[1.0.5] derated takeoff - thrust mode display

[Tom Stian]

Hello

I dident find this reported before. And wasen't this working pre 1.0.5 ?

Anyway.
When doing a derated takeoff, the thrust mode display show "TO" and not "R-TO".  If im correct they should show "R-TO" right? (being carefull if it is a pilot error again  )

 

 

 

Edited May 31, 2016 by Tom Stian

[Litjan]

Pilot error , but very understandable.

A "derate" is not a "reduced takeoff thrust".

Thanks for the report, though - keep ´em coming!

Cheers, Jan

 

[kyrre1978]

Hello,

But shouldn't the thrust display readout show something like (D-TO) if a derate is selected (like shown here) ?

I have noted that when using an assumed temperature during takeoff, the display on the engine instruments stay at R-CLB all the way up to crz level. I thought that the thrust mode display would switch to CLB when passing 15000 feet or so.

Br.

Kyrre Andersen

[Litjan]

3 hours ago, kyrre1978 said:

Hello,

But shouldn't the thrust display readout show something like (D-TO) if a derate is selected (like shown here) ?

I have noted that when using an assumed temperature during takeoff, the display on the engine instruments stay at R-CLB all the way up to crz level. I thought that the thrust mode display would switch to CLB when passing 15000 feet or so.

Br.

Kyrre Andersen

No, there is no D-TO to my knowledge. Checking the thrust rating on the CDU is essential to avoid having the wrong "max thrust" selected. On my airling, a derated thrust was never used, not sure if other airlines even do that...

The effect of R-CLB washes out to 15.000 feet, but I am not sure if the label will change from R-CLB to CLB... something I never consciously watched in 10 years of flying the plane. Simply don´t know and the manual doesn´t state it clearly, either.

Jan

 

[Tom Stian]

On 1.6.2016 at 9:18 AM, Litjan said:

Pilot error , but very understandable.

A "derate" is not a "reduced takeoff thrust".

Thanks for the report, though - keep ´em coming!

Cheers, Jan

 

Damn.. is hard to be a desktop pilot.

 

4 hours ago, kyrre1978 said:

Hello,

But shouldn't the thrust display readout show something like (D-TO) if a derate is selected (like shown here) ?

Kyrre Andersen

 

Here are the different modes

 

[kyrre1978]

11 hours ago, Litjan said:

No, there is no D-TO to my knowledge. Checking the thrust rating on the CDU is essential to avoid having the wrong "max thrust" selected. On my airling, a derated thrust was never used, not sure if other airlines even do that...

The effect of R-CLB washes out to 15.000 feet, but I am not sure if the label will change from R-CLB to CLB... something I never consciously watched in 10 years of flying the plane. Simply don´t know and the manual doesn´t state it clearly, either.

Jan

 

That's great Jan

Thanks!

Br.

Kyrre

[frankbyte]

Well...can someone explain me the difference beetween a derate and reduced take-off then? I don't get it, sorry.

[mmerelles]

1 hour ago, frankbyte said:

Well...can someone explain me the difference beetween a derate and reduced take-off then? I don't get it, sorry.

Going to the basics

The aircraft engines are rated to a certain amount of thrust, i think this bird was modeled based on 20k lbs of engine thrust each. This amount of thrust was designed to sustain under this aircraft MTOW, safety procedures an standards.

Using the engines to their maximum potential increases exponentially their tear and wear, reduces the life time, thus increasing their maintenance costs, fuel burnt, etc.

Most of the time, the aircraft is not loaded up to its maximum limits (full fuel+pax+cargo), the runway may be long enough, the temperature could be cold enough (dense air), the departure procedure may have not critical obstacles to clear, etc. etc. So, why to use the the engines up to their limits shortening their lives, risking failures, increasing maintenance costs, etc. etc?? This is were reducing takeoff engine thrust comes into play. How to reduce?

 

1. Assumed temperature (also known as flex, reduced thrust). You fool the engines by entering a higher OAT (outside air temperature) than what the real temperature is. So the engines think the air is a lot less dense than actually is producing less thrust. This provides high level of granularity and depending how high the OAT value you enter will be the thrust reduction you get.

2. Derated profile. The fmc may also provide pre-established semi-permanente fixed derating profiles. CLB-1. CLB-2. Each profile derates the engines thrust by a % of the total engine rating. 5%, etc.

 

how much to reduce? the charts will tell based on your present conditions. The whole idea is to reduce as much as possible without compromising safety, single engine climb out, obstacles, margins, climb rates, runway length, weather, etc.

