Jets really don’t like flying low...

Discussion in 'Flight Following' started by RudyP, Apr 5, 2021.

  1. RudyP

    RudyP Cleared for Takeoff

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    I’m flying from Naples FL back home to San Jose CA tomorrow and I will need to do three legs and nearly 8 hours of flying due to the headwinds (vs. two legs and 5:40 on the way here).

    Here is the time and fuel forecasts for the last leg showing I could technically get there in 2:14 if I stayed real low at 12,000 ft but I’d be sucking so much fuel, I can’t actually do that with adequate reserves. So instead I’ll go to 320 and spend an extra 25 min but with enough fuel to be safe.

    57472298-DE04-48A6-BABC-7B49F8D4E93A.jpeg

    Just thought this might be interesting to some who are aspiring or moving up to jets.
     
  2. MauleSkinner

    MauleSkinner Final Approach

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    What would it look like at, say, FL240 with the thrust pulled back to the same fuel flow as you’d get at FL320?
     
  3. ktup-flyer

    ktup-flyer En-Route

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    I'll trade you ;)
     
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  4. idahoflier

    idahoflier Cleared for Takeoff

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    Sounds like you need a bigger jet... ;-)
     
  5. kyleb

    kyleb Final Approach

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    How high can you go? Don't winds begin to diminish waay up high?
     
  6. jordane93

    jordane93 Touchdown! Greaser!

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    I just fly at whatever altitude the dispatcher says;)
     
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  7. falconkidding

    falconkidding Line Up and Wait

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    Yeah on the beechjet it sucks down gas at low altitudes. For anything over 300 miles i'm at 40 or 41. Best TAS is somewhere around 30 but I'd rather burn 1050pph and have the reserve vs save a few min and be worried about fuel. You could also try lower and pull the fuel flows back. It can be worth it depending on the headwind. I'll pull back to 600 a side and that will give me a close enough speed to barber pole that I don't look too weird lol.
     
  8. Rebel

    Rebel Filing Flight Plan

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    "Go to 320 and spend an extra 25 min but with enough fuel to be safe." Excellent aeronautical decision making.
     
  9. RudyP

    RudyP Cleared for Takeoff

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    The Mustang isn’t exactly overpowered so that could help a little but probably not much.

    Don’t we all?

    FL410 and yes, often they do but not tomorrow.. Headwinds >100 kts in the high 30s/low 40s at the end of my trip.
     
  10. Sluggo63

    Sluggo63 Cleared for Takeoff

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    I guess it’s all relative. Here’s our fuel plan from the other day. Your total burn is just a bit more than our fuel to taxi to the runway.
    E1A7F5FE-1931-4135-84CB-F36B84F7819E.jpeg

    But, then again, I’m not paying for the gas. When I’m footing the 100LL bill, my GAS factor goes way up.
     
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  11. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    Question, do some airlines (or operators) elect not to taxi on one engine? Some I’ve been on will and others won’t. Seems like it would save a considerable amount of fuel on those long taxi’s - no?
     
  12. Sluggo63

    Sluggo63 Cleared for Takeoff

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    I can’t speak for all of them, it at our airline, the standard procedure is “Less Than All Engine Taxi” (since we have some three-holers, we don’t call it single engine taxi).

    That’s if there’s no restrictions. Some airports will require you to start them all to taxi. Also, we don’t LTAET is engine anti-ice is required to be on during ground ops. Also certain hydraulic or electrical MELs will force an all-engine taxi.

    Our engines require a 3-minute warm up time before takeoff, so if it’s going to be a short taxi, I’ll start them both up during the pushback so I’m not holding short waiting for the engine warmup time to expire.

    But, you’re right. It saves quite a bit of fuel for the long taxi routes.
     
  13. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    Interesting, thanks!
     
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  14. dmspilot

    dmspilot En-Route

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    It is usually decision by the PIC based on operational factors. Starting both early may be required for engine warmup (ours can be as long as 8 minutes in cold weather). Or to burn extra fuel when the aircraft has been overfueled slightly.Or a short taxi is anticipated but does not come to fruition. Or both engines are needed for the ice protection system during icing conditions on the ground.
     
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  15. GRG55

    GRG55 Final Approach Gone West

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    I'd be so traumatized by something like this if I was flying that jet I'd probably start thinking irrational thoughts...like reverting back to a piston airplane or something...:eek: :D

    Seriously, keep the informative posts coming. This sort of stuff is interesting as I don't think I would have ever even thought it was a factor until I saw the numbers you posted. Learn something new here every day! :thumbsup:
     
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  16. MauleSkinner

    MauleSkinner Final Approach

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    It’s not about being overpowered...it’s about whether or not you gain more than 24 knots of true airspeed for the same fuel flow in that 8000 feet to offset the increase in headwind.
     
