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Engine Start With No Bleed Air

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HiI was testing some systems and found that I was able to start engines with no apu bleed air.In fact they also started with the apu set to off as well as fuel pumps.I was wondering if the pnumatics are just dummies and will it stay that way or be fixed?This is in 2002 with su1.Best RegardsTed

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I experienced similar issues.1. I am able to start engines with fuel pumps off.2. I am able to start engine #2 with bleed air from engine # 1 when both packs are set to high, isolation valve is open (which is normal of course in this situation) and there is NO APU bleed air. Is this possible in real life?3. Will ground start (ground air in stead of APU bleed air) be implemended in an upcoming patch/version?Regards,

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Yup, I started the engines just a few minutes ago with bleed air completely off.What a disappointment... just a dummy pneumatic system. Oh well, I can get over that, but I really wish it worked correctly.James

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I just tried this... I am unable to start then engines without an air source. The engines won't start unless the APU is running, the APU bleed is on amd the crossfeed valve is open.I'm using FS9 with the SU1 patch.I am able to start then engines with both packs set to high... this doesn't seem right though.

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seems to work fine for me APU off =no startapu on = no start isol= off bleed = offapu= on apu bleed on isol off eng 1 starts this is normal eng 2=no startisol= auto eng 2 starts make shure you have the patchand you can start the engs with the packs on with the real NG

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Seems to be a air pressure leak into the left system no matter what you do, this lets the left engine start. The right will not start without the air flow so it seems like a bit of a bug. You will not even hear the starter engage on the right engine until the air is turned on.Ray

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Yup, my apologies to PMDG. It works correctly when you follow the normal start procedure. I used CTRL-E before writing my prior post, and that's when it didn't work right.James

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"1. I am able to start engines with fuel pumps off."This is ok, Floris.... Most Boeing aircraft engines can be started without the tank fuel pumps being on. This is because the engines have their own fuel pumps. These suck the fuel out of the tank into the engine. Gravity also helps. Although it is a mandatory requirement for the engines to be able to do this in all phases of flight, some engine pumps may have difficulties at high thrust settings to keep up the flow. BTW, I have seen engines started by engineers when they have forgotten to switch the fuel tank pumps on for ground test runs. :( If the PMDG engines can't start without the tank pumps, then this would be a bug ;-)"2. I am able to start engine #2 with bleed air from engine # 1 when both packs are set to high, isolation valve is open (which is normal of course in this situation) and there is NO APU bleed air. Is this possible in real life?"Probably not recommended, but it may be possible. In real life, the APU boosts pneumatic manifold pressure dramatically when the start switches are turned... to specifically assist with the start. The 737NG Airconditioning System does interface with the engine starting system, but I haven't yet been able to determine why (I'll have to do some more reading ;-)).On the subject of Pack airflow control (not bleed air control):Pack air flow control is quite complex (as always). There are actually 3 flow modes (Note that the flow rate values and logic may be different on later series aircraft...e.g. on the -800) :1. Low Flow Mode = 55 ppm (pounds of air/minute). This is normal with a pack running in AUTO and one or both Engine Bleed switches selected to ON.2. High Flow Mode = 80 ppm. This happens in Auto Mode in the air (and T/E flaps UP) if one of the packs is shut down manually (crew action) or automatically (if one pack overheats). I believe this should also happen if the Pack switch is selected to HIGH.3. APU High Flow Mode = 100 ppm. This is a special case. If the pack switch is in HIGH FLOW, the aircraft is on the ground and the APU is running (greater than 95%rpm) and the APU bleed switch is ON, the pack goes to its highest flow rate. I'd say that the engines would have the most problems starting in this mode, but that is not to say they won't. On some Boeing aircraft, the Packs go to high flow mode on the ground automatically... and it is even recommended that at least one be switched on to reduce the shock to the pneumatic system.Hope this helps.Cheers.Ian.

