Complex real time systems, such as those envisaged for autonomous vehicle control, are expected to exhibit: adaptive and dynamic behaviour, resilience to software/hardware failures and graceful degradation, under conditions of overload. Two objectives need to be met before such properties can be realised. First, critical services must be guaranteed to provide results of a minimum acceptable quality and reliability by their deadlines. Second, the utility of the system needs to be maximised. We present an approach to meeting the above objectives. This approach combines the benefits of both fixed priority preemptive and best effort scheduling: offline analysis is used to guarantee that critical timing requirements will be met, whilst at run time, a simple adaptive threshold policy arbitrates between competing optional components, enhancing the system utility obtained.