Recently I received a good question about air traffic control procedures regarding approaches and departures. The question is: Why are some airports just one IFR arrival or departure at a time, while others permit a string of intermixed traffic? Well, lend me your ears (eyes) and let me tell you.
Why? Just Why?
The reason this is such an important question is the ability for ATC to permit a string of intermixed traffic obviously makes that airport much more efficient than those airports only permitting one at a time. Conversely, airports that don’t allow that string of intermixed traffic wind up with backlogs of aircraft trying to get in and out of them.
One good example of an inefficient airport is our editor’s own Santa Fe, New Mexico, airport. KSAF is an active Class D airport that has both commercial and GA traffic, and the GA traffic ranges from training to bizjets. It often gets delays for IFR aircraft coming in and out of the airspace. Radar services for approaches are provided by Albuquerque ARTCC (En-route Control, “Center”).
Despite the airport having a traffic radar site located directly on the field, air traffic control is unable to conduct simultaneous approaches and departures due to specific rules restricting this. The airport is obviously busy. It is located almost adjacent to Albuquerque Class C airspace, and as I mentioned, it has a radar site on the field. So why can’t ATC procedures allow for simultaneous approaches and departures?
The problem lies in the type of radar used, the rules regarding en-route control with Class D airports, and the weather radar provided by nearby radar sites.
It’s the Rules
I think a good place to start is an overview of the rules associated with multiple approaches and departures. This will give us a good baseline to talk about some of the more complex technicalities involved with these operations.
The rules for handling multiple IFR arrivals can vary depending on the availability of radar services and the type of airspace. While Class B and C airports benefit from continuous radar service and dedicated controllers, Class D, E, or G airports face additional challenges due to the absence of continuous radar coverage.
However, through the use of visual separation, tower controllers’ discretion, and timed approaches at controlled airports, pilots and air traffic controllers can navigate these challenges and ensure the safe and efficient flow of IFR traffic.
The procedures for managing multiple IFR arrivals differ depending on the radar services available at an airport and whether or not it has a tower. Class B and C airports have radar service available to the surface, with the control tower staffed by a dedicated controller. In these cases, aircraft can be spaced closely on approach to a runway, and multiple aircraft can be cleared for an approach and/or departure simultaneously.
On the other hand, Class D, E, or G airports do not have continuous radar service. At Class D airports, the tower cannot provide radar separation, and at class E or G airports, pilots will be instructed to switch to the advisory frequency (constituting termination of radar services). In these situations, maintaining separation becomes more challenging.
For busy Class D airports like Santa Fe, timed approaches can be used. Timed approaches require an operating tower, specific weather conditions, and the use of an instrument approach (visual approaches are not permitted unless alternative approved separation methods are used). In a non-radar environment, timing is used to ensure separation between arrivals, and this is extensively covered in the 7110.65 (ATC rules and procedures).
Radar
Where a radar environment is always maintained, the minimum distance between aircraft over the FAF/OM can be used. If there is a significant difference in performance between aircraft, such as a fast corporate jet following a Cessna 172, the spacing distance would need to be increased accordingly.
It is important to consider the possibility of both aircraft executing a missed approach procedure as well. The spacing over the FAF/OM should allow for the establishment of diverging courses in the radar environment. Timed approaches can only be used at towered airports, where the tower ensures separation between arrivals and coordinates missed approaches and IFR cancellations.
At Class B and C airports, radar service is available to the surface, allowing for precise monitoring and control of aircraft movements. In these cases, the control tower is staffed by either a dedicated controller or the overlying approach controller, who can provide radar separation and guidance to arriving aircraft. This enables aircraft to be spaced closely on approach to a runway, and multiple aircraft can be cleared for an approach to the same airport simultaneously. The controllers must maintain constant communication with the pilots, ensuring a smooth flow of traffic and safe separation between aircraft. If there is reason to believe that communication cannot be maintained, alternate instructions must be given to the pilot.
On the other hand, Class D, E, or G airports do not necessarily have continuous radar service. At Class D airports, the tower is not equipped to provide radar separation, while at Class E or G airports pilots are instructed to switch to the advisory frequency (radar termination, as mentioned), where they rely on their own navigation and communication with other pilots to maintain separation.
It is important to remember to cancel your IFR clearance as soon as safely practical. The sooner you cancel your IFR clearance, the sooner another aircraft can be cleared in or out of the airport. However, don’t ever feel pressured to cancel, especially if there’s weather, it is a new airport, or there’s any uncertainty about landing safely.
Class D airports do have the capability to clear an aircraft for departure while another aircraft is on approach because tower controllers are ensuring separation on the runway. Additionally, they are ensuring visual separation by physically observing both aircraft.
In the case of a busy Class D airport with a high volume of IFR arrivals, timed approaches can be employed. Timed approaches are commonly used to facilitate a continuous stream of IFR arrivals, even in poor weather conditions. However, the use of timed approaches is limited to several technicalities involving approach control that we won’t get into here. Suffice it to say that there are many complex variables involved in what controllers can and cannot do.
But One In, One Out?
So let’s talk a little bit about nontowered airports, and the reasons for one in one out. At nontowered airports, there is a gap in ensured separation between IFR aircraft on approach/departure and their destination airport/IFR departure clearance. Air traffic control must maintain some form of separation between IFR aircraft (constantly) from the moment they takeoff to the moment they land.
