CaptainSandl
I was purposely vague as we only operate one model of Airbus, the A300, I was hoping that someone would jump in and explain the real logic. Anyway this is from the A300 FCOM. (Unfortunately my message will be without diagrams.)
Explanation of Takeoff Performance Calculation.
The performance calculation is optimized. This means that considering the runway, the obstacles (if any) and the environmental conditions, the maximum takeoff weight, associated with the optimum flap setting is the resulting parameter. Conversely, a max TO temperature will result if the chart is entered with the actual takeoff weight.
Considering a given runway and wind, given temperature and pressure conditions, given flap setting, the max weight that can be lifted varies with the V2/Vs ratio.
(Mutt) Draw a graph, weight increasing on the left side. V2/Vsmin and V2/Vsmax along the bottom. Within the graph you have two lines, an increasing “2nd segment limitation” as the speed increases and a decreasing “Runway Limitation” as the speed increases. The intersection of these two lines gives you the “Optimum V2/Vs” on the bottom and “Max takeoff weight” on the left. Still with me? (end Mutt)
As a general rule, the optimization leads to balance ASD and TOD, which means having a single V1. However, on very long runways, the airplane being limited by the 2nd segment climb gradient, it is possible to have a wide range of of V1’s, V1min being limited by TOD and V1max by ASD.
So what does this mean to a crewmember? Well all that he sees is a takeoff chart that looks something like this…
************************************************************ *******************
* S15/F00 * Default * 36 *
************************************************************ *******************
LEVATION= 0.0(FT) CODES LIMITATIONS : A300-620/AA/PW-7R4H1 :
:T.O.R.A. = 3000.0(M ) 1-STRUCTURE 5-TYRE SPEED : FAR BB77 11-Aug-01 :
:A.S.D.A. = 3000.0(M ) 2-2ND-SEGMENT 6-BRAKE ENERGY : :
:T.O.D.A. = 3000.0(M ) 3-RUNWAY 7-RWY 2 ENGINES: :
:SLOPE = 0.00( %) 4-OBSTACLE 8-FINAL T.O. : QNH=1013.25 (MB):
:-----------------------------------------------------: :
: : AIR COND. OFF :
: : ANTI-ICING OFF :
:----------------:------------------------------------: :
:TREF = 30 DC : WIND (KT) : WITHOUT REVERSE :
:TMAX = 55 DC : MTOW (KG) CODES : :
:-------: : MEAN V1 - VR - V2 (IAS.KT) : DRY RUNWAY :
: O A T :--------:------------------------------------:-----------------------:
: DC : 0 :
:-------:-------------:-----------:
: 30.0 : 165000 1-1 :
: : 158 164 167 :
:-------:-------------:----
: 35.0 : 165000 1-1 :
: : 159 165 167 :
:-------:-------------:----
: 40.0 : 163574 3-3 :
: : 159 164 167 :
:-------:-------------:--------
This was created using the normal Optimum defaults for Airbus. If I were to use a fixed V1/VR ratio as Boeing does, we would get this:
************************************************************ *******************
* S15/F00 * Default * 36 *
************************************************************ *******************
:----------------:------------------------------------: :
:TREF = 30 DC : WIND (KT) : WITHOUT REVERSE :
:TMAX = 55 DC : MTOW (KG) CODES : :
:-------: : MEAN V1 - VR - V2 (IAS.KT) : DRY RUNWAY :
: O A T :--------:------------------------------------:-----------------------:
: DC : 0 :
:-------:-------------:-------------:
: 30.0 : 163532 3-3 :
: : 150 164 167 :
:-------:-------------:-------------:
: 35.0 : 159295 3-3 :
: : 149 162 165 :
:-------:-------------:-------------:-
: 40.0 : 155152 3-3 :
: : 147 160 163 :
:-------:-------------:-------------:
Apologies for the formatting. Look at the 35° temperature. You will see that using optimum V-speeds allows you to takeoff with 165,000 kgs, where using a fixed V1/VR ratio does not.
So from this long winded answer, you will hopefully see that Airbus allows you the option to do improved climb all the time. Boeing gives you a fixed ratio with improved climb corrections.
Now I’m going off to hide before my good friend
John_Tullamarine jumps on me for using more physical runway than technically required.
Finally, I don’t work with the Airbus, what’s written above is my understanding and may not be completely correct. If anyone is able to correct me, I would be much obliged. This data is also based on FAR’s
Mutt