Aviation Topic of the Week
Original by Michael Oxner, July 20, 2003
Updated June 20, 2022


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This week's topic:
IFR Flight Part 5d: Simultaneous Instrument Approaches

I received a question a while back regarding simultaneous instrument approaches. I had intended to do a Variety Q & A style page where I would attempt to give short answers to many short questions (I know, as if I could explain myself in just a few words!) and I got to thinking about this one. This really isn't a short answer question. Because there are a number of situations, all with a few variables, it could be it's own topic. And it fits in nicely in this part of the IFR Flight series. So here goes.

Definition
Background
Situations
    Aircraft Utilizing the Same Approach
    Aircraft Using Converging Approaches
    Aircraft Using Parallel Approaches
        Dependent Parallel Approaches
        Independent Parallel Approaches
Conclusion

Definition

Simultaneous Instrument Approaches means at least two aircraft cleared to fly instrument approach procedures at the same airport at the same time. Visual and Contact approaches from the IFR world are IFR approaches for IFR aircraft, but both use visual reference during the approach phase and not just the landing, so they don't count for simultaneous instrument approaches. They've been covered in a previous topic, so I'll refer you to that one for more information on these.

Background

Normally, IFR aircraft can only be cleared for approaches one at a time. When the first one lands, the second one can receive its approach clearance. Until that time, it has to be delayed somehow. The nature of the delay can depend on the ATC environment. If surveillance (such as radar) coverage is non-existent in the area, ATC will likely be issuing a holding clearance. If surveillance coverage and workload permit, ATC may vector the second aircraft wide in an attempt to have it reach the final approach course at such a distance that an approach clearance can be issued by the time the first one is has landed.

The distance between the aircraft can be great in such a case. In both of these cases, ATC will have to keep the second aircraft above the altitude specified in the published missed approach (unless alternate instructions have been issued to the preceding aircraft). To keep the second aircraft in a place where it can intercept the glidepath on an ILS without having to dive for it from above when the first aircraft lands, this may mean 10-15 NM between aircraft.

At controlled airports (those with a Tower in operation), there may be more flexibility and efficiency. By default, in IFR conditions, there isn't much more that can be done. In VFR conditions, there are rules that can be applied, such as the transfer of responsiblity for control for an IFR arrival from the IFR controller (in the IFR ATC Unit, typically an Area Control Centre or ACC) to the airport controller. This means that the typical standards for separation of IFR aircraft will no longer be applied at the airport environment (ie, the Control Zone). This has the effect of increasing efficiency for operators.

An IFR aircraft operating in visual conditions (outside of Class A or B airspace) may elect to "cancel IFR". This means they are relieving the IFR controller from providing separation between them and other IFR aircraft. The aircraft is still technically on an IFR flight plan, but no longer requires authorization from the IFR air traffic controller for any manoeuvers the pilot wishes to perform. Note that this does not relieve the pilot of having to follow directions of the Tower controller within the Class C or D Control Zone.

In all of the above cases, while an aircraft may be vectored and instructed to track a localizer or other final approach course guidance, the approach clearance is withheld until the preceding aircraft is no longer IFR traffic for the IFR controller.

Recognizing that this kind of setup simply cannot work at a high-volume airport, something else must be done. Enter the Terminal Control Unit, or TCU. With the IFR ATC unit providing Terminal Control Service, a whole new world of efficiency opens up. With a few extra precautions for safety in terms of equipment, a little extra training to handle some new procedures, and a greater "specialization" in airspace, the Terminal Controller is left to focus on one particular piece of airspace, perhaps only serving one particular airport. This allows the application of much more precise operations than can be handled from an enroute controller.

Spacing is typically reduced to as little as 3 NM between aircraft, and multiple IFR arrivals can be cleared to fly instrument approach procedures (IAP) at the same time. ATC is still responsible for separation (unless pilots are in a position from which they can assume responsibility for visual separation), but it's much more efficient and controlled than just the one-at-a-time method applied elsewhere. Holding due to traffic is practically non-existent under normal circumstances, and separation between arrivals can, in some places, be reduced further to 2.5 NM.

Situations

There are several situations a pilot may encounter when simultaneous instrument approaches are being conducted.

  1. Multiple aircraft intending to use the same approach at the same airport
  2. Multiple aircraft destined for the same airport, but using converging runways
  3. Multiple aircraft destined for the same airport, but using parallel runways

Each situation has its own set of rules.

