Defining precision

An aircraft only needed an ILS receiver and an experienced aviator with good eyes at the controls. However, for decades after World War II, most military pilots—especially those flying single-seat fighters—didn’t enjoy even the luxury of an ILS approach. An F–100, for example, carried only receivers for tactical air navigation (TACAN) and maybe an ADF approach. There was one navigation tool that gave military pilots a step up on their civilian counterparts that was better than an ILS: Precision approach radar (PAR) in military use, more commonly called a ground-controlled approach (GCA).

GCA approaches were available to any aircraft since they demanded no extra onboard equipment, only a decent UHF or VHF communications receiver. Each GCA required highly skilled air traffic controllers to interpret a unit’s radar displays to be able to talk an aircraft down. Controllers could give pilots both their range to the runway centerline and threshold in addition to how well established they were on a three-degree glidepath only the controller could see. Think of GCA as a poor man’s ILS system capable of helping a pilot safely find the runway in even the worst weather. GCA controllers of course assumed a pilot was able to fly the heading and descent instructions they were given via radio. (PAR and surveillance radar approaches still exist at some airports but are rarely used.)

How GCA worked

My initiation into the world of GCA arrived during the height of the Vietnam War when the U.S. Air Force offered me a slot in air traffic control school as an enlistment enticement. I didn’t need much coaxing since it also kept me out of a rice paddy. I took to ATC school like I’d found my soulmate. The job taxed my brain and was also my first real job at an airport with airplanes. No matter that it was a Royal Air Force station in east England constantly pestered by many of those glorious century series fighters. My base—RAF Wethersfield—was home to two squadrons of supersonic F–100 Super Sabres.

The GCA unit comprised a couple of windowless house trailer-sized units placed 150 feet to the left side of the landing runway. Did I mention it was nearly pitch black inside so we could clearly see those old cathode ray tube radar displays? Because the GCA unit sat so close to the runway, we could open a side door and watch the 100s take off, their afterburners spewing plumes of yellow-white flames behind. But our proximity to the runway also made a GCA unit a dangerous place should a pilot lose control on the runway. The control tower, set well back from the runway, acted as our watchdogs. If they saw anything dangerous headed our way, they hit a klaxon horn that we affectionately called the bailout bell. If it rang, we ran like hell. One time my buddy was inside the GCA when the klaxon blew. He opened a runway-facing door just in time to watch an F–100 slide by in the grass after landing gear up.

Because the GCA unit sat so close to the runway, we could open a side door and watch the 100s take off, their afterburners spewing plumes of yellow-white flames behind.Each GCA unit included three sets of radar scopes. If I looked up from my chair, I’d see the round airport surveillance radar (ASR) display that showed air traffic within 30 miles of the base. It was updated every 20 seconds. Tilt my head down and I was face-to-face with the actual PAR displays that showed a single aircraft relative to the runway centerline as well as to their vertical position on the glideslope. These scopes updated about twice per second so together, an aircraft’s movement left or right or up or down was easy to track.

The controller working the ASR scope vectored an aircraft, sometimes a flight of two, to an eight-mile final at about 2,000 feet agl. That’s when I’d join the same frequency for a radio check. “Pop twenty-two, this is your GCA final controller. How do you hear me?” Once communications were established, the ASR controller turned off that frequency and the rest was up to me.

“Pop Twenty-Two, fly heading 275 to join the Runway Two-Eight centerline. Maintain two thousand feet. Prepare to begin descent in three miles.”

About the six-mile point I’d tell them, “Pop Twenty-Two begin descent.” The pilot would ease the power back to capture a three-degree rate of descent. The pilot flew only the headings and altitudes I gave him.

“Pop Twenty-Two, five miles from touchdown. On course, on glidepath.” A few seconds later the pilot might hear, “Pop Twenty-Two, going slightly below glidepath, adjust your rate of descent. On course.”

At the three-mile point tower would speak the landing clearance in my earphones and I’d relay it to the pilot. “Pop Twenty-Two, check wheels down, Runway Two-Eight cleared to land. Turn right heading two eight zero, two miles from touchdown, on course, but now slightly above glidepath. Wind two nine zero at ten. Turn left heading two seven eight. Report the runway lights in sight.”

As the aircraft approached nearer, I was able to announce when they were over the approach lights. About this time, I’d usually hear, “Pop Twenty-Two has the runway in sight.” My response was simple. “Pop Twenty-Two, take over visually and land.” In especially poor visibility or strong winds we’d usually hear, “Nice job GCA. Thanks.” If the pilot saw nothing, of course, it was missed approach time.

In early fall, England’s cool damp air created the densest fog I’ve ever seen. If the pilot was short on fuel and really needed to get down, minimums didn’t matter much. I watched our chief controller—a former P–61 pilot in World War II—talk an F–100 down in nearly zero-zero and even plant them within a few feet of the runway centerline.

When I opened the trailer door to watch the airplane go by, I couldn’t even see them through the fog. If not for the whine of the Sabre’s Pratt & Whitney J57, no one would even know there was an aircraft landing.

Rob Mark is a pilot, aviation journalist, and the publisher of JetWhine.com. He is a frequent contributor to AOPA Pilot Turbine Edition.