What Do Control Towers Control?

Minneapolis Center controllers handed Stancie and me off to the temporary control tower at Tarkio, Missouri, with a clearance to fly a visual approach. We, and many dozens of other pilots, were arriving at Tarkio for the annual EAA Chapter 1405 Wing Nuts Flying Circus.

The Wing Nuts show is a little different, no, make that a lot different, than a normal EAA chapter fly-in. At Tarkio the stealth bomber B-2 flew over the runway so we could all see that exotic boomerang shape. Two FA-18s flew very long and impressive performances. And how many other local air shows have Sean Tucker performing?

The reason the Wing Nuts show is so well attended by pilots and performers is because Tarkio is the home airport of Missouri Congressman Sam Graves. Congressman Graves is on the Small Business Committee and the House Transportation Committee, both groups are crucial to all of aviation. And Congressman Graves is an avid aviator flying all sorts of airplanes including the T-6, Stearman, and other antiques and classics. Congressman Graves supports aviation, and we in aviation support him, and that makes the Wing Nuts show the biggest “little” air show in the country.

Because of the huge popularity of the fly-in at the single runway Tarkio airport a temporary control tower is wheeled in. Putting a tower at a normally non-controlled airport, and then flooding the airport with traffic, started me thinking about what controllers in a control tower really control.

The short answer is that the tower controls the runway. The primary function of the tower – local control in formal ATC speak – is to separate aircraft using the active runways. Controlling the airspace around the runway is a different situation.

At larger fly-ins temporary control towers are put in use, like in Fond du Lac, Wisconsin, during AirVenture. Courtesy: Bob Simmeron. Above photo courtesy: eviro.aero

The FAA has a very specific set of rules of how aircraft on an active runway must be separated to prevent collisions, and it is the job of the tower to enforce those rules. The most fundamental rule is that only one aircraft can be using the runway at the same time, so it’s the tower controller’s job to make sure the runway is going to be clear before approving pilots to land, take off, or cross the active runway.

Keeping airplanes apart on the runway is obvious, and makes sense. But to maximize use of a runway without compromising safety, tower controllers can issue clearance to one pilot to land if another is still on the runway but is rolling for takeoff. Or a landing clearance can be issued if the aircraft ahead is down safely and rolling out to clear the runway before the other lands. And in some circumstances on long runways pilots of airplanes of lower performance and less runway requirement can be cleared to land if the landing airplane ahead is far enough down the runway.

What a tower controller does to separate airplanes in the air around the airport is less clear. Even though we call the airspace around an airport with an operating control tower “controlled airspace,” the fact is that if conditions are VFR it is up to each pilot to see and avoid other traffic. That also goes for pilots still flying on an IFR clearance as we were when arriving at Tarkio.

Tower controllers do their best to point out known traffic to other pilots in the area. They have full authority to issue directions such as “follow the Cherokee on left downwind.” They can also clear pilots to enter the traffic pattern at a specific location, and that is a clearance we are obligated to fly.

Tower controllers can help establish safe separation for airplanes in the air by issuing clearances such as, “I’ll call your turn to base,” when they can see an airplane a pilot flying downwind doesn’t. The controllers can also tell pilots to fly a 360 if they see spacing on final is not sufficient, or they can request pilots to fly a minimum safe airspeed.

Though tower controllers are doing their best to help pilots separate themselves in the airspace around the runway, they are not “controlling,” or “separating” traffic in the same way that happens when we fly under positive control such as while in the clouds or above 18,000 feet.

Temporary towers are also used by non-FAA fly-in organizers to control large parking areas such as in the Warbird area at EAA AirVenture Oshkosh.

When we fly under positive control we are separated from other controlled airplanes by maintaining an assigned altitude, and assigned course or heading. Pilots cannot deviate from those ATC clearances without declaring an emergency. Controllers use radar identification and procedures – the assigned course and altitude – to keep airplanes under positive control safely apart. And the system works. There hasn’t been a collision between two aircraft under positive control in decades.

At many airports tower controllers now have a radar display that supplements their visual observations and they can give pilots traffic advisories. Those radar advisories are a huge help, but are still not positive separation in the formal sense. The advisories are intended to help us see the other airplane so that we can then establish our own visual separation.

But at Tarkio on Saturday it was back to the basics of a “tower-controlled” airport where the controller did his best to see airplanes entering the pattern and instruct pilots to fall in behind airplanes he could see, but in the end the only positive clearances he could issue were to actually use the runway.

