Reduced thrust take-offs
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Reduced thrust take-offs
Hello everyone,
The questions I have concern reduced take-off thrust. Our example aircraft will be a 767-300 with a ZFW of 116.0 (kilos). The runway is dry and is 10,000 feet long. I understand that to help preserve the life of the engine as well as noise abatement a reduced TO thrust is often used.
1. I have read that a reduced TO must not be used if the calculated V1 is less than the Vmcg. Please explain to me what the Vmcg is.
2. How do you determine if you are using TO-1 or TO-2 reduction?
3. Do you use CLB-1/CLB-2 reduction always with the TO reduction (or instead of TO reduction with shorter runways and greater weight)?
4. Finally, if using a TO-1 reduction would that always be paired with a CLB-1 as a TO-2 with a CLB-2?
Currently, I am a lowly PPL student and don't need to worry about these prodecures just yet but I try to read as much as I can so that I can have a slight head start when the right time comes.
As always, I appreciate your answers and help.
Cheers!
The questions I have concern reduced take-off thrust. Our example aircraft will be a 767-300 with a ZFW of 116.0 (kilos). The runway is dry and is 10,000 feet long. I understand that to help preserve the life of the engine as well as noise abatement a reduced TO thrust is often used.
1. I have read that a reduced TO must not be used if the calculated V1 is less than the Vmcg. Please explain to me what the Vmcg is.
2. How do you determine if you are using TO-1 or TO-2 reduction?
3. Do you use CLB-1/CLB-2 reduction always with the TO reduction (or instead of TO reduction with shorter runways and greater weight)?
4. Finally, if using a TO-1 reduction would that always be paired with a CLB-1 as a TO-2 with a CLB-2?
Currently, I am a lowly PPL student and don't need to worry about these prodecures just yet but I try to read as much as I can so that I can have a slight head start when the right time comes.
As always, I appreciate your answers and help.
Cheers!
Bottums Up
xfeed
I've no experience with the 767 but Q1 pertains to any multi except inline thrust, such as a Cessna 337, push-me pull-me.
Vmcg is Velocity minimum control ground, and is the minumum airspeed where one can control the direction of the aeroplane aerodynamically (rudder) without leaving the confines of the runway.
I've no experience with the 767 but Q1 pertains to any multi except inline thrust, such as a Cessna 337, push-me pull-me.
Vmcg is Velocity minimum control ground, and is the minumum airspeed where one can control the direction of the aeroplane aerodynamically (rudder) without leaving the confines of the runway.
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I don't know the 767, and procedures vary among airlines. However, in general, using our 744 as an example:
1. Not true. Just increase V1 to Vmcg. Besides, reduced thrust will reduce Vmcg as well, since asymmetric thrust is less.
2. Use the Runway Performance Manual and Airplane Performance Manual (or Operational Performance System computer) to determine what is the least thrust that can be used. If TO2 can be used, use it.
3. While CLB1/2 is selected automatically with the respective TO1/2, it can be overridden. Often CLB will be used when there is a mandatory altitude on the Deaprture Procedure (e.g., HKG).
4. Usually, but not always.
1. Not true. Just increase V1 to Vmcg. Besides, reduced thrust will reduce Vmcg as well, since asymmetric thrust is less.
2. Use the Runway Performance Manual and Airplane Performance Manual (or Operational Performance System computer) to determine what is the least thrust that can be used. If TO2 can be used, use it.
3. While CLB1/2 is selected automatically with the respective TO1/2, it can be overridden. Often CLB will be used when there is a mandatory altitude on the Deaprture Procedure (e.g., HKG).
4. Usually, but not always.
You need to specify if it is a normal reduction (based on higher assumed temperature to indicate required thrust), or if it is a derated thrust.
Not familiar with the 767, for the Airbus A340:
V1 cannot be lower than Vmcg, since in case of engine failure, you need to be sure you can keep the aircraft on the runway when you continue to accelerate above V1 (with TO thrust on the "alive" engine). Vmcg is a minimum speed to make sure your rudder will give you enough directional control to cope with the engine failure, while rolling on the ground.
The difference is that with normal reduction (assumed temperature/flex temperature), Vmcg is calculated based on the max TO rating for that engine. So in case of engine failure at V1, you could bring the thrust up to the max rating, and you still would have sufficient directional control with the rudder during the ground roll while continuing to accelerate to rotation speed.
