It doesn't seem to matter that the phone is powered down, as shown by this incident on a South West airlines flight where the Note 7 caught fire after it was powered down.
Here's an excerpt from an interview by The Verge on this incident [1]:
"Green said that he had powered down the phone as requested by the flight crew and put it in his pocket when it began smoking."
Here's an excerpt from an interview by The Verge on this incident [1]:
"Green said that he had powered down the phone as requested by the flight crew and put it in his pocket when it began smoking."
[1]: http://www.theverge.com/2016/10/5/13175000/samsung-galaxy-no...