a "thrust reverser" is a part of the engine of a plane. it changes the flow of air through the engine so that it ends up trying to push the plane backwards instead of forwards. the first thrust reversers on the 737 were not very good. they were said to lift the aircraft off the runway when they were used. this meant that the wheels were not being pushed down onto the ground very much, which made the brakes less useful. in 1968, boeing improved the thrust reverser. this improvement was put onto all aircraft after march 1969. aircraft which were made before the improvement could have it installed. boeing also improved the flaps. this improvement allowed the flaps to be used more during takeoffs and landing. all these changes meant that the 737 could hold more cargo and fly further. in may 1971, after the 135th aircraft was built, all of these improvements, as well as better engines and more fuel, were put into the 737-200. this gave it 15% more payload and range than the first 737-200s. this type of 737-200 became known as the 737-200 advanced.