We address delay faults: faults that cause a software component to take more time for completing an action than a given deadline. Such faults are particularly of interest in real- time mission-critical control applications that use general-purpose computing platforms to compute setpoints. A violation of real- time constraints associated with setpoints can result in failure. Existing benign and Byzantine fault-tolerance architectures do not tolerate delay faults. We discuss the challenges involved in tolerating such faults. Then, we list the requirements on the real- time systems that pave the way for our solution: Axo. We describe how Axo masks delay faults, and we conclude with open issues.