Even though I was still flying on an IFR clearance my duties to separate myself from the many other airplanes arriving was no different. The tower cleared us to enter a right downwind for Runway 36, even though the wind was building from due south. As we flew our clearance to enter the pattern the pilot of a T-6 – correctly so – refused the clearance to land on Runway 36 because the south wind was now approaching 10 knots. That’s a lot of tailwind for any airplane, and too much for any careful pilot of a heavy taildragger, so the T-6 pilot was absolutely correct to insist on using Runway 18.

The tower announced a runway change, so I rolled into a tight right turn to be on left downwind and found myself ready for a turn to base about the same time the tower controller saw me and cleared me to land No. 2 behind a Cessna on final.

The airspace around Tarkio seemed to anything but controlled as pilots were announcing their arrival from all directions and at a variety of altitudes and airspeeds. But guys in the temporary tower sorted airplanes out as they neared the runway and I didn’t see, or hear of, any close calls. The tower control system works, but we pilots need to remember that until we get close to the runway, maintaining safe separation is still our job.

This entry was posted in Airmanship, Flying for Fun, Mac Clellan's Left Seat Blog, Safety. Bookmark the permalink.

16 Responses to What Do Control Towers Control?

  1. Hubertus von Heynitz says:

    This is not really true !

    1 July 2002; Bashkirian Airlines Tupolev 154; near Ueberlingen, Germany: The aircraft was involved in a midair collision involving a DHL 757 while both aircraft were flying at about 36,000 feet (about 11,000 meters). Debris from both aircraft fell in an area near Lake Constance on the German-Swiss border. Both crew members on the DHL 757 and the 57 passengers and 12 crew members on the Tupolev 154 were killed. The 57 passengers on the Tupolev 154 included 44 children. The 757 departed Bahrain and was continuing on to Brussels after a stop in Bergamo in northern Italy. The Tupolev 154 was on a trip from Moscow to Barcelona and had made a stopover in Munich shortly before the accident.

    • Matt Recupito says:

      Also the September 29, 2006 mid air between Gol 1907 and the Embraer Legacy over Brazil, but I assume Mac’s frame of reference was the US NAS. In that context, no we’ve not had a mid air collision of planes under positive control for a long time. Furthermore, for all the hand-wringing over sleeping controllers and pilots distracted by laptops, I believe we haven’t had a single fatality from commercial aviation in the US for over a decade. If only we in GA could come close to that record…

      • Matt Recupito says:

        I realized I should rephrase. “We haven’t had a passenger fatality…” I remember the unfortunate accident when a child riding in a car was killed when a SWA 737 overran the runway at MDW and struck the car.

        • John Smith says:

          Actually, there wasn’t a single commercial fatality in 2010. But there were several commuter accidents that caused loss of life in the years preceding, most recently I think was the Colgan Q400 in Buffalo, NY. It has been quite some time since there has been a major air carrier fatality accident in the U.S. and we have been unarguably in the safest period in history in that respect. It’s pretty remarkable.

  2. Mac says:

    Hi Hubertus and Matt,

    You’re right, I was only writing about U.S. airspace controlled by the FAA. I should have been more clear. And I don’t mean to be an ugly American, but the rest of the world does not have an ATC system that can match what has been created in the U.S.


    Mac Mc

    • Borneo Pilot says:

      I fly in Indonesia and the ATC system here can be very frustrating. Whether it’s being held for takeoff because a plane is inbound for landing but 8 minutes out in VMC (no exaggeration) or separating planes by miles and miles that are flying VFR, it’s inefficient to say the least. Controllers can typically only handle 4-5 planes at most (and this is on approach control, not 4-5 planes in the pattern!), and yet there have still been instances where they clear two planes, head-on, to the same altitude. So, you’re not being an ugly American, the US controllers truly are the best!

  3. Pingback: Maintaining Separation – It’s Your Responsibility | High Altitude Flying Club

  4. Bob Ketchem says:

    The tower controllers were Air Guard members doing a training weekend. They were doing their best, but they were over whelmed. Landing C-130′s evenly spaced and all coming from the same direction is much different than the Tarkio traffic this past weekend. It would have been a good safety move to have at least one experienced, high volume, controller on duty with them.

  5. Rich Dugger says:

    There was a GA flyin at Grissom Air Base in Indiana about 25 years ago and we went up in the tower during the departures and the military controllers were really on edge.

    They were freaking out because the planes were departing so close behind each other. At least the guy in charge was. The two ladies actually doing the work were pretty good with it all.

