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 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