeding the same duct (Neither really knows which has priority).Check valves aside, the DUAL BLEED light is a reminder to the pilots not to advance the throttles with the APU bleed valve in the open position. There is the question of "Is the engine protected from APU bleed?" (as well as the other way around). When a start switch is moved to GND, the real NG APU bleed system goes into overdrive (40+ psi from new APU's)... and the mere 20psi the engine is putting out at idle is not going to win in this situation. Some of the engine valves may act as check valves to prevent the APU blasting air backwards through the (already) running engine, but again, this process seems to be governed by simple mechanical/pneumatic means (reliable, but not completely foolproof) rather than electronic means (a-la the 767)."Furthermore, is it correct to say that the APU should not be active (play a role) in the start phase and that the second engine should not close automatically the bleed valves? If not so then there should not be any bleed air for the crossbleed start. Or am I missing something here?"I'm not sure what you're trying to say here, either. It's not really the best of topics to discuss when there are no accompanying diagrams. Are you familiar with the NG's bleed system and have seen PMDG behaving differently to the real aircraft... or have you seen a discrepancy between something I've said and PMDG's behaviour?Perhaps I can clarify what I am trying to say...Putting the start switch to GND closes (via electronic logic) only the engine bleed valve on the engine being started (apparently). I deduced this from aircraft wiring diagrams. This protects the engine being started from high back pressures from both the APU and the other engine's bleed system.During a crossbleed start (when the APU valve is not selected open), the running engine will be providing pressure to the entire bleed system. When the running engine is put to the correct rpm for crossbleed start, the non-running engine's start switch is put to GND. This opens up the non-running engine's start valve so that bleed air from the running engine can flow to the non-running engine's starter motor. The non-running engine's main bleed valves (not start valve) don't need to be open to start the engine (You don't start an engine by blasting bleed air backwards through an engine's core :-)).Hope this makes sense?Cheers.Ian.

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Hi Ian,Had a hard time finding the thread again. It shows that our discussion may be a bit boring for many PMDG users.Thanks a lot for your explanations which not only make a lot of sense but also answered fully my questions. By the way I found a thread in the PIC Forum which is addressing the same issues and in which you heavily contributed.Thanks againDISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michael

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Further info...Found this paragraph in my notes regarding APU protection from engine bleed back flow....(Paraphrasing)"An APU automatic shutdown occurs if the APU load compressor (bleed) air flow decreases to approximately zero for 6 seconds. However, the APU Engine Control Unit commands the APU Bleed Air Valve closed when the air flow decreases to approximately zero for one second. With the Bleed Air Valve closed, bleed air can not reverse flow from the engine to the APU load compressor. This prevents most shutdowns due to reverse air flow."This means that the APU has two lines of defence against back pressure from the engine (i.e. the APU Check Valve and the APU Bleed Air Valve).Cheers.Ian.

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Hi Ian,So in fact the NG 737 series work along the same lines as the 767 and 777 series. It looks from the informations you gave here that they are identical for the three aircraft types.Michael

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"So in fact the NG 737 series work along the same lines as the 767 and 777 series. It looks from the informations you gave here that they are identical for the three aircraft types."Roughly speaking, Michael...I'm not familiar with the 777 and other types, but all modern Boeings will have reverse flow protection in some form or another.In the case of the 767, the APU valve is immediately commanded closed with electronics whenever the engine bleed valves are open (and start is not in progress). On the 737NG, however, the APU bleed valve is not directly commanded to close if either/both engine valves are open: Abnormal duct pressure has to be sensed before the command is given. If APU bleed pressure is greater than engine pressure, it will not be told to close at all.Engine start changes the logic. Each aircraft type seems to have its own unique setup during this period.Cheers.Ian.P.S. I'd be interested to know what the DUAL BLEED light does on the real aircraft if you had the engines at full thrust (forcing the APU Bleed Valve to close if was selected open). The DUAL BLEED light logic looks at APU Bleed Valve position (not switch position). If it is closed, the light should go out.

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