Effect of Reverse Thrust
Stilton is absolutely correct. In the case of the Qantas accident at Bangkok the use of FULL REVERSE in the prevailing conditions would have resulted in a reduction in landing distance of 422 metres, even with only 25 Flap. Had 30 Flap and FULL REVERSE been used the landing distance would have been reduced by 575 metres. As it was braking did not commence until 1625 metres past the threshold on a 3500 metre runway.
Threshold crossing speed was 168 KIAS (Vref + 19 knots) and threshold crossing height was 32 feet higher than planned. The increased speed and height over the threshold added 512 metres to the landing distance which would have been required otherwise. FULL REVERSE could have made the difference between remaining on the runway and exiting the end at 88 knots as happened. As Stilton correctly says, REVERSE does not have the same effect on a dry runway as on a wet or, in this case, a contaminated runway. Manufacturers do not go to the trouble and expense of fitting up Reverse Thrust systems for no reason.