When you takeoff with derated thrust, Vmcg will be lower since the max TO rating is lower. This allows a lower V1 as well, which can be interesting in some cases. However, as a pilot, if the engine fails at V1, you cannot bring the thrust up to the max TO rating, since this would increase Vmcg as well. If at that point your speed is still below that Vmcg, you lose control since the rudder at this lower speed is less effective and you can only cope with the derated thrust amount, not the max thrust.
Not familiar with the 767, for the Airbus A340:
V1 cannot be lower than Vmcg, since in case of engine failure, you need to be sure you can keep the aircraft on the runway when you continue to accelerate above V1 (with TO thrust on the "alive" engine). Vmcg is a minimum speed to make sure your rudder will give you enough directional control to cope with the engine failure, while rolling on the ground.
The difference is that with normal reduction (assumed temperature/flex temperature), Vmcg is calculated based on the max TO rating for that engine. So in case of engine failure at V1, you could bring the thrust up to the max rating, and you still would have sufficient directional control with the rudder during the ground roll while continuing to accelerate to rotation speed.
When you takeoff with derated thrust, Vmcg will be lower since the max TO rating is lower. This allows a lower V1 as well, which can be interesting in some cases. However, as a pilot, if the engine fails at V1, you cannot bring the thrust up to the max TO rating, since this would increase Vmcg as well. If at that point your speed is still below that Vmcg, you lose control since the rudder at this lower speed is less effective and you can only cope with the derated thrust amount, not the max thrust.
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xfeed,
I think that you need to clarify whether you are referring to Reduced Thrust (Assumed Temperature, Flex etc.) or Derated thrust. There is a world of a difference, and this is a "need to know" before the question may be correctly addressed.
Regards,
Old Smokey
I think that you need to clarify whether you are referring to Reduced Thrust (Assumed Temperature, Flex etc.) or Derated thrust. There is a world of a difference, and this is a "need to know" before the question may be correctly addressed.
Regards,
Old Smokey
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From the 767-300 QRH the highest VMCG value I can find is 116 knots. This is at a pressure altitude 1000' below sea level and at -54 celcius. The lowest is 91 knots, 8000 pressure altitude and +40.
In "normal" ops, V1 is way above these values and VMCG only comes into the equation on lightweight ferry sectors.
In "normal" ops, V1 is way above these values and VMCG only comes into the equation on lightweight ferry sectors.
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Thanks for the info, everyone.
Old Smokey,
I am assuming that the information I was reading was Reduced Thrust as opposed to Derated Thrust. However, I must plead ignorance since I don't know what the differences are nor how they are determined for take-off. If this could be explained a bit I would greatly appreciate it.
Cheers,
xfeed
Old Smokey,
I am assuming that the information I was reading was Reduced Thrust as opposed to Derated Thrust. However, I must plead ignorance since I don't know what the differences are nor how they are determined for take-off. If this could be explained a bit I would greatly appreciate it.
Cheers,
xfeed
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xfeed,
Derated thrust applies to those engines / aircraft where several different thrust ratings are available. A great many advantages exist for this, both economic and operational. Looking first at the operational applications.
All engines, of course may be operated to their 100% rating. Important performance considerations such as Vmcg / Vmca are determined according to the Rated Thrust. This will have a direct impact upon V1. If derates are possible, the aircraft is virtually re-certified as though it had lower thrust output. The aircraft that I operate (B777) has full Takeoff thrust (TO), an 8% reduction (TO-1), and a 20% reduction (TO-2). To each of these thrust ratings there is a seperate Vmcg / Vmca, The aircraft may then be re-certified as though it were 3 aircraft in one, with 3 different thrust ratings, Vmca's and Vmcg's. As V1 must be equal to or greater than Vmcg (Vef actually), then using the lower rating will allow better accelerate-stop performance (lower V1), and, in the case of wet or contaminated runways where Vmcg is an increasing problem, allow operations (at the lower Vmcg) that might not be possible at higher thrust settings (increased yaw following engine failure). It is in this area where SOMETIMES it is possible to achieve a higher Takeoff Weight when using a lower thrust. If an engine were to fail when operating, for example, at full TO-2 and at or near V1, the thrust must not be advanced beyond TO-2. (Because V1, Vmcg, and Vmca are based upon TO-2).
