Takeoff thrust is calculated by a computer (or by looking up tables), since a lot of factors are considered:
- Takeoff weight
- Effective runway length
- Runway surface condition (dry / wet)
- Air temperature
"Effective runway length" is calculated from runway length, wind direction, wind speed, and runway slope.
Since we do not know the takeoff weight of the specific flights, I will make an assumption here and set it to 80,000 KG, which is the highest figure I have in the reference table.
At a temperature of 20 degrees Celsius, takeoff weight of 80 tons, effective runway length of 3000 meters, the speeds from the table are: V1 at 145, Vr at 153 and V2 at 155. This is the takeoff with max thrust, which according to Wikipedia, each engine is delivering 120 kN of thrust. The thrust to weight ratio would be 0.15. Converting the variables to SI units, I calculated that the time to reach V1 to be exactly 10 seconds.
This calculation is, obviously, far from reality, as it assumed:
- Thrust application is instantaneous.
- The aircraft maintained a constant 1.5 ms-2 acceleration. In reality, aerodynamic drag will come into play as the aircraft picks up speed.
- Max thrust is used on takeoff, which is rare in operations.
Moving on to shorter runways, the shortest runway length in the table I have is 2250 meters. It states that at this length, the maximum takeoff weight is 74.2 tons, and the limitations which resulted in this figure is runway length and Vmu(Minimum Unstick speed), the lowest speed at and above which the aircraft can safely lift off the ground with sufficient control surface authority and without a tail strike.
From the few data points I have, it seems that on a 1500 meters runway, the maximum takeoff weight would be around 63~65 tons.
So to answer your question:
- If they used the same thrust, then the takeoff weight on the short runway is around 78% of that on the long runway.
- If they kept the same weight, that would mean the pilots have extra thrust at their disposal on a longer runway. They can choose a derated takeoff thrust from the options -4%, -8%, -12%, -16%, -20% and -24%.
Source: A318/A319/A320/A321 Performance Training Manual