Climb Questions
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Climb Questions
I am currently trying to understand how different levels of climb derate should be applied.
The obvious method is to use TOW bands for each derate level, but with minimum rate of climb constraints.
Any comments on this approach?
Any particular departures where climb derates are not possible, or limited?
The obvious method is to use TOW bands for each derate level, but with minimum rate of climb constraints.
Any comments on this approach?
Any particular departures where climb derates are not possible, or limited?
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I am trying to determine whether the general policy used is broadly similar for various aircraft types (irrespective of size and engine type)
In particular, what determines the level of climb derate that can be used?
Is derate used provided TOW is low enough to hit a minumum time/distance to height target with the derate level selected?
I am then aware that the departures from some airports make it advantageous to get as high as possible as quick as possible. For which airports is this as issue?
In particular, what determines the level of climb derate that can be used?
Is derate used provided TOW is low enough to hit a minumum time/distance to height target with the derate level selected?
I am then aware that the departures from some airports make it advantageous to get as high as possible as quick as possible. For which airports is this as issue?
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Hmmmm .....
Well, the first thing you need to know is about Aircraft Performance. This is a complicated subject, and represents one or more of the exams that pilots have to sit for their professional pilot licences. This tells you what an aircraft is supposed to do, performance-wise, when it takes off.
Aircraft performance at take-off relates to: amount of runway used if take-off is continued; amount of runway used if take-off is stopped with an engine failure at a critical speed; amount of ground covered to clear an obstacle that is 50' above the height of the runway (or 35' if wet); ensuring a climb gradient that clears obstacles in the take-off path by a given amount, again assuming an engine has failed; use of anti-icing and bleed systems, consideration of runway gradient, air temperature, pressure, wind and so on.
What you come up with is an RTOW - regulated take off weight. This is the maximum weight of the aircraft that will allow it to comply with all these legal requirements.
However, the aircraft will often be operated at considerably less than this RTOW. In the case of both B737 and A320, "derating" (on the day) operates by determining the highest temperature at which the aircraft could take off at and telling the engines to pretend that this is the actual temperature (which commands a lower power setting). On the A320/321, the temperature is called the "flex" temperature; on the B737, the "assumed temperature."
There is also a difference between max power, which is typically used at most for short bursts (at the start of a go-around, or full-power take off), max continuous power, which is typically used for single engine work, and climb power which is the normal "full power" away from the ground.
In short, it's hideously complicated and technical - I hope my answers aren't too misleading! I expect someone will correct me if they are.
Well, the first thing you need to know is about Aircraft Performance. This is a complicated subject, and represents one or more of the exams that pilots have to sit for their professional pilot licences. This tells you what an aircraft is supposed to do, performance-wise, when it takes off.
Aircraft performance at take-off relates to: amount of runway used if take-off is continued; amount of runway used if take-off is stopped with an engine failure at a critical speed; amount of ground covered to clear an obstacle that is 50' above the height of the runway (or 35' if wet); ensuring a climb gradient that clears obstacles in the take-off path by a given amount, again assuming an engine has failed; use of anti-icing and bleed systems, consideration of runway gradient, air temperature, pressure, wind and so on.
What you come up with is an RTOW - regulated take off weight. This is the maximum weight of the aircraft that will allow it to comply with all these legal requirements.
However, the aircraft will often be operated at considerably less than this RTOW. In the case of both B737 and A320, "derating" (on the day) operates by determining the highest temperature at which the aircraft could take off at and telling the engines to pretend that this is the actual temperature (which commands a lower power setting). On the A320/321, the temperature is called the "flex" temperature; on the B737, the "assumed temperature."
There is also a difference between max power, which is typically used at most for short bursts (at the start of a go-around, or full-power take off), max continuous power, which is typically used for single engine work, and climb power which is the normal "full power" away from the ground.
In short, it's hideously complicated and technical - I hope my answers aren't too misleading! I expect someone will correct me if they are.
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YP, many thanks for your response.
Apologies for any vagueness in my post, but my main interest here is climb derate.
Aircraft Performance is a major part of my work, but climb derate usage policy seems to vary significantly depending on who you talk to. My understanding is that climb derate is more of an operational decision as opposed to take-off derate which clearly has major safety implications.
PS - Before someone else chips in - I believe that the screen heights for dry & wet runways are 35ft and 15ft respectively.
Again thanks of taking the time to respond.
Apologies for any vagueness in my post, but my main interest here is climb derate.
Aircraft Performance is a major part of my work, but climb derate usage policy seems to vary significantly depending on who you talk to. My understanding is that climb derate is more of an operational decision as opposed to take-off derate which clearly has major safety implications.
PS - Before someone else chips in - I believe that the screen heights for dry & wet runways are 35ft and 15ft respectively.
Again thanks of taking the time to respond.
