744 start knob problem

[Guest fmpjb@hotmail.com]

Hello,I've been using the 744 and 744F for about 1 year now, and haven't had a problem, however, when I run through the normal engine startup sequence for some reason the engine start knob (on the overhead panel) will not illuminate when i attempt to pull it out properly (this happens in both the VC and 2d). It has worked normally for quite a while, so perhaps I've missed something? or forgot to do something basic?Thank-you for your time, and any help is much appreciated!Michael

[Guest fmpjb@hotmail.com]

sorry, problem fixed, it was something basic!

[Chaos81 110]

Mind posting what the fix was, so if others have this issue they will know.

[Guest fmpjb@hotmail.com]

certainly, for some reason, in the PMDG menu under various, the Ground Air/Power Available in the Ground Conditions was not checked. When I checked it, the engine start knobs pulled out and illuminated again.Michael

[craig_read 43]

>certainly, for some reason, in the PMDG menu under various,>the Ground Air/Power Available in the Ground Conditions was>not checked. When I checked it, the engine start knobs pulled>out and illuminated again.>>>MichaelYour problem here is bleed air.. it's nothing to do with the menus.. you're just unaware of what bleed air does to start the engines.. Bleed air comes from essentially 3 different sources on the 747-400.. 1 - External power.. this drivers the bleed air compressors and provides bleed air..2 - APU.. this is a small turbine in the tail of the aircraft which is used to provide electrical power which in turn drives the air compressors and provides bleed air..3 - The engines.. these provide bleed air from their own compressors when they are running..In your case.. you were trying to start the engines with no bleed air awailable.. The reasons for this are as follows:1 - There is no power available to drive the compressor for the bleed air.. this could mean no APU or not External Power supply.. fix this by providing power.. via either APU or external power..2 - There is no bleed air available.. this could be that the APU generator is isolated from the system and not providing bleed air to the whole system.. fix this by opening the valve from the APU bleed air to the rest of the system to allow flow..The bleed air is hot compressed air which when let through the turbines of the engine rotates the engine.. once the engine reaches a specific RPM internally on the first spool.. (N1) then the fuel controllers will automatically introduce fuel into the combustion chambers which will ignite as a result of one of the ignitors.. this will fire up the engine.. Once running.. the engine ignites the fuel using compression and temperature.. rather than actual ignition.. ignition is used to stabilise the running of the engine once it's going.. ensures no stalls or flameouts.. Hence.. why this feature is used in bad weather and on take off.. to ensure the engine runs correctly..I imagine.. bleed air is your problem.. and was your fix but you didn't realise it.. I would read the manual with regards to bleed air to get a better appreciation.. or take a look at some of the tutorials..CheersCraig

[Guest fmpjb@hotmail.com]

Excellent Explanation Thank-you Craig

[Qavion 19]

Craig, you can't turn a Boeing/Airbus engine directly using compressed air. To begin with, the air can't get past the bleed air check (1-way) valves in the engine bleed plumbing (to get to the compressor blades of an engine). To start an engine, you have divert some of the bleed air from the ground source/APU or engines through different plumbing to a Starter Motor (via the Start Valve).A Starter Motor converts bleed air pressure into rotary mechanical motion. In the Starter Motor, an air turbine is spun at incredibly high rpms by the bleed air, which drives a gearbox in the starter motor(the output of which is a much more sane rpm). The starter motor then drives the ENGINE Accessory Gearbox. The engine accessory gearbox (as well as powering hydraulic pumps, oil pumps, electrical generators, etc) is mechanically connected to the N2 stage of the engine (N3 on an Rolls Royce engine) via an angled gearbox.When the N2's reach 50%, the start valve is closed and the starter motor is removed from the "loop" so to speak. The N2 rotor(under its own "steam") then drives the Engine Accessory Gearbox (and not vice versa).Here's a diagram I prepared earlier (for a Roller engine)....http://forums.avsim.net/user_files/170882.jpgHope this makes sense.CheersQ>(Edit) "1 - External power.. this drives the bleed air compressors and provides bleed air.."Not true. External power and external air are completely different things. There are no electrically-powered "bleed air compressors" on the 744. External air is compressed air from a non-aircraft source. This is fed directly into the aircraft's bleed air ducts (crossfeed manifold)

[CF104 0]

Hey Craig,>>1 - External power.. this drivers the bleed air compressors>and provides bleed air..>Sorry but there is no such beast as a "bleed air comppressor". Bleed air is taken from the comressor discharge air of the APU and is used for such things as engine starting, air conditioning, leading edge devices, hydraulic pumps, etc.... The engines also provide bleed air from different stages of the compressor for normal systems usage.>2 - APU.. this is a small turbine in the tail of the aircraft>which is used to provide electrical power which in turn drives>the air compressors and provides bleed air..See my reply above.>>3 - The engines.. these provide bleed air from their own>compressors when they are running..Just to clarify, the bleed air is taken directly from the engine compressor section.I think Qavion provided answers for the rest of your post.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

[craig_read 43]

aaahhhI stand corrected..Cheers guysCraig