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JulianUK
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A Derated Take Off Question - Some Confusion

Fri May 26, 2006 2:14 am

Ok, I admit. I don't understand it - well this bit actually. Surely engines are less efficient in hot temperatures, therefore by setting the temperature higher you will be asking the engines to do a lot more work so how can it be derated?

J

http://www.b737.org.uk/assumedtemp.htm The CFM56-3 and -7 are flat rated at ISA+15C ie 30C. This means that they are guaranteed to give (at least) the rated thrust at the full throttle position when the OAT is below this temperature. Above this temperature, they will give less thrust because the air is less dense.

On occasions when full thrust would be more than is safely required eg light aircraft, long runway, headwind etc. we can choose a thrust setting below full thrust by telling the engines (via the FMC) that the OAT is much higher than it actually is. This higher temperature is called the assumed temperature.
 
Ryanair737
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 6:32 am

Quoting JulianUK (Thread starter):
Ok, I admit. I don't understand it - well this bit actually. Surely engines are less efficient in hot temperatures, therefore by setting the temperature higher you will be asking the engines to do a lot more work so how can it be derated?

Using the assumed temperature method means that you're telling engines a temperature which is higher than the actual OAT at the time. Telling the engines this temperature makes the N1 or EPR reduce therefore your increasing the life of the engines quite substantially. Flightcrew are quite keen to do this whenever possible, for the reason given above.

It really is that simple!  Smile

Regards
Ryanair737
 
JulianUK
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 6:55 am

But surely if I take off from a runway with a 34c outside air temperature compared to a 1c temperature i need a lot more thrust so a lot more N1 basically...therefore if i set it for a 34c take off the n1 would have to be more to produce more thrust?
 
seanp11
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 7:28 am

Except the N1 limit doesn't take how much work it will take to get you into the air into effect. It takes the altitude and OAT into effect, you will simply take more runway to get into the air.
 
3DPlanes
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 7:47 am

Quoting JulianUK (Reply 2):
i need a lot more thrust

You might want more thrust, but due to the temp limits, you can't get more thrust. Temp limited thrust is another reason why hot ops make for longer takeoff runs and slower climbs (density altitude issues aside).

On a 34c day, I'd bet they won't de-rate the engines, unless they're light, or at sea level, or have a long runway. The engines simply give all they can, limited by max temp.

On a 1c day, the engines will produce more thrust - and if you have weight/density/runway in your favor, then why ask for the max?


I recall watching an 727 takeoff from Denver in July. It was maybe 200' or 300' up when it crossed the far threshold, maybe 5 degrees nose up or so...

(Which begs the question, how did they keep those JT8s from over-temping? No FADECs... Did they not run the throttles all the way up?)

-3DPlanes
 
F14D4ever
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 11:31 am

Quoting 3DPlanes (Reply 4):
(Which begs the question, how did they keep those JT8s from over-temping? No FADECs... Did they not run the throttles all the way up?)

I suppose there was an OAT bias in the hydromechanical control. Maybe something like the bimetal bar in a toaster? The bar takes a different shape, and so actuates some mechanism, based upon its temperature.
 
F14D4ever
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 11:40 am

Quoting JulianUK (Thread starter):
Surely engines are less efficient in hot temperatures

But derating is not about efficiency; it's about decreasing firing temperature. As ambient temp. increases past the corner point, the firing temperature (T41 in cycle thermo parlance) is the limiting factor. Above corner point, engine thrust is decreased so as to not exceed the desired T41 that's been specified to deliver guaranteed engine life.
Imagine a plot of thrust vs. OAT. below corner point, thrust is constant - horizontal line. Above corner, slope is negative - thrust falls off with increasing OAT. With flex, the pilot specifies a fictional OAT that puts him on the downward-sloping part of the thrust-OAT plot, at a lower thrust level than that delivered at actual OAT.
 
113312
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 12:01 pm

Most of these guys have explained it correctly but you still might not understand the concept. If you takeoff today at maximum rated power on a long runway at less than maximum weight, you know that you will leap off the ground after a short run and climb out with great performance. But, using maximum power means high internal temperature and pressure within the engine thus reducing service life.

