Does an eTA apply for private flights to Canada from the UK? (Via Greenland).
The Entry requirements by country page has this to say
The following travellers need an Electronic Travel Authorization (eTA) to board their flight to Canada. However, these travellers do not need an eTA if entering by land or sea.
It doesn't make a distinction between scheduled or chartered flights.
If you go to the visa tool and enter your country, you will see this:
Fly to or transit through a Canadian airport:
You need to apply online for an Electronic Travel Authorization (eTA).
depending on your nationality and again nothing else. Same on the eTA page
visa-exempt foreign nationals who fly to, or transit through a Canadian airport, need an Electronic Travel Authorization (eTA)
This is quite clear in my opinion.
Finally, common sense applies: can you imagine the outrage if people rich enough to charter a plane were exempted? Osama bin Laden was estimated to have a net worth of $50 million. Also, what about tour operators who charter a plane for a group of people...? The loopholes would be unending.