Thrust on during flare...Q for AIRBUS test pilots...

[737Jock]

Insomniac, There is no problem.

I'm not saying that thrust for speed is incorrect, it works. But its still a simplified version of what actually happens.

Thrust is energy that can be added or removed from the airframe by pushing the thrust levers foward or aft. We use it to balance the forces on the aircraft.
If the aircraft is flying a stable path, vertical speed and airspeed do not change.
If in this stable flightpath I then add thrust (add energy) either the airspeed or the vertical speed will change until a new equilibrium (a new stable flightpath) is found. The pitch determines if the airspeed changes or if the vertical speed changes. Obviously this new equilibrium (flightpath) is not what we desire, its either too fast or too shallow.

There are also external factors that can increase or decrease the energy of the aircraft. An updraft will add energy to the airframe, if we don't do anything the pitch attitude will either convert this extra energy into airspeed (too fast) or into vertical speed (too shallow). This is obviously undesired so we need to restore our original stable flightpath. We can only do this by reducing the energy of the aircraft, by the same amount that was added by the updraft. Therefore we reduce thrust.

So when I encounter an updraft, I reduce thrust as I know I will need to decrease the energy of the aircraft to maintain the stable path.
If I encounter a downdraft I add thrust as I know I will need to invrease the energy of the aircraft to maintain the stable path.

Any examples where I'm still slowing down or speeding up are not stable flightpaths. Therefore I can utilise the energy in a different way.