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Engine Start With No Bleed Air

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On Some aircraft (I Forget if it is ERJ or CRJ) SOP is to use the APU to provide cabin AC and pressurization on TO/LDG so that the full thrust of the engines is available for take off and go-around(as read in Flying Magazine recently). AFAIK, the only time one might use the APU for bleed air under 'normal' conditions with 2 good engines is on a takeoff or landing with the icing systems aciviated, and even that may depend on the runway that you are using.Tim

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"You say that the APU is an on-demand system. But what about a crossbleed engine start? When you do a crossbleed engine start, you will have to give thrust to the running engine to provide bleed for the non-running engine (APU is off in this situation).In the PMDG this scenario isn't needed. You can just start engine number 2 without giving extra thrust to engine number 1."Hi, FlorisSorry for the long delay. As you've probably realized, I've been having some difficulties with this one.... I've been "scratching my head" trying to figure out how the 737NG handles this for the last few days. The current notes I have available to me don't fill in enough of the gaps... so the answer may come in installments, perhaps some with corrections.Bleed duct pressure with the engines running is dependent on two things:1. how much air the engine can supply2. how much pressure the pneumatic system control valves will allow into the distribution ducts.Re the latter....The engine bleed pressures is limited by two inline valves. Below is a basic chart which shows what the pressure should be in the ducts with aircraft packs in normal mode and the engines at certain rpms.http://members.ozemail.com.au/~b744er/737/...eVsN1Rising.gifThe step in the graph is due to the fact that the two valves open and close at different times (Note that the graph is a different shape when the engine is decelerating, but I won't go into that now).In the graph, the aircraft is on the ground, and probably no bleed-user systems are turned on other than the packs. The Boeing Manual tells us that engine N2 idle rpms are adjusted to certain values depending on number of different things, including flight phase (That is, if the aircraft is on the ground, in the air, or making an approach, the idle speed will change). The engine has to be kept above certain rpms for....:1. the proper generation of electrical power2. adequate engine component cooling3. proper Environmental Control System ops (i.e. pack stuff)4. Cowl Thermal Anti Ice operation5. faster engine acceleration when Go Arounds are commanded.Please note: Even though the N2 idle speed is being varied, the thrust levers are not moved from idle stop. The EEC's are doing all the required adjusting to the engine to boost the speed.Now, back to the original question. You can see from the graph that at 20%N1 with two packs running, it looks like you won't be able to reach your nominal 25% start value, because the air from the engines is being limited to about 20psi as per the graph. If you turned the packs to HIGH, although the engine system might adjust the flow of bleed air, to run the packs, it might only keep the duct pressure to 20psi, because that is what is designed to do at idle speed. 20psi, however, may be enough to start an engine on the day with everything else running smoothly. You never know until you try. BTW, the most likely reason to carry out a crossbleed start would be because the APU Bleed system was unserviceable (or the APU itself) and not being able to start two engines at the gate (otherwise you would simply use the same (external) source of bleed to start the second engine that you used to start the first engine). To crossbleed start an engine, it will be necessary to manually select the Bleed Isolation Valve to OPEN (or else satisfy the Isolation Valve AUTO logic. i.e. In our hypothetical situation, with Both Packs on, you would have to have the Bleed Switch for the engine being started, selected to the OFF position. As previously mentioned, Isolation Valve position is determined by ENG and PACK switch position, not by bleed availability).Here is a little more on the 737NG bleed system for the beginners (or for those who need a refresher course) :-).....I don't know if you'all are familiar with the bleed system, but bleed air for the bleed ducts comes from various engine compressor stages. A "stage", by the way, is simply a row of blades on an engine spool. The PMDG 737NG's engine is a two spool engine... hence the "N1" and "N2" annunciation.... with lots and lots of stages). Each engine stage is assigned a number. Bleed air comes from the 5th and the 9th stages of the N2 (or high pressure) compressor on this particular engine.After the air is bled off the engine, it has to be controlled to a certain temperature/pressure to prevent damage to the ducts and, of course, the user systems. The pressure in the ducts is controlled by 2 in-line valves. These are the "High Pressure (HP) Valve" (which accepts pressure from the 9th Stage of the N2 compressor) and the "Pressure Regulating Shutoff (PRSOV) Valve" (which accepts both the pressure fom the HP valve and the 5th stage of the N2 compressor).http://members.ozemail.com.au/~b744er/737/...BleedSystem.gifWhen an engine is at low rpms, the engine stage with the higher pressure is used first because it is able to supply higher pressures, sooner. When the engine builds up speed, all the stages increase in pressure. The 9th stage pressure builds up very rapidly and would probably blow up the pneumatic