The 777 engines are simply massive, with the GE-90’s generating upto 115,000lbs of thrust, per side.

When cleared for take-off we run the engines to 55% N1 and allow them to stabilise. They are very large and thrust takes several seconds to build, especially from idle power. As well as being kinder to the engines to allow them to spool (rather than opening them up to the maximum immediately) if one engine spools slightly faster than the other, any asymmetric thrust will be balanced before opening them up to take off thrust. This is important as at low speeds corrections with the rudder are aerodynamically ineffective.

Additionally, stabilising the engines gives the pilots a chance to examine engine parameters at a fair thrust loading, to establish that they are behaving normally and it is safe to continue. In particular we are checking there are no surges, or disturbances of airflow which would indicate unseen compressor or turbine damage and that fuelling is normal.

This standard procedure also caters for a wide variety of conditions, for example icing or rain and high winds.

In some aircraft, other systems become active after the power levers reach a certain position: for example on the DHC8-Q400 the auto-feather system arms after a PLA (power lever angle) of around 50 degrees, so a pause to establish this has happened (and a call from Pilot Monitoring, ‘stable, auto-feather armed’) is also made.