PARALLEL APCHS. VISUAL SEPARATION DIFFICULT TO MAINTAIN UNTIL TOUCHDOWN.

Narrative

ACCEPTED A VISUAL APCH TO RWY 28R SFO AT ABOUT 8 MI. APCH POINTED OUT TFC ON RWY 28L AND GAVE US A REDUCTION IN SPD TO 170 KTS. WE SLOWED TO MINIMUM APCH SPD IN ATTEMPT TO REMAIN BEHIND AND VISUAL ON TFC ON RWY 28L AND IT APPEARED WE WOULD BE ABLE TO DO THAT BUT AT ABOUT 100 FT AGL WE PULLED ALONG SIDE THE TFC ON RWY 28L AND WHILE I WAS ABLE TO MAINTAIN VISUAL ON TFC (FROM L SEAT; FO FLYING THE ACFT) AT THE LAST MIN ON FINAL WE WERE NOT ABLE TO REMAIN BEHIND THE ACFT ON RWY 28L. IT SHOULD BE NOTED THAT THE OTHER ACFT WAS A COMMUTER TYPE WITH A HIGHER PROFILE GLIDE PATH AND LNDG LONGER THAN WE WERE; THIS HELPED US MAINTAIN VISUAL LONGER THAN WE COULD HAVE NORMALLY. IT IS VERY DIFFICULT TO MAINTAIN VISUAL SEPARATION AND JUDGE CLOSURE RATES ON THIS APCH; PARTICULARLY WITH DIFFERENT ACFT TYPES AND DIFFERENT PROFILES TO THE RWYS. UNDER THESE CIRCUMSTANCES I WILL BE RELUCTANT TO ACCEPT A VISUAL APCH AGAIN. CONSIDERING THE DIFFERENT SPDS WE SHOULD HAVE HAD MORE SEPARATION WHEN CLRED FOR THE APCH AND I SHOULD HAVE RECOGNIZED THAT EARLIER.