Availability Bias is a tendency to over-value something just because you were able to recall it or you remember something well. The underlying assumption is that if we remember it so well, it must be really important. That can be far from reality. If we see 2 recent stories of airplane crashes in recent times, we would perceive a higher probability for another one to occur. One part of this bias is how well you can recall a similar incident or related memory. The more recent such a memory is more chances that you'll recall it better. The second part is the frequency. How many such incidents can you recall. That has a direct effect on what we perceive. In both cases the ease of retrieval is quite important. You would easily remember and retrieve an accident that you were a part of compared to another one that was narrated to you.