B737NG engine start
 

Hello All,

I have a question regarding engine start procedures on the 737NG series aircraft. Specifically I'm interested in what are
a) the minimum requirements (technologically speaking)
b) operational requirements
for fuel quantity in the center tank to complete engine start?

As far as I know, the engines are normally pressure fed from the center tank, unless it's quantity decreases to near 0; and are then normally fed from the respective main tanks. I'm aware of operational procedures restricting center tank pumps to be switched off at the various minimum center tank fuel levels for the various phases of flight; I'm more interested to know are there any limitations both a) and b)-wise regarding engine start with center tanks empty?

Thanks for your input!

Regards,
Zoltan Hegedus

If the center tank is empty, the pumps would be switched off prior to engine start. Fuel would come from the main wing tanks then.

IR

Hi
Yes, however I'm wondering does any reason a) or b)-wise exist which would prohibit starting the engines under normal conditions with center tank empty.
Thanks

As far as I know the answer to both questions is No.

The only limitation that comes close is if there is more that 453KGS of fuel in the CTR tank, wing tanks must be full.

IR

The wings do not have to be full, but in that case the center tank fuel must be included in the zero-fuel weight.

Taken from the Boeing 737NG Flight Crew Operating Manual.

"Main tanks 1 and 2 must be full if center tank contains more than 453 kgs."

IR

also, if you have more than 2300 kgs in the center tank, you can start engines with the center tank and main tanks pumps on, however the center tank pumps must be selected off before t/o.

Hi All,

Errrr......"Tropical Wave" - my understanding is that this restriction applies only when the center fuel tank has LESS than 2300 kgs of fuel at startup!

Confirm !!!!

Cheers, FD :eek:

i stand corrected....center tank fuel pumps off for t/o,if less than 2300 kgs....i should read my posts before posting.Cheers

Hi Tropical Wave...

I hate to do this again, but......

Errr.....the fuel restriction of 2300 kg applies to that amount at engine start....not takeoff!

I wish I had my books with me to check myself!!

Cheers, FD:\

not with us,we can start with below 2300 kgs...however, when we begin to taxi, we select the center tank pumps off.

fuel & engine start
 

Just to clarify:

Fuel tank selection is not important for engine start. If there is fuel available in any tank, that fuel will be used.
If there is fuel in the centre tank and the centre pumps are on, due to a higher pressure operation (to allow centre tanks to be drained first), the engines will draw fuel from this tank. Otherwise, the fuel would be drawn from the wing tanks.

The need for the wing tanks to be full with more than 453 Kgs in centre tank is a structural limit (or recommendation from boeing).

Also, the centre pumps are switched off with less than 2300 kgs (varies depending on company SOPs) for start and take-off simply to avoid annoyance warnings of 'low-pressure' warnings in the centre tank due to the position of the detectors and gravity in a climb !

Hope that helps !

Thanks very much indeed for the replies.
Cheers

Old Boeing 737 SOP :

- centre tank fuel pumps OFF when Qty < 2300 kg

New Boeing 737 SOP :

- (before start) centre tank fuel pumps ON if fuel available
- (during taxi) centre tank fuel pumps OFF when Qty < 2300 kg

And of course, we always forget to switch them OFF during taxi...

B737MRG ......Thats all correct according to our SOPs.Thanks TW

Checking in late but i hope this will help..
 

As far as i can understand there is no limitation regarding the the amount of fuel required, in the center tank to start the engines. How ever we must keep in mind that the CENTER TANK AD states that there must be minimum 2300 kgs of fuel in the Center Tank for T/O and this is only to avoid the fuel pumps from being exposed due high body angle. As we follow the AD procedure of turning the pump off at the first indication of low pressure, if the same is followd on ground, i do not see any reason not to use the center tank for starting engines even if the QTY is pretty low. :)

Have'nt heard from 'flight detent' as regards his Co's SOPs on,fuel required in center tank to start engines.Cheers TW

Hi, thanks for the reply.

Based on that, have Boeing's published fuel loading procedures changed, from having to fill main tank 1 and 2 first, and then the center tank? (main tanks having to be full if center tank contains 453 kg or more)

If the planned fuel load for a given flight is below the sum of the main tank capacities, is it required to have the center tank contain 2300 kgs?

By the way, does anyone have exact information on the quantity of unusable fuel in each tank?

zth - some misleading info here - the fuel 'minima' for the centre tank (with unmodified pumps) are only applicable when the pumps are 'ON'.

You can fly sector after sector with nothing in the centre tank if you wish as long as you don't use those pumps.

NO requirement for ANY fuel in the centre tank for starting - on any Boeing 737.

Possibly the longest reply on PPRUNE (I'm expecting an award !!!)
 

I'll just repeat most of what i said before !!! (Also add more !!!)

There is NO need for any fuel in the centre tank if the flight does not require it ! THe engine start uses same fuel as during any other phase of flight.

If the amount of fuel required is greater than the capacity of the wing tanks, THEN fuel is loaded in the centre tank.

If there is more than 453 Kgs of fuel in the centre tank, for stuctural reasons, the main (wing) tanks must be full before more fuel is loaded into the centre tank. The structural reason is that due to the position of the gear, when on the ground/during landing, the stresses upon the wing root are too great with all weight in centre (between gear) rather than in wings (outboard of gear, holding wings down, therefore relieving stress upon the wing root).

When all pumps (2 for each tank) are operating, the centre tank pumps (on the classic) operate at a higher pressure than the wing tank pumps. (on NG, the plumbing/valves create the higer pressure required) This encourages the engines to draw fuel from the centre tank. The fuel pumped by the wing tank pumps is simply returned to the main tanks. When the fuel in the centre tank is exhausted or the centre pumps are switched off, the system is plumbed to simply draw fuel from the wing tank pumps.
If the situation occurs where the pumps are no longer powered or the pilots switch off all fuel pumps then the engines have the capability of 'sucking' fuel from their respective wing tanks to maintain a constant fuel supply. If that were to happen, the fuel in the centre tank is left redundant and the structural implications can not be helped !

Now to consider the case when the wing tanks are full but the centre tank contains >453 kgs but <2300 kgs.
The fuel pumps/pressure 'sensors' are located towards the front of the centre tank. When there is less than 2300 kgs in the centre tank, at take-off and during the initial stages of the climb when there is a high body-angle, the pumps/sensors may become exposed.
This situation would cause a 'low pressure' caution on the flight deck. If this were to occur on take-off/early climb, the distraction may cause problems.
The reason for the large proportion of SOPs to ask for Centre pumps off with less than 2300 kgs in the centre tank is to avoid this undesired distraction - to the best of my knowledge, there is no other reason.

The centre tank pumps can still run without fuel in the centre tank without resulting in damage. Of course it is still advisable to turn off pumps with an empty tank to avoid wearing out pumps as they will be running without the colling/lubrication provided by the fuel !

The 737 also has a 'scavenge' feature. This means that when then centre pumps are switched off (regardless of contents of centre tank), the pressure created by the pumps in main tank 1 will draw out remaining fuel from the centre tank. On the classic, this process will run for 20 mins (although it can be restared and run for another 20 mins by recycling centre pump switches). On the NG, it will run for the remainder of the flight BUT will only begin once the quantity in main tank 1 drops below 50%.


Finally, unusable fuel quantites :

Classic Wing tanks (each) 272 kgs
Centre tank 419 kgs

NG Wing tanks (each) 152 kgs
Centre tank 398 kgs


If this doesn't help, read it again !!!

Hope this actually helps !!!!!!!!!!! :ok: