Which tool for take-off performance calculation?

[Alpha Floor 237]

TOPCAT only has a 'beta' profile for the 200LR/F, no 300-ER. 

 

For what it's worth, I have access to a real world 777 EFB (300-ER/F/200-LR in GE and RR engines) and have tested it against both the TOPER and TOPCAT calculators. The results varied wildly, as you might expect. The TOPCAT one spat out the most accurate results in terms of speeds and margin but the derate N1 figures were a long way off and were up to 10 or 15 degrees lower than the real one. Using these figures we managed to drag the gear through the lights at Dublin on the departure - whoops. 

 

The 777 FMC automatically generated V speeds are incredibly accurate however, within a knot or two of the Boeing EFB for 'balanced' field conditions. Great job, PMDG.

 

 

Good to know! Did you mean "higher" by any chance? Lower would put you on the safe side. EDIT: You were talking about N1% and I was thinking about the Assumed Temperature, my bad.

 

I personally don't bother spending hours on performance calculations. When I'm light doing a short haul flight I use 50 or 60ºC. When I'm near MTOW I don't use derates... 

 

I use a rule of thumb for my fuels as well: 10 to 20 tons + 8 * hours of flight

 

I know this is not the most realistic, but whatever...

[Gregg_Seipp 570]

I always disliked TOPCAT...I was the garbage in side of that equation and always felt lost in it (primarily fly GA).  Taking a step back, what do real world pilots use?  Is it on their paperwork? 

[Airline2Sim 489]

Top left is the real Boeing one with no thrust derate, so TO. Top right is TOPER. Bottom is TOPCAT. 

 

Note that I used F20 as that is what the real one wanted to use. TOPER does not give the optimum flap setting, but allows you to choose one. TOPCAT will choose the optimum, but in this case chose F15 and TO-1 so I manually adjusted it to F20 and TO.

 

[Gregg_Seipp 570]

 

 

Top left is the real Boeing one with no thrust derate, so TO. Top right is TOPER. Bottom is TOPCAT.



Note that I used F20 as that is what the real one wanted to use. TOPER does not give the optimum flap setting, but allows you to choose one. TOPCAT will choose the optimum, but in this case chose F15 and TO-1 so I manually adjusted it to F20 and TO.

 

That's a lot simpler...but I don't see any options for derated TO?  Look at me using fancy words.

[PinkPony 171]

 

 

That's a lot simpler...but I don't see any options for derated TO?  Look at me using fancy words.

Next to the RTG (Rating) field: TO (other options being OPTIMUM, or user selected TO-1, TO-2).

In the output field: SEL TEMP 57 C. 

Result of the calculation being: D-TO 57 C, as in: Full TO rating with a assumed temperature of 57 C. 

Note: Fixed takeoff de-rates TO-1, TO-2, and fixed Climb de-rates: CLB-1, CLB-2 are Boeing customer options.

The worlds largest B777 operator for example did not purchase the TO-1, TO-2 option. Thus only using full TO in conjunction with assumed temperature. (They did purchase the CLB-1, and CLB-2 option).

 

[Boeing or not going 578]

 

Next to the RTG (Rating) field: TO (other options being OPTIMUM, or user selected TO-1, TO-2).

 

In the output field: SEL TEMP 57 C. 

 

Result of the calculation being: D-TO 57 C, as in: Full TO rating with a assumed temperature of 57 C. 

 

Note: Fixed takeoff de-rates TO-1, TO-2, and fixed Climb de-rates: CLB-1, CLB-2 are Boeing customer options.

 

The worlds largest B777 operator for example did not purchase the TO-1, TO-2 option. Thus only using full TO in conjunction with assumed temperature. (They did purchase the CLB-1, and CLB-2 option).

 

 

I read I think on this forum, an advisory from Boeing, about engine wear when using de-rated climbs - David Lee

[Gregg_Seipp 570]

 

 

Next to the RTG (Rating) field: TO (other options being OPTIMUM, or user selected TO-1, TO-2).

In the output field: SEL TEMP 57 C.

Result of the calculation being: D-TO 57 C, as in: Full TO rating with a assumed temperature of 57 C.

 

Thanks.  Can someone remind me what the Selected Temp does?  If I recall, using a higher temp than actual temp causes the engines to use more power and vice versa?

[Alpha Floor 237]

If I recall, using a higher temp than actual temp causes the engines to use more power and vice versa?

 

Hi Gregg,

 

Not quite: A higher Assumed Temperature than the actual temperature (OAT) will decrease the take-off thrust reference. This is used when conditions allow for a decreased thrust setting (good weather, low weights, long runways etc) and increases engines life.

 

More info here: http://www.b737.org.uk/assumedtemp.htm

[Gregg_Seipp 570]

 

 

Hi Gregg,

Not quite: A higher Assumed Temperature than the actual temperature (OAT) will decrease the take-off thrust reference. This is used when conditions allow for a decreased thrust setting (good weather, low weights, long runways etc) and increases engines life.

More info here: http://www.b737.org.uk/assumedtemp.htm

 

Seems backward to me but it is what it is.  The other question is how to you figure out what assumed temp to use.

