A few years ago, in a former life as a mountaineer, I was in a group that had arranged for a plane pick up from a glacier at the end of the trip. We had arranged a lat/long for our rendezvous point, but when we got there we found the area full of cravasses and unsuitable for a plane landing. We moved to the other side of the glacier instead, but when the plane came it was looking for us on the wrong side. The glacier was over 10 miles wide, and we could see the plane but the pilot could not see us. We let off a red smoke, which reverted the situation - he could see us, but we could not see anything at all. It was after it all cleared that we discovered that the smoke makes everything in range red, including us, the tents, and the snow. We weren't in distress, but we would have been a bit distressed if the plane had gone away again, being over 100 miles from the nearest settlement.
Not had to fire one personally but was once involved looking for a motor boat who had broken his engine and called Mayday.
He did'nt know where he was but had sailed from Conwy. The Coastguard pursuaded him to fire his flare, a red hand held. It was clearly visible from my position near Conwy No2 buoy. He was well off Great Orme at least 3 miles away. This was on a bright day too.
yes had to use a red hand held flare from an open boat as both vhf and moblie were u/s
it was mid afternoon on a wet overcast day
spotted very quickly by several persons problem solved