really slow descents . . . . save fuel and go faster?

Discussion in 'Flight Following' started by comanchepilot, Nov 18, 2017.

  1. comanchepilot

    comanchepilot En-Route

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    Ok everyone - we've all got ipads and phones at least with flight planning software etc.

    If you climb to your cruise altitude and are on a 400nm leg - your GPS tells you that your descent to destination is 50fpm and that number goes up as you get closer to your destination.

    So if you waited until you had 100fpm descent to destination - and you then set up a 100fpm rate of descent, you'd pick up at 5 knots most likely - at the same fuel flow - or you could go the same speed and throttle back - I did that once and could maintain the same groundspeed and burn 0,7gph less.

    obviously with terrain and airspace obstacles maybe its the not the most practical . . . but it does provide an interesting flight planning choice.

    Climb to 8500, start a 100fpm descent to 500 - its 80min - maybe its a way to stretch a fuel supply!
     
  2. Direct C51

    Direct C51 Pre-takeoff checklist

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    That method is more efficient than chopping the throttle and diving in at the last minute. My preferred method is to stay up high, with a higher TAS, leaned out, hopefully with a solid tailwind. Then keep the power in and push for a 500 FPM descent 16 minutes out, in your scenario. That's about the most efficient way, in time and fuel. You can always lower fuel burn by throttling back to most economical power setting, which applies not just in the descent, but also in cruise.

    I haven't really thought through the numbers, but I have a hunch that climbing, just to start descending again for the slight increase in ground speed is less economical than just staying down low and not burning all that fuel in the climb, at climb air speeds. If you are going to climb that high, might as well use the thinner air to your advantage for awhile.
     
  3. Kritchlow

    Kritchlow Final Approach

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    I'm guessing a piston aircraft is different...

    In a jet the most efficient way is to chop power at TOD and glide in.
     
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  4. luvflyin

    luvflyin En-Route PoA Supporter

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    I read somewhere a long time ago that the most efficient way is to never level off. Climb until your halfway and then descend the rest of the way. That's probably rarely practical but it makes sense from a purely 'the numbers' standpoint. Wind, terrain, airspace, traffic and all that stuff make it usually not feasable.
     
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  5. Clark1961

    Clark1961 Touchdown! Greaser!

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    Yeah, our fuel consumption doesn’t change as much with altitude.

    As Mark pointed out if ya run the numbers cruise-climb and then cruise-descent is theoretically the most efficient. ATC talks to me like I’m weird if I try that profile.
     
  6. mscard88

    mscard88 Touchdown! Greaser!

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    That's not the reason.
     
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  7. Clark1961

    Clark1961 Touchdown! Greaser!

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    I’m used to the “ where do you think you’re trying to go today?” questions. Start descending half way to your destination and they get ... err, umm, strange.
     
  8. mscard88

    mscard88 Touchdown! Greaser!

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    I don't see why they would. They might ask if you're having a problem.
     
  9. Clark1961

    Clark1961 Touchdown! Greaser!

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    ATC can be a lot like POA in some ways. Some controllers have expectations and have a tough time accepting things that don’t conform to those expectations.
     
  10. FastEddieB

    FastEddieB Final Approach

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    Just a reminder...

    If on a clearance to climb or descend, this is what the AIM says:

    “d. When ATC has not used the term "AT PILOT'S DISCRETION" nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC.”
     
  11. steingar

    steingar Touchdown! Greaser!

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    I didn’t buy a fast airplane to fly slow. I point the nose down and get going. I just try to stay below Vne. Not the easiest thing to do in my aircraft.
     
  12. Shepherd

    Shepherd Pattern Altitude

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    Isn't everyone doing this already?
    Oh my.
     
  13. FastEddieB

    FastEddieB Final Approach

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    But I think the question is, is it better to come down at a speed slightly above cruise for a long distance, or to come down really rapidly all at once.

    A lot of factors in play, but I think the fact that drag increases as the square of the increase in velocity makes longer, more gradual descents more efficient aerodynamically.

    Personally, my 496 reminds me when I reach the point where a 500fpm descent will put me at 1,500’ AGL 3 miles from my destination. I then fly down the faux glideslope representation onscreen. Seems like a nice compromise.
     
    Last edited: Nov 18, 2017
  14. Clark1961

    Clark1961 Touchdown! Greaser!

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    Yeah, I’ve got VNAV set on the 430w. Gotta be a little careful with it in the hills...
     
  15. FastEddieB

    FastEddieB Final Approach

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    Oops! I meant on the 496 in my Sky Arrow, which gives a visual “glideslope”.

    But I think I had the 430’s in my Cirrus similarly programmed.

    Going back and editing my post.
     
  16. Kritchlow

    Kritchlow Final Approach

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    Regardless, different types must have different sweet spots as far as efficient descent rates and speed. Obviously ten feet per minute for five hours isn't going to be the answer.
     
  17. Clark1961

    Clark1961 Touchdown! Greaser!

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    As a side note Concorde used cruise climb once it was in cruise. Dunno what it’s descent profile was. Prolly something like get out of my way imma coming down and will be fuel emergency if’n ya mess with me...
     
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  18. dmspilot

    dmspilot Pattern Altitude

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    Descending from 8,500 at 100fpm means you'd be spending half an hour below 3,000 where the airplane is not very efficient. So I would bet the answer to the OP's questions is "no".
     
