Saturday, September 01, 2007

Fuel Optimization

Over at Flight Level 390, some of Dave's readers are having a discussion about how he makes decisions to add "uh oh" fuel and whether he suffers any consequences as a result of adding fuel that costs the airline money. At his airline, dispatchers determine the optimal fuel load considering such factors as weather, fuel price differential, and payload. If the fuel is cheap at your destination or if you'd need to bump payload to tanker fuel, the optimum fuel load will probably be close to the minimum legally required fuel load. When the Captain's "spidey sense" starts twitching, as Dave's did for the flight in question, they have the power to add more fuel. This is one of the areas that Captains are being paid to exercise the judgment that comes from years of experience, and most airlines are pretty good about not griping if that extra fuel means bumping payload because the cost of diverting for fuel enroute would be much higher.

At my airline, fuel optimization has been a huge issue over the last few years as fuel prices have skyrocketed. We do things quite a bit differently from Dave's airline. Here, the Captains order fuel for themselves with little input from dispatch, with the request that they try to save the company money when possible. There are some captains who always just take min total plus a few hundred pounds, but most are pretty good about working out the optimal fuel load.

There's a large disparity between fuel prices at our hub cities and those at most outstations, so it almost always makes sense to tanker fuel on outbound legs. This is much more of a no-brainer on the Megawhacker than most jets, because unlike jets, the extra weight does not result in appreciably more fuel burn. The most common explanation I hear is that jets fly a given mach number and increased weight requires more engine power to maintain that mach, whereas we just set cruise power and accept whatever airspeed we get (or pull the power back to stay under Vmo). I think there's more to it than that, because you'd think that all other things being equal, increased weight results in decreased airspeed and higher fuel burn for the leg. I think the real difference is that most jets are flying close to their maximum altitude, where extra weight increases induced drag appreciably. The Megawhacker always flies far below its aerodynamic maximum altitude (it's restricted to FL250 by lack of O2 masks and the pressurization differential limit); our angle of attack is only a degree or so in cruise, so extra weight doesn't increase induced drag much.

Whatever the explanation, it always makes sense to tanker when we're flying somewhere that fuel is more expensive, so we simply figure out how much we can get away with taking. Maximum takeoff weight (MTOW) on the Megawhacker is 65,200 lbs, and maximum zero fuel weight (MZFW) is 58,000 lbs, so you'd think that we'd always be safe taking 7400 lbs (7200+200 lbs taxi fuel). However, many of our shorter legs are weight limited not by MTOW but by the Maximum Landing Weight of 62,000 lbs. If this is the case, you add the expected fuel burn to 62,000, and that gives you your new max takeoff weight. You subtract MZFW of 58,000 from that, and this gives you the most fuel you can take while protecting max payload. If this number is below min total and you have a full boat, you may need to bump bags or pax.

This method protects maximum payload by assuming you'll be at MZFW, but that's seldom the case with the Megawhacker. Even with 76 passengers, it's rare to have so many bags that we bump up against MZFW. Every pound below MZFW is a pound of fuel we could be tankering. The trick is to guess how much payload we're going to have before boarding, when we write the fuel slip. To that end, the company has begun providing load planning sheets, printed 25 minutes before departure, which estimate passenger load and cargo weight. Most captains leave a little wiggle room for last minute passengers and jumpseaters. You add the expected payload to aircraft Basic Operating Weight (BOW), subtract that from most limiting takeoff weight, and that gives you fuel load. When passenger loads are light, we might fuel all the way up to our maximum capacity of 11,700 lbs.

It is, however, possible to tanker too much fuel. You need to take the next leg into consideration. If you're doing a short turn (SEA-GEG-SEA for example), you don't want to put so much fuel on that you arrive back in the hub city with 4000 extra pounds of fuel! Worse, if you have a lot of payload on your next leg, you could find yourself with way too much fuel and be forced to bump a lot of people or bags. Our dispatch release tells us how many people we can expect on the next leg, and the more conscientious dispatchers put down the expected min total fuel as well. The ideal is to tanker enough fuel to arrive with more than min total, but not so much that you risk bumping payload.

It seems like a complicated process, but you get pretty good at it after a few weeks on the line, to the point you just "know" what to tanker by looking at your release and load planner. The company recently added a fuel optimizer program to the PDAs they issue to Captains, but it turns out that the program doesn't work nearly as well as the old method. The really effective way to do this would be to just take the pilots out of the picture like Dave's airline does; let the airline decide when to tanker. That way, the only decision the Captain needs to make is whether the company's number gives him/her warm & fuzzies, and if it doesn't, exercise their prerogative to add some extra.


joel p said...

Hey Sam-

How do you determine the weight of the airplane? Is there some kind of gauge in the flight deck that tells you the weight of the aircraft, or do you estimate based on the number of passengers on board and their estimated weight?


Sam said...

Basic Operating Weight is the weight of the aircraft with operating fluids at maximum levels (engine oil, hydraulic fluid), any unusable fuel, and crew members with their bags. This is known (measured by maintenance) and shown on our dispatch release. To this we add passenger weight based on a standard weight of 189 lbs (194 in the winter) or half that for a child under 12 years of age, and bags based on an average of 30 lbs or an average of 60 lbs for each bag actually weighing over 50 lbs. The sum total gives you zero fuel weight. This is normally done by a computer program on the Captain's PDA; we're normally spared from computing W&B manually but sometimes have to do it if the captain forgot to charge the PDA's battery. You can count on a delayed departure when that happens!

Aviatrix said...

My weight and balance is done on my laptop in MS Excel. It gives me a little graph of the C of G during the flight, so I can see which tanks I have to use first to keep it in bounds.

At another company I used the PDA method and then had to look up the calculated C of G on a laminated card.

Anonymous said...

Makes me glad that fuel is free when I fly in flight sim :).

Have not had a chance to congratulate you on your recent job switch. It is funny because I too moved from the PNW to a destination in the east about a month ago myself. In fact, judging from your new employer, you probably will be making frequent stops at my new "home" airport- DTW. I guess there was actually a chance that your new base could have been very near to where I am now!