The exponential growth in data traffic and dramatic capacity demand in fifth generation (5G) has inspired the move from traditional single-tier cellular networks towards heterogeneous cellular networks (HetNets). To face the coming trend in 5G, the high availability requirement in new applications, needs to be satisfied to achieve low latency service. In this work, we present a tractable multi-tier multi-band availability model to examine the high availability in carrier aggregation (CA)-enabled HetNets. We first derive a closed-form expression for the availability in CA- enabled HetNets based on the signal-to-interference- plus-noise model. By doing so, we formulate the joint subcarrier and power allocation problem, to maximize the availability under the power constraint. The optimization problem is non-convex problem, which is challenging to solve. To cope with it, the genetic algorithm (GA) is proposed to optimize availability through joint subcarrier and power allocation. The average availability in CA-enabled HetNets improves with decreasing the number of UEs, and increasing the power budget ratio interestingly.