Luftfahrer
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Fuel Planning Boeing 737NG

Sat May 25, 2013 2:19 am

Hello everyone,

I have been researching accurate fuel planning for flights operated by commercial aircraft, with particular focus on the Boeing 737NG series. The information available on the web along with some discussions are quite insightful, though some questions have remained for me. Maybe someone can shed some light on them? Since knowing something in theory often proves difficult when applied in practice, I have added a case example for better understanding. These are random even numbers (that hopefully make sense!) on a flight from airport A to airport B with airport C as alternate.

0200 TAXI FUEL
3000 TRIP FUEL
0150 TRIP CONTINGENCY FUEL (5% of TRIP FUEL)
1200 ALTERNATE FUEL
0060 ALTERNATE CONTINGENCY FUEL (5% of ALTERNATE FUEL)
1000 MINIMUM RESERVE FUEL
____
5610 FOB (fuel on board) **all weights in Kilograms (Kg), aircraft Boeing 737-800*

Basic question: Would this fuel planning be approvable? The way I see it, if everything goes well en-route, the fuel consumption on this flight would be +/- 3200 Kg (taxi+trip+factoring in contingency), so the FOD (fuel over destination) = 2410 Kg. So far, so good. However, not everything goes well in aviation all the time. There are weather-related delays, sudden airport closures (as we have seen at LHR yesterday), and so on. For that reason, I would like to consider the worst case. I.e. using all the taxi fuel, trip fuel + contingency due to stronger winds than expected, a go-around to due to bad visibility at airport B, a subsequent holding over airport B using the minimum res fuel, and finally a diversion to airport C that also consumes the contingency. That would mean the tanks would be pretty much empty having arrived at airport C – quite a worrisome thought! Would it therefore the acceptable/recommendable to take on some extra fuel just to avoid that situation, even when weather or lots of inbound traffic are likely not going to be a factor?

My second concern is how to set up the FMS (flight management system) properly. The PERF INIT page allows the entry of a "reserve fuel" value. My take is that the system warns the pilot when the computed fuel at DEST (rather than ALTN DEST) falls below that value. My question is, what should I enter here? Logically, I would say that (at least) 2260 Kg would be applicable (min res + alternate + cont fuel). These reserves should not be touched beforehand, so to speak. Should, for example, a holding be expected, maybe it would be good to put some extra fuel in the tanks right away.

I realize that flight planning including fuel calculation is a wide field occupying whole departments at airlines, so any insight is greatly appreciated!  
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YYZatcboy
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RE: Fuel Planning Boeing 737NG

Sat May 25, 2013 5:18 am

You need to land at your alternate with your 30 minute reserve. (Canadian rules and regs), so yes the tanks would be pretty much empty, and low fuel alert lights would be on. On the NG that works out to pretty much 3200lbs reserve. That is a perfectly acceptable amount to land with so no there is usually no consideration given to padding that amount. remember, any fuel you take with you costs you more fuel to carry, and is payload that you could have taken instead.

The crew enter the Reserve fuel plus the alternate burn to the PERF INIT RES line.

Airlines will have different policies about having enroute reserves and how much they will be over and above the minimum requirements. Finally both the dispatcher and the captain can add in extra fuel.
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Starlionblue
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RE: Fuel Planning Boeing 737NG

Sat May 25, 2013 12:54 pm

Quoting Luftfahrer (Thread starter):
Would it therefore the acceptable/recommendable to take on some extra fuel just to avoid that situation, even when weather or lots of inbound traffic are likely not going to be a factor?

You're forgetting the final reserve as YYZatcboy says. In EASA final reserve is:
- For jets. 30 minutes at holding speed at 1500ft AGL over the destination.
- For props. 45 minutes.


The breakdown looks like this.
- Taxi fuel.
- Trip fuel from dep to dest
- Contingency, equal to 5% of trip or 5 minutes at holding speed, whichever is greater
- Alternate fuel, from missed approach at dest to alt
- Final reserve, as per above
- Extra fuel (AKA Commander's discretion), for example for the expected hold before landing at LHR
(- Tankered fuel).


That's the normal case. There are special cases such as for isolated airports, where (if memory serves) you have to carry 2h of "alternate" fuel but that includes final reserve. And then there's Decision Point Procedure, where you technically don't have enough alternate fuel but you recalculate en route. This is used for long oceanic passages for example.

Quoting Luftfahrer (Thread starter):
The PERF INIT page allows the entry of a "reserve fuel" value.

Methinks this would be the mandated final reserve plus extra fuel.
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Luftfahrer
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RE: Fuel Planning Boeing 737NG

Sun May 26, 2013 4:57 pm

Thanks guys! It makes a lot more sense now.   Since I cannot use the final reserve fuel for holding at the destination (=airport A in my example), wouldn't it make sense to take on some dedicated holding fuel so that I can circle in the air waiting for the weather to clear (or anything else that prevents a landing)? If not, then I would pretty much have to fly right to the alternate, correct? Unless, if applicable, I can use the Captain's discretionary fuel (which I would distinguish from the holding fuel here).

Quoting Starlionblue (Reply 2):
Methinks this would be the mandated final reserve plus extra fuel.

I've been giving this some more thought and I can think of two options:

-Entering all reserve fuels (basically FOB minus taxi, trip and trip contingency as they are slated to be used)
-Entering all reserve fuels expect those that aren't legally needed (i.e. no Captain's discretionary fuel, holding fuel, etc.)

Perhaps there is not a single valid solution, making it subject to each airline's procedures.
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Starlionblue
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RE: Fuel Planning Boeing 737NG

Sun May 26, 2013 5:05 pm

Quoting Luftfahrer (Reply 3):
Since I cannot use the final reserve fuel for holding at the destination (=airport A in my example), wouldn't it make sense to take on some dedicated holding fuel so that I can circle in the air waiting for the weather to clear (or anything else that prevents a landing)?

That's what extra fuel (AKA Commander's Discretion) is for. Note that I listed regulatory minimums. Nothing stops an airline from having an SOP with more fuel as standard. Some airlines will always load on 20 minutes of extra to add to the contingency fuel, or it will depend on the route.

Asia to LHR flights that arrive in the early morning are a good example. Slots before 0600 are very expensive and few in number. So if you get there early, you'll hold. But so will everyone else. And then at 0600 all hell breaks loose as everyone gets ready to land. More holding. Nice work, NIMBYs...
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Luftfahrer
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RE: Fuel Planning Boeing 737NG

Sun May 26, 2013 11:31 pm

Quoting Starlionblue (Reply 4):
That's what extra fuel (AKA Commander's Discretion) is for.

Thanks, understood. I read something about a dedicated holding fuel so I wasn't sure if this would actually fall under the Captain's discretionary reserve. Regarding airline SOPs, the German wikipedia says that Lufthansa apparently always schedules 1000 Kgs of taxi fuel, which is probably more than other carriers do. My goal of this thread was to gain more general knowledge on fuel planning, which I definitely have.   Thanks again! I feel a lot better prepared now.
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Starlionblue
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RE: Fuel Planning Boeing 737NG

Mon May 27, 2013 6:18 am

Quoting Luftfahrer (Reply 5):
Regarding airline SOPs, the German wikipedia says that Lufthansa apparently always schedules 1000 Kgs of taxi fuel, which is probably more than other carriers do.

It depends on the plane. That sounds like a 747-400 number. Other airlines would use a similar number for the same plane.
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