Aircraft Utilizing the Same Approach

The first situation involves one airport, one approach, and more than one aircraft wishing to use it at the same time. This is the simplest situation. The background section above really takes care of most of the information associated with this topic. The only concern not addressed is what aircraft do in the event of a missed approach. Many missed approaches go straight ahead to a fix or NAVAID, and if successive aircraft were to head for the same fix or NAVAID, this would present a problem. In the event of missed approaches, alternative missed approach instructions may have to be issued. This can be done directly by the IFR unit before sending a pilot to tower frequency, or can be relayed by tower to a specific aircraft who initiates a missed approach. All that's required is instructions that keep the aircraft away from the final approach of the following aircraft and give something to work with on the other side, like departure versus departure separation would accomplish.

One thing that has changed in recent times regarding missed approach instructions includes the slight change in criteria. In the past, approach procedures had only one set of instructions. The point was that they had to be written in such a way as to allow an IFR aircraft concluding an instrument approach procedure to reposition for another approach in the event of a communications failure prior to reaching ATC for furhter instructions. Recent criteria changes allow for a set of primary instructions and a set of instructions to be flown in the event of a comm failure. For example, the pimary instructions might read, "fly runway heading for vectors and climb to 3,000 feet. Contact ATC for further," while the comm failure instructions may instruct a pilot to return to the FAF to commence another approach.

Aircraft Using Converging Approaches

Next, we'll look at the situation involving one airport, but more than one approach in use. ATC rules state that this has to be approved and agreed upon between the IFR unit and the tower concerned in an interunit arrangement defining roles and responsibilities of each. Aircraft have to be informed that the procedure is being done, and each has to be assigned a runway on or shortly after initial contact. From a piloting perspective, a pilot simply flies as usual. ATC will, as in the previous case, be interested in what happens in the event of a missed approach -- the Missed Approach Procedure for one runway could direct an aircraft toward the approach path of the other. The controller will either assign alternate missed approach instructions right away, or issue something through the tower if a pilot goes around.

From the ATC side of things, it's a bit more complex. Except in fine weather, where the tower can assume control and safely reduce spacing, IFR separation must be applied until one of the aircraft lands or otherwise is no longer traffic for the IFR unit. This means that a controller looking at applying a 3 NM radar minimum, that spacing must be ensured right down to the runway, and ensure that it continues to be provided for if one aircraft overshoots. For example, a tough situation arises at CYQM, the Greater Moncton/Romeo LeBlanc International Airport. There is an ILS on 06, and one on 29, and these runways actually intersect at an angle of 45°. If the first aircraft is approaching on runway 29 and all that was provided for was 3 NM spacing between this one and the arrival using runway 06 on the assumption the this aircraft will land, once that arrival for runway 29 goes around, there is practically nothing left between the aircraft on the converging tracks. ATC must take into account the airport layout and the final approach courses and apply extra spacing as needed.

As mentioned earlier, if it's VFR, the tower can help by assuming control of one or both, thereby alleviating the pucker factor for the IFR controller if things are getting close. Of course, a plan has to be in mind so if one or both of the aircraft involved misses, something safe can be communicated to each of the aircraft. At the very least, traffic information should be passed to both pilots to enure they're aware of the situation.

One important note about this: This has nothing to do with Land and Hold Short Operations (aka LAHSO). LAHSO is an airport issue, and not an IFR approach issue. That is something worthy of its own topic.

Aircraft Using Parallel Approaches

Last, we'll glimpse the environment where parallel runways are involved. This one becomes a lot more complicated than the last two situations, but is also a lot more rare, only applying to the largest and busiest airports since they're the only ones that will have multiple runways aligned in a parallel fashion.

To begin with, there are two types of parallel approach operations, and each one has rules and specifics to work with. The two types are dependent and independent parallel approaches.

Dependent Parallel Approaches

We'll look at dependent approaches, first. Once again, these are largely the same from a pilot's perspective: Fly the approach, and be extra sure you're where you are supposed to be. Wandering far from the final approach course could affect separation with other traffic on the other approach. What does ATC do? The specifics depend on the distance between the runways. The basics are as follows.