Reduced thrust is when the engine is operated as though the environmental temperature was higher than actual, that is, thrust is reduced below the rating (TO, TO-1 or TO-2) in the interests of saving engine stress. This is only possible when the Actual Takeoff Weight is less than the limiting weight at the rated thrust. There is no allowance for reduced Vmcg / Vmca due to the Reduced thrust, and if necessary the applicable RATING may be used following engine failure if desired, e.g. for Takeoff using TO-1 with an assumed temperature of 44° when the actual temperature is 20°, thrust may be advanced to TO-1 but not TO. If thrust was advanced to TO in this case, loss of directional control may result.
Economic advantages exist for both. Operating at reduced thrust increases engine life, thus reducing maintainance costs, and, as a safety bonus, decreases the likelyhood of engine failure. Operating at De-Rated thrust has the further economic advantage of increasing the guarantee from the manufacturer, the guarantee depending upon the recorded use of TO, TO-1, or TO-2. There is no change to the manufacturer's guarantee for the use of Reduced thrust.
That's manufacturer's guarantees, my guarantee is that if you're conducting a Takeoff at a De-Rated thrust, lose an engine at V1, and advance the thrust to full TO, you will in all likelyhood lose the aeroplane.
Lots of info on other forums on these topics, that's my attempt to put it into a nutshell.
Regards,
Old Smokey
Derated thrust applies to those engines / aircraft where several different thrust ratings are available. A great many advantages exist for this, both economic and operational. Looking first at the operational applications.
All engines, of course may be operated to their 100% rating. Important performance considerations such as Vmcg / Vmca are determined according to the Rated Thrust. This will have a direct impact upon V1. If derates are possible, the aircraft is virtually re-certified as though it had lower thrust output. The aircraft that I operate (B777) has full Takeoff thrust (TO), an 8% reduction (TO-1), and a 20% reduction (TO-2). To each of these thrust ratings there is a seperate Vmcg / Vmca, The aircraft may then be re-certified as though it were 3 aircraft in one, with 3 different thrust ratings, Vmca's and Vmcg's. As V1 must be equal to or greater than Vmcg (Vef actually), then using the lower rating will allow better accelerate-stop performance (lower V1), and, in the case of wet or contaminated runways where Vmcg is an increasing problem, allow operations (at the lower Vmcg) that might not be possible at higher thrust settings (increased yaw following engine failure). It is in this area where SOMETIMES it is possible to achieve a higher Takeoff Weight when using a lower thrust. If an engine were to fail when operating, for example, at full TO-2 and at or near V1, the thrust must not be advanced beyond TO-2. (Because V1, Vmcg, and Vmca are based upon TO-2).
Reduced thrust is when the engine is operated as though the environmental temperature was higher than actual, that is, thrust is reduced below the rating (TO, TO-1 or TO-2) in the interests of saving engine stress. This is only possible when the Actual Takeoff Weight is less than the limiting weight at the rated thrust. There is no allowance for reduced Vmcg / Vmca due to the Reduced thrust, and if necessary the applicable RATING may be used following engine failure if desired, e.g. for Takeoff using TO-1 with an assumed temperature of 44° when the actual temperature is 20°, thrust may be advanced to TO-1 but not TO. If thrust was advanced to TO in this case, loss of directional control may result.
Economic advantages exist for both. Operating at reduced thrust increases engine life, thus reducing maintainance costs, and, as a safety bonus, decreases the likelyhood of engine failure. Operating at De-Rated thrust has the further economic advantage of increasing the guarantee from the manufacturer, the guarantee depending upon the recorded use of TO, TO-1, or TO-2. There is no change to the manufacturer's guarantee for the use of Reduced thrust.
That's manufacturer's guarantees, my guarantee is that if you're conducting a Takeoff at a De-Rated thrust, lose an engine at V1, and advance the thrust to full TO, you will in all likelyhood lose the aeroplane.
Lots of info on other forums on these topics, that's my attempt to put it into a nutshell.
Regards,
Old Smokey
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When looking at your TO performance manuals (e.g 767). Would you first choose your derated thrust takeoff section of the manual e.g TO-1 or TO-2. Then when you are asked to find out, which category TO you are doing (A, B, C etc) by finding the pressure alt. and temperature; if you are going to use reduced TO thrust would you just increase the temperature to your assumed temperature?