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The general concept is that the CLB rating selected is tied to the takeoff thrust, the lower the takeoff thrust, the lower the CLB thrust. For the majority of airports which we operate to it really doesnt matter that it takes a few more minutes to get to cruise altitude.
We operate with fixed climb derates depending on the aircraft type, for some Boeings we use 10% and 20%. The climb derate is selected by the FMC in conjunction with the Thrust Rating used and the delta difference between ambient temperature and assumed temperature.
On older Boeings we used to use a climb rate of 500fpm as the change over point from CLB2/CLB1/CLB. On newer FMC equipped aircraft, the CLB derates will wash out around 14000 feet.
On another US product we are in the process of selecting climb derates manually, the policy will go something like, "when using DER1 and temp delta is above 10C, use CLB2. With a temperature delta between 0-10C, use CLB1. When using Max Takeoff Power, use CLB".
This is a general concept which works for the majority of our airports. For some such as GVA heading southeast on the SID, we can be better off using max climb due to distant height requirements on the SID, therefore avoiding a quick hold after takeoff.
You are correct in saying that the screen heights are 35/15 feet.
Mutt.
We operate with fixed climb derates depending on the aircraft type, for some Boeings we use 10% and 20%. The climb derate is selected by the FMC in conjunction with the Thrust Rating used and the delta difference between ambient temperature and assumed temperature.
On older Boeings we used to use a climb rate of 500fpm as the change over point from CLB2/CLB1/CLB. On newer FMC equipped aircraft, the CLB derates will wash out around 14000 feet.
On another US product we are in the process of selecting climb derates manually, the policy will go something like, "when using DER1 and temp delta is above 10C, use CLB2. With a temperature delta between 0-10C, use CLB1. When using Max Takeoff Power, use CLB".
This is a general concept which works for the majority of our airports. For some such as GVA heading southeast on the SID, we can be better off using max climb due to distant height requirements on the SID, therefore avoiding a quick hold after takeoff.
You are correct in saying that the screen heights are 35/15 feet.
Mutt.
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Noise Unit,
I am still not exactly sure which part of this subject you are trying to understand here (or why), but to add to Mutt & YP’s answers:
An airline will generally operate the aircraft’s engines at the lowest derate that is practical because it puts the engines on a cheaper maintenance schedule. For instance an airline may operate its 737 fleet with engines derated to 22K in the summer when loads are higher and 20K in winter when they are lower. This may also be accompanied by a change in the declared MTOW to reduce landing & nav charges (in the winter). The different engine ratings require a whole new set of performance tables and is a big job.
Take-off Derate
The Captain may on the day decide that he either needs more power, eg to get past an obstacle (say 22K when the a/c is notified as 20K) and depending on the airline he may either have to ask his Ops for a one-off authorisation or it may just be a tech log entry. Either way he must use 22K perf data. In some rare cases he may need less power eg if he is Vmcg limited, this is usually much easier insofar as nobody needs to be asked but again you must use the perf data applicable to the engine rating.
Climb Derate
This is done almost every sector either automatically as a consequence of using an assumed temperature thrust reduction (aka Flex) or manually by selecting CLB-1 or CLB-2 in the FMC after you have cleaned up and don’t require full climb power. It is more of an airmanship thing to reduce your climb thrust to reduce engine wear (good for the company’s bills and reduces your chance of having some asymmetric practice!). NB this is not a true “derate”, just a reduction of climb thrust.
If you haven’t already seen it, I have produced a more in-depth article on this at
http://www.b737.org.uk/assumedtemp.htm
This has photos of all the FMC pages used to make these selections, it is based on the 737 but is valid for all jet types.
S & L
I am still not exactly sure which part of this subject you are trying to understand here (or why), but to add to Mutt & YP’s answers:
An airline will generally operate the aircraft’s engines at the lowest derate that is practical because it puts the engines on a cheaper maintenance schedule. For instance an airline may operate its 737 fleet with engines derated to 22K in the summer when loads are higher and 20K in winter when they are lower. This may also be accompanied by a change in the declared MTOW to reduce landing & nav charges (in the winter). The different engine ratings require a whole new set of performance tables and is a big job.
Take-off Derate
The Captain may on the day decide that he either needs more power, eg to get past an obstacle (say 22K when the a/c is notified as 20K) and depending on the airline he may either have to ask his Ops for a one-off authorisation or it may just be a tech log entry. Either way he must use 22K perf data. In some rare cases he may need less power eg if he is Vmcg limited, this is usually much easier insofar as nobody needs to be asked but again you must use the perf data applicable to the engine rating.
Climb Derate
This is done almost every sector either automatically as a consequence of using an assumed temperature thrust reduction (aka Flex) or manually by selecting CLB-1 or CLB-2 in the FMC after you have cleaned up and don’t require full climb power. It is more of an airmanship thing to reduce your climb thrust to reduce engine wear (good for the company’s bills and reduces your chance of having some asymmetric practice!). NB this is not a true “derate”, just a reduction of climb thrust.