  19. hindsight2020

    hindsight2020 Pattern Altitude

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    The specific fuel consumption of the powerplant largely will dictate what descent profile is more fuel efficient, if that's what you're trying to solve for.

    In the case of piston engines, it doesn't matter, the SFC is fairly constant across all altitudes, so there's no economy beyond maximizing groundspeed for the lowest power setting practicable in all phases of flight. That becomes an aerodynamic consideration, not a powerplant one.

    For turboprops, the profile actually looks like staying at cruise altitude until no kidding on top of the airport, then chopping to flight idle and using the drag of the prop(s) to get ya down. In the aggregate, that leads to less fuel consumption than a cruise descent. I was incredulous until I did it in the T-6A. This is a combination of the specific fuel consumption behavior of the turbine (which favors staying high), combined with the inefficiency of the propeller application in the descent (which doesn't favor flight idle). This profile is of course impractical for non-emergency flight, but there ya go.

    Turbine fans are the ones which benefit from idling TOD idling descents in earnest. This is due to the worsening SFC at lower altitudes, plus the aerodynamic efficiency of not having unducted powerplant components, especially at the speeds the turbine aircraft is designed to cruise and descend at.
     
  20. jordane93

    jordane93 Final Approach PoA Supporter

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    This. I just calculate for a ~500fpm descent and keep the throttle in and watch the airspeed increase.
     
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  21. nauga

    nauga Pattern Altitude PoA Supporter

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    No descent with a power setting above idle is going to increase range. Fuel burned in the descent would be more efficiently used at optimum cruise conditions. To make an extreme example of it, cruise at altitude at best-range conditions until you run out of gas, then descend at L/Dmax (best glide) with the prop stopped. Max time at optimum conditions, then extend the range with no fuel consumed.

    Nauga,
    and his Rutowski climb
     
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  22. Hank S

    Hank S En-Route

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    Me too, brother. Ground and airspeed increase significantly for the same cruise fuel burn initially, but as I descend and manifold pressure increases I walk the throttle back to maintain cruise power (and higher IAS & GS). Of course, I also walk mixture forward to maintain cruise EGT. Then I need a couple of miles at reduced power to bleed off excess airspeed for pattern entry.

    If only I had fuel flow readings, I could do the math and see which is best from empirical results. Instead, my fuel readings come from the gas pump, and that makes it hard to compare between different flights.
     
  23. jonnyjetprop

    jonnyjetprop Line Up and Wait

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    This is what jets do on short legs. Not sure if it's the best profile for piston aircraft.

     
  24. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    Pistons are different. The difference in fuel burn in a jet at high vs low altitude is vastly different from a piston.
     
  25. Checkout_my_Six

    Checkout_my_Six Final Approach

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    lessee....where can you go fast and use the least amount of fuel? Stay there and come down quickly....using potential energy and fuel burn wisely. That's what I do....when available. :D
     
  26. Salty

    Salty Cleared for Takeoff

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    Why'd you climb up that high in the first place if you didn't want to be there?
     
  27. jkaduk

    jkaduk Cleared for Takeoff

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    While an interesting exercise I suspect the difference between chop and drop and descend at 100 FPM makes very little difference. Further, I would look to operations that are attempting to make a profit to tell what is most efficient.
     
    Last edited: Nov 20, 2017
  28. SCCutler

    SCCutler Administrator Management Council Member PoA Supporter

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    As a practical matter, basing in a large class Bravo area like I do, I rarely have the option of when to send anyway.

    The other very real consideration is that, for most of the year, I like to stay high for as long as I can, so I am staying in the cool air and spending less time bouncing around in the bumpy hot air below.
     
  29. Bill Jennings

    Bill Jennings Final Approach

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    Yup, in the 201 I do a 500fpm descent and keep cruise power and cover the ground at 3mi/min. It helps make up some time lost on climb.
     
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  30. WillFly4Food

    WillFly4Food Filing Flight Plan

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    It’s nice to have a long gentle high speed cruise at -300 fpm, easy on the pax ears, too. But if the descent is into headwinds or lumpy convective air currents, I’ll just stay high, then slow down and descend.

    Sometimes when you’re descent cruising at 3 mi/min it can become a challenge to get down in time when you have to suddenly manage bumps, speed, engine cooling, fpm, etc. Gotta think ahead and stay ahead of the airplane/situation.
     
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  31. Hank S

    Hank S En-Route

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    If it's too bumpy, reduce power and slow down.

    What gives me trouble is knowing when to start down with a huge (30 knot or more) tailwind. Can't trust the GPS Time Remaining, the wind will slack off at some point during descent. It doesn't happen often enough for me to figure out, I always end up at TPA far from my destination, or I realize that's what is happening and level off, distant with greatly reduced tailwind.
     
  32. comanchepilot

    comanchepilot En-Route

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    in the Comanche it's 20" of MP and 500 fpm down - I'm over 165 knots usually.
     
  33. denverpilot

    denverpilot Taxi to Parking

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    I fly a 182. There’s nothing efficient about it and that’s just life. LOL. And you’re already late for lunch chasing the kids with the turbos up here, so just leave it at full rental power and get there. :)