duct system if it wasn't for the High Pressure HP Valve opening and closing. This "modulation" limits the duct pressure to approximately 32 (+/-6)psi... until the 5th stage catches up at higher engine rpms. At this point, the HP closes and the 5th stage takes over... and duct pressure increases with rising engine speed until it reaches approximately 42 (+/- 8) psi (at which point, the Pressure Regulating Shutoff Valve (PRSOV) caps the pressure).With the thrust levers at the aft stop, the engine speed is controlled to certain minimum idle values. However, the maintenance manual also tells us that the EEC adjusts the "commanded" N1 above idle value for the amount of bleed air the aircraft takes from the engine. If bleed air demand increases, N1 speed decreases to compensate for the additional load. i.e. This keeps engine hot section limits for the current engine thrust rating..."The things that the EEC looks at for N1 control are...Right Pack on or offRight Pack in normal or high flowLeft pack on or offLeft pack in high or normal flowIsolation valve open or closedBleed valve for opposite engine open and engine runningWind anti-ice on or offCowl thermal anti-ice on or off.An Engine Electronic Control (EEC) computer, mounted on each engine, is given the task of managing the flow of air through the engine and controlling the flow of fuel to the combustion chamber. This is to ensure that things like engine thrust and aircraft bleed demands are properly catered for (i.e. without overheating or stalling the engine).You may get a better picture of what happens to an engine when you "bleed" or "tap" air from it's compressor stages (to operate certain aircraft systems) if you know how the particular user system uses air.For example, with an engine starter motor, the bleed air has to impart a lot of energy into a turbine to make it spin at crazy rpms... the rpms being converted into rotary mechanical motion to rotate the engine N2 (to initiate the start process). The air just doesn't stop at the starter, it is ejected out of it after it has imparted its energy to the turbine blades... If it didn't come out of the starter, the starter motor turbine blades wouldn't turn.With the Anti-Ice systems, the bleed air is fed into ducts and cavities in the leading edge slats (inboard three slats on this particular aircraft) and the engine cowls (forward of the big fan), then it is ejected overboard through small holes/vents.Airconditioning Packs use bleed air also. All the air which goes to the packs is put out by the packs (after it has been cooled) into the cabin, but then it is slowly leaked out of the Cabin Pressurization Outflow Valve (and Forward Overboard Valve) at a controlled rate.All this air leaking overboard is sapping both air & energy from the main engine compressors. With less flow and compression happening, you're not going to get as good combustion happening in the combustion chambers as you would normally... and the turbine stages won't spin as fast as they should... ultimately slowing down the engine. It is the EEC's task of preventing the slow-down of the engine."P.S. One question: In which situation is there to less bleed air to start the engines??"Apart from when things are not working properly, probably when the ambient conditions won't allow the engine or APU compressors to provide enough bleed pressure. Engine output decreases with air temperature and pressure/altitude. The APU bleed air output varies dramatically with outside air temperature and pressure/altitude also). Generally the colder and more dense the air is, the more pressure it can supply. Pneumatic carts/trucks are known not to provide sufficient bleed for large loads and you will see a rapid drop in pressure under increasing loads. All bleed systems have their limits. You will eventually see pressures drop if you completely overload the system (Packs High, both engines started at once, Thermal Anti-Ice switched on, etc).As Bluecoater, Stuart Law, would say.... Hope this helps more than it hurts? Cheers.Ian.

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Last summer I had the expereince of sitting through a cross-bleed start on a 733 in CLT- Obvious that it will be so when you hit the seat and you're plugged in to external AC and a start cart :) (loud bugger, and that's before they load it for the initial start)Anyhow, we taxied out of the way on one engine and the crew ran it up for proper duct pressure to facilitate the start of #2.I still don't know how they started at the outstation (KAVL) with no start cart that I could see or hear. Still one of the shortest jet flights I'd been on- flight time was on the order of 30 minutes. almost as short as KCVG-KSDF.The NG having a FADEC on the APU makes life easier for everybody :) It just takes a little getting used to not seeing the dcut pressure all of the time on the APU as it is on the earlier gen 737's.Tim

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I am amazed they flew a 733 to Asheville! Maybe the Dash 8s were all busy or something.I am from Lenoir so I know CLT and AVL reasoably well.

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I was on one of the last 733's to AVL last Summer. The following week, USA switched to Dash 8's- Considering that there were 12 pax on the flight I don't know why they kept the mainline jets that long. We cruised at 10000 from CLT, and 11000 to CLT. At least the last time I flew from CVG-SDF we went to 18000! but that was 10 years ago on a DAL 727.

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I wonder if it will go regional jet eventually, as that seems the way US Air, is heading.

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