Regulations dictate the minimum climb performance and minimum runway length for both accellerate stop and accellerate go considerations. If you takeoff using less than maximum thrust, you would be simulating the performance as if you were at a maximum weight, maximum temp or shorter runway condition. We use the assumed temperature method to simulate those conditions from a performance standpoint. The net result is that we takeoff with less thrust but perform as if we were using maximum power at maximum weight on a hotter day!
 
Tristarsteve
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RE: A Derated Take Off Question - Some Confusion

Fri May 26, 2006 8:22 pm

Quoting 3DPlanes (Reply 4):
Which begs the question, how did they keep those JT8s from over-temping? No FADECs... Did they not run the throttles all the way up?)

No, on older engines it was up to the crew to set the correct take off throttle position. Usually a bug was set on the EPR or N1 gauge, by hand from a set of tables, and the throttles advanced until the needle aligned with the bug. The crew must then watch the N2 and EGT to make sure they stay below the red line. Of course on a B727 you have a flight engineer to do this. The fuel control system has crude overtemp controls but they are there for emergency and not normal use.
Nowadays the FADEC does it all for you.
 
3DPlanes
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RE: A Derated Take Off Question - Some Confusion

Sat May 27, 2006 5:54 am

Quoting TristarSteve (Reply 8):
Usually a bug was set on the EPR or N1 gauge, by hand from a set of tables, and the throttles advanced until the needle aligned with the bug.

So, given that the engines take a bit to spool up, an occasional overshoot seems likely. Must have been an a real kick having to pull power during the takeoff run...

I'm sure the crews got a 'feel' for where to push the levers to get a certain N1.

-3DPlanes
 
JulianUK
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RE: A Derated Take Off Question - Some Confusion

Sat May 27, 2006 9:04 pm

Thanks the answers are great, but surely now in the modern aircraft why are we having to "confuse" the settings by setting a temperature to "force" the N1 settings to be lower than normal - I would have thought that most flight management computers should take into account everything now and you don't have to put in ficticious figures, just put in everything as it is and it will sort it out for you rather than doing this assumed method?
 
Ryanair737
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RE: A Derated Take Off Question - Some Confusion

Sun May 28, 2006 4:45 am

Quoting Julianuk (Reply 10):
Thanks the answers are great, but surely now in the modern aircraft why are we having to "confuse" the settings by setting a temperature to "force" the N1 settings to be lower than normal - I would have thought that most flight management computers should take into account everything now and you don't have to put in ficticious figures, just put in everything as it is and it will sort it out for you rather than doing this assumed method?

I don't think it would be quite as simple as that, that as why the method hasn't changed. That would be asking the computer to be doing everything that the flight crew calculate manually before every flight. I don't think the computers would be able to do this successfully working on a select number of figures. I guess technology isn't that advanced yet, possibly one day it might happen.

Ryanair737
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Tue Jun 06, 2006 11:05 am

Quoting Julianuk (Reply 10):
Thanks the answers are great, but surely now in the modern aircraft why are we having to "confuse" the settings by setting a temperature to "force" the N1 settings to be lower than normal - I would have thought that most flight management computers should take into account everything now and you don't have to put in ficticious figures, just put in everything as it is and it will sort it out for you rather than doing this assumed method?

The key comment here is "computers should take into account everything as it is and it will sort it out"...is a key comment. Think about that for a moment; if one programs an assumed temp takeoff, of say, 40 centigrade when the OAT is 27, then that allows last minute bags and pax to be added to be added without changing "bug" speeds, so the system as designed is fairly robust in order to meet runway, 2nd stage climb, and single engine climb parameters.

A lot of people have questions on this forum (wheel spin up, for ex) that, while valid, have been considered before most that pose them were born. The technology is very advanced, but it is predicated on safety and profit/cost. No small protion of the profit scenario is to be able to get off the gate on time, with enough margin to be safe. Typically, safety and cost are opposing parameters...and no computer can account for the fact that Mrs Biggemeyer stayed in the bathroom and missed her flight, along with her husband (at the last minute)...A computer cannot predict that, so assumptions must be made in order to make the system work.
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Tue Jun 06, 2006 11:10 am

Quoting EssentialPowr (Reply 12):
The key comment here is "computers should take into account everything as it is and it will sort it out"...is a key comment

UUhh...Just a key comment!
 
