<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by AC*SE:
Customs is an issue. Only a small handful of flights to the US are not pre-cleared--those are the flights from smaller centres (e.g. YHZ, YSJ, YYJ and YQB)
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Add YLW
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and the flights from third country carriers who don't pay for pre-clearance, (e.g. CX YVR-JFK).
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Poor, if not completely inappropriate example. A substantial number of pax (if not the vast majority) on the CX flight are travelling from HKG to JFK. Preclearance at YVR would require all pax bound for JFK to clear at YVR (since I doubt U.S. authorities would allow a mixture as the Australian authorities do). A better one would be AC for Canada-HNL flights since clearance at HNL is required now. TS pays for preclearance in YVR for HNL flights.
The vast majority of flights leave from the pre-cleared stations. You cannot mix passengers in sterile transit with US bound passengers on a pre-cleared flight--so either the entire flight is sterile, making it useless for US bound passengers, or you require transit passengers to pre-clear.
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For full implementation the obvious candidate is LHR. You could easily sterilize the far pier of T3--after all, AA and UA pretty much own it. </font>
There was a pilot scheme a few years back (or so it was announced), but it wasn't extended into a permanent system.