TO1;TO2;CLB1 and CLB2
They are all thrust limits used by the thrust management function of Boeing FMCs.
TO - Full rated take-off thrust.
TO1 - A fixed derate, typically 10% (airline selectable).
TO2 - A fixed derate, typically 20% (airline selectable).
CLB - Full rated climb thrust.
CLB1 & CLB 2 - Climb thrust derates that are gradually removed as the aircraft climbs. CLB1 typically uses a 10% derate of CLB thrust to 10,000ft, then increases thrust linearly with altitude to CLB thrust at 30,000ft. CLB2 typically uses a 20% derate of CLB thrust to 10,000ft, then increases thrust linearly with altitude to CLB thrust at 30,000.
The numbers quoted above may change for different airlines or aircraft types, but the concept is similar.
TO - Full rated take-off thrust.
TO1 - A fixed derate, typically 10% (airline selectable).
TO2 - A fixed derate, typically 20% (airline selectable).
CLB - Full rated climb thrust.
CLB1 & CLB 2 - Climb thrust derates that are gradually removed as the aircraft climbs. CLB1 typically uses a 10% derate of CLB thrust to 10,000ft, then increases thrust linearly with altitude to CLB thrust at 30,000ft. CLB2 typically uses a 20% derate of CLB thrust to 10,000ft, then increases thrust linearly with altitude to CLB thrust at 30,000.
The numbers quoted above may change for different airlines or aircraft types, but the concept is similar.
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The principle Buzzbox describes is correct, with slight number changes in between airframes.
Essentially what you are doing is using the actual weight of the aircraft and using only the amount of thrust required for that weight to depart rather than the full engine rating on each departure.
Selecting a lower rating as part of preflight (reducing your engine output for departure) automatically tries to achieve later on reduced climb performance as well.
The reduced engine thrust is still capable of performance capabilities required for obstacle clearance gradients, so no safety issues regarding performance are an issue here.
Thus manufacturers propose it as a fuel reduction technique for departures using a combination of both fixed derate and assumed temperature method (Boeing), increasing engine EGT margins and prolonging engine lifecycle.
Essentially what you are doing is using the actual weight of the aircraft and using only the amount of thrust required for that weight to depart rather than the full engine rating on each departure.
Selecting a lower rating as part of preflight (reducing your engine output for departure) automatically tries to achieve later on reduced climb performance as well.
The reduced engine thrust is still capable of performance capabilities required for obstacle clearance gradients, so no safety issues regarding performance are an issue here.
Thus manufacturers propose it as a fuel reduction technique for departures using a combination of both fixed derate and assumed temperature method (Boeing), increasing engine EGT margins and prolonging engine lifecycle.
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It is not a fuel reduction technique, in fact reducing thrust increases fuel consumption, however it reduces maintenance (and usually lease) costs quite a bit.
To get the best of both reducing maintenance cost and reducing fuel consumption a few carriers have now changed their SOP to reduce take off thrust as much as possible and manually select full climb thrust. Takes a bit getting used to increase thrust every time at thrust reduction, but apparently it achieves both aims.
To get the best of both reducing maintenance cost and reducing fuel consumption a few carriers have now changed their SOP to reduce take off thrust as much as possible and manually select full climb thrust. Takes a bit getting used to increase thrust every time at thrust reduction, but apparently it achieves both aims.
One other important concept with using derate is that min control speeds are based on the derate, therefore it is not permissible to use full thrust on the good engine in event of an engine failure like you can do with just reduced thrust.
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It also means that sometimes selected climb thrust means an automatic power increase over takeoff thrust depending on the selections made.
Usually we use max derate(TO2) and max ATM. The cleared altitudes and weights govern the decision for what climb thrust is pre-selected while still on the ground. Heavy weight frequently results in full climb thrust pre-selection. At lighter weights, especially with a low initial altitude clearance will result in a CLB 2 selection. Once a significantly higher altitude clearance is received, full climb thrust is typically selected for a more fuel efficient profile although at light weights we sometimes stick to CLB2 as CLB gives a high enough pitch attitude combined with low fuel levels to result in forward fuel pump low pressure lights.
Usually we use max derate(TO2) and max ATM. The cleared altitudes and weights govern the decision for what climb thrust is pre-selected while still on the ground. Heavy weight frequently results in full climb thrust pre-selection. At lighter weights, especially with a low initial altitude clearance will result in a CLB 2 selection. Once a significantly higher altitude clearance is received, full climb thrust is typically selected for a more fuel efficient profile although at light weights we sometimes stick to CLB2 as CLB gives a high enough pitch attitude combined with low fuel levels to result in forward fuel pump low pressure lights.
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We use the TASS (assumed temp) method on the NG which in most cases will result in TO D2 (22K) but full CLB is recommended, having said that if you are light and have a low initial speed limit or level off then CLB 2 will often be preferable e.g. CLN departures from 26L at LGW. On this departure it is very important not to overshoot the DET inbound radial on the first turn to the East and we recommend that speed is limited to 190 Knots for this turn, a light weight aircraft with full CLB will give a high ROC and with a stepped climb to not above 4000 is likely to increase the risk of a TCAS RA and a high body angle.