F14D4ever
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 1:50 am

Quoting Julianuk (Reply 10):
... I would have thought that most flight management computers should take into account everything...

The key enabling piece of information would be, I think, actual gross aircraft weight. There are fundamentally two ways to obtain that: a) weigh the aircraft on a scale and uplink the result, or b) install a system of transducers in the landing gear that would compute the actual gross weight. Results from a) or b) would then be sent to the flight management computer, which could then compute the desired amount of derate and transmit to the FADEC. Meanwhile we have a couple guys sitting in the cockpit who are eminently capable of doing all the necessary estimations & calculations. I'm not sure an automatic system would survive a cost/benefit analysis.
 
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CCA
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 1:58 am

Quoting Julianuk (Reply 10):
Thanks the answers are great, but surely now in the modern aircraft why are we having to "confuse" the settings by setting a temperature to "force" the N1 settings to be lower than normal - I would have thought that most flight management computers should take into account everything now and you don't have to put in fictitious figures, just put in everything as it is and it will sort it out for you rather than doing this assumed method?

One day you will, if not now, at the moment the Aircraft FMCs don't have the required performance programmes, they're either done manually from charts, via a laptop, or through the ACARS using the company computers, these figures are fed into the FMC. Now aircraft like the 777 with the Boeing Flight Bag can probably compute the figures onboard using the flight bag computer as it will have the performance figures, but I'm not sure if it can be synced to the FMC. As with the A380 I'd imagine it's version of the Boeing flight Bag will probably allow it to send the data directly to the FMGECs.

They will probably use the same system of Assumed temp though as it's a good indication to flight crew of how much of a derate is allowed. For example if the temp is 33 degrees and the computer spits out a derate of 34 degrees you know there isn't much of a derate, you're 1 degree from MAX thrust. If you have a 52 degree derate you know there's plenty of thrust still available.


CCA
 
JulianUK
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 2:15 am

So can anyone give an example of say a take off in 34c on a 6500ft runway and a take off in 2c on the same runway - what are the figures for anyone that has a particular type of aircraft chart to hand on the N1 and other settings?
 
SlamClick
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 5:00 am

Quoting JulianUK (Reply 16):
So can anyone give an example of say a take off in 34c on a 6500ft runway and a take off in 2c on the same runway - what are the figures for anyone that has a particular type of aircraft chart to hand on the N1 and other settings?

Best I can give you at the moment is this. While it is derived from an actual chart, it has not been updated or revised and is not to be used in actual flight operations. Don't try this at home, I am a professional.

This data is derived from charts relating to a B-727-200 with JT-8D-7 engines which means it is a bit underpowered by today's standards.

First off, the runway length you cite. We don't do it that way. For many years now Part 121 operators have needed an analysis for each runway they use. So the runway length is not necessarily of primary importance, it is part of a package. A sea level 6500' runway with no obstructions in the departure corridor and no slope is an entirely different animal from a 6500' runway at 4500' elevation where the temperatures are high and, with a 1.9 degree downslope toward obstacles. So we get a runway analysis for, say Runway 19L at Las Vegas and a separate one for 01R. Even though it is the same strip of concrete it is a different takeoff case.

So using the one good example I have close to hand is for a specific eleven thousand foot runway at an actual airport somewhere in the US. It is situated just below a thousand feet above sea level. Our -7 powered 722 will perform like this:

At 2°C it will take off at its maximum certificated takeoff gross weight of 185000 lbs. At 34°C it will only lift 175080 lbs. That is based on runway limits.

Climb limits for the same airport are: At 2°C 185000 lbs, at 34°C - 170090 lbs.

So, while we are restricted by the runway at 34°C we are even more limited by climb performance. In fact that is probably going to be the limiting factor for our takeoff on that day.

