There's absolutely no reason why a start without boost pumps running will be slow. Fuel flow during start is relatively low, lower than ground idle. If the engine will run without boost pressure it will start. The reason the boost pumps are there is to prevent the suction from the engine pumps causing pump inlet pressure to drop below the vapour pressure of fuel and thus cause cavitation. Kevin Hall

Now I think about it, I have to say your right !!!. my mistake.

Wow, that's some insane insight there. I wonder how that BSV would work though - all I see is that the nozzles are connected to one common ring line that goes around the engine core - how would it be possible to select single nozzles there? And aren't the nozzles two stage after all to cater for different fuel needs?

The burner staging valve (BSV) opens to send fuel to the ten staged fuel nozzles. The EEC software logic schedules BSV operation The EEC controls the Hydromechanical Unit (HMU) to operate the BSV. There are going two fuel lines to the fuel nozzle's, one unstaged fuel line and one staged fuel line When there is a low fuel/air ratios controlled by the EEC the BSV closes, at a high fuel/air ratios the BSV opens But this a old system that is removed from most engine's.

The engine had 3 different configurations, various service bullettins simplified the system.The initial design hadBefore it the configuration with burner selector valve has dual flow design, with 2 types of nozzle: bleed nozzles and cooling nozzles, both dual flow. The pilot manifold goes to both nozzles, a main 1 manifold goes to the cooling nozzles, while main 2 goes to the bleed nozzles. So we have 3 conditions, pilot fuel only, pilot + main 1 and all together.In this config. we have 10 bleed nozzles and 10 cooling nozzles.We can have a configuration of 20/0 (idle power), 20/10 (medium power), 20/20 (high power)The system was then simplified by a service bulletin from boeing.This configuration presents a burner staging valve that sends or not fuel to the staged nozzles (10 nozzles) while the unstaged (10) are always on if the shut off valve is open.So, this configuration uses a single flow nozzle configuration.When at low power the valve is closed, all the fuel is metered and sent to the unstaged nozzles, they will have a stronger flow as they are only 10. In this configuration the stronger fuel spray prevent a flame out. When fuel air ratio increases the EEC sends a command to the HMU to de-energize the solenoid wich opens the valve, all 20 nozzles are now working.A later service bulletin modified also this system.In the latest version all 20 nozzles works directly with metered fuel from HMU.

And somewhere around FL250, the engines may start to have degraded performance if no electric pumps are operating.

So, at higher altitudes, you MUST have the electric fuel pumps on -- if they fail, you would need to decend to an altitude where the mechanical ones are sufficient. ?

The reason the boost pumps are there is to prevent the suction from the engine pumps causing pump inlet pressure to drop below the vapour pressure of fuel and thus cause cavitation. Kevin Hall

What are the effects on the engine if "cavitation" is present ? Can this occur on the ground at airports with high elevations, if the boost pumps are not used ? ----------------------------------------------------------- Lastly, if Boost pumps are on, and there is a mechanical pump failure, what are the effects, and would one even know, without turning off the electric boost pumps. ? If one is always using the electric boost pumps, how would one know if the mechanical pumps were operational, ( untill there was a electrical failure, or some other incident that cause the electrical pumps to go offline) ? With no display of actual Fuel pressure (as in many other aircraft), and just relying on the "simplified" "Low Pressure" light, are there still ways to determine the above, or has that ability been sacrificed in the "simplification" of the 737 cockpit ? Comming from the GA world, I am new to these large aircraft, but had always assumes there were detailed systems status displays, that could be called up, that would display this more detailed system information. ?Is this so on the 737 --- I must admit I have not got very far into the manuals yet Heavy reading and very prone to send one to sleep, very quickly !!

If the engine driven pumps quit, you'll know because you'll get a cascade of lights as the engine dies. The electric pumps just make sure fuel gets there* easily, the engine driven pumps are the heart. *There = to the engine driven pumps.

Engine pump are driven by the engine, and the pump pressurize fuel for the nozzles that let the fire stay into the combustion chamber and mantain alive the engine, but, if you lose fuel pressure to the nozzles the engine will shut down.Boost pumps are low pressure pumps, they are not able to feed the nozzles requirements in terms of pressure.

What are the effects on the engine if "cavitation" is present ? Can this occur on the ground at airports with high elevations, if the boost pumps are not used ?

Cavitation would mean there were pockets (cavities) of fuel vapour in the liquid fuel. A fuel pump can't pump vapour so the engine would soon flame out in such a condition. Kevin Hall

Wow, guys, thanks to all. This has been a tremendously detailed answer.My wife just asked me, why I had to know all those things (atop of all those things I have to know to get the fridge filled in the real world).Well, honestly, I do not need to.But I love to! This is just a great forum. Next I will turn off the pumps at FL260 and see what happens.Judging from all the effort that they put into this plane, I guess the engines will take notably longer when started without the boost pumps running (if there is an effect in RW). I remember Rob stating that the FMC would take slightly longer to initialize depending on the temperature of the flight deck. [As my wife is not around now: it's crazy isn't it? How many parts of this plane do they actually simulate?] THANKS again to all of you. Clemens Kuehn

On ground this difference in starting times can be very little depending also on fuel level (more fuel, more weight, more pressure at the bottom of the tank where the pumps take fuel).All depends also in air temperature and the fuel density... there are a lot of things, I don't think PMDG has reached this kind of details.