Never Again Online: Muted frequency, muted readback

March 1, 2004

A deep low over south-central Texas had settled in to stay for a while. It was bringing saturated Gulf of Mexico air up in a gradual arc across the southern Gulf States to the lower Midwest. There a high-pressure area centered over the prairie provinces joined forces with the Texas low to drive this mass of moisture westward, up the nearly imperceptible slope of the Great Plains.

Some 5,000 feet higher and 450 miles west of the Missouri River valley, Cheyenne Airport in Wyoming, where I was working as an air traffic controller, sat enshrouded in upslope fog and drizzle. Cheyenne is nearly at the end of the line for wet air masses heading west. By the time this air reaches the summit of the Rockies just to our west, it's cried out. Coming down the other side, it dries its tears and begins to warm happily.

But here, at our airport, it does cause trouble. The ILS, like the cattle on the surrounding prairie, has its tail to the upslope wind. Because of the high terrain to the west, landing straight in on the ILS is to the west. This is dandy for our occasional winter blizzards with their west winds, but not for the frequent spring upslope.

I was working approach control in our nonradar-equipped control tower. The square tower cab is perched atop an otherwise five-story building at the extreme west end of the airport and skewed so as to align the side containing the operating console with Runway 30. The approach control position sits centered on the console, with the local controller — the tower position — to the right and the ground controller to the left.

I was running timed approaches that morning, and we were fairly busy. For a timed approach, the controller assigns a time for a pilot to depart either the final approach fix inbound, or the outer marker inbound. The pilot must adjust his or her flight path accordingly to arrive at the fix at the appropriate time, or the controller can give a vector that sequences the aircraft at the given interval.

Manual approach control, as the nonradar variety is called, is all clocks and altimeters and courses to be flown. One must look at little strips of paper and visualize the big block of airspace, with aircraft coming and going, and know where they are without seeing any of them. Controllers need better spatial awareness than when looking at a radar screen. Pilots, too, must be constantly alert to the ever-changing picture.

There were several general aviation singles in the stack. Many instrument instructors in the large metropolitan area to the south took advantage of our less congested airspace to give their students experience in a nonradar environment. That, coupled with this day's no icing, actual IFR weather, brought them up in a long string.

I was waiting for the last cleared aircraft to report the outer marker inbound when a Boeing 737, carrying the markings of an airline now just a memory, called on frequency. I cleared the airliner, Flight 53, to hold east of the outer marker, to maintain 12,600 feet, to expect approach clearance in 21 minutes, and advised Flight 53 it was number six for the approach.

While I was so engaged, the aircraft already cleared for the approach called the tower at the outer marker, inbound. With this report, I keyed both UHF and VHF and cleared Army 53, a helicopter holding at the bottom of the stack, to descend to 7,600 feet and cleared him for the ILS approach, to cross the outer marker at a time that was three minutes later than what had just been reported by the previous aircraft, and to contact the tower at the outer marker inbound.

The Army pilot began reading the clearance back on UHF. Nearly simultaneously an aircraft began transmitting on VHF. I instinctively muted my VHF receiver so as to hear the Army pilot's readback. That done, I unmuted VHF and transmitted, "Aircraft calling approach control on VHF, say again." There was no reply. I tried again, with the same result. Perhaps someone realized he had transmitted on the wrong frequency.

Starting at the bottom of the holding stack, I began clearing each aircraft in turn to descend 1,000 feet. I received no response from the top aircraft, airline Flight 53. Probably talking to company, I thought.

Because of the slow speed of the helicopter, I had planned to assign a six-minute interval on the outer marker crossing time to the next airplane, a single-engine Cessna. I waited patiently for the Army's call to the tower.

At the appointed time, two aircraft, Army 53 and airline Flight 53, one on UHF, the other on VHF, called the tower. Over the outer marker. Inbound.

Jolted, the local controller and I stared at one another, eyes wide, mouths agape. The first reaction was one of utter, helpless disbelief. Absolute denial. No!! This cannot be! I did not hear that right!

Then, very quickly, the situation was accepted as fact. Now, what to do. Without radar, there is no way of telling who is in front and who is behind. While both should be on the glidepath, there is no way to ascertain the altitude of either.

Which one should we send around? If the Army flight started a missed approach, the helicopter could pull up right into the path of the big Boeing. If the airliner went around, he could climb into the helicopter riding slightly high on the glidepath. Or create vortices unmanageable by the rotary-wing craft. Have one turn right and the other left so as to fly away from each other? Or into each other?

Our stunned silence was brief. We had each thought these unthinkables. We also knew if they were, in fact, both over the outer marker at about the same time — and they did not collide! — and the helicopter didn't become uncontrollable in wake turbulence while on instruments! — the speed difference would ensure the jet landed first, with plenty of space and time for a safe landing by the helicopter.

The local controller did the only thing he could: advised each of the other's presence and told them both to continue. Without an explanation, I extended the expect-approach-clearance time for the rest of the airplanes in the stack.

A couple of minutes later the Boeing broke out of the 400-foot overcast, on glidepath, approximately two miles east of the tower. We could barely see the white-with-red-stripe airliner through the fog and drizzle. The local controller called the Army helicopter and asked his altitude. I don't remember now just what it was, but it told us two things: He was still flying, thank God, and he was about halfway in from the outer marker. I have seldom felt such great relief.

Turning my attention back to the stack, I cleared the bottom aircraft to descend to initial approach altitude, and for the approach with that six-minute crossing interval, and then started the rest of the stack down in turn.

Then I swapped places with the fellow working ground control. Flight 53 and I had a long conversation.

The crew apologized for presuming the clearance was for them. They had heard "five-three" and said, "That's us." They read back the clearance, which, of course, I did not hear because I had muted the frequency. Getting no timely response from me, they changed to tower frequency. By the time I asked the aircraft transmitting on VHF to "say again," they were gone. Oh, how I wish they had asked, "Did you get that readback, approach?"

When I played the tape recording back later, there it was: their readback, loud and clear. The recorder got the transmission whether I muted it at my position or not. At that time, there was no way of telling if the frequency had been muted at any of the operating positions.

I apologized for broadcasting simultaneously on both UHF and VHF, and then muting my VHF receiver. Transmitting on both was our not-too-subtle way of telling folks we were busy. Muting the one was my way of not having to ask the Army pilot to repeat his readback. My muting VHF led directly to the potential catastrophe. We each accepted blame.

As we talked, the enormity, the horrible possibilities, of what had transpired grew more clear and larger. The 737 had descended in the holding pattern, through the altitudes of three other aircraft in the same holding pattern, and at some point had passed or been very close to a fourth aircraft.

The lessons to be learned for today? Pilots: Always get an acknowledgment of your readback. Controllers: Never mute any frequency you are transmitting on.

And six people, three in the cockpit and three in the tower cab, all vowed, "Never again!"

Thomas F. Gribble, AOPA 289612, is a retired air traffic controller who holds airline transport pilot and flight instructor certificates and has logged more than 8,000 hours.

Additional information on dealing with air traffic control communications can be found at the following links:

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Posted Wednesday, March 03, 2004 12:08:46 PM