Now this same chart gives nominal power settings for takeoff but they cannot be used for reasons I don't want to waste bandwidth on. Just FYI it says:

At 2°C t/o EPR = 1.99 and N1 = 92.3%
At 34°C t/o EPR = 1.89 and N1 = 93.9%

Again, those cannot be used. I just included them because my EPR chart does not give an N1 so it is just for reference.

The takeoff thrust chart says an EPR at 2°C of 1.99 on the pod engines (1 and 3) and 2.01 on number 2. At 34°C it says about 1.88 and 1.91. Quite a reduction there, eh?

The climb EPR chart gives this;

At 2°C about 1.93 and 1.95 and at 34°C about 1.73 and 1.75

Again, all of these are gee-whiz numbers but they should give you some idea...
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 7:50 am

Quoting CCA (Reply 15):
One day you will, if not now, at the moment the Aircraft FMCs don't have the required performance programmes, they're either done manually from charts, via a laptop, or through the ACARS using the company computers, these figures are fed into the FMC. Now aircraft like the 777 with the Boeing Flight Bag can probably compute the figures onboard using the flight bag computer as it will have the performance figures, but I'm not sure if it can be synced to the FMC. As with the A380 I'd imagine it's version of the Boeing flight Bag will probably allow it to send the data directly to the FMGECs.

I think you are missing the point that F14D4ever and I are making...The most accurate way to calculate a/c performance is to know exactly what the a/c weighs. A/c are not weighed explicitly, nor is a system of transducers used to calculate a weight (there is a package available for the ERJ at the moment). That figure is never available until the doors (pax and cargo) are closed. Doors closed means push; there is therefore little to gain, statistically, in having a very accurate weight, b/c performance #s are padded to account for last minute bags, etc...in order to minimize iterations of performance calculations, and therefore delays. There is plenty of capability in the industry now to load performance #s directly in to the FMC; it's done every day. Weight is estimated, conservatively, in order to keep the system moving, on time, and therefore enhance profitability.
 
saturn5
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 8:48 am

There is a lot of thermodynamics involved here to understand the problem fully. Remember the famous Carnot's law about efficiency of the engine - (T1-T2)/T1. It is very much part of the whole equation here. Turbine engines happen to be a case of complex thermodynamics.
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 12:59 pm

Quoting Saturn5 (Reply 19):
There is a lot of thermodynamics involved here to understand the problem fully. Remember the famous Carnot's law about efficiency of the engine - (T1-T2)/T1. It is very much part of the whole equation here. Turbine engines happen to be a case of complex thermodynamics.

Yeah, yeah...thermo. Heat engines. Enthalpy. Latent heat. Carnot cycle...the human body happens to be a case of complex thermo. What isnt?

Oh well. Runway performance is really more physics, if you want to get discipline specific...but it's all just math, right?

BTW, along with the weight issue, a robust weight and balance system must also include some capability to account for atmospheric changes. If the performance for an a/c is detailed down to the nth degree, what happens if the barometer drops, of the wind shifts? These are other reasons that simple, robust assumptions are made in terms of airliner performance. Safety is ensured, and yet the system remains flexible. That is the point.
 
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CCA
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 2:49 pm

Quoting EssentialPowr (Reply 18):
Doors closed means push; there is therefore little to gain, statistically, in having a very accurate weight, b/c performance #s are padded to account for last minute bags, etc...in order to minimize iterations of performance calculations, and therefore delays. There is plenty of capability in the industry now to load performance #s directly in to the FMC; it's done every day. Weight is estimated, conservatively, in order to keep the system moving, on time, and therefore enhance profitability.

I think you are confusing the Aircraft performance with RTOW performance.

The Company I fly for we usually enter the weight to the nearest 100kg and there may be a pad or there may not, we don't pad it for last minute bags. and we will get new RTOW figures for detrimental changes in QNH/Temp. This statistically will over time save $$$$.

Our performance figure are based on specific runways not a generic one, (to get the most accurate RTOW figures) it also has several taxiway/ runway combinations, so an airport with 4 runways may have more than 24 performance charts, that's not including brakes deactivated or thrust reverser deactivated etc.