For example if you were at a pressure altitde of 0 and the actual OAT was 20 degrees, then you would be doing a category A TO according to the 767 manuals. However if you were choosing to use 45 degrees assumed temperature for thrust, would you just pretend as if the OAT was 45 degrees? This would push you up to a catagory B TO.
Thanks
Kristian
For example if you were at a pressure altitde of 0 and the actual OAT was 20 degrees, then you would be doing a category A TO according to the 767 manuals. However if you were choosing to use 45 degrees assumed temperature for thrust, would you just pretend as if the OAT was 45 degrees? This would push you up to a catagory B TO.
Thanks
Kristian
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Mutt, We limit the use of TO-2 to contaminated runway use only (specifically icy runways). Thus it is rarely used, as the icy runways of the world are a long way from home and we'd never lift the weights required for the long haul. Company policy is to use TO-1 with Assumed temp reduction up to TO-1, and then TO with Assumed temp reduction up to TO as weight increases. Company policy is to limit the use of TO-2 to contaminated runways only.
Out of interest, with the greater likliehood of Sandy / Dust covered runways in the Middle East, do you use "Wet" runway performance for these runways? (Our policy is Yes). If so, are you satisfied that, in accounting for the reduced friction coefficient that this is Adequate, Insufficient, or Overkill?
Kristian17, If the Actual Takeoff Weight is less than the maximum for using full TO-1, then you may use TO-1 with a reduction, or TO with a reduction, with TO-1 the preferred option. The Assumed (Flex) temperatures would be different, but the Net thrust produced the same. For a made-up example, TO provides 100 Units of thrust, TO-1 provides 90 Units of thrust, but your ATOW only requires 85 Units of thrust on a 20° day. This might be achieved at TO with 50° Assumed temperature, or TO-1 with 30° Assumed temperature. In both cases, actual thrust is the SAME, therefore either choice will give the same all-engines Takeoff performance. The advantage of TO-1 lies in improved guarantee from the manufacturer. Accelerate-Stop performance will be identical. Continued Takeoff performance following Engine Failure will be identical if thrust is not increased (it need not be), but less at TO-1 than at TO if thrust is advanced to the rating.
xfeed, With regard to your subsidiary questions regarding CLB, CLB-1, and CLB-2, usually these are coupled with the Takeoff selection automatically, i.e. TO/CLB, TO-1/CLB-1, and TO-2/CLB-2. Note that I said USUALLY, the FMC goes one step smarter and will sometimes 'assign' a lower CLB rating depending upon the weight and environmental circumstances. The pilot can over-ride this, as is often done to meet a 'tough' altitude constraint during departure, e.g. Takeoff at TO-1 at Assumed temperature 50°, but selection of full CLB to meet altitude constraints. For the aircraft that I fly, CLB, CLB-1, and CLB-2 all become full CLB after passing 10000 feet (even though CLB-1 or CLB-2 is still annunciated). Another currently running thread regarding B777 IFSD rate indicates Takeoff followed by early climb as the most engine failure prone occasions, fully justifying the Climb thrust reduction in addition to the Takeoff thrust reduction.
With all of the discussion regarding thrust De-Rate and Thrust Reduction on this and other threads, I'm surprised that no-one has raised questions relating to APR (Auxiliary Performance Reserve), the opposite of De-rate, where thrust is increased beyond the nominal full Takeoff setting following engine failure.
Regards,
Old Smokey
Out of interest, with the greater likliehood of Sandy / Dust covered runways in the Middle East, do you use "Wet" runway performance for these runways? (Our policy is Yes). If so, are you satisfied that, in accounting for the reduced friction coefficient that this is Adequate, Insufficient, or Overkill?