Apart from reduced engine wear both methods will reduce noise significantly. TASS must not be used on contaminated runway, or when wind shear is likely or with a pronounced temp inversion, typically +10c change in the first 1000 ft
To further save engine wear and noise we also normally use improved climb which results in a much longer take off run and higher v speeds but a higher TASS which lowers the engine temp (the engine EEC thinks its warmer out side than it is) your ground run is much longer so i tend not to use this at places where the bird strike risk is high e.g. FAO/AGP/AMS
Apart from reduced engine wear both methods will reduce noise significantly. TASS must not be used on contaminated runway, or when wind shear is likely or with a pronounced temp inversion, typically +10c change in the first 1000 ft
To further save engine wear and noise we also normally use improved climb which results in a much longer take off run and higher v speeds but a higher TASS which lowers the engine temp (the engine EEC thinks its warmer out side than it is) your ground run is much longer so i tend not to use this at places where the bird strike risk is high e.g. FAO/AGP/AMS
Last edited by LNIDA; 9th Jul 2014 at 04:43.
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FWIW, I sincerely doubt that neither engine wear, noise, nor fuel consumption is reduced by using a "double derate" combination of fixed derate plus ATM. Reducing thrust that drastically is SOLELY a beancounter's method of reducing the average monthly derate for the fleet so the company pays less for the engine time.
There is no other sane reason to reduce takeoff thrust to well below Climb thrust, and often below CLB2 thrust. If CLB can be used continuously (Max Cont is usually very close to CLB), and can be used routinely without "excess" engine wear, then there is NO reason to have a takeoff thrust below the desired climb thrust. There MAY be a slight reduction in noise on the runway, which is NOT a critical noise area, but with the increase in takeoff roll and decrease in climb rate, the decrease in safety margin is NOT worth it. Decreased climb rate may also serve to increase noise fartherr from the runway due to the lower altitude and longer time with gear and flaps extended. Increased time on the runway and at low altitude also INCREASES exposure to any bird activity.
There is no other sane reason to reduce takeoff thrust to well below Climb thrust, and often below CLB2 thrust. If CLB can be used continuously (Max Cont is usually very close to CLB), and can be used routinely without "excess" engine wear, then there is NO reason to have a takeoff thrust below the desired climb thrust. There MAY be a slight reduction in noise on the runway, which is NOT a critical noise area, but with the increase in takeoff roll and decrease in climb rate, the decrease in safety margin is NOT worth it. Decreased climb rate may also serve to increase noise fartherr from the runway due to the lower altitude and longer time with gear and flaps extended. Increased time on the runway and at low altitude also INCREASES exposure to any bird activity.
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We'll let that one slide this time
Alas when reading Airbus and Boeing manuals, fuel reductions on departure can be achieved reducing the fuel flow during departure by using a combination of Fixed derate and an assumed temperature method...
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Correct, fuel flow is reduced......meaning lower EGT and less wear. But lower fuel flow means less thrust which in turn means a longer roll and combined with a Derated climb means it takes longer to reach the optimum cruise alt. This extra time in the climb burns extra fuel and hence fuel costs up, engine costs down.
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My understanding is, the lower the thrust for takeoff(within some particular limit I suppose) the lower the engine wear. The higher the climb thrust, the shorter the time to reach cruise altitude which reduces overall fuel burn.
As a comparison, I was told(so it may not be true) by an F-15 pilot that using afterburner to go straight up to cruise altitude was a more efficient profile than a normal cruise climb. To be confirmed by those in the know.
As a comparison, I was told(so it may not be true) by an F-15 pilot that using afterburner to go straight up to cruise altitude was a more efficient profile than a normal cruise climb. To be confirmed by those in the know.
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It really does not change the overall fuel cost that much. Sure, climb fuel is increased using reduced trust, but your cruise fuel is less, as cruise time is shorter.
As far as I know, climb, climb1 and climb2 become the same at 15000 ft in the B737NG.
As far as I know, climb, climb1 and climb2 become the same at 15000 ft in the B737NG.
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As far as I know, climb, climb1 and climb2 become the same at 15000 ft in the B737NG.
Intruder:
but with the increase in takeoff roll and decrease in climb rate, the decrease in safety margin is NOT worth it.
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It really does not change the overall fuel cost that much. Sure, climb fuel is increased using reduced trust, but your cruise fuel is less, as cruise time is shorter
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According to Boeing we save around 40kg of fuel each sector where we can do an unrestricted climb at full climb power instead of using the automatically selected reduced climb thrust.
When did you last have one of those in e.g. London TMA?
Hence you reduce climb performance as well, to reduce rate of climb without the crew intervention by requirement of using V/S continued througout the climb.
Reducing the amount of power up/down required with each clearance.
Powering up further requires more fuel than 40kg per flight saved with an unrestricted climb.
When unrestricted climb is possible... use full climb thrust!
Last argument in favour of reduced thrust use on takeoff, it LIMITS the damage when a failure occurs (such as effects of a bird strike when compared at a lower or higher speed engine core and fan)