If you haven’t already seen it, I have produced a more in-depth article on this at
http://www.b737.org.uk/assumedtemp.htm
This has photos of all the FMC pages used to make these selections, it is based on the 737 but is valid for all jet types.
S & L
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Yeh a derated climb thrust is for less wear and tear on the engines but it appears to be an 'FMC-only generated' option (for 737 anyway), as theres no published charts or tables from which to determine a suitable derate (clb1 or clb2). Only 'Max Climb N1' tables are provided.
Interestingley enough a derated climb produces a higher total fuel burn from a to b, esp if strong tailwinds exist above 15,000 feet. So if my fuel at destination is going to be a problem (and I have reasonabley strong tailwinds known to be above FL150), Ill use full climb power and max enroute ROC speed until Im in the 'strong tailwind' band. More so if strong headwinds exist below 10,000 ft.
Depending on sector length Ive saved up to 100-150 kg but of course thats at the expense of flight time.
Interestingley enough a derated climb produces a higher total fuel burn from a to b, esp if strong tailwinds exist above 15,000 feet. So if my fuel at destination is going to be a problem (and I have reasonabley strong tailwinds known to be above FL150), Ill use full climb power and max enroute ROC speed until Im in the 'strong tailwind' band. More so if strong headwinds exist below 10,000 ft.
Depending on sector length Ive saved up to 100-150 kg but of course thats at the expense of flight time.
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Mutt
To clarify my interest.
What conditions determine whether climb derate is selected? I believe that aircraft weight and (to a lesser degree) temperature determine the level of climb derate theoretically available.
Is the following correct?
TO derate is a function of aircraft weight, temperature, humidity and the runway conditions (runway length, stopways, clearways, elevation, wind, obstacles, slope, wet/contaminated etc).
Climb derate is a function of aircraft weight, temperature (which varies less at altitude) and occasionally terrain avoidance (like GVA).
I am aware of the Boeing concept of defaulting the climb derate to the TO derate (Is this the same for Airbus aircraft?). I also realise that it is possble to manually select the level of climb derate.
Thus, what criteria are considered when manually selecting climb derate in each of the three situations?
i) RTOW limited take-off (full TO thrust required) with a relatively low TOW (short runway)?
ii) Aircraft at MTOW but operating from a very long runway (significant TO derate possible)?
iii) If take-off derate is selected using flexTO or Assumed Temperature Method (rather than DERATE 1 or 2)?
Capt SandL
Thanks for the link - very interesting.
To clarify my interest.
What conditions determine whether climb derate is selected? I believe that aircraft weight and (to a lesser degree) temperature determine the level of climb derate theoretically available.
Is the following correct?
TO derate is a function of aircraft weight, temperature, humidity and the runway conditions (runway length, stopways, clearways, elevation, wind, obstacles, slope, wet/contaminated etc).
Climb derate is a function of aircraft weight, temperature (which varies less at altitude) and occasionally terrain avoidance (like GVA).
I am aware of the Boeing concept of defaulting the climb derate to the TO derate (Is this the same for Airbus aircraft?). I also realise that it is possble to manually select the level of climb derate.
Thus, what criteria are considered when manually selecting climb derate in each of the three situations?
i) RTOW limited take-off (full TO thrust required) with a relatively low TOW (short runway)?
ii) Aircraft at MTOW but operating from a very long runway (significant TO derate possible)?
iii) If take-off derate is selected using flexTO or Assumed Temperature Method (rather than DERATE 1 or 2)?
Capt SandL
Thanks for the link - very interesting.
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some numbers
Gentlemen.
Some number crunching (admittedly for an a330)
Taking a 6.6 flight hour average mission, average aircraft weight,
average takeoff derate, cruise derates predicted by AIRBUS performance programmes, and based a 70USD labor cost:
selecting derated climb level 1 brings around 4% DMC saving vs. Max Climb,
selecting derated climb level 2 brings around 5.5% DMC saving vs. Max Climb.
Reference DMC (with Max Climb) for a 6.6 flight hour mission is close to 140 USD per flight hour and per engine.
however when it comes to deciding specific derate policy out of specific airports with known average takeoff weight and temperatures, coupled with known altitude constraints. Then things get interesting.
rgds
O/z
Some number crunching (admittedly for an a330)
Taking a 6.6 flight hour average mission, average aircraft weight,
average takeoff derate, cruise derates predicted by AIRBUS performance programmes, and based a 70USD labor cost:
selecting derated climb level 1 brings around 4% DMC saving vs. Max Climb,
selecting derated climb level 2 brings around 5.5% DMC saving vs. Max Climb.
Reference DMC (with Max Climb) for a 6.6 flight hour mission is close to 140 USD per flight hour and per engine.
however when it comes to deciding specific derate policy out of specific airports with known average takeoff weight and temperatures, coupled with known altitude constraints. Then things get interesting.
rgds
O/z
