Full T/O power on take off!
Join Date: Dec 2011
Location: laptop city
Posts: 23
Likes: 0
Received 0 Likes
on
0 Posts
speeds
Guys as to the eventually of losing airplane control during an assumed temperature takeoff if increasing power in case of single engine .
Why not while doing the assume use the speed for the full rated take off ? i do not know about bigger planes but on 737 claasics speed differences are usely 5 kts at max
Why not while doing the assume use the speed for the full rated take off ? i do not know about bigger planes but on 737 claasics speed differences are usely 5 kts at max
Last edited by mamad; 11th Dec 2012 at 21:14.
Join Date: Apr 2008
Location: nowhere
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
O.K.....I guess I will give it a go.
It appears that different aircraft types may have different procedures and limitations. Perhaps even the same type with different engines on the same aircraft type or certifying authorities.
On the 744 with CF-6 engines we have three choices of thrust ratings. The first is TO which is full rated thrust. We also have TO-1 and TO-2 which are derated thrust settings in the order of 10% and 20% respectively.
In order to save wear and tear on the engines, it is company policy for the pilot to use the maximum derate possible unless circumstances dictate otherwise such as reported windshear, visibility below a certain amount(or perhaps some other reasons such as the captain for whatever reason decides to use full thrust) in which case we use full rated thrust. Of course runway length, weight and other factors determine how much of a derate we can actually take.
When a decision is made to use a derated thrust setting, we are choosing to change the maximum certified thrust for takeoff for that aircraft. One of the consequences of doing this is that VMCG changes with a change in thrust. If VMCG changes then min V1 changes. So now if we are using a lower V1 we get an engine failure at this lower V1 speed, firewalling our EEC overboost protected engines to max will have a serious effect on directional control.
So if we are rolling down the runway with all things normal and for some reason you decide to increase thrust, your V1 speed is not accurate anymore. Of course if you have a vehicle crossing the runway ahead of you or a windshear or discovered you used the wrong takeoff data and need the thrust, of course you will use the extra thrust. The chance of an engine failure at this time is almost zero. Trying to get full rated thrust by changing the FMC rating on the roll would seem to be a bad idea. It would be distracting, time consumung and I wonder if your V-speeds would disappear.
Of course there is also the Assumed Temperature Method(ATM) of thrust reduction which is also designed to save engine wear and tear. Like derated thrust, it can be used alone. Or it can be used together with the derate(which is actually the company policy). A maximum of 25% reduction in thrust is available from the rated or derated thrust you have chosen to use. TO-2 is a maximum certified rating that is already 20% below TO thrust so if you use that combined with ATM which allows another 25% reduction, you have quite a large overall thrust reduction from full TO thrust.
But....unlike a derated thrust reduction, using ATM does not change the VMCG speed for this kind of thrust reduction. You use the same VMCG speed that your chosen rated thrust setting would use with no ATM thrust reduction. Therefore if using only the ATM method, you can advance the power fully forward(with overboost protection from the EEC's) and your V1 speed is the same and you are flying within certified limits. ATM has some other restrictions such as not being allowed on contaminated runways and one company I worked for did not allow it after de-icing although another company does. I seem to remember the old company not allowing for tailwind ops but haven't seen that restriction on the present machine. What is your policy?
I think the ATM method was the initial method of of thrust reduction. We did not have derates on the JT-8D aircraft I flew. So then why did these derates come into play instead of just allowing more assumed temperature beyond 25% thrust reduction. I believe that using a 20% derate thrust reduction with a lower V1 as compared to a 20% ATM thrust reduction with the same V1 will allow a better payload. Of course the airlines are happy with the derate because now they can have really large thrust reductions when the two are combined.
I have to admit that I am not sure now what happens on the 744 if you hit TO/GA a second time on the ground. I was under the assumption that there was no effect. Have to look into it.
It appears that different aircraft types may have different procedures and limitations. Perhaps even the same type with different engines on the same aircraft type or certifying authorities.
On the 744 with CF-6 engines we have three choices of thrust ratings. The first is TO which is full rated thrust. We also have TO-1 and TO-2 which are derated thrust settings in the order of 10% and 20% respectively.
In order to save wear and tear on the engines, it is company policy for the pilot to use the maximum derate possible unless circumstances dictate otherwise such as reported windshear, visibility below a certain amount(or perhaps some other reasons such as the captain for whatever reason decides to use full thrust) in which case we use full rated thrust. Of course runway length, weight and other factors determine how much of a derate we can actually take.
When a decision is made to use a derated thrust setting, we are choosing to change the maximum certified thrust for takeoff for that aircraft. One of the consequences of doing this is that VMCG changes with a change in thrust. If VMCG changes then min V1 changes. So now if we are using a lower V1 we get an engine failure at this lower V1 speed, firewalling our EEC overboost protected engines to max will have a serious effect on directional control.
So if we are rolling down the runway with all things normal and for some reason you decide to increase thrust, your V1 speed is not accurate anymore. Of course if you have a vehicle crossing the runway ahead of you or a windshear or discovered you used the wrong takeoff data and need the thrust, of course you will use the extra thrust. The chance of an engine failure at this time is almost zero. Trying to get full rated thrust by changing the FMC rating on the roll would seem to be a bad idea. It would be distracting, time consumung and I wonder if your V-speeds would disappear.
Of course there is also the Assumed Temperature Method(ATM) of thrust reduction which is also designed to save engine wear and tear. Like derated thrust, it can be used alone. Or it can be used together with the derate(which is actually the company policy). A maximum of 25% reduction in thrust is available from the rated or derated thrust you have chosen to use. TO-2 is a maximum certified rating that is already 20% below TO thrust so if you use that combined with ATM which allows another 25% reduction, you have quite a large overall thrust reduction from full TO thrust.
But....unlike a derated thrust reduction, using ATM does not change the VMCG speed for this kind of thrust reduction. You use the same VMCG speed that your chosen rated thrust setting would use with no ATM thrust reduction. Therefore if using only the ATM method, you can advance the power fully forward(with overboost protection from the EEC's) and your V1 speed is the same and you are flying within certified limits. ATM has some other restrictions such as not being allowed on contaminated runways and one company I worked for did not allow it after de-icing although another company does. I seem to remember the old company not allowing for tailwind ops but haven't seen that restriction on the present machine. What is your policy?
I think the ATM method was the initial method of of thrust reduction. We did not have derates on the JT-8D aircraft I flew. So then why did these derates come into play instead of just allowing more assumed temperature beyond 25% thrust reduction. I believe that using a 20% derate thrust reduction with a lower V1 as compared to a 20% ATM thrust reduction with the same V1 will allow a better payload. Of course the airlines are happy with the derate because now they can have really large thrust reductions when the two are combined.
I have to admit that I am not sure now what happens on the 744 if you hit TO/GA a second time on the ground. I was under the assumption that there was no effect. Have to look into it.
Last edited by JammedStab; 24th Dec 2012 at 12:00.
