737NG low fuel QRH ction

[BOAC]

I asked this question on another thread (R&N), but no response. Capt S&L or someone please!

It appears that the 'LOW' indication is set at 907kg by Boeing (for ETOPS). It is changeable to 453kg by operators for non-ETOPS I understand.

The QRH calls for a drill at the 'LOW' indication, presumably to ensure adequate fuel flow to both engines. If this is 'necessary' at 907kg (un-modded), why can it be ignored on a modded a/c and only completed at 453kg, or is there a special instruction to carry it out at 907kg on those?:confused:

[Ashling]

The QRH is only actioned when the light comes on irrespective of whether its 453 or 907 aside. Its triggered by a wing tank reaching one of those figures not a total amount although in practise 907 or 1814 will be the least you have when the first light comes on.

As to why its one thing for ETOPS and another for non ETOPS I can't tell you I'm afraid. No experience of ETOPS to draw on so I can't even hazard a guess. Maybe its something to do with reserves in order to be ETOPS at the time? as 907 very roughly equates to final reserve. What I can say is that below a set amount of fuel you have to switch the electric hydraulic pumps off on the ground, I think its 1400 there or there abouts, due to cooling and it does seem a touch odd that the low fuel warning is not set at or above this with a QRH ref to remind you to switch them off.

[BOAC]

to remind you to switch them off - that's what they pay us for - don't need a light!

Anyone know the answer?

[Ashling]

BAOC I did actually answer part if not the main part of your question which is why I posted.

I hope you find the rest of your answer.

[CaptainSandL]

Hi BOAC,

Before I answer, for those that don’t know, 907kg & 453kg may appear to be precise but they equate to 2000lbs & 1000lbs. Read into these very round numbers what you will! See here (http://www.b737.org.uk/fuel.htm#Fuel_Gauges) for photos of these displays.

I asked Boeing about the origin of the 907kg back in 2003, their answer was as follows:
“The Fuel LOW indication is one of three fuel status indications required on the B737 NG for compliance to ETOPS Advisory Circular No. 120-42A, Paragraph 8b(2,iii), dated 12/30/88. This circular mandates that the fuel management system alert the flight crew of a low fuel condition. The alert should commence at a total fuel quantity available condition equivalent to no less than one-half hour operation at maximum continuous thrust (MCT). Analysis has shown that 1815 kilograms/4000 pounds fuel is sufficient to feed both engines at maximum continuous thrust for 30 minutes. The LOW indication therefore illuminates when fuel in the related main tank is less than 907 kilograms/2000 pounds.”

They went on to say:
“We have recently completed a flight test to determine a minimum fuel value for non-ETOPS operation. The test data suggest we can continue to move forward in our investigation of a change to allow operation at lower fuel levels. We are reviewing the results to determine the feasibility of revising the current B737 NG Fuel LOW Indication NNC for non-ETOPS operations.”

The flight tests must have gone well because in June 2005 the 453kg/1000lb option became available with the introduction of CDS Block Point 04. A quick quote from the SL does not really give us the reason behind the change:

“Low Fuel Threshold
This option enables the display of an amber Low Fuel alert message if the fuel remaining in either/both main wing tanks falls below 1000 lbs. The default display of the Low Fuel alert message is when fuel quantity decreases below a 2000 lbs limit. The 1000 lbs option cannot be used concurrent with ETOPS operations.”

My guess is that operator demand for a lower LOW fuel threshold drove Boeing to review the 907/2000lb figure. During this flight testing it was found that at least one of the pumps would remain covered by fuel down to lower levels than previously thought necessary. Remember each engine has three pumps; a forward, an aft and an engine driven pump, any one of which will keep the engine running at MCT. Hence Boeing could persuade the FAA that the LOW fuel warning could be reduced. The higher ETOPS limit was probably retained because it was deemed that the warning should come on earlier as you may be further from a suitable airfield, but again that is just my guess.

Rgds

S&L

[BOAC]

Thanks both. The query still remains unanswered, however . Why, non-ETOPS, is it necessary to action the QRH on un-modded a/c at 907?
the feasibility of revising the current B737 NG Fuel LOW Indication NNC for non-ETOPS operations - I guess they didn't bother?

I am surprised that they 'allow' operations with crossfeed closed and no restrictions on pitch/acceleration at 500kg a side, however. By then I would be walking on eggshells:eek: