It is possible for http to be down with the ftp still up. Depending on your luck, you may experiance very little, or quite alot of down time. I personally have had quite acceptable uptime, but others have complained about it being quite bad. But bear in mind people are more likely to come to the forums with a problem rather than to report good uptime. If the downtime gets too much for you, you need to move to a paid plan for better reliability, or if you dont want to pay for that, you'll have to risk another host