When wing anti-ice is selected, the N1 or EPR limit is automatically reduced, and the idle N1 or EPR is automatically increased. What does this statement mean ???

  • $\begingroup$ I understand about descent but what about in flight cruise $\endgroup$ Mar 5, 2021 at 9:35

1 Answer 1


When engine or wing anti-ice are needed, the required amount of bleed air increases. In order to provide enough bleed air at the required pressure, the engine idle speed must be increased. That way, the compressor runs faster, which provides more high pressure air to the bleed valves. Note that this will result in more idle thrust as well, which can have an impact on descent performance as discussed for the Boeing 737 here: What is the impact of thermal Anti-Ice on descent performance?

For the A320, this is already taken into account for the descent calculation:

The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary.

(Airbus A320 FCTM - Normal Operations - Descent)

When more bleed air is taken out of the compressor stage of the engine, the EGT (exhaust gas temperature) will increase because more fuel flow is required to maintain the same N1 speed. This implies that the N1 or EPR limits must be decreased in order to protect the engine, meaning the lower limit will result in the same EGT as before.

  • 1
    $\begingroup$ When more bleed is taken out, the fuel control system has to schedule more fuel to the combustor to ensure the same level of performance (i.e. thrust) is maintained, not because of less air reaches the combustion chamber (that will not happen). An engine with increased bleed extraction or increased power (electrical, for electric anti-ice systems) off-take results in an increased EGT and increased spool speed of scheduling extra fuel, this will reduce the margin to the max allowable EGT. $\endgroup$
    – 0scar
    Mar 5, 2021 at 13:27
  • 1
    $\begingroup$ @0scar You're right, thanks for the comment. I updated the answer. Feel free to edit if it's not quite right yet. $\endgroup$
    – Bianfable
    Mar 5, 2021 at 13:33

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