When is FTP over port 21 secure?

Port 21 is the control port for the original and unsecure FTP protocol.   Credentials and payload are exchanged over the network unencrypted.

However file transfer over port 21 can be made secure with the newer FTPS protocol. FTPS or  FTP over SSL/TLS can enable encryption of both the Control and Data Connections.  Port 990 and 889 are defined for 'implicit' FTPS whereby both Control and Data are encrypted: port 990 for Control, 889 for Data.   If the client connects on port 990 the assumption is that SSL/TLS encryption will be performed i.e. the port number signifies security. 

However 'explicit' FTPS runs over port 21 (the port used by original and unsecure FTP protocol).  FTP clients who connect on port 21 and require encryption, must send AUTH SSL or AUTH TLS to the server. If the server complies it will take steps to complete an SSL/TLS handshake.   Selective use of encryption enables transfers to be secured according to need, with benefit of greater speed for unencrypted exchanges.