This article dates from 2016, but I'm sure things have only gotten worse since then.
The NSA and the GCHQ are able to intercept data from passengers traveling on board commercial aircrafts.
. . .
At the end of 2012, in a presentation, the Government Communications Headquarters (GCHQ), the British equivalent of the NSA ... disclosed a ‘top secret strap’, the term used for the highest level of classification, the content of the Southwinds programme, set up to gather all the activity, voices and data, metadata and content of the calls on board aircraft. The zone was still restricted to the regions covered by the Inmarsat satellites: Europe, the Middle-East and Africa.
The data collection was done ‘practically in real time’ and an aircraft could be followed every two minutes. To spy on a telephone, all that was required was that the aircraft be cruising at an altitude of 10,000 feet. As the signal transited through the satellite, the intercept technique was done by secret aerial stations on the ground. The simple fact that the telephone was switched on was enough to give its position, the intercept could then be crossed with the list of passengers registered and the number of the aircraft to assign a name to the user of the smartphone. The GCHQ could even, remotely, interfere with the working of the phone; as a result the user was forced to redial using his or her access codes. The British Intelligence services intercepted the identification codes (login and password) at the same time.
. . .
Today, approximately one hundred companies permit in-flight use of telephone. ‘Customers now consider it normal, even necessary, to remain connected in flight’ stated the Air France management. The aviation security authorities have all approved the use of GSMs on board aircraft and the experts estimate that the years 2016, 2017 and 2018 will go down in history as the years of the in-flight mobile phone, in particular with the long-term installation of in-flight Wi-Fi.
This will further extend the scope of espionage by aiming at ‘several hundreds of thousands of people’ to be closely monitored, according to the NSA projections. This implies a population which goes far beyond the targets involving terrorism alone. The political or economic surveillance of passengers in Business or in First Class on long-haul flights is of interest to many more services.
There's more at the link.
The potential for state-sponsored industrial espionage is obvious. I wonder how many business visitors to China have had their negotiating positions revealed by such techniques, to be passed on to their prospective Chinese partner companies so that they can negotiate from a position of strength? I don't know whether US or British government entities would do the same, but I wouldn't be surprised to find at least some leakage taking place.
I've spoken many times in these pages about our loss of privacy, and how angry it makes me. However, I've come to realize that I'm simply a dinosaur in my attitudes to that sort of thing. I don't intend to change, but I'm definitely out of step with our world, particularly younger people who seem to have no problem at all publishing the most intimate details about themselves in public. Privacy, it seems, is now conspicuous more by its absence than by its reality.
This is, of course, yet another reason to avoid air travel unless absolutely necessary.