Compressed air is taken from the HP compressor stage of a gas turbine engine and is used for various purposes: de/anti-icing, air conditioning, engine start. This compressed air driven from the compressor is known as bleed air. When selecting the bleed air ‘ON’ some of the air will be sucked out of the normal flow inside the engine and will induce some inconveniences and risks. In fact, the mass flow of air will be reduced, which will increase the engine’s overall temperature, especially the turbine temperature since less air is available for cooling.

It also increases specific fuel consumption and the EGT and the power output (or thrust) will be decreased, in other words, aircraft, and more precisely engine performance will greatly be decreased. Because of all this, it is normal to see PACKS OFF take-off, since all the available power is needed during this critical stage of the flight. Conducting a PACKS OFF take-off also has the advantage of reducing the required thrust and the noise footprint.