What is the first step you should take if the DME malfunctions at or above 24,000 feet MSL?

The appropriate course of action when facing a DME malfunction at or above 24,000 feet MSL is to notify Air Traffic Control (ATC) and request a lower altitude. This step is crucial for safety and compliance with regulations. DME malfunctions can lead to navigational inaccuracies, particularly at high altitudes where reliance on instrument navigation is essential.

By communicating with ATC, you gain access to updated information on your flight route and can receive guidance on safe altitudes that ensure continued navigational accuracy and the safety of the flight. Additionally, ATC can provide assistance in maintaining safe separation from other aircraft and may even provide alternative navigational help.

While continuing to your destination under VFR conditions can be tempting, it may not always be feasible or safe in the context of an instrument flight (IFR) environment. Performing repairs is not appropriate during flight, as this could jeopardize safety. Changing the flight plan to avoid high altitudes might not be practical, especially if it leads to non-compliance with ATC instructions or creates unnecessary complications. Thus, notifying ATC first is the most logical and safest response.