Jet Fuelishness


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 ┬átypically 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.

sna 10-9

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.

737 landing crop

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.

sunset contrail

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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19 Responses to “Jet Fuelishness”

  1. I know we’re taking off at dusk and the entire thermodynamics of the air mass will change dramatically.
    OK, I NEED to know! Other than the de-coupling from the PBL at about 850 mb, and the LLJ kicking in (mostly warm season), how does the thermodynamic change of the atmosphere–and here I am assuming just general cooling–change the flying characteristics of your jet? Is it a buoyancy or air density issue? Thanks!

    • Good question. Actually, it has little to do with the aircraft and everything to do with the behavior of the air mass as the sun sets and the air cools. Circulation then becomes less intense and also, diminished convection, adiabatic effects and even orographic effects, some of the root causes of bad rides.

      If the bad ride reports were a couple hours old, which they typically are, they reflect conditions in late afternoon at the height of the sun/temperature induced effects. Thta won’t be worth planning around at dusk.

      From the higher altitude, I’ll send automated position reports back to Dispatch recording wind speed, direction and turbulence, so they will have more accurate information to plan later flights both into and out of the coastal area.

      • Very cool!, and fun to think about. Do most commercial aircraft have ACARS onboard?

      • I’m not sure. But we’re in constant datalink contact with Flight Ops Technical.

        I got a printer message recently that asked “what’s your engine vibration readout on the #2 engine? It’s showing high down here.”

        They also monitor fuel flow, fuel burn, winds, turbulence, altitude and speed in real time.

  2. I’ve been reading your work for a few months – and so far, this is my favorite bite-sized look at your world. I’d consider it mandatory reading for travelers, just so they can get a good glimpse into the management and decision-making that go into every pilot’s job.

  3. Great article Professor. I remember when you, me and Huge were coming back from Alaska and after an initial tail wind we came across a 100+ MPH Headwind coasting out of Washington State. As I recall Huge was really sweating it and thought we might have to divert for fuel. Ah, those were the good old days. :0)

  4. Another good one, Chris. I’ve never flown into SNA, but it must be an interesting experience, watching grandma’s teeth fly two or three rows forward when the auto brakes activate. Do you warn your pax about the firmer than usual braking? I’ve heard about those fuel calculations before (Seem to recall it as ‘tankering,’ but whatever.) Getting it right, down to the last pound is yet another reason that they call you Captain. After twenty-two years in the left seat, you’d think they would give you a larger airplane. Perhaps it is a QOL issue and you don’t want a larger one… -C

    • Yes, I have the pilot seniority to hold the 777 captain seat, but that would mean being gone for three days at a time rather than home every night as I am now. Plus, it’s not a pay raise because they only fly 75-80 in 15-16 days, while I fly 90+ in 13 days. And I would have to forego my university salary. So, I’m happy on the 737.

      No, I don’t warn folks about the deceleration. We do it pretty smoothly and if they’re surprised, they obviously haven’t paid much attention to the airport they booked.

  5. […] life of an airline pilot, zero fuel weight. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own […]

  6. Randy Sohn Says:

    And Chris, thinking back to a l-o-n-g time ago when I was flying DC-9s, to tell’ya the truth, I was much more concerned about the noise abatement climbout with the power cutback and what someone in back was thinking during that when taking off south at John Wayne airport.

    • Yep Randy, I remember when we first took the DC-9-80 in there in the 80s. A certain Chief Pilot we both know whose initials are “Tattoo” reportedly coughed up a half order of fries and nearly had a stroke the first time he experienced a demo of the power cut at level off.

      Glad to be flying a quiet Boeing now with no power cutback and a strong climb.

  7. Randy Sohn Says:

    Re >>Glad to be flying a quiet Boeing now with no power cutback<<

    "Now, ain't THAT the ever-lovin' truth". Concur!!!!!!!!!

  8. Giving us a look into these kinds of doing-the-job realities that flesh out the life of Captain so that pax have only what’s in their own lives to think on is one of the things that make your blog worthwhile. Especially so for those of us who sometimes wonder what goes on in the front office.

    I finished recently “The Ice Diaries” which fleshes out Capt. Anderson’s account in “Nautilus 90 North” with the Cold War backstory, and “Unkown Waters” wherein Capt. McLaren recounts the first under-ice survey of the Siberian continental shelf in ’70.

    The more I read the larger the word Captain grows.

  9. Hi Chris,

    As a student pilot at Embry Riddle, I have yet to receive a concrete answer on the age old question, is it more economical to fly higher?

    The majority of pilot know it is more fuel efficient to fly higher, but it also takes fuel to get up that high, so is it really more fuel efficient?

    I spoke with another AA 737 pilot who said if you can get up there its economical to do it, saying he’s flown from DFW to AUS in the mid 30s. (I just cannot picture, flying uphill then transitioning almost immediately into descent as a way to save fuel)

    Also on a side note, at AA, are they flying their routes at slower airspeeds to cut costs? I frequently fly the KMIA-MKJP route which used to take roughly an hour. I flew it in August and it took closer to 1hr 40min

    • It’s not that simple. Yes, the fuel flow is lower at a higher altitude. That normally offsets the slight additional burn to climb, especially on a longer flight.

      But if the headwind at 390 is 124 (as it was yesterday when I flew DFW-SAN) versus 75 at 270, you’ll save flight time and fuel too at 270. So we did.

      No, we’re not flying at slower speeds because that doesn’t necessarily cut costs. The optimum cruise Mach is based on a number of factors–depending on loads, weight, crew costs, altitude and route, faster may even be cheaper.

      The DFW to AUS parabolic arc is an example of pure efficiency: that’s where optimum descent angle meets optimum climb angle. That gives you half of your flight coasting downhill, rather than cruising lower at a high fuel flow, with a short descent.

      Block times are an aggregate figure, based largely on predicted/historical winds. I can’t explain the drastic difference you cited on a particular flight, but no, flying an hour and forty instead of an hour sure doesn’t save any money.

  10. Taking off at dusk ? At Orange County ? I suppose the noise abatement procedures don’t apply to you…

    • It’s pretty clear that you have no clue what the noise restrictions are, much less “noise abatement procedures” (hint: they’re two different things). Check FlightTracker from 4-7pm out of SNA and report back. You can hide behind your fake email address as usual if you’re feeling scared.

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