 

hope this helps.

[Tom Stian]

After Jan told that derated and reduced thrust is not the same, I had to find something out about this. In addition to @mmerelles nice explanation I found a short version from a other forum. 

Assumed temp (reduced thrust) – take-off with less than 100% thrust but you can push the thrust up to 100% if needed.

Derated – take-off with less than 100% and that’s all you have (can not or should not push the thrust up.)

Or maybe this one is better.

When take-off power is based on reduced power, whatever the method for establishing the reduced power, all limiting speeds are based on full rated power. Vmcg and Vmca are the most limiting speeds.

Fixed derate, however, is like taking one engine off the plane and installing one of greater or lesser thrust rating. Vmcg and Vmca are based on the rated power being used, the full thrust or the fixed de-rate.

Edited June 3, 2016 by Tom Stian

[Tom Stian]

But why is the thrust mode display showing R-CLB at from 400ft and above when I only have derated? ^^
Its from the same flight as the screenshots from my first post.

https://youtu.be/2eSTeEOBarY?t=24m19s

edit:
Or maybe CLB-1 is a reduced thrust upon the derated engines. ?

Edited June 3, 2016 by Tom Stian

[frankbyte]

18 hours ago, mmerelles said:

Going to the basics

The aircraft engines are rated to a certain amount of thrust, i think this bird was modeled based on 20k lbs of engine thrust each. This amount of thrust was designed to sustain under this aircraft MTOW, safety procedures an standards.

Using the engines to their maximum potential increases exponentially their tear and wear, reduces the life time, thus increasing their maintenance costs, fuel burnt, etc.

Most of the time, the aircraft is not loaded up to its maximum limits (full fuel+pax+cargo), the runway may be long enough, the temperature could be cold enough (dense air), the departure procedure may have not critical obstacles to clear, etc. etc. So, why to use the the engines up to their limits shortening their lives, risking failures, increasing maintenance costs, etc. etc?? This is were reducing takeoff engine thrust comes into play. How to reduce?

 

1. Assumed temperature (also known as flex, reduced thrust). You fool the engines by entering a higher OAT (outside air temperature) than what the real temperature is. So the engines think the air is a lot less dense than actually is producing less thrust. This provides high level of granularity and depending how high the OAT value you enter will be the thrust reduction you get.

2. Derated profile. The fmc may also provide pre-established semi-permanente fixed derating profiles. CLB-1. CLB-2. Each profile derates the engines thrust by a % of the total engine rating. 5%, etc.

 

how much to reduce? the charts will tell based on your present conditions. The whole idea is to reduce as much as possible without compromising safety, single engine climb out, obstacles, margins, climb rates, runway length, weather, etc.

 

hope this helps.

Hi

Thanks for the explanations. In fact i knew everything about the first part. I know why this is needed. My question was more about the difference of bothe methods. So if i understand it correctly, both do the same but for the one we can enter every temperature and the thrust will be reduced according to it (assumed). And in the sceond one those values are already pre-defined and fix (derated)?

Edited June 3, 2016 by frankbyte

[Tom Stian]

17 minutes ago, frankbyte said:

And in the sceond one those values are already pre-defined and fix (derated)?

No, not as I understand it. With reduced power (assumed temp) you still have the 20k rate engine available. But with derate you actually downgrading the engine (reducing horsepower ). If you derate to 18.5k the Vmcg and Vmca will be based on that derating and will be different if you was using 20k. 

Bare in mind. This is what I belive. I could have misunderstood this (also).

Edited June 3, 2016 by Tom Stian

[frankbyte]

Hm...it's quite complicate to understand for me. And first i was thinking that those 20K were 20 Kelvin (temperature) but it is the thrust of the engines in 1'000 x pounds? Because since the reduced thrust also has to do with fooling the real temperature, that's really quite confusing for me. I'm sorry to say i am still confused beetween the difference of those two modes

[frankbyte]

OK, i understand a little bit better after having read your post edited 7 hours ago Tom. So with the derated take off we have a max predefined power and can't go over it even if we lost one engine and had to goe over 100% with the second one? That just wont work?

But why is the sense of doing that? Still confusing....

[mmerelles]

10 minutes ago, Tom Stian said:

No, not as I understand it. With reduced power (assumed temp) you still have the 20k rate engine available. But with derate you actually downgrading the engine (reducing horsepower ). If you derate to 18.5k the Vmcg and Vmca will be based on that derating and will be different if you was using 20k. 

Bare in mind. This is what I belive. I could have misunderstood this (also).

