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Assumed Temperature De-rate  
User currently offlineAjaaron From United Kingdom, joined Nov 2000, 113 posts, RR: 0
Posted (13 years 9 months 3 weeks 1 day 3 hours ago) and read 3164 times:

Would anyone please confirm if my understanding of the folowing is correct:

To preserve engine life, less than full power is used, obviously where a/c weight, ambient temp, flap setting, and length of runway allow for take off.

On the 744 and the 777 as well as a few other a/c one can do an assumed temperatrue derate, entering a temp into the t/off data page in the FMC before departure.

My question is:

The actual temperature figure entered, will that result in the thrust that will be produced on a day when that is the ambient temp?

Obviously maximum thrust that can be produced decreases with an increase in outside temperature, so with a greater assumed temp entered into the FMS, does that means less thrust on take off - If someone would confirm that or correct any errors, i'd appreciate it - its bugged me for years! Thanks in advance!

19 replies: All unread, jump to last
 
User currently offlineAAR90 From United States of America, joined Jan 2000, 3471 posts, RR: 47
Reply 1, posted (13 years 9 months 3 weeks 1 day 1 hour ago) and read 3109 times:

By jove, I think he's got it!  

I think of it this way. I'm basically lying to the engine controllers telling them it is hotter than it really is. They in turn will reduce the amount of fuel injected to match the "assumed temp" I gave them. That, in turn reduces the amount of thrust available in derated mode.

Naturally, full engine capability remains (if needed) by simply pushing throttles forward at any time. All performance requirements, including engine failure, are met at the calculated reduced thrust setting.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineAjaaron From United Kingdom, joined Nov 2000, 113 posts, RR: 0
Reply 2, posted (13 years 9 months 3 weeks 9 hours ago) and read 3088 times:

Thanks! - Im a a happier man now!

User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Reply 3, posted (9 years 5 months 3 weeks 6 days 11 hours ago) and read 3042 times:

How do you calculate the assumed or flex temperature?

User currently offlineGoldenshield From United States of America, joined Jan 2001, 6005 posts, RR: 14
Reply 4, posted (9 years 5 months 3 weeks 6 days 10 hours ago) and read 3026 times:

When I work my releases, it prints out a section for reduced rate takeoffs. Basically, the way it breaks it down, is it gives you an assumed tempurature, and then gives you the max weight that you may use a reduced takeoff with. If you go above that weight, then a regular power takeoff is needed.


Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 5, posted (9 years 5 months 3 weeks 6 days 10 hours ago) and read 3025 times:

Quoting LMML 14/32 (reply 3):
How do you calculate the assumed or flex temperature?


Normally we would let the box do it for us.

Here is the basic logic. I am about to take off at 130,000 lbs gross weight. The runway is very long and we are near sea level and it happens to be 15 degrees C. How heavy could I be (theoretically) and still make this takeoff while meeting the FAA-required runway length, climb gradients, etc.

So we could look at the runway analysis page and it says that for KXYZ airport, runway 26R at the actual temperature we can meet the runway limits at 150,000 lbs and the most restrictive climb segment at 148,000 lbs. So the 148,000 for climb is the more restrictive and that we have 'surplus' performance.

Now we look back up (or down) the table for this runway and we find the hottest temperature at which we could take off at our actual weight. This might be up around 40oC or thereabouts and remember that it is actually only 15oC so we tell the box that it is 40 and it derates the engines.

We still have all the required performance including meeting climb gradient with an engine failure and without putting the remaining engine back up to full takeoff thrust. And we still have that option - going back to Max thrust if we lose one.

The method is pretty simple and has a good level of safety built in.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Reply 6, posted (9 years 5 months 3 weeks 6 days 10 hours ago) and read 3016 times:

Do you factor in runway conditions, QNH and wind? When does the box cancel your input and revert to real OAT?

User currently offlineSchooner From United Kingdom, joined Oct 2001, 139 posts, RR: 0
Reply 7, posted (9 years 5 months 3 weeks 6 days 8 hours ago) and read 2991 times:

Runway conditions (Dry, Wet or Contaminated), QNH, wind (head, tail or zero wind), any MEL related performance reductions & whether or not wing/eng anti-ice is used are all taken into account. The selected temp is normally cancelled once Climb thrust is selected.

Cheers.



