Does it cost fuel to perform a step-climb?
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Does it cost fuel to perform a step-climb?
I was wondering if there's any fuel penalty for performing a step-climb, ie. would the actual climb of 2000' (or 4000') burn additional fuel, which would be later on offset by cruising closer to OPT altitude?
My reasoning is NO, because during climb the a/c would gain additional potential energy (ie. altitude), which can be then redeemed in descent. Also, while climbing, the engines operate at max CLB thrust, where their efficiency is better than on lower thrust setting... Does that sound right?
Cheers,
S.
My reasoning is NO, because during climb the a/c would gain additional potential energy (ie. altitude), which can be then redeemed in descent. Also, while climbing, the engines operate at max CLB thrust, where their efficiency is better than on lower thrust setting... Does that sound right?
Cheers,
S.
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I was once told a rule of thumb... If you could spend 10 minutes cruising at e higher optimum altitude then it would be worth it.
So I guess this would indicate that you would burn extra fuel in the climb, but this would be offset as long as you were at this optimum altitude for a period of 10 minutes before starting on down again.
So I guess this would indicate that you would burn extra fuel in the climb, but this would be offset as long as you were at this optimum altitude for a period of 10 minutes before starting on down again.
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Another 'ancient' rule of thumb is 'Do not climb unless you are more than 1 hour from landing'.
Some typical 737-400 figures at 48T:
Climb from FL 320 to Fl 360 Fuel Burn 100kg
Cruise at 320 2240kg/hr
Cruise at 360 2100kg/hr
IE Save 100kg for the hour. (min of 40 mins cruise with 20 mins descent). Factor in improved descent distance at idle as well.
Some typical 737-400 figures at 48T:
Climb from FL 320 to Fl 360 Fuel Burn 100kg
Cruise at 320 2240kg/hr
Cruise at 360 2100kg/hr
IE Save 100kg for the hour. (min of 40 mins cruise with 20 mins descent). Factor in improved descent distance at idle as well.
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An extract from the Boeing 737 (NG) FCTM, page 4.10.
Fuel for Enroute Climb
The additional fuel required for a 4,000 foot enroute climb varies from 300 to 500 lbs (135 to 225 kgs) depending on the airplane gross weight, initial altitude, air temperature, and climb speed. The fuel increment is largest for high gross weights and low initial altitudes. Additional fuel burn is offset by fuel savings in the descent. It is usually beneficial to climb to a higher altitude if recommended by the FMC or the flight plan, provided the wind information used is reliable.
Note: The fuel saved at higher altitude does not normally justify a step climb unless the cruise time of the higher altitude is approximately 20 minutes or longer.
No try telling the last part to captains who obsess with the fuel league and insist on going up to FL410 a few minutes before descent...
Fuel for Enroute Climb
The additional fuel required for a 4,000 foot enroute climb varies from 300 to 500 lbs (135 to 225 kgs) depending on the airplane gross weight, initial altitude, air temperature, and climb speed. The fuel increment is largest for high gross weights and low initial altitudes. Additional fuel burn is offset by fuel savings in the descent. It is usually beneficial to climb to a higher altitude if recommended by the FMC or the flight plan, provided the wind information used is reliable.
Note: The fuel saved at higher altitude does not normally justify a step climb unless the cruise time of the higher altitude is approximately 20 minutes or longer.
No try telling the last part to captains who obsess with the fuel league and insist on going up to FL410 a few minutes before descent...
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Did a 1000nm leg yesterday, we werent that heavy and optimum FL was about 390 but me skipper elected to stay at FL330. At first I wondered why but then checked the winds and realized why. Headwind at FL330 was between 40 n 50 knots the whole way increasing at higher levels.
Sometimes it pays to fly way below optimum.
Sometimes it pays to fly way below optimum.
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Aircraft FCOMs and FCTMs discuss step climbs in the context of zero or constant wind. A good flight planning system understands the incremental fuel and cost for the step climb and compares it to the fuel savings associated with a potentially better wind profile.
