blueloo
Minimum thrust/Maximum reduction depends on several factors including aircraft and engine combination, company preference and regulating authority.
Using one operator's B747-400 for example:-
Fixed Derates of
10% and 20% are available which may then be subject to Assumed Temperature Derates of
up to 25%. This gives a maximum reduction of
up to 40% from Rated Thrust.
Using 100,000 lb as Rated Thrust makes the sums simple for this example:-
Max Thrust. 100,000 - up to 25% Assumed Temp Reduction = 75,000
10% Derate . 90,000 - up to 25% Assumed Temp Reduction = 67,500
20% Derate . 80,000 - up to 25% Assumed Temp Reduction = 60,000
Oh for a B747-400 with 4 x 100,000 lbs of thrust ! like
off a shovel
You will understand that even on a 4km runway, any significant thrust reduction from that required will fail to accelerate the aircraft sufficiently for normal flight.