Hello M K Somanathan, It might be the reason If a pilot lands air craft within 50% of the run way then that might cause only 3.4 accidents per million landing. As we know 3.4 DPMO is achieved at six sigma quality, We have to look at the technical specifications of safely landing the aircraft. How closely the pilot meet those specification every time, that indicates his sigma level. Here I considered that only impacting factor is runway. USL=55% length of Run Way (Beyond that for sure all will go to heaven) Lower the safer landing, Say less than 50% With Mean of 40% and Std. Deviation of 0.033333333 You get Zst=6 and DPMO=3.4