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Gents-Any of you guys who haven't done CTRL+E that can still get the engines to start without bleed pressure- please drop a note into this thread...i've tried to duplicate- but can't....(And for the fellow who thought the pneumatics were "dummy" shame on you! The pneumatics took almost 30 days to program....) :-lol

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Ian, thanks for your explanation.Robert, I certainly CAN'T start my engines WITHOUT bleed air, so that is ok.But I can start my engines when I have a duct pressure of +- 20 psi.I thougt the engines need a minimum pressure of 30 psi to start.(APU ON, PACKS HIGH)Same for a crossbleed start (#1 engine running, APU off, crossbleed open, pressure of +-25psi. I real life, you will have to give thrust to increase the duct pressure)(Maybe I am wrong, as I am not an expert ;)I really like to know something about this.Regards Floris

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RobertIn other words you are saying that the use of any external hardware such as Go-Flight will result in engine starts regardless of APU/Bleed Air state?Ok I`ll go along with that as this is my option but I feel no shame whatsoever in asking about it and feel that you should not be able to start the engines if there is no air source no matter how you try to start them.IanI understand about engine driven pumps but was wondering what drives them during startup.Is it bleed air as well or power takeoff from the compressor or what exactly?Also you said that tank pumps are used in critical phases of flight.When is it safe to shut them off?Transistion alt.cruise alt?I`m also curious as to when the apu is shutdown and restarted to provide backup electrical/hyd during a normal flight?Best RegardsTed

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APU bleed press:737classic with -129 APU, you will notice a pressure drop during engine start.737NG APU bleed is controlled by demand. Operating the eng. start switch will increase press from approx 20 psi to approx 30 psi.engines may be gravity fed from wing tanks - up to certain flight level (can`t remember), with boosterpumps off. Centertank must have the boosterpumps operating to feed the engines. With centertank pumps inop, the remaing fuel must be added to zero fuel weight= increased landing weight.APU generator:APU does normally not run during flight, but one ecxpt.:APU must be running through flight if one of the engine driven generators are off bus. Need two generators to dispatch the 737. eker

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"I understand about engine driven pumps but was wondering what drives them during startup. Is it bleed air as well or power takeoff from the compressor or what exactly?"Hi, Ted.The engine driven hydraulic pumps are driven by an "accessory gearbox" (hidden under the engine cowls) which is mechanically driven by the rotation of the N2 engine rotor (via a number of drive shafts and another gearbox). If you had a super strong wind blowing when you are sitting on the tarmac, the N2 rotor would start rotating... and you would get "some" hydraulic pressure.The engine starter motor converts APU bleed air pressure into rotary mechanical motion. The engine starter is connected to the accessory gearbox also, so drives the N2 via the same drive shafts. The faster the starter turns the gearbox and N2 rotor, the more hydraulic pressure is generated (Of course, when the pump produces the the correct pressure, which may be well below engine idle speed, the pump pressure is capped at that correct pressure.... It doesn't keep on producing pressure until the flaps/gear blow off :-)"Also you said that tank pumps are used in critical phases of flight.When is it safe to shut them off?Transistion alt.cruise alt?"It's not really safe to turn them off. The tank pumps are left running continuously (until the tanks are empty/almost empty for safety reasons (you never know when you'll need a sudden burst of speed). ;-) "I`m also curious as to when the apu is shutdown and restarted to provide backup electrical/hyd during a normal flight?"Me too!I haven't got that far into the 737NG ops manual yet. On many twinjets, you don't need to run the APU in flight unless something on the aircraft has broken down (but is covered by the MEL's). In many cases, you'll find that the APU is only used for actual inflight emergencies (as a back-up in case one of your engine fails). When twinjets fly over long stretches of water, sometimes it's necessary to prove (on a regular basis) that your APU can start without any problems during the flight ("ETOPS requirements"). In other cases, the APU on some types of aircraft may be required to give you an immediate supply of backup electrical power during Cat III Autolands (should an engine generator fail)Anyway, I hope this helps some...Cheers.Ian.

