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B737 Which Fuel Pumps to turn on when starting APU?

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B737 Which Fuel Pumps to turn on when starting APU?

Old 14th Aug 2019, 18:07
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B737 Which Fuel Pumps to turn on when starting APU?

Hello everyone,

I am currently undergoing ground training for B737NG and have come up with question regarding APU ops.
FCOM really does not specify which fuel pump is to be turned on when starting APU,
The company procedure states that, "If fuel is loaded in the center tank more than 1,000 lbs, position the CTR FUEL PUMPS L switch ON to prevent a fuel imbalance before takeoff. Otherwise turn on AFT FUEL PUMPS 1 or FWD FUEL PUMPS 2."
I am sure everyone has many different views and understandings on fuel pump operations I am having hard time why those pumps are chose ones? to be turned on.
My understanding is that, turning on AFT fuel pump1 will feed fuel through linings all the way to the APU however it is consuming fuel from left tank which means that fuel will be imbalanced before takeoff. Absolutely no idea why company states "or FWD FUEL PUMP2.
tae9141 is offline  
Old 14th Aug 2019, 20:54
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APU start = #1 tank, aft pump. IIRC it has the lowest feed point and any moisture would gather there.
ampclamp is offline  
Old 14th Aug 2019, 21:23
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ampclamp is correct, his statement is true for when there is little fuel in tank.

When main tank 1 is full (not below ~1000kg/2000lbs then it doesn't matter which pump s used, as both are suitably submerged.
When centre tank full is available (above minimum quantity for ground use as er your aircraft specification) use CTR fuel instead to avoid an imbalance.

Simple...

FWD FUEL PUMP 2?
Never unless you've got crossed open and AFT FUEL PUMP 1 is selected OFF
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Old 14th Aug 2019, 21:26
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Some planes are equipped with an additional DC-powered fuel pump. IIRC, it's in the L Main tank as well. I think it's powered on with the APU start switch. Not a common option, I believe.
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Old 14th Aug 2019, 21:31
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More than happy to be corrected as correctness is not assured

the reasoning for running a pump is to give positive fuel pressure to the APU fuel control unit to preserve its lifecycle. Which pump you use depends on fuel load and flight deck occupancy by a pilot because as far as I’m aware, there are limitations to running centre tank pumps without anyone in the cockpit.
As a standard, #1 Tank is where apu fuel feed comes from, that is, the left side of manifold/crossfeed.
standard pump used is #1 Tank AFT. used for the feed in low range fuel quantity config’s.
Alternatively, some people use #1 Tank FWD - the benefit here is that when the #1 tank quantity is less than 1990kg, and there’s a bit of fuel in the centre, the centre tank fuel will be scavenged. This works best when you operate the classics as well as NG.
The centre tank L pump is used where fuel loads are greater than roughly 9tonne and can be run where fuel in the centre is greater than a specificied quantity and the cockpit is occupied. This is to prevent imbalance in the wings which I’ve seen once in 10years (easily fixed as long as it’s caught).

the right side of the manifold/crossfeed can be used and behaves in exactly the same way, except you have to open the crossfeed and remember to close it.

good luck with your training! There is a terrific 737 website which is tantamount to being authoritative, but isn’t. It’s website address ends with .org.uk if you google it.
have a read.
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Old 15th Aug 2019, 01:30
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Originally Posted by QuarterInchSocket View Post

Alternatively, some people use #1 Tank FWD - the benefit here is that when the #1 tank quantity is less than 1990kg, and there’s a bit of fuel in the centre, the centre tank fuel will be scavenged.

Does the 737NG have a center tank scavenge system?
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Old 15th Aug 2019, 04:51
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As far as i know, yes. Atleast the CL's and NG's ive been exposed to do. See your company books to determine if your fleet how your fleet is configured.

cheers
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Old 15th Aug 2019, 05:52
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The FCOM at my carrier has clearly suffered from a local edit. It says “any AC powered fuel pump” which is clearly incorrect with the crossfeed valve in the (normally) closed position.
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Old 15th Aug 2019, 05:56
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Hi Dave

Running the FWD pump in the LH wing tank will siphon (jet pump) residual fuel in the centre tank after the centre pumps go low pressure. Conditional on the LH wing tank quantity being less than about 2 tonnes.
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Old 15th Aug 2019, 08:52
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Jesus lads, just turn them all on and don’t worry any further!
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Old 16th Aug 2019, 03:34
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Thanks, ampclamp. I'm not a pilot. I worked as an engineer on other powerplant aspects of the NG back in the 90s and I am familiar with Boeing's scavenge systems in general, but somehow had it in my head that the NG did not have a CWT scavenge system. Obviously incorrect, though. Thanks for the polite explanation.
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Old 16th Aug 2019, 09:03
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ok so right side forward fuel tank is used as some of you mentioned, Is there a reason why it is turned on so?
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Old 16th Aug 2019, 09:11
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Originally Posted by tae9141 View Post
ok so right side forward fuel tank is used as some of you mentioned, Is there a reason why it is turned on so?
no. The APU fuel source is the left side of the fuel manifold, normally supplied only by the pumps on the left side of the panel. Ie: either left main tank pump, or the left centre tank pump. You would have to open the fuel crossfeed valve to use any other AC powered pump. I have never seen a dc pump on the 737, but mention was made of such an option earlier.

The APU fuel pump will also self-feed, but with some longevity issues.
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Old 16th Aug 2019, 10:00
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As per Australopithecus 's comment above, LEFT side of fuel installation feeds APU, unless crossfeed is open position, as per my post at beginning of this thread.

In practical terms, in the OP's interest, two options exist when arriving at the aircraft in the beginning of your duty on it:
  1. GPU connected, therefore no need for APU start from Battery power, leave APU off and start when required for dispatch, all fuel pumps are then ON as required by fuel amount carried;
  2. No GPU connected, start APU using Battery power, (no AC power available for fuel pump yet) then put all fuel pumps on as per checklist item before dispatch;
Longevity issues are realistically only a factor when using APU for extended use.
  • During a TRANSIT turnaround, MAIN TANK fuel pumps remain ON in all cases, so APU will continue to feed from left side of fuel system, and as all pumps are ON there is no need for this discussion item;
  • On last flight on aircraft, when arriving on stand with APU ON, and GPU gets connected within a minute or so, this is not defined as extended use, so during the SHUTDOWN sequence following, the need to leave any fuel pump ON in this case is not required.
  • Should you arrive on stand and there is no sign of ground crew and/or GPU/FPU, leaving one fuel pump running ensures longevity of the APU while it is being used, once the shutdown sequence of the APU has been started by turning APU OFF, the last fuel pump can be turned OFF as AC power is lost.
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Old 16th Aug 2019, 11:29
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Can’t be much of a ground school. They still have instructors?
Small cog is offline  
Old 16th Aug 2019, 13:02
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Originally Posted by Australopithecus View Post
... I have never seen a dc pump on the 737, but mention was made of such an option earlier. ...
All Lufthansa 737 were equipped with it...
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Old 18th Aug 2019, 22:33
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Originally Posted by Skyjob View Post
Longevity issues are realistically only a factor when using APU for extended use.
Politely disagree. Lifecycle of the fuel controller is affected due to operating irrespective if for a short transit or long turnaround.

there are many applied techniques, we’ve covered most of them. Moral of the story is - select pump/s based on healthy knowledge of the system, limitations and op procedures.
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