Hi all, last night (it was late) I noticed, that you can start your engines with all fuel pumps off and even the low pressure lights on. I wonder if this is supposed to happen. Just loaded the plane with "short" panel state, selected 1/3 fuel and started the engines. The switches were in off position, engaging them turned off the low pressure light and started the respective sound (or shall I say noise?), so I think they are working correctly. Do the engines get fuel without the pumps on? Thanks in advance. Cheers Clemens Kuehn

Yes they do. Gravity feeding is your friend. Although I do have to say they will survive prolonged inverted flight which probably should not be the case after all so you might still have a point.

And what is with the APU? When loading the short turn stage it runs without any pumps on. I can hardly imagine that the gravity will feed the APU with fuel as it's loacated above the fuel tanks.

And what is with the APU? When loading the short turn stage it runs without any pumps on. I can hardly imagine that the gravity will feed the APU with fuel as it's loacated above the fuel tanks.

The APU is capable of drawing fuel from tank 1 via suction when no fuel pumps are operating.

Which is why it's standard procedure to leave one pump on to ensure positive pressure. Yes it's correct it will just run without, but most of the time you'd leave #1 tank AFT pump running to serve the APU. Some NGs will also have a dedicated APU pump in the tank, DC operated.

Thank you for the quick answer. In the preflight checklist it says that the fuel pumps must be turned of, so do I really have to turn them off even though the APU might be running?

Do the main engines and the APU, not have mechanical Fuel pumps as well. ?

Thank you for the quick answer. In the preflight checklist it says that the fuel pumps must be turned of, so do I really have to turn them off even though the APU might be running?

No, that wouldn't make sense. Check SP.6.3, you're asked to turn on one AC pump for prolonged APU operation, this is well before the preflight checklist. And the preflight actually never says anything about the pumps at all. The BEFORE START checklist calls for pumps to be on, so there shouldn't be any interference with the checklists.

The engines have a dual mechanical pump that is linked to the gearbox, the pump contain a low pressure centrifugal pump and an high pressure geared pump (mainly for servo users).APU have a similar pump, but to initially pump fuel (remember that pumps are AC, and APU starts with only battery) the NGs have a DC fuel pump that works whenever low pressure is detected or AC power is not avaiable.This DC pump is present as an option also on 737 classic.The 737 wich don't use the DC pump can still start their APUs without pump. So, the answer to the question is " the engine and APU can start and run without pumps".

Do the main engines and the APU, not have mechanical Fuel pumps as well. ?

Yes, most jet engines have 2 mechanical pumps. The low pressure pump takes the fuel from the tank/fuel pumps and boosts it around 150psi. The fuel then routes to the high pressure fuel pump and is kicked up to about 1200psi. This high pressure is needed to help atomize the fuel and proper spray pattern from the nozzles. The low pressure pump will draw fuel from the tank when the fuel pumps are not running. The fuel pressures I state are generic.

The fuel pumped by the HP fuel pump goes to the HMU (hydro mechanical unit) this component provides servo pressure to its users, ex. to move the variable stator vanes, and has a metering valve that control the fuel that goes to the nozzles. Before continuing the fuel needs to pass the HPSOV (the fuel shut off valve that is indicated on the overhead with a blue light, not the spar valve) then goes to the BSV (burner selector valve) that send fuel to selected nozzles depending on power request.

There are two fuel pumps on the engine as mentioned before, the Low Pressure fuel pump and a High pressure fuel pump, the Low pressure fuel supplies fuel pressure to cool the oil of the IDG and than it goes to the Oil/fuel heat exchanger where the fuel is heated up for better combustion and also to cool the engine oil, after that the fuel goes to the High pressure pump to boost up the fuel pressure for the servo system and for combustion, the servo system has actuators that operate's under high fuel pressure. The engine accessory Gearbox (AGB) turns the drive shaft to operate the fuel pump's. The AGB start to turn when the starter valve is opened and Bleed air is supplied to the starter valve to start the engine. So when the engine is starting then also the fuel pump;s are starting to operate. For a better and quicker engine start, it's highly recommeded to supply fuel with the fuel boost pumps. But it's possible to start the engine without fuel pressure only it will be a slowwww start !!!!.

then goes to the BSV (burner selector valve) that send fuel to selected nozzles depending on power request.

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?

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

There are two fuel pumps on the engine as mentioned before, the Low Pressure fuel pump and a High pressure fuel pump, the Low pressure fuel supplies fuel pressure to cool the oil of the IDG and than it goes to the Oil/fuel heat exchanger where the fuel is heated up for better combustion and also to cool the engine oil, after that the fuel goes to the High pressure pump to boost up the fuel pressure for the servo system and for combustion, the servo system has actuators that operate's under high fuel pressure. The engine accessory Gearbox (AGB) turns the drive shaft to operate the fuel pump's. The AGB start to turn when the starter valve is opened and Bleed air is supplied to the starter valve to start the engine. So when the engine is starting then also the fuel pump;s are starting to operate. For a better and quicker engine start, it's highly recommeded to supply fuel with the fuel boost pumps. But it's possible to start the engine without fuel pressure only it will be a slowwww start !!!!.

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