Engine Start With Fuel Pumps Off?

[Guest jase439]

Just curious, but I noticed that the aircraft's mains will start with or without the fuel pumps being enabled. Seems like previous versions would not allow engine start if the fuel pumps were off. Is gravity feed sufficient for starting this aircraft?Thaks,Jason Barlow

[Mats_J 2]

Jason,Yes, gravity feed is enough. The engines themselves has feed pumps that will supply the respective engine with the fuel needed. Basically... ;-)Cheers,

[Guest jase439]

Are the AC pumps just there for added redundancy in case the suction pumps in the mains fail? Do the AC pumps have a functional purpose in the sim (apart from extinguishing the LOW PRESSURE annunciator on the overhead)?Thanks,Jason Barlow

[Guest boxjockey99]

The pumps are there for 3 reasons:1) to prevent cavitation of the engine driven pumps during start. Cavitation being a nasty vibrating type situation caused by air going into the pump as well. This causes foaming of the fuel and all sorts of stresses on the engine driven pumps. While the engine driven pumps are sufficient to suck the fuel through with a little assistance from gravity, the booster pumps help them out and supply pressurised fuel all the time! 2) in flight any manoever that changes the G's in the aircraft could affect fuel flow and thus cause the engine to flame out!3) at high altitudes if the fuel is not supplied under pressure the engine will probably flame out also when set at a high power setting. I recall on the F27 (granted its a turboprop) the booster pumps were needed for all operations above 12,500rpm and 5000ft. Anything more than this and the engines would probably flame out! Normal operation of the engine was up to 15,000rpm incidently so 12,500 is only a smidge above idle! I have no idea about their function in FSKRis

[Qavion 19]

"Cavitation being a nasty vibrating type situation caused by air going into the pump as well."Interesting.... Where is this air coming from? I'm thinking if air is already in the tank-to-engine plumbing, would forcing fuel from the tanks with tank pumps help the situation?Thanks.Cheers.Ian.

[Guest N401SS]

It's not really air that gets into the pump, but small pockets of partial vacuum caused by the extremely low pressure. Think of it this way, the engine driven pump is pulling fuel from the tank and if the engine is at higher power settings, i.e. pulling more forcefully, then there is the possibility for the fuel to cavitate. This really just means that the engine pump is trying to pull the fuel through the line faster than it can physically move, and subsequently little pockets of vacuum form in the line/pump which can cause the pump to stall or starve the engine of fuel(for lack of a better word/description). If you're using the fuel pumps, the fuel is getting force fed to the lines, and since it is at a relatively higher pressure it is therefore much less likely to cavitate in the fuel lines/engine pump.I hope that makes sense. :( I also hope I'm not too far off base with the description.Nick

[Guest jase439]

Nick,Thanks for the highly informative post. I wonder: is any of this behavior modelled in the PMDG '37? So far, I haven't experienced any "flame out" conditions at altitude with or without the AC pumps - they seem to be placebos in this simulation. But, perhaps I am mistaken?Jason

[Mats_J 2]

Jason,I'm afraid that MSFS sets the limitations for simulating flame-outs.Cheers,

[Guest jase439]

Bummer. So I guess the short answer is no, the AC fuel pumps are strictly eye candy and have no functional impact on simulation state?J

[Qavion 19]

Thanks for the tips, Nick. :-)Fluid dynamics is a complete mystery to me ;-)Cheers.Ian.