The Aircraft performance (Drag and Fuel Flow Factors) is in the FMC, all the RTOW figures for all the runways that the Aircraft may operate into are certainly not. That's why our ACARS RTOW figures are so accurate, the pad has been reduced to the minimum to allow maximum derates.

A manual RTOW may give a derate of 40 but a ACARS RTOW may give 50 degrees.

CCA
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 10:18 pm

Quoting CCA (Reply 15):
One day you will, if not now, at the moment the Aircraft FMCs don't have the required performance programmes, they're either done manually from charts, via a laptop, or through the ACARS using the company computers, these figures are fed into the FMC.

That's exactly what I'm not confusing it with. Your quote above- regardless of whether its referring to takeoff, climb, crusie or descent- is what I disagree with. Figures will always have to be provided to the FMC, but the required perfromance programs are certainly there, and have an overall cost function available...the cost index, which changes parameters for all phases of flight.
 
SlamClick
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 11:25 pm

Quoting EssentialPowr (Reply 20):
, what happens if the barometer drops,

In my experience these things are considered. We are basing our calculations on actual reported weather at that airport. If it is ten o'clock in the morning at PHX and the day is going to be hot, many would go to the next higher temperature on the chart, anticipating that increase before pushback.

The airport analysis chart I used for Reply #17 has increment/decrement for winds, specific to each runway. For example on one runway it may allow you to add 60 lbs. for every knot of headwind but require you to subtract 550 pounds for each knot of tailwind. Many types of charts have such notes. On the takeoff thrust chart it said you could not interpolate between temperatures or exceed the actual or assumed temperature. So if you were assuming 34°C and the chart only has rows for 30 and 35 you must use 30.

The last system I used was fully automated, downlinked from Ops. It included the statement that it was valid so long as the altimeter was [0.1 less than actual or higher] and the temperature was a certain value or colder. Most of my career I never took advantage of headwind increments, but accepted them as a safety margin.

The closest I ever came to max allowable was about twenty pounds. I could see that my copilot was not happy with that. (We were departing BUR) so I told him that right after takeoff he should sneak a quick peek as we cross the airfield fence. I said if we lose an engine at V1 we will be at fifty feet, if we don't we will be at 200. We didn't and we crossed the fence at 200RA.

The charts are very good!

edit: Hey, this is my 6000th post. Maybe I should go start a thread about that. Well, maybe not.

[Edited 2006-06-07 16:26:43]
 
David L
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RE: A Derated Take Off Question - Some Confusion

Wed Jun 07, 2006 11:58 pm

Quoting SlamClick (Reply 23):
Hey, this is my 6000th post. Maybe I should go start a thread about that.

You forgot the ten exclamations marks. Amateur.  Smile
 
fr8tdog
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RE: A Derated Take Off Question - Some Confusion

Thu Jun 08, 2006 1:56 am

Flex thrust ... why do we do it?.
quite simply put, it's a program designed by the engine manufactures to extend the engine beyond TBO.

Most engines are "leased" separately from the aircraft. (power by hour or flat rate's)
the program if entered into by the Lessee, guarantee's that a certain percentage of take-off's will be "reduced thrust" they will be able to operate the engine longer. While they cant make it mandatory for the pilots to do every T/O a flex-thrust, they encourage it.

Every engine is over-engineered to a point that statistically the number of engine failures are at an acceptable level. This means that an engine is designed to do full power takeoffs through-out its life span, meaning that the engine must be overhauled at a certain hour limit (TBO).

If the carrier is in an approved program they may operate that engine to a point beyond TBO. Generally this is a Temp limit, however if the engine operates within temp parameters, there is still an hour limit beyond TBO for internal components.

example: TBO for X engine is 3500hrs. If the operator is in the flex-program then they may operate the engine beyond this limit and go into a "monitored" program, allowing them to operate the engine until engine goes beyond ITT limits for a full power run (IE, standard day, temp of xxx specified by manufacture) or until reaching 4200 hrs. then the engine must be overhauled.

one last thing, when an engine reaches a point where the ITT's reach their limit on a full pwr take off, it is classified a zero-margin engine and falls under further scrutiny for inspections and how its operated. Again these programs are different for different manufacturer's and aviation laws in various country's.



