The manoeuvres pilots use to prevent a collision in the air
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Air Traffic Control around the world is more advanced than it has ever been. There are strict rules on how close aircraft can fly to ensure that air travel is as safe as possible.
However, like all good safety systems, there should always be multiple layers of backups should the main procedures fail. Should ATC inadvertently allow two aircraft to become too close, there is one final system at the pilot’s disposal to save the day.
The need to correctly identify aircraft came during WW2 when formations of returning Allied bombers would be infiltrated by German aircraft. Radar systems at the time were in their infancy — able to detect aircraft, but unable to confirm who they were. A formation of aircraft flying towards the U.K. would come up as a blip on the screen, but there was no way of determining if they were friend or foe.
To help determine if an inbound radar blip was friendly, British scientists developed a system called the parrot. Aircraft were given a unique two-digit code which identified them as a particular aircraft.
When returning to the U.K., controllers on the ground would instruct pilots to turn their system on by asking them to “squawk your parrot.” With the system turned on, the controller on the ground could confirm that the aircraft was friendly. The phrase “squawk” is still in use by controllers and pilots today.
Primary and secondary radar
The radar systems used during the 1940s are what’s known as primary radar. The antenna emits a high energy beam into the air, which bounces back off aircraft in the sky. This returning “ping” is then displayed on a screen to the radar operator. However, it’s functionality is really only limited to the direction and distance of the aircraft, hence the need for the parrot system.
Secondary radar (or secondary surveillance radar — SSR) is far more advanced. Not only does it detect the position of the aircraft, but it also requests a range of other information from the aircraft, such as unique identity and altitude using the aircraft’s transponder.
What is a transponder?
The parrot system used in WW2, in conjunction with the American “identification friend or foe” system, developed into the advanced transponder systems, which all airliners have today.
Not only are they used by ATC to determine the position of aircraft, but they are also used by aircraft to determine the position of other aircraft around them. Transponders on modern aircraft are also able to transmit important data to ATC such as airspeed and altitude.
Before each takeoff, pilots are issued with a departure clearance. Not only does this define the departure route which the pilots must fly, but it also specifies the transponder code which the pilots must set, or as it’s more commonly known, the “squawk code”.
This code is specific to that flight at that moment, not the aircraft itself. The same flight the next day will most likely have a different squawk code.
As we fly from one area of the world to another, we often have to change our code due to confliction with another flight’s squawk code. To do this, ATC quite literally asks us to “squawk” the new number they give us.
Types of transponder operation
When operating the transponder system, there are a number of modes available to pilots.
Mode C, or “Mode Charlie”, not only sends back the aircraft’s position and identification, but it also includes the aircraft’s altitude to the closest 100 feet.
Mode S, or “Mode Sierra”, builds on the advances of Mode C but takes the system to a whole new level. Aircraft equipped with Mode S transponders are able to share a far greater range of data with ATC, including:
- Callsign (e.g. “UAE001”)
- Altitude to the nearest 25 feet
- Indicated airspeed
- Magnetic heading
- Rate of turning
This extra information gives ATC a superior picture of what the aircraft is doing, without having to waste precious seconds asking the pilots for the information. It’s particularly useful to approach controllers who can use the exact speed of an aircraft to determine if it is going to get too close with the one ahead.
Not only do aircraft transponders share information with ATC, but they also share information with other aircraft. Whilst pilots don’t have a need for most of this information, on that one day in a million, it could save their lives.
Traffic Collision Avoidance System, or TCAS, uses the aircraft’s transponder to broadcast its position to other aircraft. The receiving aircraft then take this information and display it on the navigation display to the pilots.
The TCAS system generates an invisible “safe zone” around the aircraft based on time to a potential collision. This zone isn’t fixed but varies depending on how quickly targets are moving towards the aircraft.
The system constantly makes calculations on the trajectory of all the aircraft that it is tracking, ensuring that none of them conflicts with the “safe zone”. If it looks like a particular aircraft is heading towards a position where they could become a threat, the system escalates the warnings to the pilots as appropriate.
Depending on the level of threat, there are four ways in which other aircraft can be displayed to the pilots.
Aircraft that are in the vicinity but are not close to being a threat are depicted by a hollow white diamond. Depending on the aircraft type, this may be displayed along with the height of the traffic, relative to the pilot’s own aircraft. For example, “-20” means that the aircraft is 2,000 feet below. “+10” means that the aircraft is 1,000 feet above.
When another aircraft comes within six miles and 1,200 feet of the pilot’s own aircraft, the hollow diamond becomes solid white. Whilst the other aircraft isn’t an immediate threat, its proximity means that it could become a threat if either aircraft changes its flight path.
As the normal safe separation is 1,000 feet vertically, proximate traffic is fairly common, particularly when flying in the lower altitudes around the approach and departure. As both other traffic and proximate traffic are common occurrences and pose no threat to the aircraft, they are displayed on the screens without any aural alert.
