Don't forget aircraft use high lift devices at take off / landing too - flaps and slats to increase the lift available at lower speeds. Consequently, the greatest lift is available during approach, when all the high lift devices are deployed to their maximum settings.
If you think of it this way:
An L1011 can weigh about 225tons at take off.
The engines can only produce about 70 tons of thrust.
To make the 225 tons go upwards, the aircraft structure would have to generate 155 tons of lift even if the engines pointed straight down as they couldn't overcome the weight of the aircraft.
Last edited by glum; 29th Oct 2012 at 23:37.
Reason: Number stoopidity...