Advice needed on perf calc 744

[Ephedrin 418]

Hello everyone,

I would like to ask you "pros" out there something I ran into last night. I am NOT posting this in the 747 forum as I don't feel this is a support question as TOPCAT is completely off any PMDG intervention but a general understanding issue based on my incomplete knowledge. 

For my VA I wanted to do a cargo flight in the 747-400F from Frankfurt EDDF to Hong Kong VHHH. Loading the scheduled cargo and using the Simbrief calculated required fuel I found myself close to 400t. I then reduced my cargo for a MTOW takeoff and went over to TOPCAT for my usual performance calculation. Using the "optimum" option I got this. I am aware of the differences between thrust deration and assumed temperatur. So using a 20% weaker engine for a MTOW takeoff on a clean and dry runway really makes no sense to me. Is there a deeper sense behind it or is this simply wrong? Any advice would be great! 😃 When I set the thrust config to TO it gave me an assumed temp around 50°C. 

 

I remember having seen a video on YT of a DLH 747 takeoff to SFO or LAX and during the departure briefing the captain mentioned they used "full power, no derate, no assumed temp" because they were close to MTOW. I can't recall the weather situation though.

 

[downscc 1,327]

5 hours ago, Ephedrin said:

So using a 20% weaker engine for a MTOW takeoff on a clean and dry runway really makes no sense to me. Is there a deeper sense behind it or is this simply wrong? Any advice would be great! 😃 When I set the thrust config to TO it gave me an assumed temp around 50°C. 

Using DTO 50C makes sense to me too.  I have always avoided derates in the quad jets because the cases where they are called for are fairly rare, such as a wet runway where Vmcg may make it necessary to derate to lower Vmcg such that there is a V1 for the available length.  Note ATM does not lower Vmcg since full thrust is always available whereas when using derate the maximum is the derated thrust.  I'm 80% sure I've got this right but I'm not in the "pro" category you addressed this to.

I just departed KMIA rwy 09 in a B748F lightly loaded so I guessed at DTO 50C with CLB-1 and I rotated midway down the runway haha... looking forward to a TO calculator.

[Ephedrin 418]

23 minutes ago, downscc said:

such as a wet runway

yip, that's why I was wondering.. The weather was absolutely fine, light wind on the nose, no reason why I should lose control of the airplane if an outboard engine failed.. So no reason to derate. 

Assumed 50C is okay, I think, 4000m is plenty of space, V1 was actually a bit lower but that's fine. I guess I will try it out if I can really brake her down at MTOW and V1 minus a little bit When I do my calculations using TOPCAT.. 😄

Edited September 28, 2018 by Ephedrin

[captainsazzman 83]

1 hour ago, Ephedrin said:

yip, that's why I was wondering.. The weather was absolutely fine, light wind on the nose, no reason why I should lose control of the airplane if an outboard engine failed.. So no reason to derate. 

Assumed 50C is okay, I think, 4000m is plenty of space, V1 was actually a bit lower but that's fine. I guess I will try it out if I can really brake her down at MTOW and V1 minus a little bit When I do my calculations using TOPCAT.. 😄

Problem with TOPCAT and PFPX is the lack of support and updates. Not impressed. 

[iwebber 207]

8 hours ago, Ephedrin said:

So using a 20% weaker engine for a MTOW takeoff on a clean and dry runway really makes no sense to me. Is there a deeper sense behind it or is this simply wrong?

If you allowed TOPCAT to use a derate and assumed temperature then it's merely doing as it's told. The airline I fly for only uses derates for contaminated runways but I understand other airlines use it routinely. I suspect that 50C TO is about the same EPR as 31C TO2.

There's no right or wrong about it, it's just about what you want (or IRL, what the company tells you to do), with 4000m you can expect a pretty reasonable derate at most weights (especially with no obstacles in the climb out) so you pays your money and takes your choice.

Hope this helps,

Ian Webber

[PinkPony 171]

9 hours ago, Ephedrin said:

For my VA I wanted to do a cargo flight in the 747-400F from Frankfurt EDDF to Hong Kong VHHH. Loading the scheduled cargo and using the Simbrief calculated required fuel I found myself close to 400t. I then reduced my cargo for a MTOW takeoff and went over to TOPCAT for my usual performance calculation. Using the "optimum" option I got this. I am aware of the differences between thrust deration and assumed temperatur. So using a 20% weaker engine for a MTOW takeoff on a clean and dry runway really makes no sense to me. Is there a deeper sense behind it or is this simply wrong? Any advice would be great! 😃 When I set the thrust config to TO it gave me an assumed temp around 50°C. 

Assuming 396t MTOW takeoff off 07C/25C at EDDF, D-TO +40C ish is reasonable.  4000m of pavement and a few distant obstacles will allow for a good assumed temp, even at MTOW. 

Are you referring to the pilots eye LH456 video? FRA-LAX would never go out at MTOW on a passenger -400. In that video they were at 349.5t and used D-TO 55C (wet runway). At the time LH's OPT (on-board performance tool) did not calculate TO-1, TO-2. Something they began doing more recently. For the same weight off 25C on a CAVOK day, it would output D-TO 1 35C today. 

 

 

 

 

[berts 97]

29 minutes ago, iwebber said:

The airline I fly for only uses derates for contaminated runways but I understand other airlines use it routinely. I suspect that 50C TO is about the same EPR as 31C TO2.

