@ fisbangwollop: "Selcal only works on HF..." that's not true, onboard decoder works as well with VHF (mutt is right).
"...Try listening to the RF noise on HF during a 7 hour flight and you would be driven to drink..!!!..." Yes, onboard HF don't have squelch but it seems to be an aviation industry standard only, not a standard for SSB operation: "voice squelch" is intensively used on the marine radios and others...