  6. Denny Cunningham says:

    Mac, good piece, just a small point of clarification:

    You said, “And in some circumstances on long runways pilots of airplanes of lower performance and less runway requirement can be cleared to land if the landing airplane ahead is far enough down the runway.”

    Well, yes. But for most of the airplanes EAA members fly, this will be the rule, not the exception– only larger airplanes and jets are guaranteed a clear runway. If you’re flying a light single, the tower controller can have a dozen airplanes on the runway when you land– as long as there’s at least 3000 feet between the last one and you, at the time you cross the runway threshold, it’s legal. If you’re flying a twin, the minimum distance requirement is 4500 feet.

    And if you’re landing at an airport operating under a waiver due to a special event (Oshkosh and Sun ‘n’ Fun are the best known, but smaller events with towers or temporary towers can get waivers, too), those numbers become 1500 feet for singles, and 3000 feet for twins.

  7. Mac says:

    Thanks, Denny, for adding details. There are similar requirements for land and hold short clearances on crossing runways.

    In the great old days when we law abiding tax payers were allowed to land at Washington National, I did frequently. Tower controllers there would often clear pilots of piston airplanes such as my Baron to land on Runway 33, or sometimes Runway 22, while the airline jets were using the main north-south runway. There is not enough room on the crossing runways for a land and hold short clearance, but the tower would ask me if I could do that. The way it worked is that I would say yes, the tower would say “confirm you can hold short after you have touched down.” Once I touched down and was rolling I would call the tower and say I would hold short of Runway 1, and that became my clearance. I guess it was a sort of “waiver” of the formal rules, and it worked great. The guys who ran the National tower were the best in the business at getting the most from an airport with very limited capacity. I say “were” because it has been nearly 10 years since I was kicked out of National so I don’t know how the tower functions there these days.

    Mac Mc

  8. Bart R says:

    REKLAW, TX Up to 400 airplanes, one 3400 ft grass strip, with a significant hill at about the halfway point, 25 years, no tower, temporary or othewise, so far so good, no wrecks.

  9. Tom Stehler says:

    I did my young eagles flight, and although I’m not quite fifteen I’m doing the online ground school. I have one question about the 1500′ runway spacing rule. In the event of a brake failure of the rear aircraft, would the front aircraft be responsible for getting out of the way? Would the rear pilot have to make a desperate go-around attempt? What exactly is the procedure?

  10. Mac says:

    Hi Tom,

    For light airplanes such as a Cessna 172, there must be 3,000 feet of spacing between a landing airplane and an airplane rolling out on the runway ahead, not 1,500 feet. The minimum spacing of airplanes on the same runway is based on the airplane ahead rolling out and turning off, not on the following pilot slamming on the brakes. Since all airplanes on the runway are moving in the same direction it all works very well.

    I’m happy to hear you are doing the ground school and wish you the best with your flight training.

    Mac Mc

  11. michael cosgrove says:

    Great, Mac, you just muddied up a lot of brains. “The most fundamental rule is that only one aircraft can be using the runway at the same time… but… tower controllers can issue clearance to one pilot to land if another is still on the runway… a landing clearance can be issued if the aircraft ahead is down safely… pilots of airplanes of lower performance and less runway requirement can be cleared to land if the landing airplane ahead is far enough down the runway.” Clear the water. More than one aircraft CAN be using the runway and MAY use a tower-controlled runway IF issued a clearance to do so. At non-towered airports, no clearance is required for more than one aircraft to use the runway at the same time. Nota bene CFR 14 PT 91.13!

  12. Mac says:

    Hi Michael,

    Okay, how’s this? Only one airplane can use a portion of the runway at the same time. That portion must be long enough for the airplane landing, and any airplane ahead, to remain safely apart. At a tower controlled airport–the subject here–controllers get to determine how long that portion of the runway must be. The controllers have minimum distances they must enforce, but are not required to clear another airplane to land or takeoff if they have any doubts about the safety of the operation even if the required minimum distance is available.

    The reality is that at most airports that have enough traffic to support a control tower there is a mix of airplane types. Even a piston twin is required to have half again as much runway separation as a light piston single. Of course heavier airplanes demand even more runway under the rules. Instead of trying to eyeball these various distances to remain within the FAA rules, tower controllers stay on the safe side–for their career and our safety–and clear only a single airplane to use the runway at once.

    So back to the original fundamental–I should have said that only one airplane can use the same portion of the same runway at the same time. Now, how long is that portion is where we get into details.


    Mac Mc

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