  17. EvilEagle

    EvilEagle Cleared for Takeoff

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    Some of them do... it just depends on who's paying the gas bill. ;) _DSC3076-2.jpg
     
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  18. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    Same.
    Have done many, many experiments in different headwinds and in every case up to 100kts HW, it has turned out to always be better to go high.
    The fuel savings is always better than to be low in a lesser headwind.
    So many calculators on various apps and websites that are offered during preflight planning are wrong!
    During the climb you can level at a lower altitude and watch Fuel @ Destination, write it down. Ask for 4K higher, level, recheck, write it down. Continue on up, and I cannot recall a time it was worse.
    Also, I have noticed powering back ie 100% N1 to 90% does not help tremendously like in a piston plane.
    CE-525.
     
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  19. RudyP

    RudyP Cleared for Takeoff

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    What I meant is that the Mustang being relatively underpowered gets slow when you pull the thrust back.. And that leads to issues with ATC and pilot sadness when looking at TAS. The other day, I got asked if I was on a speed restriction by a new controller while I was at max continuous thrust.
     
  20. Tarheelpilot

    Tarheelpilot Final Approach

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    This conversation is one that reminds me how lucky I was to start my career sideways on the 727. Learned so very much in that seat. I’m also glad I was only there for a year :)
     
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  21. RudyP

    RudyP Cleared for Takeoff

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    Love this! I’m really glad to not be footing that fuel bill!
     
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  22. EdFred

    EdFred Taxi to Parking

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    I wish I knew enough poor people I could steal from to afford a jet. :D
     
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  23. RudyP

    RudyP Cleared for Takeoff

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    If you’re willing to do that, might as well emulate the real pros (IRS) and steal from everyone and the rich in particular... then you can afford really awesome hardware (see EvilEagle’s picture!)
     
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  24. Plano Pilot

    Plano Pilot Line Up and Wait

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    We almost always taxed out on one engine in the Sabreliner 60 and Falcon 20's that I flew. Each engine on the Sabreliner burned about the same amount of fuel at idle on the ground as max cruise at FL410. The fuel burn on the ground is not as bad in the Citation Ultra that now fly. The only time we taxi out on one is when we are expecting a delay.

    With the APU running the second start in the Falcon 20 was easy. Without an APU you have to turn off the avionics during the second start and then get them going again before takeoff. It depends on the avionics how much you have to reprogram, almost everything in the Ultra. No APU in the Ultra or the Sabreliner that I flew.
     
  25. MauleSkinner

    MauleSkinner Final Approach

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    BTDT, but if pulling the thrust back makes you go faster over the ground, who cares about TAS?
     
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  26. EvilEagle

    EvilEagle Cleared for Takeoff

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    Here here, we never burn less than 800# in the 330 for taxi. Normally it's more like 1400#.

    In the F-15 we planned on min #2000 fuel burn to get through gear up.
     
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  27. Larry in TN

    Larry in TN Pattern Altitude

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    What your table doesn't show is the dramatic increase in true airspeed as the altitude increases. Not only is there a big drop in fuel burn, but you're going much faster at that lower fuel burn.

    Some fleets might not do many (any?) single-engine taxi due to the amount of thrust required from that single-engine to move a heavy airplane. Too much chance of causing damage to others and objects from the jet blast.

    Even on fleets that routinely single-engine taxi, it will depend on the circumstance. Length of taxi, weight of the aircraft, condition of the taxiways, and whether engine anti-ice is needed for the taxi. Then there are some Captains who like to taxi with all engines running

    For those who didn't do the math, that's 15,724 pounds-per-hour (~2,300-2,350 gph) for the trip. B777?
     
  28. Tantalum

    Tantalum Final Approach

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    This thread really puts all the "but my plane can do 150 knots at 10 gph, yours burns 30 to go 170 knots" debates in perspective!

    :rofl:

    can't have any of that!
     
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  29. Pugs

    Pugs Line Up and Wait

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    Yep, in the Prowler the 4000 lbs of 2 drop tanks of gas got you to a mid-alt (220-240) top of climb. Adding a third tank to the centerline didn't get you much more due to increased drag.
     
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  30. kgruber

    kgruber En-Route

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    John Deakin had a post, (which I can't find) about how he saved fuel in the Japan Airlines B-747. If he took off without full fuel, he could immediately climb to a high altitude and save fuel for the trip. But if, as JAL almost always required, he had full fuel............he would get stuck behind other airliners on the same route and had to step climb. Usually, this resulted in a higher total fuel usage for a given flight.
     