There are two fixed derates from full thrust available in the case of the RR G-rated engines fitted to the PMDG 744, namely 10% and 20% and they relate to the selection of TO1 and TO2.  As you correctly state, Ian, TO1 is intended for use on contaminated and VMCG limited runways, whereas TO2 is intended for use on 3 engine ferry operations.

These two Derates should not be confused with the use of Reduced Thrust.  Unless the aircraft's Performance Manual states otherwise, reduced thrust should always be used whenever the actual takeoff weight (ATOW) is less than the performance limit for a given runway and the main reason for doing this is to increase engine life.  By entering the takeoff data page with the adjusted ATOW it is possible to derive the reduced thrust setting (EPR) and the Assumed Temperature at which the takeoff could be accomplished at that weight. The calculations for the  amount of reduced thrust and reference speeds are done automatically by the FMC; so provided the correct data is input the pilots shouldn't have a problem taking off! 😊

[LukasBA 1]

9 hours ago, berts said:

There are two fixed derates from full thrust available in the case of the RR G-rated engines fitted to the PMDG 744, namely 10% and 20% and they relate to the selection of TO1 and TO2.  As you correctly state, Ian, TO1 is intended for use on contaminated and VMCG limited runways, whereas TO2 is intended for use on 3 engine ferry operations.

These two Derates should not be confused with the use of Reduced Thrust.  Unless the aircraft's Performance Manual states otherwise, reduced thrust should always be used whenever the actual takeoff weight (ATOW) is less than the performance limit for a given runway and the main reason for doing this is to increase engine life.  By entering the takeoff data page with the adjusted ATOW it is possible to derive the reduced thrust setting (EPR) and the Assumed Temperature at which the takeoff could be accomplished at that weight. The calculations for the  amount of reduced thrust and reference speeds are done automatically by the FMC; so provided the correct data is input the pilots shouldn't have a problem taking off! 😊

 

On the 747 you, as you correctly state, want to reduce thrust as much as possible to increase enginelife, but there is not really any specific rules (AFAIK) to when and how to use it. 

For example: having a TO2 with 34 degrees assume could be the same as a TO1 with 48 degrees (hypothetical numbers, I don't have the EFB in front of me) the main difference, as I think you state as well is from what thrust setting the VMCG & VMCA is calculated. VMCG is calculated always from the full thrust setting, so with TO1-48, your VMCG will be calculated from full TO1, with TO2-34 it will be calculated from full TO2. 

The EFB should take VMCG limits into accounts though, and give you the right derate for the specific conditions. TO2, is good for 3 engine operations, but not exclusively. on the RR TO2 is used all the time in real life. It's just easiest. If I have a VMCG limited runway I would also use TO2 if possible other conditions taken into consideration. 

[Ephedrin 418]

Guys,

thanks a LOT for your answers.. I've waited a bit until jumping in again now, as this discussion is REALLY a great opportunity to learn a lot from your knowledge about the use of deration and assumed temps. So basically, as I tell TOPCAT to give me the "optimum" it simply does what it's been asked to: it reduces the thrust outcome as much as possible using what it has.. If I wanted a special treatment of the available options due to weather, 3 engines ops etc depending on the airline, I would have to know that myself and predefine the parameters. Thank you!

[DEHowie 269]

The reason many airlines refuse to use derate as opposed to flex/Atm is the somewhat ambiguous legal grounds.

Under FAA rules the max flex you can use is to be no more than 25% off the original max rated thrust.

So for instand you could not use a flex temp that would reduce a 50000Lb thrust engine to less than 37500Lb’s thrust.

Now if you derate to 42000Lbs thrust then flex down to 25% off that so max thrust 31500Lb’s which is way less than the rated 50000 less 25% ie 37500Lbs then depending on who you work for and there outlook on what the rated thrust of the engine is ie 50K or 42K it gets very grey.

Hence many of the worlds airlines refuse to use anything other than flex.

Lawyers would have a field day..

 

Edited September 29, 2018 by DEHowie

[berts 97]

On 9/29/2018 at 7:45 AM, LukasBA said:

 

On the 747 you, as you correctly state, want to reduce thrust as much as possible to increase enginelife, but there is not really any specific rules (AFAIK) to when and how to use it. 

For example: having a TO2 with 34 degrees assume could be the same as a TO1 with 48 degrees (hypothetical numbers, I don't have the EFB in front of me) the main difference, as I think you state as well is from what thrust setting the VMCG & VMCA is calculated. VMCG is calculated always from the full thrust setting, so with TO1-48, your VMCG will be calculated from full TO1, with TO2-34 it will be calculated from full TO2. 

The EFB should take VMCG limits into accounts though, and give you the right derate for the specific conditions. TO2, is good for 3 engine operations, but not exclusively. on the RR TO2 is used all the time in real life. It's just easiest. If I have a VMCG limited runway I would also use TO2 if possible other conditions taken into consideration. 

Lukas, 

I was referring specifically to the B744 and not the -8.  Different operators will sometimes use different operating procedures for the same aircraft type and these procedures will also change over time; especially if they are found to be necessary following an investigation into a serious incident or accident.  However, the performance information in my earlier post is accurate for the 744's I am familiar with.