If there are displays of this kind of aircraft, we make a mental note of them but say nothing more. They are so common that if we don’t see any other or proximate traffic on our screens, it’s an indication that there may be something wrong with the TCAS system.
However, every so often, we get the next level up — a traffic advisory (TA).
Traffic advisory — (TA)
In most situations, a traffic advisory will result from proximate traffic changing its flight path to one which breaks the TCAS system’s “safe zone”. Traffic advisories are normally triggered when a threat is 48 to 20 seconds from a potential collision.
The whole point of a traffic advisory is to alert the pilots that an aircraft which was previously not a problem, is now worthy of their attention. It consists of the traffic target going from a white diamond to a sold yellow circle and an audio alert of “TRAFFIC! TRAFFIC!”
However, it’s important to note that a TA is based on the rate of closure, it does not take account for what the pilots are actually planning to do. The most common cause of a TA is when climbing and descending near to an airport.
Due to the large volume of traffic, ATC will often issue climb and descent clearances of just a couple of thousand feet at a time. Even if a climbing aircraft is going to level off a safe 1,000 feet below the aircraft above, if the aircraft is climbing too quickly, it could break the “safe zone” and both aircraft will issue a TA to its pilots.
This type of scenario is the most common source of TAs. That said, traffic advisories are still fairly infrequent in that an airline pilot may experience a TA once every couple of months.
Resolution advisory — (RA)
If the TCAS system determines that the separation from the threat may not be sufficient, it increases the warning level to the highest alert — a resolution advisory, or “RA”. This consists of the traffic display changing to a solid red square, an aural alert and a command to change the pitch of the aircraft. A resolution advisory will be issued 35 to 15 seconds from a potential collision.
The key to a successful RA is that the TCAS systems on both aircraft communicate with each other and co-ordinate their commands. If one aircraft instructs its pilots to climb, the other will instruct its pilots to descend. Once an RA has been issued, it is imperative that the pilots obey the TCAS instruction and ignore any climb or descent commands given by ATC.
Resolution advisories are very rare. Most pilots will go a whole career and maybe have a few. In the 15 years I have been flying, I’ve had just the single RA.
As other traffic and proximate traffic are common occurrences, there is no need for pilots to say or do anything when they appear on the screen. In an approach to a busy airport, discussing other traffic with each other may be useful to build situational awareness and create a mental picture of what other aircraft are doing in relation to your own aircraft.
When climbing out of a busy airport, if other traffic is observed nearby, particularly just 1,000 feet above the altitude to which you’ve been cleared, a good crew may wish to reduce the rate of climb so that they do not activate a traffic advisory.
If a traffic advisory is activated, depending on the aircraft type, there’s still very little for the pilots to do. On the Boeing 787 Dreamliner, which I fly, the flight manual directs both pilots to look for traffic using the traffic display as a guide.
A good crew may use this moment to confirm which pilot is currently responsible for flying the aircraft (Pilot Flying — PF) with a call of “TCAS — I have control”. They can then look at their screen and call out the position of the traffic and its relative height. The PF can also use this time to mentally rehearse their actions should the situation escalate.
The other pilot (Pilot Monitoring — PM), can then look out the window to see if they can visually identify the threat aircraft.
If the TA progresses to a RA, things have got serious. Depending on the trajectory of each aircraft, the TCAS system on both aircraft will communicate with each other and calculate a vertical escape path for each aircraft.
When issuing an RA command, there are three initials actions which the TCAS system could instruct the PF to do:
- “CLIMB! CLIMB!”
- “DESCEND! DESCEND!”
- “MONITOR VERTICAL SPEED”
At the same, the display on both the primary flight display screen and in the head-up display change, providing a target to aim for. If a climb or descent is needed, the PF disconnects the autopilot and the auto-throttle and smoothly adjusts the pitch to satisfy the target set by the TCAS system.
If the instruction is “MONITOR VERTICAL SPEED”, the PF can leave the automatics engaged and just monitor the situation.
The job of the PM is not only to ensure that the PF is correctly complying with the TCAS instruction but to also inform ATC that they are manoeuvring to obey a TCAS RA. This should stop ATC from inadvertently issuing a conflicting instruction.
If both sets of pilots correctly execute their TCAS manoeuvre, a collision will be avoided. All that is left to do is return to the original cleared altitude and inform ATC.
Very rarely does TCAS need to be used, but on the rare occasion it does, it can mean the difference between a safe flight and a serious accident. To ensure that we are ready for this once in a career event, pilots practice TCAS scenarios in the simulator every six months.
Even if this incredibly rare event does happen on your flight, you will most likely be unaware it is happening. It’s just another task your unassuming pilots see as part of their role in transporting you safely to your destination.
Featured photo ADEK BERRY/AFP/Getty Images
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