speedbird034

From the "BIG BOEING FMC USERS GUIDE" by Bill Bulfer (page 78, to be specific)
 
"1) The purpose of the entry in the RESERVES field is to initiate the FMC message, INSUFFICIENT FUEL.
2) While you must dispatch and takeoff with at least the reserve fuel stipulated by regulation (FAR121.639 and FAR121.645), there is no regulatory requirement to land with that amount.
 
Some operators publish guidelines for the minimum fuel on board for landing. These values fall in the range of 15 to 30 minutes of maneuvering fuel. One operator refers to this as Minimum Desired Landing Fuel (MDLF). Some pilots have personal minimums in excess of these numbers. The point on which we may agree is that the Captain is responsible for determining the minimum fuel that he or she must have on board at the destination or the alternate.
 
Boeing and most operators specify that "FAR Reserve" Fuel be entered in the PERF INIT page. While the warning message INSUFFICIENT FUEL, would be valid at break release on takeoff (the point at which the flight "officially" begins) it might nnot provide a meaningful warning after the airplane had detoured through a thunderstorm or two.
 
Additionally, there can be significant differences between FAR fuel on a domestic flgiht and on an international flight. In equivalent terms, the '...45 minutes at normal cruising fuel consumption' (calculated by one operator at FL250) can be as little as 30 minutes sing the same criteria and the provisions for a close-in Redispatch. One operator's procedure solves the inconsistency dilemma between domestic and international reserves: Minimum RESERVE fuel entries: The Captain may choose a larger value than those listed here, but should avoid values which result in spurious INSUFFICIENT FUEL messages:
 
1) If no alternate is required, enter Minimum Desired Landing Fuel or FAR fuel whichever is greater.
2) If an alternate is required, enter burn to most distant alternate, plus MDLF.
3) If no alternate is named, but Minimum Arrival Fuel is required (such as Sydney or Hong Kong), enter that value
4) If alternate is deleted or added enroute, update the reserves fuel line.
 
End Quotation"
 
So what does this mean practically? A couple of things:
 
1) There is no "set" amount that is to be entered...it is not something that is on the flight plan per se, since there are variations as to what could be entered
2) The Captain is the one who makes this decision and updates the reserves line in the PERF INIT page as changes are required en route (e.g. add or delete an alternate).
 
From a dispatch standpoint, we operate with FAR fuel, which we otherwise call MIN FUEL. You will see a min fuel line (MIN T/O) on the PFPX flight plan. MIN FUEL is defined as trip fuel, reserve fuel, hold fuel and alternate fuel. EXTRA fuel is simply fuel that is added by the dispatcher or captain as additional fuel which is onboard but not required fuel. For domestic and international operations, this is the minimum fuel that must be on the aircraft at takeoff. One pould (or kg) less and you go back to the gate and get more fuel.
 
A simple way of thinking about reserve fuel is "what fuel MUST I have when I land?" A diversion decision is a great place to think about this. There is always a point in a flight by which you must make a decision to divert if you can't get into the airport to which you are flying. I do diversion calculations almost daily and the fuel required to get to the alternate plus the reserve fuel are the absolute minimum required fuel at the time of the diversion decision.
 
For the captain, this is BINGO fuel and if you think in terms of...."When I hit this number, I must divert or be on the ground at my destination".
 
SO...my recommendation would be to do this math:
 
FAR RSV (or whatever reserve is dictated if you use EU Rules or JAR OPS) plus ALTN fuel (if an alternate is specified). The total of those two figures from your flight plan is your "RESERVES" that you enter on the PERF INIT page of the FMS.
 
Put another way:
 
An example.... Boeing 737-800 Winglets
Fuel numbers as follows: KLAX-KSFO ALTN KSMF
 
TRIP                  5446  /  00:55
ALTN KSMF       2230 /   00:22
FAR RSV           3775  /   00:45
MIN FUEL         11584  /  02:02
EXTRA               2406  /  00:30
TAXI                     750  /  00:30
RELEASE:       14600  /  03:02
 
The amount you would put into the PERF INIT page in the FMC:
 
BLOCK FUEL: 14.6
RESERVES:      6.0 (6001) ALTN + FAR RESV.
 
If you use the concept of BINGO fuel as the basis of your RESERVES entry in the FMC, you should never get an INSUFFICIENT FUEL message until your onboard fuel hits 6.0....
 
..and you will always know when you need to be on the ground OR if you have an alternate included, by when (in lbs/kgs) you must divert.
 
In closing, I know I've put a lot of information here, but there is an important concept here, not just an entry issue....you ALWAYS want to know your BINGO fuel number, since if you get to this point, you better be on the ground or diverting somewhere. 
 
Hope this explanation is slightly clearer than mud.
 
Cheers,
Dave