Chain dropping off front rings

Was in Djouce at the weekend and noticed my chain kept dropping off the front rings on downhill sections (not pedaling). Front derailleur was set to middle ring and rear to smallest gear. Any ideas??:confused:
you could probably take out a few links of the chain to reduce the slack or select gears that will keep your chain tensioned.

However, especially when riding hardtail on rough descents, you will drop your chain every now and then and there is not much you can do. That's why downhillers etc. have chain guides and tensioners installed on their bikes.
yeah, the tighter the chain the less likely it is to drop..
so try having it in the ring for descents
I'd avoid using the smallest gear on the back unless you need it, keeps the chain tighter..