"We tried various price points but we weren't happy if only a small percentage of passengers were using this so decided at the beginning of October, 'Let's see what happens when [it is] free'."
Since going free there had been a four-fold increase in wi-fi use with most people using social media sites.
"We can't see what they're doing but the most popular sites are Facebook and WhatsApp."
Streaming big files to passengers' devices was difficult.
"We're some way off technically speaking and cost-wise," he said. "The mathematics of transferring data to an aircraft travelling at 500mph at 40,000ft is pretty tough."
The airline's Airbus A380s use Inmarsat satellites for the service.
Brannelley said personal devices and phones could be used when aircraft reached between 10,000 to 20,000ft.
Airlines need permission to operate any radio equipment in a country's airspace.