To qualify for ZFT one needs a number of hours on a "heavy"jet. I can't remember the exact number maybe 1000 hours on a jet say 25 tonnes.
Yep BBK's pretty much got it in one. Last time I looked the generic BA requirement to qualify for ZFT training was something like 2000+ hours total, with 1000+ on a transport category jet of >= 25 tonnes or, subject to head office approval, 1000 hours on a multi-crew turbo prop of =>50 tonnes.
May have changed of course so worth asking the question.