Untouched and Alive
User currently offlineUndehoulli From , joined Dec 1969, posts, RR:
Reply 8, posted (9 years 5 months 3 weeks 6 days 4 hours ago) and read 2951 times:

I know in the ERJ-135/145 you enter the ambient temperature before takeoff (actually before taxi I think) along with if you will have anti-ice on or off, and select your takeoff power setting (see later). The FADEC's have their own temperature sensors, the temperature you enter must be within 10-deg of the actual temperature that the FADEC's are sensing.

Depending on the aircraft (and thus the engine type..A1/3, A1, A1/P, A1/E) you can select different takeoff power settings such as ALT-TO (Reduced) and TO-1 (Full) for the A1; ALT-TO and TO for the A1/P and A1/3 (and you can do TO-RSV in the event of an engine failure), and ALT-TO, TO, and E-TO with the A1/E which can go to TO-1, TO-RSV, and E-TO-RSV with an engine failure. On takeoff, all you have to do is place the thrust levers in a detent that is about 90% of the total forward travel of the levers. In this detent, the FADEC's automatically produce the required thrust based on the settings the crew made before takeoff. If an engine fails, the operating engine will automatically go to the respective engine-out setting (such as TO-RSV, TO-1, TO-RSV, and E-TO-RSV).

WOW - I was confused in ground school and probably still am since I haven't been around an ERJ for almost 7 months. So directly interpreted I guess the ERJ doesn't really use assumed temperatures, but they still do use runway analysis charts which list, based on temperature and runway for each airport, the maximum takeoff weight with anti-ice on and off for each type of takeoff, and if that takeoff is restricted, how (obstacle, weight, climb performance).


User currently offlineTroubleshooter From Germany, joined Feb 2005, 423 posts, RR: 5
Reply 9, posted (9 years 5 months 3 weeks 6 days 2 hours ago) and read 2932 times:

An ERJ pilot once told me that ALT-T/O 1 is about 90% of T/O 1 power (assuming same take off data setting). Could not find a reference for that in my manuals. Can anyone here confirm that?


This job sucks!!! I love this job!!!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 10, posted (9 years 5 months 3 weeks 5 days 19 hours ago) and read 2917 times:

Quoting AAR90 (reply 1):
Naturally, full engine capability remains (if needed) by simply pushing throttles forward at any time. All performance requirements, including engine failure, are met at the calculated reduced thrust setting.


With Vmc being an exception. Vmc will be calculated with the ‘maximum available takeoff power or thrust on all engines’ as per the FAR. That’d be TOGA.

The Me-262 Schwalbe replicas are interesting in this respect. The original engines are replaces with modern ones, enclosed in an adapter which converts their external interface to that of the original engine. The maximum thrust of the new engines are significantly higher though. To get the aircraft certified with the original Vmc without hassle, they have a forward power lever position corresponding to the maximum thrust of the original engines. The additional thrust is only available by holding the power levers forward of this position against a spring, which returns them as you let go of the levers.


As for the ERJ derates, I’d assume the various fixed derates (such as on the ERJ) are set by maintenance just as on the 744. They are software constants after all. Nothing more, nothing less. Anyone who has the facts?


The beauty of assumed temp derates, as opposed to percentage of EPR/N1/whatever derates, is that you already have takeoff charts compensated for higher temperatures.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineTinPusher007 From United States of America, joined Feb 2005, 977 posts, RR: 1
Reply 11, posted (9 years 5 months 3 weeks 1 day 14 hours ago) and read 2790 times:

My question is if the FMC is as sophisticated as it is, why do you have to lie to the computer at all? Why can't you just tell it to derate the take-off thrust outright?


"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
User currently offlineUAL Bagsmasher From United States of America, joined Sep 1999, 2146 posts, RR: 10
Reply 12, posted (9 years 5 months 3 weeks 1 day 11 hours ago) and read 2781 times:

On the CRJ when we do high power runs, we enter the OAT into the FMS performance page. It then spits out a 'bug' on the N1 indicator on EICAS with the maximum N1 speed we are allowed to use. The max N1 speed varies considerably depending on the OAT. Hope this makes sense as I am on a couple hours sleep right now. Wink

User currently offlineAvionicMech From United Kingdom, joined Jan 2005, 315 posts, RR: 3
Reply 13, posted (9 years 5 months 3 weeks 1 day 10 hours ago) and read 2781 times:

Quoting TinPusher007 (reply 11):
My question is if the FMC is as sophisticated as it is, why do you have to lie to the computer at all? Why can't you just tell it to derate the take-off thrust outright?