Kristian17, If the Actual Takeoff Weight is less than the maximum for using full TO-1, then you may use TO-1 with a reduction, or TO with a reduction, with TO-1 the preferred option. The Assumed (Flex) temperatures would be different, but the Net thrust produced the same. For a made-up example, TO provides 100 Units of thrust, TO-1 provides 90 Units of thrust, but your ATOW only requires 85 Units of thrust on a 20° day. This might be achieved at TO with 50° Assumed temperature, or TO-1 with 30° Assumed temperature. In both cases, actual thrust is the SAME, therefore either choice will give the same all-engines Takeoff performance. The advantage of TO-1 lies in improved guarantee from the manufacturer. Accelerate-Stop performance will be identical. Continued Takeoff performance following Engine Failure will be identical if thrust is not increased (it need not be), but less at TO-1 than at TO if thrust is advanced to the rating.
xfeed, With regard to your subsidiary questions regarding CLB, CLB-1, and CLB-2, usually these are coupled with the Takeoff selection automatically, i.e. TO/CLB, TO-1/CLB-1, and TO-2/CLB-2. Note that I said USUALLY, the FMC goes one step smarter and will sometimes 'assign' a lower CLB rating depending upon the weight and environmental circumstances. The pilot can over-ride this, as is often done to meet a 'tough' altitude constraint during departure, e.g. Takeoff at TO-1 at Assumed temperature 50°, but selection of full CLB to meet altitude constraints. For the aircraft that I fly, CLB, CLB-1, and CLB-2 all become full CLB after passing 10000 feet (even though CLB-1 or CLB-2 is still annunciated). Another currently running thread regarding B777 IFSD rate indicates Takeoff followed by early climb as the most engine failure prone occasions, fully justifying the Climb thrust reduction in addition to the Takeoff thrust reduction.
With all of the discussion regarding thrust De-Rate and Thrust Reduction on this and other threads, I'm surprised that no-one has raised questions relating to APR (Auxiliary Performance Reserve), the opposite of De-rate, where thrust is increased beyond the nominal full Takeoff setting following engine failure.
Regards,
Old Smokey
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I appreciate all the comments and answers all of you have provided.
If I may, I am still just a bit confused about one thing: when you are performing your cockpit preparations, what makes you decide (runway contamination aside) whether you will have a derated TO-1 or a reduced TO? Is this simply an SOP (to help preserve the life of your engines, noise abatement, etc.) or are there other factors that you consider, such as your weight, runway length, no MEL or ATC restrictions on the departure?
Here is an example I can use:
You are departing EGLL from RWY27R (about 12,850' long) & CPT3F.CPT departure. (This departure has a restriction of FL060 at the CPT08 about 23nm from EGLL, if memory serves).
Weather: 260/10 9999 FEW045 18/04 1021 NOSIG
Aircraft (depending upon what you fly) : B763 GWTO 158.0kg; ZFW 112.0; B744 GWTO 340.0kg ZFW 212.0; B772 GWTO 253.0kg ZFW 176.0
I know that Heathrow has got some pretty serious noise abatement issues so perhaps that is a consideration for a reduced or derated TO? What other things may be taken into consideration with the above example?
I hope my examples make sense.
Cheers,
xfeed
If I may, I am still just a bit confused about one thing: when you are performing your cockpit preparations, what makes you decide (runway contamination aside) whether you will have a derated TO-1 or a reduced TO? Is this simply an SOP (to help preserve the life of your engines, noise abatement, etc.) or are there other factors that you consider, such as your weight, runway length, no MEL or ATC restrictions on the departure?
Here is an example I can use:
You are departing EGLL from RWY27R (about 12,850' long) & CPT3F.CPT departure. (This departure has a restriction of FL060 at the CPT08 about 23nm from EGLL, if memory serves).
Weather: 260/10 9999 FEW045 18/04 1021 NOSIG
Aircraft (depending upon what you fly) : B763 GWTO 158.0kg; ZFW 112.0; B744 GWTO 340.0kg ZFW 212.0; B772 GWTO 253.0kg ZFW 176.0
I know that Heathrow has got some pretty serious noise abatement issues so perhaps that is a consideration for a reduced or derated TO? What other things may be taken into consideration with the above example?
I hope my examples make sense.
Cheers,
xfeed
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xfeed,
A pilot's reasoning would go pretty much like this -
(1) For the known Takeoff Weight and environmental conditions, ascertain whether or not the Takeoff can be accomplished using TO-1.
(2) If it can be accomplished using TO-1, ascertain the thrust reduction possible using the Assumed Temperature / Flex Thrust procedures, all the way to full TO-1 if necessary.
(3) If it can not be accomplished using TO-1, ascertain the thrust reduction possible using the Assumed Temperature / Flex Thrust procedures for TO, all the way to full TO if necessary.