The use of simultaneous parallel instrument approaches must first be approvedby Transport Canada. Equipment must meet the minimums for such an operation, the controllers have to be trained to apply it and so forth. The aircraft must be informed that such an operation is taking place (this can be done on an ATIS). Each aircraft must be vectored to the localizer at an angle of 30° or less, and they must intercept the localizer at least 2 NM from the point where final descent will begin. While this is actually standard for vectoring an aircraft to a final approach course, there is no provision in this case for vectoring an aircraft inside this point. This is not allowed for parallels. Each aircraft must also be informed of the landing runway to be used on initial contact, and the normal radar minimum separation (3 NM since this is only done in a terminal environment) must be maintained until both aircraft are established on their respective localizers. Known factors which may affect an aircraft's ability to track a localizer, such as wind shear, must be considered before letting the aircraft get closer than 3 NM, as well. How close can a controller permit aircraft to be?

Between 4,300 and 9,000

For parallel runways that are at least 4,300 feet apart, but not more than 9,000 feet apart, a controller can permit two successive arrivals on the same localizer to be 2.5 NM apart, but you must maintain 2 NM diagonally between aircraft on the separate localizers.

Between 2500 and 4300

If the runways are less than 4,300 feet apart, but not less than 2,500 feet apart, the aircraft are still required to be 2.5 NM apart on the same localizer, but they can be as close as 1.5 NM apart diagonally between the localizers. Note here that the scanned image contains an error, showing the distance between runways as being between 2,500 and 4,000. The text accompanying it read as 4,300, which is the correct value.

In either case, ATC must closely monitor the spacing to ensure that each aircraft tracks its localizer precisely until the controller is no longer responsible for separation. This means that until one has landed, tower assumes control of one, or an aircraft accepts a visual approach clearance with instructions to maintain visual separation from the preceding traffic, the controller responsible for monitoring aircraft on final must maintain the applicable separation minimum through speed control and judgment calls.

Independent Parallel Approaches

All of the basics for the dependent approaches listed above apply here, too, but there are more details that come into play. There is no lateral minimum between the localizers, so aircraft can be side-by-side on their respective localizers. Hence the name "independent". Each aircraft must be vectored to the localizer with the same conditions mentioned above being met, with one addition: The aircraft must be provided with at least 1 NM of straight and level flight prior to localizer interception. The reason is to minimize the chances of "shooting through" the localizer and violating what is called a "No Transgression Zone", or NTZ. This is a zone of airspace where aircraft from neither localizer are allowed to operate. Also, an aircraft must explicitly be issued an altitude to maintain until established on the localizer. The aircraft must also be sent over to the airport controller's frequency, or the approach monitor's frequency if one is established, prior to the aircraft commencing descent.

There is another ATC position involved with independent approaches and it was alluded to above: the approach monitor. This is a dedicated position whose job it is to monitor the airport controller's frequency, ready to transmit instructions at a moment's notice to ensure that aircraft either remain within their "Normal Operating Zone" or NOZ, but most importantly that all aircraft remain clear of the NTZ. If an aircraft runs close to the edge of its NOZ, this controller immediately gives vectors to return the aircraft to its localizer. If an aircraft penetrates the NTZ, the approach monitor immediately gives traffic avoidance instructions, like a turn, to the other aircraft on the parallel approach. The reason it's designated this way is that there is so little time to react. Statistics say it takes a controller two radar updates to detect a change in track. Terminal surveillance radars in Canada turn just once every 5 seconds (4.8 actually), so that means ten seconds elapse before a controller is likely to detect a change in track. At the approach speeds common at airports where such a procedure is taking place, that 10 seconds lets an aircraft cover a fair amount of ground. Instructions have to be immediate, clear and concise to ensure safety on the approaches.

An airport approved for such an operation will also be approved for use of the 2.5 NM separation minimum between successive aircraft established on final approach where they are operating on the same localizer.

Conclusion

Most of the airports frequented by VatSimmers normally do allow for simultaneous instrument approach procedures. Where some pilots fly that don't do these in the real world, you often don't see larger volumes of simulated traffic running there, so you don't get involved in it anyway.

Simultaneous instrument approaches are basically the same from the pilot's viewpoint as they are for being cleared as the only traffic on the approach. The pilot flies the ILS just as normal. Some extra situational awareness would go a long way toward safe flight. Stick close to the localizer and be smooth on the approach, and don't be too quick to make speed changes. It's ATC who has more to be concerned with while conducting simultaneous instrument approaches, whatever the situation involves.




I hope I have done this topic justice. If I haven't, please feel free to mention this fact to me, and where my shortcomings are. I'm not afraid to learn something new. My e-mail address is mo@xlii.ca. Thanks again for reading!