Bottom line the FCTM and many rules of thumb are poor decision making tools.
Bottom line the FCTM and many rules of thumb are poor decision making tools.
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As above...in Boeing FMCs you can type in the alt and if other environmental data is correct it will give you a percentage saving. Personally I don't even look at it unless there is at least half an hour of the cruise remaining.
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The Boeing Performance and Planning Manual provides you with pretty acurate data.
However you need to review both the "Step Climb Table" and the "Wind Trade-off Table" too.
However you need to review both the "Step Climb Table" and the "Wind Trade-off Table" too.
You can get a pretty good idea by noting the extra fuel flow (above cruise fuel flow) over the time it takes to climb. That will give you the cost. Now have a look at the cruise control tables (if you have them) and check how much fuel you will save to compensate for the extra fuel used in the step-climb.
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If you could spend 10 minutes cruising at e higher optimum altitude then it would be worth it.
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Another 'ancient' rule of thumb is 'Do not climb unless you are more than 1 hour from landing'.
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Another 'ancient' rule of thumb is 'Do not climb unless you are more than 1 hour from landing'.
Even those 10 min or 1 hr will make you loose fuel if you start your descent early or late ( twice as much if late)...(ToD point depends on GW, wind, RWY change, descent-speed, cabin descent rate... )
If ToD is correct, even 1 sec on cruise ( not 10 min) will save fuel.
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The discussion was about step climbs, not screwing up descents!
I do not understand this - can you explain?
I do not understand this - can you explain?
Originally Posted by gg
If ToD is correct, even 1 sec on cruise ( not 10 min) will save fuel.
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"Another 'ancient' rule of thumb is 'Do not climb unless you are more than 1 hour from landing'."
I retired nearly 14 years ago so my memory might be faulty. But I do seem to recall that after take off the normal practice was to climb even though I might be back on the ground within the hour
Jack
I retired nearly 14 years ago so my memory might be faulty. But I do seem to recall that after take off the normal practice was to climb even though I might be back on the ground within the hour
Jack
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very much depends on how you climb.
If you can cruise climb with no minimum rate required then all climbs are good.
If you have a minimum rate to achieve then that rate will dictate when a climb becomes beneficial.
(assuming still air)
If you can cruise climb with no minimum rate required then all climbs are good.
If you have a minimum rate to achieve then that rate will dictate when a climb becomes beneficial.
(assuming still air)
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Hijack..........I know what you mean - in 't good old days, climbing up for flights of 1 hour or less never happened until you had to to get into radar contact for the IMC recovery..................
Good one Jack!! 
Generally speaking on most heavy Jets I've flown unless you are a long way below optimum then climbing in the last hour ( considering you'll be descending in about 30 mins anyway ) isn't worth it.
Generally speaking on most heavy Jets I've flown unless you are a long way below optimum then climbing in the last hour ( considering you'll be descending in about 30 mins anyway ) isn't worth it.
Last edited by nitpicker330; 21st Jul 2012 at 12:10.
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what algorithms is the box using?
Goodonyer Bloggs,
Such blind faith in a box (in your jet) that does not even take into account the aircraft bleed status, relies on a predicted capability of only 100 fpm for max altitude, has no means of temperature prediction and has no installed limitations for manoeuvre margin between low and high speed buffet boundaries... but then, you were trained by the "it's OK to let the feet cross" experts, if I remember correctly.
Ask Clarrie what happens if you follow your advice...
I had hoped you would have said: "know the limitations of what and how the box calculates things, because it accepts no responsibility for the outcome"!
Stay Alive,
If you've got accurate route winds and temps in the FMS and it says go up, go up!
Ask Clarrie what happens if you follow your advice...
I had hoped you would have said: "know the limitations of what and how the box calculates things, because it accepts no responsibility for the outcome"!
Stay Alive,