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  31. Groundpounder

    Groundpounder Pattern Altitude

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    What is your burn and time at 410? The difference in fuel burn at higher altitudes on my work airplane almost always makes up for higher headwinds.
     
  32. EvilEagle

    EvilEagle Cleared for Takeoff

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    I never flew the whale but in the 757, 767 and the 330 we take off heavy and can't get to "top of climb" but the FMS continually calculates optimum altitude based on a myriad of factors including total weight. Typically it'll have us starting in the mid-low 30's and be above 40k by the time we get through half of our fuel. Step climbs are the best way to save fuel in the big jets I've flown.

    Just speculating, the whale is fast - cruising near/at/just past .9 mach on crossings (second hand info; but I have seen them fly by me while I'm going .83M). We got held up one time by a BA A320 leaving Kennedy. He messed up the hold short restrictions while taxiing and ended up cutting us off on the ground, we had to step climb behind him all the way to the NA Tracks; that little bus climbs slow and flies slowish so it cost us (767ER) a lot of fuel and time staying behind him. I'm guessing he was commenting that getting step climbed for traffic was a big issue, but not that step climbs inherently cost them fuel. The whale plans on step climbs for long routes to save fuel as well.
     
  33. Rockymountain

    Rockymountain Pre-takeoff checklist

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    Turbo props Don’t take as much of a hit as jets down low. In the M600, can easily make Naples to San Jose with one fuel stop. The best compromise over ground speed in fuel burn for one fuel stop turns out to be 16,000 feet. 9:47 flight time.

    Figuring in one fuel stop versus two fuel stops, the M600‘s gonna be rounding error in total travel time versus the mustang, especially when you factor in the extra hassle of the additional flight planning and the fatigue that occurs with multiple legs. Having met you though, squeezing you into the cockpit of the M600 would be a little challenging

    44B7DBF3-CE7F-4E1D-B7A2-6CB1B679904A.png
     
  34. Ted

    Ted The pilot formerly known as Twin Engine Ted

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    When I was flying the MU-2, economy cruise (96% RPM) at FL250 (service ceiling) was around 52 GPH combined, and power cruise (100% RPM) at 14k feet was around 85 GPH combined. In between those two had fuel burns in between. Takeoff was around 120 GPH combined. Normally in the winter I'd end up somewhere in the FL180-200 range heading westbound, which was a sweet spot for best speed and fuel economy.

    I would step climb with the heavy loads when hot, starting at FL230 and then going to 240 and 250 (I never had an issue getting wrong direction at 240 heading eastbound).

    Making a 1,000 nm westbound was almost always doable non-stop. But I'd definitely have been making 2 stops on Rudy's westbound flight home in the MU-2.

    One other thing that comes into play more with turbines than pistons because of the mandatory items (like hot section inspections) is that often it makes sense to burn more fuel to get there faster, because when you look at the total cost, the fuel is cheaper. That gets more complicated when you look at a total ownership picture.
     
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  35. SoCal RV Flyer

    SoCal RV Flyer En-Route

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    OK, if I really want a jet I'm gonna have to build one. There's a guy who flies his Sub Sonex cross-country to Oshkosh...he says "duration is about 1.5 hours and then it's a glider." He cruises at 17,500 ft and plans his fuel stops carefully. :D

    About as practical as a jet pack, but looks like a fun toy. 304-mile range, 240 mph TAS, 40 gal. of fuel.

    SubSonex_Air-to-Air_7786-AirVenture_2018-20.jpg
     
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  36. Ghery

    Ghery Touchdown! Greaser!

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    Interesting read. I'll be flying a C-172P later this afternoon and all this is academic in comparison. :)
     
  37. Groundpounder

    Groundpounder Pattern Altitude

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    Here is what it would look like in a Challenger 605. Initial level off at FL360 due to weight, step to FL380 later in the flight.

    ForeFlight Web.jpg
     
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  38. Plano Pilot

    Plano Pilot Line Up and Wait

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    Some times it makes sense to take a longer route around the winds. A few times it has saved me a fuel stop flying New York to Dallas by taking a more northly route to stay out of the worst of the headwinds, it does not happen often.

    As Ted said you also need to factor in the hourly maintenance cost. Some times it makes more sense to stay low and take the fuel burn hit to save time on the airplane, as long as you do not have to add a stop for fuel.
     
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  39. MauleSkinner

    MauleSkinner Final Approach

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    I always figured a fuel stop was about 15 minutes of extra flying time. You can buy a fair chunk of fuel for that same money. ;)
     
  40. kgruber

    kgruber En-Route

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    Otherwise known as "pressure flying!"