 

BOTH methods are there not to use 100% of the engine thrust capabilities, because working the engines up to their 100% capabilities periodically shorten their lives considerably, risk failures operating at maximum temps, increases their maintenance costs exponentially.

 

DERATE method.

The aircraft manufacturer pre-program derated profiles available to the pilot via the FMC. As said above on my first post they act as a semi-permanent % derate. The engines IXEG modeled on this bird have 20k (20,000 lbs of thrust each). Selecting CLB-1 you will have a pre-set fixed 5% reduction during TO and CLB, selecting CLB-2 you will have a 20% reduction.

CLB-1 washes out about at 15k ft, CLB-2 washes out about FL300 if my memory is correct.

 

REDUCED method (also known as flex, assumed temp)

You have a range of OAT temperatures you can enter to fool the engines and get reduced thrust to save the engines as well,  this offers much more granularity to the exact present conditions weight, temps, etc. And yes under this method you may recurre to 100% thrust if required.

[FloB]

More than you want to know:

http://www.theairlinepilots.com/forumarchive/quickref/reducedandderatedthrust.pdf

 

 

[slai]

As an engineer by training and no aviation background, this subject was very confusing to me so link had just enough info I wanted.  Here are a few things that helped me get a grip.

 As previously mentioned, DERATE is just like having a completely different airplane with different FAA certified performance rating charts that pilots have to reference.  You are not even allowed to cancel the derate and go full power (may cause too much asymmetrical thrust) unless it is a dire emergency (wind-sheer or going into the side of a mountain).  No wonder Jan says he never used it- why give yourself a less capable aircraft?  There are good edge cases- heavy cargo loads was mentioned.  I guess the engine instruments do not indicate a DERATE cause you are already using a different set of performance charts etc. as if you had a different plane.  The engine instruments don't tell you it is working with GE 20K engines in the actual case, you have to verify this on acceptance of the aircraft I believe.

The "fool the engine to think it is very hot outside"  explanation for the Assumed Temp REDUCED method, although simple, was very confusing to me.  A hypothetical example was easier for me to understand.  Engines can generally produce max thrust up to 30 degrees Celsius.  Beyond that performance starts to degrade for reasons already give prev.  Makes sense, GE designs an engine to work well in all parts of the world.  At 50 degrees Celsius,  though, the GE 20K engine is only able to produce say 15K thrust.  So presuming it is actually only 20 degrees Celsius outside, the FMC reads the Assumed Temp value of 50 degrees Celsius and calculates a Takeoff throttle setting of 75% N1- thus the engines will  produce 15K of thrust on takeoff- the same amount as 100% N1 but 50 degrees outside.  The pilot by the way, is free to increase the throttle to 100% N1 at any time.

So the main question in my mind, was why doesn't the pilot just pick a reduced takeoff N1 percentage directly?  Well, as I understand it, it has to do with workflow.  For a given runway length, altitude, etc. the pilot looks up in the performance charts (since DERATE has its own performance charts, you could also combine REDUCED with DERATE) and finds the maximum temp that falls within the safety margins.  This maximum temp is the largest assumed temp you are allow to set.  The Assumed Temp method is conservative too because the plane performs better in all aspects at 75% N1 at 20 degrees than it does at 100% N1 at 50 degrees.  In addition, this method is good because performance drops off linearly from say 30 degrees to say 50 degrees so the Pilot has pretty large range of values to select from and know what he/she will be getting.

Finally, I learned that the throttles only command a percentage of N1.  The engine in turn sets a value for N2 and you get some not exactly known amount of thrust depending on altitude, temp, engine age, etc.  So why not use a throttle that sets thrust?  Some planes do but it is hard to measure accurately with a low incidence of instrument failure.

Edited June 5, 2016 by slai

[Morten]

17 hours ago, slai said:

Makes sense, GE designs an engine to work well in all parts of the world.  At 50 degrees Celsius,  though, the GE 20K engine is only able to produce say 15K thrust.  So presuming it is actually only 20 degrees Celsius outside, the FMC reads the Assumed Temp value of 50 degrees Celsius and calculates a Takeoff throttle setting of 75% N1- thus the engines will  produce 15K of thrust on takeoff- the same amount as 100% N1 but 50 degrees outside.  

Close, but it will actually have more than 75% thrust because the (real) air density is higher.  Also true airspeed will be lower because the (real) ambient temp is lower.  So you will actually get off the ground sooner than if it really was 50C outside   

[slai]

1 hour ago, Morten said:

Close, but it will actually have more than 75% thrust because the (real) air density is higher.  Also true airspeed will be lower because the (real) ambient temp is lower.  So you will actually get off the ground sooner than if it really was 50C outside   

Glad I'm at least close!  This truly is a "Dr. Dr." subject.