Noise abatement is NOT an issue in the choice between TO and TO-1. As mentioned earlier, if weight was below the TO-1 limit, we could use either with an appropriate thrust reduction, leading the exactly the same actual thrust used and noise generated.
Other factors to be considered? - For example, if wind-shear is a possibility, use full TO with no reduction, and to hell with the economics (and the noise).
Regards,
Old Smokey
what makes you decide (runway contamination aside) whether you will have a derated TO-1 or a reduced TO?
(1) For the known Takeoff Weight and environmental conditions, ascertain whether or not the Takeoff can be accomplished using TO-1.
(2) If it can be accomplished using TO-1, ascertain the thrust reduction possible using the Assumed Temperature / Flex Thrust procedures, all the way to full TO-1 if necessary.
(3) If it can not be accomplished using TO-1, ascertain the thrust reduction possible using the Assumed Temperature / Flex Thrust procedures for TO, all the way to full TO if necessary.
Noise abatement is NOT an issue in the choice between TO and TO-1. As mentioned earlier, if weight was below the TO-1 limit, we could use either with an appropriate thrust reduction, leading the exactly the same actual thrust used and noise generated.
Other factors to be considered? - For example, if wind-shear is a possibility, use full TO with no reduction, and to hell with the economics (and the noise).
Regards,
Old Smokey
Moderator
.. the other consideration to which OS alludes ... is that flexing is limited in its permitted scope ... so, if the circumstances permit (low aircraft load, long runway, benign terrain .. etc.) one can get the maximum effect ONLY by combining flex with derate .. which then lets you go to a lower thrust for the takeoff than could be achieved by using maximum rated thrust with flex ..
Think of flex and derate as doing the same sort of things but with some differences in the rules which apply to their use.
Think of flex and derate as doing the same sort of things but with some differences in the rules which apply to their use.
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Old Smokey,
Thanks for the response. Sorry if I sound repetative in my questions, I'm just trying to understand all the information. I am surprised to learn that thrust reduction has nothing to do with noise abatement.
Just out of curiosity, what do you do on takeoff that meets the noise abatement that an aerodrome may have? I see this in EGLL NOTAMS all the time:
HEATHROW NOISE RESTRICTIONS NOTICE 2005. AIP SUPP 3/05 REFERS
Cheers,
xfeed
Thanks for the response. Sorry if I sound repetative in my questions, I'm just trying to understand all the information. I am surprised to learn that thrust reduction has nothing to do with noise abatement.
Just out of curiosity, what do you do on takeoff that meets the noise abatement that an aerodrome may have? I see this in EGLL NOTAMS all the time:
HEATHROW NOISE RESTRICTIONS NOTICE 2005. AIP SUPP 3/05 REFERS
Cheers,
xfeed
Moderator
Noise is not a simple thing to look at .. involves
(a) sound intensity
(b) distance
(c) environmental influences.
In the basics for takeoff, considering only varying thrust, you can have either
(a) a louder noise further away from the microphone
(b) a softer noise closer to the microphone.
Which is better will depend on circumstances ... so one sometimes sees quite artificial techniques to get around noise restriction problems .. eg max thrust initially followed by a significant thrust cutback during the initial climb/acceleration ..
A case of the operator doing the analysis and the sums dictate the best way routinely to meet the imposed requirements ... windshear and such like things notwithstanding.
(a) sound intensity
(b) distance
(c) environmental influences.
In the basics for takeoff, considering only varying thrust, you can have either
(a) a louder noise further away from the microphone
(b) a softer noise closer to the microphone.
Which is better will depend on circumstances ... so one sometimes sees quite artificial techniques to get around noise restriction problems .. eg max thrust initially followed by a significant thrust cutback during the initial climb/acceleration ..
A case of the operator doing the analysis and the sums dictate the best way routinely to meet the imposed requirements ... windshear and such like things notwithstanding.
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...one can get the maximum effect ONLY by combining flex with derate
The rest of the time -50A rating was used, and FLEX was applied to the 50A books to achieve maximum possible reduction.
I'm sure the same rules applied to other aircraft/engine types as well.
It may be in some jurisdictions that a nameplate change is necessary to implement the above, though. Job security for someone, I guess...