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Hi Robert,As I said in my first post I am unable to start the engines without APU running, APU Bleed on and Isolation valve open. If I understand correctly this is the proper behavior.But, I can still start the engines with the packs on high and considerably less duct preasure. There does not even appear to be a reduction in the time for N2 spool up. Is this the way it should be in the 737? I know that on my "other" sim I am unable to perform engine start with the packs tunred on.Thanks,

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"I know that on my "other" sim I am unable to perform engine start with the packs tunred on."I think you'll find that many sims are designed to give you a worst case scenario for this sort of thing, Mike, rather than model the 101 factors/parameters involved in an engine start.Another sim, for example, might not allow you to load up the APU too much and will blow up at the slightest provocation, but as has been stated, if conditions are right in the real world, you will be able to start an engine or even two with a pack on (or two). I don't know which other sims you refer to, but PS1 does it, PIC does it, etc.... even though the programmers know better than this (if not then, certainly now).Occasionally, in the real world, there will be a greater than wanted tailwind, the OAT will be very hot and the APU will be tired (and perhaps the engines, too). In this case, you definitely will have a problem starting the engines. To err on the side of caution, sim builders will model this depressing scenario as standard (even though, these negative factors may not be present on the day (in the sim)).Duct pressures quoted for engine start in manuals may be recommended figures, but real pilots do, occasionally, I've been told, make educated guesses as to whether to start an engine or not with less pressure. However, they do know that they should be extra observant, as you should be, just in case the engines are slow to accelerate... or their EGT's show too rapid a rise. But you shouldn't expect the worst to happen everytime.Even in big sims, you may not find that all the factors/variations are taken into account. In many cases, the Fault/Failure Scenarios will give you the same predictible results when the Instructor pushes his buttons. The EGT's will always rise to X degrees in a hot start, the hydraulic temps will always be X degree in an overheat, the wings will always fall off when you pull 10.15972 G's, etc, etc. Basically, it's simply a matter of economy not to model randomness in sims. (Aside) If, on the other hand, you've found a (cheapish) sim which does model things like this, I would love to know about it ;-) ;-) ;-)Anyway, I hope this makes sense?Cheers.Ian.P.S. BTW, (good) modern aircraft APU bleed air systems are usually designed to accommodate the load put on them. For example, if a Start Switch is pulled/turned, the APU will firstly increase the bleed air output by a certain amount, then fine tune that amount as required. The same may apply to certain combinations of Packs in different modes, although engine start usually takes priority over these with the greatest demand. Depending on how fast the APU can increase/adjust the pressure and how quickly the demand system can suck it up, the final duct pressure can vary by a considerable amount.BTW, in the real world, if an APU did not perform as well as we think it should on a particular flight, we don't throw it in the garbage bin... we will keep an eye on it over a period of time... and if it shows a definite (downward) trend, then we will throw it in the garbage bin ;-) We don't throw out an APU simply because one Captain tried to start 4 engines with 3 Packs running in Johannesburg (5~6000feet elev) on a hot day...unless the engineers there saw bits of white hot metal being ejected out of the tailpipe ;-)

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Ian and allThanks for taking the time to answer so informativly.I always enjoy posts that reflect RW aviation knowledge and translating it into the sim.I`m sure others do as well.Best RegardsTed

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"In this situation, an engine start should not be possible"You may be missing the point here, Floris(?)Had you turned the engine start switch before you took this screenshot? It is almost irrelevant what the duct pressure is before engine start... it is the "during" that is important. One of the real world maintenance checks after replacing an APU is to see if the bleed air pressure reaches a minimum of 10psi (no load). With your theory, no engine could ever be started. Pilots need to look at what the air pressure is during the initial stages of engine start, not before start.I think your complaint with PMDG should be that the bleed pressure is not rising shortly after the start switch has been turned. The APU bleed system is an on-demand system and the output pressure should "rise to meet demand". Note, however, this sort of thing is not modelled in similar and much more expensive simulators.http://members.ozemail.com.au/~b744er/737/BleedPanel.jpgHope this clarifies things?Cheers.Ian.

