FTP server installation

I have a problem to "NAT" a local Filezilla FTP server.

HTTP is working:

  nat {
        rule 1 {
            description HTTP
            inbound-interface pppoe0
            inside-address {
                address 10.10.0.21
                port 80
            }
            log disable
            protocol tcp
            type destination
        }

Now if I have done the same with Port 21, but I can see nothing.

No DPT=21 in (WAN_LOCAL-default-D) and no DPT=21 in my spezial (WAN_IN-4-A) log.

What happens with Port 21?