Many civilian pilots are aware, generally at least, of ground radars being used to guide airplanes to landings through the clouds. The military has used ground control approach (GCA) systems for many years. By the end of WWII and during the Korean War this was an established way of helping pilots. Radar systems housed in trailers could be relocated to different airfields, giving the military a world-wide capability despite the vagaries of weather.
The development of radar systems with precise target-tracking capabilities made GCA techniques possible. Those systems permitted trained radar-scope interpreters to guide planes down the glide path toward the runway. Accuracies to within a few feet in azimuth, elevation and distance (range) to the plane's radar echo made it possible to track and guide it to a safe landing.
But what about airfields without GCA systems? Or when GCA systems are inoperative? Is there a backup? The answer is "that depends". It depends on two key factors. First, is the airplane equipped with its own precision radar system? And just as important, is there anybody aboard who is capable of accurately guiding the plane to a safe landing? In the early days, neither capability existed, despite the ego-based claims of radar manufacturers and some radar operators who felt they were the "world's greatest".
While the GCA operator looks for the radar echo of the incoming airplane, an airborne radar operator must find the end of the runway. Because airborne radar signals striking the runway tend to bounce off and away from the pavement, like a flashlight beam reflected off a mirror, the runway's appearance on the scope is dark, not a bright dot like the GCA view of a plane. The airborne operator searches for dark lines and patterns within the clutter of reflecting ground echoes. This can be tough in some situations. Not all military radar navigators and bombardiers could master the airborne radar directed approach (RDA).
The accuracy and precision required for an airborne radar directed approach is similar to that demanded for ground-based GCA systems. But it's a tougher job to reliably locate and track the end of the runway from a moving, bouncing airplane in the weather.
Could a pilot of a single engine or twin aircraft equipped with a weather avoidance radar make his own radar approach? Is the radar good enough? And what are the techniques needed to make such an approach?
Theoretically, the answer is a qualified "yes". But in a practical sense the answer is "probably not". Consider the following factors bearing upon airborne radar directed approaches. Then compare these against your weather radar's features.
Your radar equipment must present a high resolution image, an accurate portrayal of the topographical and man-made features in and around the airport. That calls for a narrow radar beam (width under two degrees ) to preclude azimuth smearing of the images which could obscure the runway.
The radar display should include accurate range markers or a variable calibrated cursor capable of indicating distances from the airplane to the end of the runway. It is critical that range is accurately determined to one-quarter mile or less.
Short pulse lengths are also necessary to minimize range measurement errors. And it is useful to have adjustable gain and contrast controls to optimize the image for sharpness and maximum detail. Adjustable antenna tilt is not important because a narrow azimuth beam pattern often means a wide vertical pattern anyway. Remember, the target (runway) is not a radar blip. It's the absense of one within the clutter of ground returns and man-made objects.
Simple weather radars typically display slant range and not ground range to the runway. There is a difference. Remember, you're flying down the glide slope along the hypotenuse of a right triangle, one side being your altitude and the other the ground distance to the runway. If your glide slope is steep the difference between slant range and ground range is greater than if the glideslope is shallow.
The general procedure requires the pilot to know his ground range to the end of the runway as well as the heading to steer. At various ranges along the glidepath altitude and speed must be carefully maintained. Assume, for example, that groundspeed is 120 knots and a descent rate of 500 feet per minute is practical for your airplane and airfield obstacles. At five miles from touchdown you must be at 1250 feet AGL and on course. Touchdown is just 2.5 minutes away. At three miles out you must be at 750 feet and at one mile 250 feet AGL. All the while you must keep on course, aligned with the runway.
As the range to the runway diminishes the radar image of the runway widens and smears. Close-in ground clutter brightens because echoes get stronger. You may detect the edges of the runway, the rough surfaces which reflect back toward you. The smooth runway itself will remain dark because returned echoes are absent.
It's a lot of work to fly the airplane precisely, maintaining course and glideslope while holding speed constant. Altitude at each range mark is crucial. There's no opportunity for one pilot to do all this while constantly monitoring the ever-changing radar images. It takes two people, a skilled radar operator and a capable pilot.
What weather minimums should apply to this technique? In the above example a 500 foot ceiling gives the pilot just one minute to transition to a visual approach and landing. At lower ceilings the time gets much shorter and there's not enough time to make last minute corrections. You're down to just seconds.
Equipment in many military bombers and fighters is accurate enough to make such approaches possible. Crews must practice to master the techniques and to perfect the coordination required.
Should others with less capable radar systems or no RDA experience try this? Even in an emergency situation? Not really. That's what alternate airfields are for.