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Hi Ian!Thanks for the reply. Your posts are always detailed and informative. I enjoy reading them. The "other" sim I mentioned was indeed PIC767. I just tried it again now to be sure, and I am unable to start an engine with the packs on AUTO. N2 does not move at all (well got as high as 1). I assumed this was normal behaviour that there would be insufficient air to perform a start with the packs on.Before Start With Packs On AUTOhttp://forums.avsim.net/user_files/31427.jpg During Start, N2 Not Moving, Duct Preasure AT 0http://forums.avsim.net/user_files/31428.jpg

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I can't promise anything, Murf, but I might be able to get some typical 767 values for you with various systems running. The start values may be a little harder to get... I certainly don't want to overheat an APU or engine by running everything at once ;-) On a 767, for example, (I have some very good notes on this aircraft... my current 737NG notes don't tell me a great deal), the "Inlet Guide Vanes" on an APU are adjusted to compensate for the exta demand. Whenever the ADP is turned on, Packs turned on or Engines started, a signal is sent to the APU Controller to adjust the inlet guide vanes. Actually, the amount the APU compensates for these systems can be adjusted on the front of the controller (with a screwdriver). The system is very smart though, it monitors APU inlet temperatures and EGTs and compares these to Limit Tables in the APU controller to ensure that the total load is not going to overstress/overheat the APU.Main Engine Start (MES) usually produces the most dramatic change. It even ramps up the APU rpm up 101%. I don't know if PIC does this(?)The ECS (environmental control system) input is the most sophisticated. The Cabin Zone Temperature Controller sends an electrical signal to the APU controller proportional to the difference between the pilot-requested cabin temperatures and the actual cabin temperatures. The greater the amount of cooling required, the harder the APU is driven. On some 767's, the APU rpms also go up to 101% in response to ECS demands.The 737 may or may not be as sophisticated as this, but I'll see what I can come up with when I get back to work ;-)Cheers.Ian.

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P.S. By the way, the photo in my previous post was with the engines switched off (and not being started). The Isolation Valve has been set to OPEN to allow the Right Pack to operate (If the Isolation valve was in AUTO, the valve would close according to Pack and Engine Bleed switch logic). With the Isolation Valve open, the pressure in the left and right ducts equalize.I'll try and get a shot of the packs in HIGH in the coming weeks.

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Thanks Ian.Oke, the only thing not modelled is the rising pressure during engine start. So PMDG... :DYou 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.Regards(BTW, I am just interested, not complaining (yet :-wink2 )P.S. One question: In which situation is there to less bleed air to start the engines??(Sorry for my bad English)

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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"I'll have to get back to you on this one, Floris.... Certain bleed user system switch positions are monitored by the Engine Electronic Control (EEC) system, and may actually reduce engine N1's so as not to overheat the engines (e.g. Isolation Valve position, Pack switch position, Anti-Ice switch positions, etc). How the EEC's handle crossbleed starts, I can't say. On most aircraft, you would definitely have to push the throttle/s forward to start another engine.Standby! Going into ALTernate EEC mode ;-)Cheers.Ian.

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"(And for the fellow who thought the pneumatics were "dummy" shame on you! The pneumatics took almost 30 days to program....)Gee - "That's Ron.....er......I mean Ted for you LOL

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could be right if there was a good airsource hooked up(i have a students job mounting high pressure compressors(oil-free) and believe me, these babies produce alot of airpower :s)but i see your point and i think it is indeed not correct that the APU produces sufficient airflow+pressure to maintain a ductpressure that high when starting engines with both packs on high...greetz

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