So I believe the Assumed Temp N1 computed by the FMC (taking both actual and assumed temp into account) will generate an equivalent thrust rating on takeoff.  *But*- you get better "lift" and better other performance with the same amount of thrust (at the higher actual air density) due to the true airspeed effect.  Thrust does not equal performance.

http://flightsafety.org/aerosafety-world-magazine/march-2011/when-less-is-more

Effect of True Airspeed
 

Pilots who are skeptical about reduced-thrust takeoffs often sense that something very important is being taken away. However, there is absolutely no loss of any necessary performance margins involving field length, screen height,¹ climb or obstacle clearance. If the airplane’s weight and power setting satisfied the certification standards at the higher temperature, then they certainly will do so at the lower temperature.

Although the takeoff speeds used by the flight crew are indicated airspeeds, actual performance is determined by true airspeed, which is a function of air density. Because we are operating at an actual temperature that is lower than the assumed maximum, true airspeed likewise will be lower.

Because of this true-airspeed effect, we enjoy a great deal of cushion between what the airplane must do and what it actually is doing. We are, in reality, using less runway and achieving a higher climb gradient, or obstacle-clearance margin, than if the ambient temperature was at the maximum for that same weight. Depending on conditions, the effect can be considerable — on the order of several hundred feet in field length. The benefit increases as the difference between the actual and the assumed temperatures increases.

[Fan-a-tik]

The TMA (Thrust mode annunciator) will display R-TO when a TO-1 derated takeoff is selected from the N1 limit page.

The main reason to use derated takeoff thrust is to save on engine wear and maintenance cost over time. The airline I work for use the "Derate" method first and then assumed temperature over it if additional performance buffer allows for it.

I would concur with the OP that this system behavior in 1.0.5 is just a tiny bit off the real airplane, which is good as we sometime need to be reminded that this is not the real thing!

I can post a video if evidence is needed. To my knowledge, this is not an option, does not need to be enabled by pin programming and is common to all FMC software including 10.8A.

[Litjan]

7 hours ago, Fan-a-tik said:

The TMA (Thrust mode annunciator) will display R-TO when a TO-1 derated takeoff is selected from the N1 limit page.

The main reason to use derated takeoff thrust is to save on engine wear and maintenance cost over time. The airline I work for use the "Derate" method first and then assumed temperature over it if additional performance buffer allows for it.

I would concur with the OP that this system behavior in 1.0.5 is just a tiny bit off the real airplane, which is good as we sometime need to be reminded that this is not the real thing!

I can post a video if evidence is needed. To my knowledge, this is not an option, does not need to be enabled by pin programming and is common to all FMC software including 10.8A.

Hi and thanks for the input.

We never used a derated take-off on my airline, so I was not sure how the real behaviour is, and the manual isn´t quite clear on it.

If you can provide me with a reference where selecting a "full power-no TASS" takeoff at the TO-1 or TO-2 derate shows R-TO on the TMA, I would be grateful and we will change this in the future. After all, nothing should remind you that this is not the real thing!

Thanks, Jan

 

[slai]

The airline I work for use the "Derate" method first and then assumed temperature over it if additional performance buffer allows for it.

What was the advantage of using derate over assumed temp when it seems like so much more procedural work?

[Fan-a-tik]

Litjan : I have attached a video that document's the TMA behavior with the FMC N1 limit selection. I agree that the documentation is not quite clear on this particular system function.Derate.wmv

SLAI : Also attached a small example of what the difference is between assumed temp and derate. Unsure if it is clear but don't hesitate to ask question. These performance charts are developed from the aircraft AFM and FPPM manuals.

[slai]

On 6/22/2016 at 7:52 PM, Fan-a-tik said:

 

SLAI : Also attached a small example of what the difference is between assumed temp and derate. Unsure if it is clear but don't hesitate to ask question. These performance charts are developed from the aircraft AFM and FPPM manuals.

Thanks for the performance charts.  Makes picking possible for me to pick valid derates for my flights.

Also interesting about reason to use TO1- "is recognized by the Engine manufacturer . . . extended IAW (er what is that?)"

[jfjoubert]

18 hours ago, slai said:

Thanks for the performance charts.  Makes picking possible for me to pick valid derates for my flights.

Also interesting about reason to use TO1- "is recognized by the Engine manufacturer . . . extended IAW (er what is that?)"

I'm guessing it's "in accordance with"...