For many types of flight, planes are robust against radio altimeter failure. The RA is really only used as a non-essential part of instrument approaches (e.g. l

For many types of flight, planes are robust against radio altimeter failure. The RA is really only used as a non-essential part of instrument approaches (e.g. landing in heavy fog, as opposed to a visual approach where a pilot can see the terrain) and as part of the ground proximity warning system (which should never be triggered if a pilot is flying correctly). For military applications, they reduce the risk to pilots and civilians from military aircraft which fly in ways civilian aircraft would never be allowed to. Military pilots are often ordered to hug the ground when deployed in order to avoid RADAR, but this raises the risk of controlled flight in to terrain. RAs are one tool which can help avoid CFIT when flying irresponsibly close to the ground. Military aircraft would still be able to fly at higher altitudes without RAs.