Fr8t.
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Thu Jun 08, 2006 4:15 am

Flex/asumed temp takeoffs also save gas as well as extend engine life, also saving dollars. Many airlines are using higher autobrake settings in exchange for minimizing use of TRs.
 
arluna
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RE: A Derated Take Off Question - Some Confusion

Thu Jun 08, 2006 1:05 pm

Hi there,

Quoting 3Dplanes (reply9)
"So, given that the engines take a bit to spool up, an occasional overshoot seems likely. Must have been an a real kick having to pull power during the takeoff run..."

On the KC135E the pilots would push the power up about half, let it stabilize and then go to the EPR setting. This was done in order to avoid overtemping the engines. If you ever have a chance to watch an older pre FADEC jet take off you will notice from the sound that they don't go immediately to full power but have a short pause during acceleration.

Jerry

[Edited 2006-06-08 06:06:55]

[Edited 2006-06-08 06:10:51]
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Thu Jun 08, 2006 1:48 pm

Same for the 737, 75/6...

A big difference in time-on-wing b/t the 2 engines can cause very different acceleration rates, so the throttles are advanced in a 2 stage manner before the autothrottles are engaged to ensure low speed directional control.
 
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CCA
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 3:22 am

Quoting EssentialPowr (Reply 22):
Figures will always have to be provided to the FMC, but the required performance programs are certainly there, and have an overall cost function available...the cost index, which changes parameters for all phases of flight.

The point I'm trying to make is the FMCs don't have RTOW performance figures for all the runway combinations available, and without them it will never be able to work out a suitable derate. This is why RTOW figures are sourced from elsewhere, and then fed into the FMC. Which is why I said the Boeing flight bag will probably have the RTOW performance programme which is then synced to the FMC. Personally I don't think FMCs will ever do RTOW figures as the amount of memory required by having every airport/configuration which is only accessed once or twice prior to take-off is pointless.

Cost Index has no effect on Take off performance. it only affects speed (CLB/CRZ/DES) and optimum altitude.
 
JulianUK
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 3:37 am

Ok here is the interesting question I thought about.

Once you decide what derate you are actually going to use where is it recorded? I hear that some airlines give you the results over ACARS you want calculated - not sure if that is true. However let's just suppose it went wrong and you lived to tell the tale, where would you actually put what settings you had decided to use that day - is it written down or just decided verbally?

J
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 3:39 am

Quoting CCA (Reply 29):
The point I'm trying to make is the FMCs don't have RTOW performance figures for all the runway combinations available, and without them it will never be able to work out a suitable derate.

There is a database for every suitable runway, which is updates on a monthly basis and on an immediate basis for intersection takeoffs and atmospheric data via datalink (this is a generic description for US airline xyz).

Quoting CCA (Reply 29):
Which is why I said the Boeing flight bag will probably have the RTOW performance programme which is then synced to the FMC.

My point is that this external input will need to be "synced to the FMC" , as you put it, will always be there and therefore there likely won't be and organic FMC. Last minute runway changes, the final cargo numbers, missing pax, and people abnormally distributed about the cabin will always generate an external input to the FMC.
 
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CCA
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 2:03 pm

Quoting EssentialPowr (Reply 31):
There is a database for every suitable runway, which is updates on a monthly basis and on an immediate basis for intersection takeoffs and atmospheric data via datalink (this is a generic description for US airline xyz).

Sorry but that database doesn't have half the required info for RTOW calculations ie. obstacles, slope, and NOTAM changes.
 
EssentialPowr
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 2:24 pm

Quoting CCA (Reply 32):
Sorry but that database doesn't have half the required info for RTOW calculations ie. obstacles, slope, and NOTAM changes

Uhhh...have you never seen a data based that didn't contain runway info to include slope, length and width? Did I ever make the comment that real time factors, such as intersection departures and other NOTAMs (or atmospheric conditions) was available from within the FMC? Quite the contrary.

