Originally Posted by
J.L.Seagull
Initially I couldn't believe that such a basic question was being asked and still not answered, but I always like to have documented references ready before I post, and to my surprise, I couldn't find one now. I'm surprised and humbled.
However, I very clearly remember in my days as a trainee, it was clearly explained (with FCTM references) that the levers must be moved slowly enough, so that actual N1 (green needle) closely follows the lever position (blue circle) on the EWD.
Safety First issue 27, has some useful info.
Anecdotally, I've had dodgy engines that from 50% (stabilised) to FLX had enough of differential thrust to require full rudder deflection. Surprisingly even after writing it up, the reply was that the acceleration on both engines was "within limits".
Where those 'FADEC' engines? Because after a rapid thrust lever movement, FADEC accelerates (or decelerates) at a "N2 Dot" (or occasionally N1 Dot) schedule - i.e. the FADEC supplies the amount of fuel needed for the N2 (or N1) rotor to accelerate at the target speed. Coming off idle, there can be some other issues that come into play that can affect the engine accel rates (hence the 50% stabilized bit), but above that it would take an extremely unhealthy engine to not accel at the N2/N1 Dot schedule above 50% N1.
Pre-FADEC were governed differently - and various tolerances in the fuel control and engine to engine differences could dramatically affect the accel rates. FADEC pretty much did away with that - and unlike the 737 - the A320 never had non-FADEC engines.