You are right in the fact that the FMC could just set the power output of the engines to a specific figure but I think you will find that its down to the fact that it has always been done like this and you know how these 'fly boys' don't like things done differently. Haha. Only kidding guys. duck 


User currently offlineAAR90 From United States of America, joined Jan 2000, 3471 posts, RR: 47
Reply 14, posted (9 years 5 months 3 weeks 1 day 8 hours ago) and read 2774 times:

Quote:
...if the FMC is as sophisticated as it is, why do you have to lie to the computer at all?


You are not lying to the FMC. You are simply inserting data for its calculations and setting thrust using the results of those calculations. On acft with FMC's that don't directly control engine power setting(s) you use the results of the FMC's calculations to lie to the engine controller(s). Other acft have other means of setting assumed temp. derates.

Quote:
Why can't you just tell it to derate the take-off thrust outright?


Primary reason is that every airline is certified to operate at the maximum assumed temp. in all situations. Additionally, many times the desire is to not operate at the maximum reduced power but rather at some intermediate power setting. IOW, it is not possible to program the FMC with all the information required for every possible situation. The simple solution is for the user (airline pilot) to input the assumed temp. desired and let the FMC do the final math.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 15, posted (9 years 5 months 3 weeks 15 hours ago) and read 2743 times:

Quoting TinPusher007 (reply 11):
My question is if the FMC is as sophisticated as it is, why do you have to lie to the computer at all? Why can't you just tell it to derate the take-off thrust outright?


Quoting FredT (reply 10):
The beauty of assumed temp derates, as opposed to percentage of EPR/N1/whatever derates, is that you already have takeoff charts compensated for higher temperatures.


I must have posted while you were typing.  Smile

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineFDXMECH From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 16, posted (9 years 5 months 2 weeks 4 days 12 hours ago) and read 2700 times:

The purpose of the assumed temp is to have the aircraft mimic the takeoff performance (using more runway) on that hot day by reducing the T/O N1(EPR).

The reduction in T/O parameters (N1) doesn't commensurate with an increase of input (assumed) temp. As temp increases, so will the T/O max N1. This is opposite of what you see when increasing assumed temp.



You're only as good as your last departure.
User currently offlineFDXMECH From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 17, posted (9 years 5 months 2 weeks 4 days 9 hours ago) and read 2699 times:

Quote:

FDXmech
The purpose of the assumed temp is to have the aircraft mimic the takeoff performance (using more runway) on that hot day by reducing the T/O N1(EPR)


What I meant to write was having the aircraft mimic hot day performance (using more runway) by reducing T/O thrust.



You're only as good as your last departure.
User currently offlineAjaaron From United Kingdom, joined Nov 2000, 113 posts, RR: 0
Reply 18, posted (8 years 12 months 1 day 12 hours ago) and read 2304 times:

How do the engine controllers know what fuel flow/thrust to produce on an assumed temperature basis? e.g assumed tempt of say 55 deg C. - This may produce 99% N1 for example.

HOw does the software know its 99% N1?

Is this built in to the software, based on engine manafacturers tests at certification etc?


User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 19, posted (8 years 11 months 4 weeks 1 day 15 hours ago) and read 2227 times:

Quoting Ajaaron (Reply 18):
How do the engine controllers know what fuel flow/thrust to produce on an assumed temperature basis? ...

Bear in mind the controller isn't aware that it's an assumed temperature. It just accepts an OAT and, along with the other inputs, looks up the desired N1. That's the beauty of using an assumed ('locked') temperature; it's convenient because it's the language spoken by the controller.

Quoting Ajaaron (Reply 18):
Is this built in to the software, based on engine manufacturers tests at certification etc?

The airframer transmits to the engine manufacturer a 'specification table' of thrust requirements for certain flight conditions. Well before certification, and before the first engine even goes to test, the engine manufacturer uses thermodynamic cycle simulations to predict the N1 and stator positions required to produce the thrust dictated by the spec. table. Actual engine test data allows performance engineers to fine-tune these schedules generated from pretest prediction. These schedules are then handed over to the control software guys for formatting and insertion into the electronic engine control.

Does that help?



"He is risen, as He said."
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