Originally Posted by
Mad (Flt) Scientist
As an illustration, here's part of an AFM chart for Takeoff Field Length, showing how temperature and altitude interrelate - you enter the chart on the LHS with the ambient temp, read across to the pressure altitude, then downwards into another part of the chart (for weight, in fact). The further to the right you are when you go 'down', the longer the TOFL.
The distinct non-linearity on the chart lines is associated with engine flat-rating limits coming into effect - you can see that above the flat rating, the effect is roughly one major division of distance per 5 deg C, and below the flat rating its more like 20-30 deg C for the same effect.
So, the engine is flat rated at low pressure altitude and low temperature. If the engine were to develop more than full rated thrust in low/cold conditions, the pressure and force might break something... If an airplane were to operate, say, from Yakutsk or Klondike or Yellowknife in midwinter, airport elevation near sea level, high pressure winter anticyclone and, say, -50 Celsius meaning ISA-65 conditions, the thrust would, because of flat rating, be exactly the same as under ISA conditions... probably because the engine N1 would be slowed down to prevent excessive thrust in dense air. Thus, the airplane would reach a given GS in exact same time and runway distance - but the wings are not and cannot be "flat rated" and therefore the plane would rotate at the same IAS, meaning at lower TAS and GS and after shorter time and runway distance than in warm weather. Correct?