Every clearance that ATC gives to an aircraft must ensure separation from other aircraft and terrain at all times. So because there is a time from the moment the aircraft is switched to advisory frequencies at Class D, E, or G airports and the time they touchdown (or the time they depart on their clearance and the time they are radar identified), ATC is not radar monitoring the aircraft and rely entirely on the safety of an approach or departure procedure to get the aircraft safely to the ground or to the flight plan route.
If the controller clears an aircraft for approach and simultaneously clears another aircraft for departure, there is absolutely no guarantee that they won’t lose separation.
Consider, for example, an aircraft cleared out of an nontowered airport (opting to depart from Runway 26). They begin their takeoff roll but then encounter a maintenance issue and they are unable to taxi off of the active runway. Imagine they also have radio issues (admittedly unlikely but possible).
No one is aware of this situation, except the broken-hearted pilot in the busted plane stranded on the runway.
If another aircraft was simultaneously cleared for an approach to Runway 26, there is now a serious issue. There is no ensured separation because nobody knows there is a disabled aircraft on the runway. It’s because of this possible scenario that air traffic control is unable to issue simultaneous approach and departure clearances at non-towered airports.
Okay, But There’s a Tower
Now, if the airport has a tower and there are tower controllers monitoring the runway, an approach clearance may be issued to the same runway that simultaneously has the active departure clearance. The tower controller has control of the situation and can issue a go-around to the inbound aircraft to ensure separation from the disabled aircraft on the runway. This unbroken chain of control is what allows for a simultaneous approach and departure.
One caveat that plays a part in approaches and departures at Class D airports is whether or not there’s an actual approach control, or if Center is handling it.
En-route control utilizes a less accurate radar and cannot safely guide the aircraft into Class D Airspace. Because of this gap in radar coverage, Tower control needs to get permission from En-route control (Center) to release an IFR departure since en-route control may have an aircraft cleared for approach.
Approach control utilizes more accurate radar. Therefore they can guide the aircraft further along the approach and ensure separation up to the tower controller’s jurisdiction.
One workaround for en-route control is a form of coordinated separation between tower control and en-route control that allows for a simultaneous IFR approach and departure at a Class D airport.
When an IFR aircraft is cleared by Center for an approach into a Class D airport and an IFR aircraft is looking for a clearance out of the airport, Tower control can call Center to request the release of that departure. Center will ask Tower if they are able to maintain visual separation between the arrival aircraft and the departure aircraft. If Tower states that they will maintain visual separation between the two, then the Center controller can approve the departure from that airport. The Tower controller has accepted responsibility to ensure separation and both aircraft are continuously monitored and controlled by ATC.
Just about all of this can be summarized by the section of the 7110.65 (5-9-5.a) regarding Approach Separation Responsibility.
“The radar controller performing the approach control function is responsible for separation of radar arrivals unless visual separation is provided by the tower, or a letter of agreement/facility directive authorizes otherwise. Radar final controllers (Approach control) ensure that established separation is maintained between aircraft under their control and other aircraft established on the same final approach course.”
There are many more technicalities for ATC regarding approach/departure procedures, but we would need a week-long class to cover everything. One of the advanced technologies that allows Center controllers to clear aircraft for simultaneous approaches to Class D airports has to do with the modern advances in radar technology.
Class D airports that have WAAS radar (ADS-B) are actually able to accept simultaneous approaches from Center. With this more advanced technology the Center controllers are able to guide the aircraft closer in the same manner as Approach control. The standard lateral separation requirement for en-route control is five NM. With the advanced technology, such as ADS-B, en-route control is allowed to utilize three NM of lateral separation, just like Approach control.
The details of why this is the case is beyond our scope, but it has to do with satellite positioning, ADS-B, and WASS all working together to create a mosaic of radar-like information that is more reliable and is approved by the FAA for such procedures.
The FAA is always working to improve the efficiency of the NAS. Many modern technologies, such as CPDLC have already begun to have very significant impacts on air traffic operations. Particularly in the case of CPDLC (Controller-Pilot Data Link Communicaitons), the technology reduces frequency congestion and hearback/readback issues.
It has been a real game changer for en-route control. Lengthy reroutes for weather and traffic management can be issued with the click of a button, saving significant time on frequency. And in ATC as they say in the fine-dining industry, “Every Second Counts.” (Shout out to one of my favorite shows, “The Bear.”)
We have seen significant improvements to air-traffic flow with WAAS technology. One airport in particular that comes to mind is Colorado’s Eagle County Airport. KEGE is an extremely busy airport during ski season. During peak ski seasons hundreds of corporate jets, airliners, and GA aircraft flock into KEGE, Aspen (KASE), and other nearby airports every year.
Because these airports are so close together, it can be a madhouse for ATC. Before ADS-B, traffic was often put into holding patterns to wait a turn for the approach. Holding to wait for an approach is still common outside of these airports, but with efficiencies achieved in the NAS, holding times have been significantly reduced at KEGE.
There’s much more to discuss about the differences between one in / one out and simultaneous approach/departure operations, but I think we’ve covered the important basics. Believe me when I say that ATC wants the most efficient operation possible. We utilize the rules and regulations to the best of our ability so we can get you safely home as soon as possible. But as a wise man said, “the devil is in the details,” and the world of aviation is full of details.
Mac Lawler has been writing since the day he read his father’s writing, many years ago. He makes time for all the good things in life: family, fishing, travel, reading, nature, and friends. He aspires to always have meaningful things to do and good people to do them with.
