Safety Publications/Articles

Tower Or Unicom?

Choose The Right Frequency

Everybody knows about the common traffic advisory frequency (CTAF) and how it is used at towered and nontowered airports, right? Here is a NASA Aviation Safety Reporting System (ASRS) report from an instrument flight instructor who knew how the system was supposed to work. However, a minor lapse nearly resulted in a ground collision with a C-130 at a runway intersection.

The CFII and his instrument student landed at a towered airport and, after refueling, prepared for departure. As the CFII and his student taxied in, the controller advised them that the tower was closing for the night. The controller suggested that they call air traffic control on the telephone to get IFR clearance for the return flight when ready to go and said that they could announce their departure on the CTAF.

While providing the clearance over the phone, the departure controller advised the instructor of a target west of a nearby VOR at 500 feet. The instructor and student taxied for takeoff immediately. The student announced the departure on the unicom and began the takeoff when the instructor noticed lights at their 2 o'clock position with no relative motion. He enthusiastically suggested that the student abort the takeoff, and shortly afterward a C-130 rolled through the intersection of the crossing runway. Thinking that the C-130 was on the wrong frequency, the student and instructor went on their way, but the next day the CFI decided to do a little more investigation.

The Aeronautical Information Manual (AIM) provides information on air traffic control and specifically discusses traffic advisory practices at airports without operating control towers. At nontowered airports, the CTAF will normally be the designated unicom frequency. However, at an airport that has a part-time tower, the CTAF will normally become the tower frequency after the tower closes. On the airport dia- gram page of NOS charts, the header shows a star next to the tower frequency followed in parentheses by the letters "CTAF" when the tower operates on a part-time basis. This should help to clear up the confusion, but the devil is in the details and the fine print.

The CFI noted in his ASRS report that he and the student normally flew from a nontowered airport. As a result, the student assumed that unicom was the CTAF when the tower closed. It was nearly a fatal mistake. Other factors that the instructor mentioned in the report were his failure to double-check the frequency that the student was using and the lateness of the hour. Both pilots had worked a full day, and fatigue may have caused them to be a little less sharp.

Both those points, while mentioned almost parenthetically, are essential information in the almost-accident chain. Habit patterns are a big part of flying safely. We find that pilots who routinely fly from nontowered airports are more familiar with those procedures and may miss a few subtle details at busier locations. The same can be said of those who fly from towered airports and their actions at nontowered fields. Books could be written, and no doubt have, on the effect of fatigue on human performance - not just in aviation but in all high-performance activities. The bottom line is that when operating in something other than home field conditions or when on the back side of your energy curve, assume nothing and double-check everything.

My congratulations to the instructor on his decision to visually clear the crossing runway, which is something we should always do whether a tower is in operation or not, and my thanks that he had the curiosity to look up the procedure and submit this incident to ASRS so that everyone could benefit from a little refresher.

Bruce Landsberg is executive director of the AOPA Air Safety Foundation. For more information and to view airport taxi diagrams, visit the Air Safety Foundation Web site (

By Bruce Landsberg

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