Archive for zero fuel weight

Jet Fuelishness

Posted in air travel, airline pilot blog, airliner, airlines, airport, flight crew, jet, passenger, pilot, travel with tags , , , , , , , , , , , , , , on November 15, 2013 by Chris Manno

I’ve always agreed with the pilot maxim, “The only time you can have too much fuel is when you’re on fire.” But, as with all things in life, there’s a catch: first, you have to be able to lift the weight into the air, and second, you have to be able to bring the tonnage to a stop on landing.

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Two simple requirements, or so it would seem–yet nothing could be further from the truth. Let’s look at the second requirement: stopping distance.

All month I’ve been flying into John Wayne-Orange County Airport in Santa Ana. That’s by choice–I like the  typically favorable weather, plus the lack of ground traffic that makes for a quick in and out. Plus, the food options from Gerry’s Wood Fired Dogs to Ruby’s awesome turkey burgers rival the Udon, Cat Cora and Tyler Florence options at San Francisco International. But I digress.

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Today I’m flying the 737-800 from DFW to Santa Ana (SNA) and approximately 2 hours from takeoff, I’ll call Flight Dispatch and ask, “What fuel load are you planning today?” And he will say, “I don’t know.”

That’s because the flight planning system won’t issue a fuel load until one hour prior. I realize that–but as crew, we show up one hour prior and by then, the fuel is already being pumped into the jet. I want to shortstop a problem unique to SNA. That is, fuel is really expensive at some California airports, including taxes, airport assessments and surcharges. So it does make sense to “ferry” some fuel into those airports.

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That is, if I need an arrival fuel of say, typically, 5,200 pounds in order to have divert or go-around options at the destination, we fuel up to that total, then add “ferry fuel,” or an additional upload so as to require less refueling, buying less with the added fees, taxes and cost for the return flight.

Problem is, SNA has a fairly short runway (5,700 feet, versus 13,000 at DFW) making stopping distance is critical.

So, while extra fuel saves money on refueling (yes, you have to figure that it does exact a higher fuel burn inbound because of the additional weight), we still have to have a sufficient stopping margin.

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In all cases, the maximum landing weight of the jet based on the structural limit is 144,000 pounds which, on a dry runway, requires 5,300 feet out of the 5,700 feet available to stop. I discount headwinds, which are favorable, and simply disallow tailwind corrections: at 144,000 pounds, I require zero–I’m not even trifling with a 400 foot margin touching down at 150 knots.

So my effort in calling Dispatch is to intervene in the numbers game: do NOT plan max “savings” ferry fuel until you know what the zero fuel weight (passengers, cargo, empty jet–everything BUT fuel) is.

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Then subtract the zero fuel weight from 144,000 (max landing weight), deduct the planned enroute fuel burn and see what is left over–THAT , minus 2,000 pounds as a safety buffer (mine personally), and you’ll have a reasonable ferry fuel load.

The problem is, by the time I get to the jet, the “planned” fuel load–which doesn’t include the above calculation, because the zero fuel weight isn’t firm yet–is already aboard. If I do the math and find that we’ll be arriving weighing over the max landing weight, I have two choices: defuel (bad choice) before pushback or fly lower (dumb choice) to reduce the landing weight.

Both are bad options: if we defuel, that fuel must be discarded–trashed–because quality assurance standards wisely say you cannot take fuel from one aircraft’s tanks and meet the purity standards for another aircraft. So that’s money in the trash, plus a guaranteed delay to accomplish the defuel.

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The “fly lower” option works, but look what we’ve done: to “save” on return fuel, we’ve wasted thousands by flying at 24,000 feet versus 38,000 or 40,000 feet, just to squeak in under the maximum landing weight. And it’s bumpier and noisier down there among the cumulus clouds.

I always choose the second option, although I don’t always like landing at the maximum structural limit of the airframe on the shortest runway in the system. But, at least we can save the absolute maximum fuel for the return, rather than simply defueling into the trash.

On a longer runway, say LAX, stopping distance wouldn’t be a consideration, but the 144,000 pound limit is simply universal: doesn’t matter where you land, 144,000 pounds is max allowable. I need to intervene in the mathematics before the fuel goes on the jet outbound.

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The second, problem: the return. Dispatch may shave the arrival fuel to 5.0, which is sufficient, but there’s a catch. He’s planned us at a low altitude (29,000) because of chop reported in Arizona at the higher altitudes. If he’s right, at that lower altitude (FL290) I know from 38 years as a pilot that there will be both flight deviations for spacing or weather, or a choppy ride anyway.

So here’s what I personally do: I add another thousand for additional time and distance flexibility in case the turbulence forecast is correct–but I also plan to climb immediately to 39,000 feet to see for myself if the ride is choppy. That’s because I’ve just flown through that airspace inbound and know firsthand what the winds and the rides are, whereas the Dispatch and even the ATC reports are hours old. Plus, and again, this is based on over 22 years as an airline captain, I know we’re taking off at dusk and the entire thermodynamics of the air mass will change dramatically.

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So based on intuition, I’ll do the climb to 39,000 and “take the hit:” the early climb will be heavier and burn more fuel versus a later step climb, but my gut feel says we’ll regain that amount and more by cruising the longer time at the higher altitude. Notice I didn’t say 41,000, because I’m claiming a little pad because of the narrower range between high and low speed buffet at the max altitude. Plus, this time of year, surfing the jet stream at the higher altitudes will get you 510 knots or more across the ground. That’s the pay dirt of efficient flying.

Also, if I’m wrong, I did add the fuel pad up front. But I bet I’m not. The alternative is to fly lower (noisier, crowded, more weather) and experiment with the step climb–which burns fuel, too, and if you have to come back down because the ride’s bad, you’ll wish you hadn’t. But in the worst case, we’ll still land at DFW with a comfortable fuel pad.

And if I’m right, we’ll save a couple thousand pounds eastbound at the higher altitude and land fat on fuel. Fuel is time, to me, so nothing could be more important than more fuel.

Unless as I noted above, you’re on fire, or more realistically, as I’ve just explained, you’re trying to achieve the best outcome as efficiently as possible. Anything less is just plane fuelishness.

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