Ok, but basically we dont know:

1) if most airlines use Assumed Temperature or TO1/TO2 or a combination of both.

2) what the advantage of one method over the other is (we only know the disadvantage of fixed derate when you add full thrust after an engine failure. What we dont know, is why one would even use TO1 or TO2?)

3) how much (percentage?) thrust reduction you get if you select TO1 or TO2?

1. Most if not all use either the assumed temp method or a Derate. Two operators I know of combine these.

 

2. I have already explained why an operator would use a derate, engines may be leased.

 

3. TO1 10%. TO2 20% (this may vary with operators)

 

Now here is an advantage of a derate over the assumed temp method, you cannot depart on a contaminated runway using flex or assumed temp, however you can use a derate and still save engine life.

When using a fixed derate, the new thrust rating becomes the maximum thrust available. Since you have less thrust, in an engine failure, the yawing moment due to thrust asymmetry is less, therefore you don't need as much rudder authority. This means that your Vmcg/Vmca (minimum control speed) is lowered. In some situations, you have to increase your takeoff speeds due to a high Vmca which means a longer take-off roll. With a fixed derate, your Vmca is lowered so you don't need such a long take-off roll and you can depart with a higher take-off weight.

 

Now you might be thinking that using assumed temperature, you would also have lower thrust asymmetry in an engine failure. However, the rated thrust is still at 100% and you can press TOGA again to get the rated thrust. Hence, assumed temperature does not give you any benefits in terms of Vmca.

 

In fact, if you exceeded the fixed derate in a Vmca limited situation by manually advancing the thrust levers, you could potentially end up in a situation where you do not have sufficient rudder authority to counteract the thrust asymmetry.

 

The other benefit of the fixed derates is that you can use them in any situation. Assumed temperature is not permitted in some situations, such as a contaminated runway (snow, ice, slush, etc.) Assumed temperature is only permitted on dry and wet runways.

 

In response to the question as to whether fixed derates are "safe": you don't know if any  take-off will be safe until you perform your take-off analysis, either manually using charts/tables or with software.

 

In response to whether derates are "easier" to do than assumed temperature: it is exactly the same process. Even using both derate and assumed temperature together is the same process. Boeing publish a set of charts for each of TO, TO1 and TO2 take-off thrust ratings (in the FPPM; the FCOM only offers a basic set of charts/tables). From these charts, you can either work forwards to find the maximum weight you can safely take-off with, or you can use your actual take-off weight to work backwards to find your assumed temperature. If you ever find that your actual weight is above the limit weight or that the ambient temperature is above the assumed temperature, then it is not safe to take-off.

1. Most if not all use either the assumed temp method or a Derate. Two operators I know of combine these.

2. I have already explained why an operator would use a derate, engines may be leased.

3. TO1 10%. TO2 20% (this may vary with operators)

Now here is an advantage of a derate over the assumed temp method, you cannot depart on a contaminated runway using flex or assumed temp, however you can use a derate and still save engine life.

.

 

Very good point that last one!

Perfect explanation by David Zhong.

 

Regards

.

And more very good points!

 

Thx to both.

Step 3: It’s now time to find our assumed temperature. For that we have 2 sources. We can either use nice, pretty and easy tables in the FCOM, or a nice graph that we can plot on from the FPPM. For the purposes of this tutorial, I will be using the tables from the FCOM (as most people don’t have access to the FPPM).

 

 

Any questions please ask away!

 

What exactly is Pressure Altitude? Is it just field elevation (above sea level) or is there more about it?

Thanks!

What exactly is Pressure Altitude? Is it just field elevation (above sea level) or is there more about it?

Thanks!

No. Pressure Altitude is what the altimeter would display when you set it to the standard barometric setting (1013hPa/29.92inHg).

 

To correct your field elevation to pressure altitude;

 

[a] Obtain current QNH at airport (For this example, we will say 1021hPa and FE = 1000ft).

Find the difference (so 1021-1013 = 8hPa).

[c] 1hPa = 30ft. Since QNH is higher than standard, pressure elevation will be less than field elevation.

[d] Pressure Altitude = Field Elevation - (30ft x hPa differences).

[e] In this example, Pressure Altitude = 1000 - (30x8)

                                                           = 976ft.

 

Most the time the pressure altitude won't be a huge difference to the field elevation and the same table/chart in the manual will be used.

Thanks to James for this amazing thread, and to all of you others contributing, I learned alot and will try a scenario with the tables in my manuals.

Thanks indeed!

Thanks.  A suggestion when all the dust settles, could we see this guide publish to AVSim files?  Much easier than a thread IMHO.

No. Pressure Altitude is what the altimeter would display when you set it to the standard barometric setting (1013hPa/29.92inHg).

 

To correct your field elevation to pressure altitude;

 

[a] Obtain current QNH at airport (For this example, we will say 1021hPa and FE = 1000ft).

Find the difference (so 1021-1013 = 8hPa).

[c] 1hPa = 30ft. Since QNH is higher than standard, pressure elevation will be less than field elevation.

[d] Pressure Altitude = Field Elevation - (30ft x hPa differences).

[e] In this example, Pressure Altitude = 1000 - (30x8)

                                                           = 976ft.

 

Most the time the pressure altitude won't be a huge difference to the field elevation and the same table/chart in the manual will be used.

Could you not just dial in 1013 in the altimeter and that would result in pressure altitude?? More easy I would think than doing head algorithms.

Could you not just dial in 1013 in the altimeter and that would result in pressure altitude?? More easy I would think than doing head algorithms.

Yes, that would work to.

 

I have been usually calculating my TO PERF during flight-planning just for the sakes of saving time. Unless the winds are forecasted to change rapidly in the TAF, the conditions will usually be stable within the 45minutes that I flight planned within.

Hey.
Would you be intersted in posting the same tables for the 777-300ER?

I can only find some tables in the FCOM for the 300ER and only with flaps 15. Maybe Im looking at the wrong page?

Dear Machine, thankyou for your tutorial it was very helping for me since i just started learning about (777) performances. But i have a question: why is your first step to calculate the full rated limit weight "backwards" from the to1 takeoff weights table having the ETOW... i would do so: first i calculate the mtow for the runway having runway lenght (with wind adjustment), airport OAT and pressure altitude. Then i look at the to1 takeoff weights table for the corresponding max to1 weight. Then, if my ETOW is lower, i look "backwards" in the takeoff field limit table (from the FPPM) having ETOW, runway lenght and airport pressure altitude for the assumed temperature........

i may be wrong, but i think that if you look backwards at the to1 takeoff weights table you increase the weight you want to fly away instead of lowering it with a derated thrust......

thank you for your answer and sorry for my bad english

 

S

Thank you for providing a detail explanation on how to calculate takeoff speed. However, the photos/screenshot you provided, I can't see them. Could you repost those photos again? Also, where are you getting the charts from?? Thanks.