How many radio altimeters are there?

• A. One
• B. Two
• C. Three
• D. Four

Answer: (B)

Redundancy in radio altitude sensing is built into transport aircraft to keep altitude information reliable during critical phases of flight. Each radio altimeter provides height above the terrain and has its own transmitting/receiving equipment and antenna. Having two independent units means the system can continue to provide accurate readings if one unit fails, and it allows cross-checking between the two readings to detect discrepancies. This is important for safe approaches and landings in low visibility and for the ground proximity warning system to function correctly. That’s why the CRJ-550 is equipped with two radio altimeters.