Quoting EssentialPowr (Reply 22):
Figures will always have to be provided to the FMC, but the required perfromance programs are certainly there,



Quoting EssentialPowr (Reply 20):
BTW, along with the weight issue, a robust weight and balance system must also include some capability to account for atmospheric changes. If the performance for an a/c is detailed down to the nth degree, what happens if the barometer drops, of the wind shifts? These are other reasons that simple, robust assumptions are made in terms of airliner performance. Safety is ensured, and yet the system remains flexible. That is the point.



Quoting CCA (Reply 29):
. Which is why I said the Boeing flight bag will probably have the RTOW performance programme which is then synced to the FMC.

I have said there will always be factors external to the box that will need to be accounted for....and have from the start of my responses. You brought up the EFB like it eliminates the last minute changes, which it can't. It only eliminates paper in the cockpit (ideally)...
 
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CCA
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RE: A Derated Take Off Question - Some Confusion

Fri Jun 09, 2006 6:15 pm

Quoting EssentialPowr (Reply 33):
Uhhh...have you never seen a data based that didn't contain runway info to include slope, length and width? Did I ever make the comment that real time factors, such as intersection departures and other NOTAMs (or atmospheric conditions) was available from within the FMC? Quite the contrary.

yes you did imply that, and I quote

Quoting EssentialPowr (Reply 31):
Quoting CCA (Reply 29):
The point I'm trying to make is the FMCs don't have RTOW performance figures for all the runway combinations available, and without them it will never be able to work out a suitable derate.

There is a database for every suitable runway, which is updates on a monthly basis and on an immediate basis for intersection takeoffs and atmospheric data via datalink (this is a generic description for US airline xyz).

Look I think we're arguing the same points just from different perspectives.

I'm just saying A/C FMC's don't have the required performance figures/programmes installed to generate their own RTOW V1 and engine derates.

The EFB could contain a similar programme to this http://www.vistair.com/RTOWS/index.htm and be easily updated prior to each flight.

Once the data entry is complete the resulting RTOW figures could be synced to the FMC ie no paper as you said.

Things that the EFB could and do receive electronically/digitally and therefore be entered automatically to an RTOW programme are.

FUEL
Final ZFW
Final TOW (Which should correspond to the first two)
ATIS (Temp/Wind/QNH/Runway)
MEL Items

Which leads me to my original post.

Quoting CCA (Reply 15):
One day you will, if not now, at the moment the Aircraft FMCs don't have the required performance programmes, they're either done manually from charts, via a laptop, or through the ACARS using the company computers, these figures are fed into the FMC. Now aircraft like the 777 with the Boeing Flight Bag can probably compute the figures onboard using the flight bag computer as it will have the performance figures, but I'm not sure if it can be synced to the FMC. As with the A380 I'd imagine it's version of the Boeing flight Bag will probably allow it to send the data directly to the FMGECs.

Obviously the RTOW auto fill entry's on the EFB will have to be checked/modified (such as an intersection departure) by the crew before sending the data to the FMC, just as it is currently is before being manually entered.

I know you mention LMCs but we don't usually request an RTOW figure until we have the Final ZFW and therefore the final TOW.

Our RTOW figures are "accurate" to 100kgs and this is on a 747 so we get the best and most accurate derate possible to conserve the engines which obviously leads to huge savings across the fleets.

[Edited 2006-06-09 11:23:17]

[Edited 2006-06-09 11:24:38]
 
EssentialPowr
Posts: 1732
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RE: A Derated Take Off Question - Some Confusion

Sat Jun 10, 2006 3:14 am

My comment notes the database info is explicit...it is udated monthly, and the FMC updates via datalink for immediate, realtime data such as atomospheric data for the rest of performance calculations (winds aloft, qnh, temp,).

Quoting EssentialPowr (Reply 31):
There is a database for every suitable runway, which is updates on a monthly basis and on an immediate basis for intersection takeoffs and atmospheric data via datalink (this is a generic description for US airline xyz).



Quoting CCA (Reply 29):
Which is why I said the Boeing flight bag will probably have the RTOW performance programme which is then synced to the FMC

Our systems are pretty much synched, the EFB will hopefully eliminate Jepps and other paper but Runway performance must always be robust enough for last minute changes, and an EFB won't make any of these changes faster than they already are...

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