[Alpha Floor 237]

The other question is how to you figure out what assumed temp to use.

 

Haha, that's the difficult part and the whole point of this thread.

 

You could do it manually, see here, but that's not really practical. You could use programs such as TOPCAT, but in this thread it's reported that these don't work as well as one would want (see posts by Airline2Sim). Real pilots use software tools that calculate the value for them. We as simmers don't have access to those tools unfortunately.

[Gregg_Seipp 570]

 

 

Haha, that's the difficult part and the whole point of this thread.

You could do it manually, see here, but that's not really practical. You could use programs such as TOPCAT, but in this thread it's reported that these don't work as well as one would want (see posts by Airline2Sim). Real pilots use software tools that calculate the value for them. We as simmers don't have access to those tools unfortunately.

 

Okay.  So, right now in terms of tempurature, the answer is either use TOPCAT for or anything you do is a guess (perhaps, educated).  In terms of derate, perhaps, don't.  If you don't use assumed temp (e.g. use real temp) then just using the FMS numbers would be reasonable?

[Alpha Floor 237]

Okay.  So, right now in terms of tempurature, the answer is either use TOPCAT for or anything you do is a guess (perhaps, educated).  In terms of derate, perhaps, don't.  If you don't use assumed temp (e.g. use real temp) then just using the FMS numbers would be reasonable?

 

You can use 50-60ºC as a safe bet on airports that a 777 usually flies from. Experiment a little, load your plane and play around with the temps in the FMC, see what take-off N1 (or EPR) you get for each temp.

 

Take-off derates are not exactly the same thing. Some airlines don't allow them. A derate is a much more rough estimate, it doesn't let you fine-tune the thrust as an assumed temperature. Derates are usually -10% and -20% (in thrust, not N1) and must be used with caution.

 

If you don't use assumed temp, then you'll be taking-off at the full TO thrust. That's the case if you don't input any temp at all into the FMC. (nor use a derate, or a assumed temperature selector on some aircraft like the 757)

[Gregg_Seipp 570]

 

 

You can use 50-60ºC as a safe bet on airports that a 777 usually flies from. Experiment a little, load your plane and play around with the temps in the FMC, see what take-off N1 (or EPR) you get for each temp.

Take-off derates are not exactly the same thing. Some airlines don't allow them. A derate is a much more rough estimate, it doesn't let you fine-tune the thrust as an assumed temperature. Derates are usually -10% and -20% (in thrust, not N1) and must be used with caution.

If you don't use assumed temp, then you'll be taking-off at the full TO thrust. That's the case if you don't input any temp at all into the FMC. (nor use a derate, or a assumed temperature selector on some aircraft like the 757)

 

Thanks Jaime...for patiently answering my basic questions.   I don't have the T7 just yet.  Just moved to P3D3...gave up my NGX so, now with no heavy metal airplanes, thinking about it.

[kevinh 409]

Top left is the real Boeing one with no thrust derate, so TO. Top right is TOPER. Bottom is TOPCAT.

 

 

 

Note that I used F20 as that is what the real one wanted to use. TOPER does not give the optimum flap setting, but allows you to choose one. TOPCAT will choose the optimum, but in this case chose F15 and TO-1 so I manually adjusted it to F20 and TO.

TOPER looks like its interface has been modelled on the real thing. I'd be happy with selecting my own flap as I usually override TOPCAT's optimum anyway. I don't use the fuel predictions TOPCAT gives any more, but it does provide landing distance calculations which is useful for choosing autobrake setting. Sadly the calcs for the 777 (still with beta data) produce contradictory predictions and are unreliable.

[PinkPony 171]

Seems backward to me but it is what it is.  The other question is how to you figure out what assumed temp to use.

 

The GE90-110/115 are flat-rated to 30C. (as are most modern jet-transport turbines). Flat-rated meaning that on the B777-300ER for example, the GE90-115BL1 will provide its maximum rated takeoff thrust of ~115,300lbs, up to a ambient temperature of 30C. Above that temperature takeoff thrust will be reduced as temperature increases.

 

Considering that the B777-300ER is dispatched on flights ranging from 45 minutes to 16h, the range of takeoff weights is quite big. 

 

When taking OMDB's runway 30R with a available full length of ~4300m, a -300ER departing at only 285,000kgs for a ~7 hour flight (highest MTOW option offered by Boeing for the -300ER being 351,534kgs), on a 25C day, the thrust required to safely complete the takeoff and continue past V1 in case of a engine failure, will of course not be anywhere near the maximum available ~115,300lbs of thrust. For this reason and in the interest of reducing the EGT temperatures (less thrust = lower EGT) and thus reducing turbine hot section wear/tear and ultimately engine maintenance cost. 

 

With the engine providing rated takeoff thrust up to 30C, assumed temperature's can be entered with any number > 30C resulting in a reduction of thrust! In simple terms, by entering 65C for that light 285,000kgs takeoff, you are telling the FMC to fool the engines into thinking it is 65C and thus providing the accordingly significant reduced thrust! 

 

Note: The CFM56-7B series, fitted on the 737NG family, is also flat-rated to 30C.