What is the minimum altitude for autopilot operation during a standard ILS approach with an inoperative radio altimeter?

During a standard ILS (Instrument Landing System) approach, the minimum altitude for autopilot operation is determined by several factors, including the availability of critical instruments. When the radio altimeter is inoperative, the aircraft must rely on the barometric altimeter for altitude information, which is less precise in certain phases of flight, especially close to the ground.

For an ILS approach, the minimums can be higher when the radio altimeter is not functioning, as a backup system for altitude reference is not available. In this case, the minimum altitude for autopilot operation during the approach is established at 160 feet. This altitude allows for safe operation while providing a reasonable buffer above the decision height, reducing the risk of controlled flight into terrain and ensuring that adequate altitude information can still be obtained from the barometric readings.

Choosing a minimum altitude of 160 feet aligns with operational safety standards and ensures that the aircraft is maintained at a safe altitude above the ground until the pilot can take over for the landing autonomously if needed. Lower altitudes, such as 50 ft, 100 ft, or 200 ft, would either not comply with safety regulations or not account for the lack of radio altimeter data to ensure safe landings.