Which of the following is a function of flight instruments?

Flight instruments are essential tools that provide real-time data regarding various aspects of an aircraft's operation and performance. The correct choice highlights one of the primary functions of flight instruments, which is to provide information on aircraft performance. This includes data on altitude, airspeed, attitude, vertical speed, and other performance-related parameters. Such information is crucial for pilots to maintain safe and effective control of the aircraft, particularly under instrument flight rules (IFR), where visual references may not be available.

The other options present functions that do not align with the primary purpose of flight instruments. While estimating fuel consumption is important for flight planning and management, it is generally not a direct function of flight instruments, which focus more on flight data rather than fuel metrics. Maintaining communication with air traffic control relates to radio and communication systems rather than flight instruments. Lastly, navigation is not limited to daylight; flight instruments facilitate navigation at all times, regardless of lighting conditions, making the assertion that their assistance is limited to daylight insufficient. Thus, the function of providing information on aircraft performance is distinct and central to the role of flight instruments.