Thanks for all the answers, a lot more than I asked. I did realize the engines increase their thrust gradually and the aircraft already accelerates during these 7 seconds, I simply claimed that (in a simplified recti-linear flight) after these 7 seconds, the aircraft accelerated further as a new equilibrium had not yet been reached.

But thanks for all the details .

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Oh - and if you want to be nitpicky, the new equilibrium will involve a higher airspeed, as it will be at a higher altitude with thinner air, where to create the same lift, a higher speed is required

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Oh - and if you want to be nitpicky, the new equilibrium will involve a higher airspeed, as it will be at a higher altitude with thinner air, where to create the same lift, a higher speed is required

Not necessarily. It might be flying at a higher angle of attack at the same airspeed. As a plane climbs, the amount of power the engine can produce decreases along with atmospheric density. At some point, the power required for flight will equal the power available from the engines. At that point, the plane can not climb any higher.