Interesting question. The ATC orders mention only two reasons that controllers have to say "at your own risk":
- The pilot "insists" on using a closed or unsafe runway (3-3-2)
- A helicopter takeoff or landing uses a non-movement area (3-11-2, 3-11-6)
Obviously Solar Impulse 2 isn't a helicopter, so that leaves the closed or unsafe runway reason. I don't know for sure why the tower used the "own risk" phraseology - it would be great to hear from an actual controller - but some possibilities are:
- The runway was marked as closed (for Solar Impulse 2) on the tower controller's displays, and he said "at own risk" more or less out of habit
- There were vehicles on the runway, therefore it was "unsafe"
- Solar Impulse 2 is such an unusual aircraft that the controller felt he couldn't contribute anything to the safety of the takeoff, so he was simply saying "go for it, I can't do much to help you"
Of course, the problem with all those suggestions is that they should apply at all the airports, not just half of them. But I did find this quote from a controller on the AOPA forums (members only):
Although it's normal for helicopters, I issued this "departure will be
at your own risk" to an aircraft once. The runway was open, he wasn't
departing from a taxiway or anything like that. The problem was fog,
where I couldn't see the runway from the tower, and therefore I
couldn't determine whether the runway was actually "clear" for his
I don't know if this is the right response in this situation or not,
but I was a pretty new controller at the time, hadn't faced this
situation before, and figured the "at your own risk" was appropriate.
That could have applied to the Solar Impulse situation: with other vehicles and personnel on the runway, the controller can't say it's "clear". And just like the controller on the forum, perhaps some of the controllers handling Solar Impulse 2 felt that the non-standard wording was appropriate for a very unusual situation. The other controllers who did issue a takeoff clearance presumably felt differently.