Intercepted - how airline pilots handle military intervention and diversions
As I write this, the aviation world is still trying to get its head around the events which surround Ryanair flight 4978 as it was forced to divert into Minsk, Belarus (MSQ) on Sunday. The aircraft was flying over northwestern Belarus, nearing the end of its scheduled flight from Athens, Greece (ATH) to Vilnius, Lithuania (VNO), when the pilots were informed by Belarussian ATC of a security threat onboard the aircraft.
Despite being closer to Vilnius than Minsk at the time, the aircraft turned towards the capital of Belarus and landed a short time later.
According to certain news outlets including Reuters, during the diversion, a Mig-29 fighter aircraft from the Belarussian military was scrambled to escort the Boeing 737 as it made its way to Minsk. As of yet, the exact details of what unfolded at 39,000ft are still unclear. However, as alarming as the thought of a military fighter jet intercepting a civilian aircraft may seem, it is something that airline pilots are trained to deal with.
Related: Behind the scenes: What goes on in the flight deck during a diversion?
Why would an airliner be intercepted?
Imagine enjoying the end of your inflight movie, only to look out of the window to see a Top Gun style military jet sitting just metres away from the wingtip of your aircraft. As worrying as this may seem, it is incredibly rare and normally down to a loss of communications with the controllers on the ground.
As aircraft fly across the world, they cross countless different countries on their journey to their destination. Each one of these countries is responsible for the airspace above it and as a result, provide an Air Traffic Service, part of which is Air Traffic Control. In most situations, this airspace is divided up into different control areas or sectors. This is done not only because countries can have a large geographic area, but also because the airspace is vertical as well as lateral.
The uppermost areas provide a lower workload for Air Traffic Control Officers (ATCOs) as most aircraft are just passing through. As a result, these sectors can cover a large geographic area - sometimes the size of the entire country. The closer to the ground you get, the higher the workload for ATCOs as they will be handing aircraft climbing and descending as they leave and arrive around the main airports. To ensure flight safety, the busier the sector, the smaller the geographic area it covers.
To enable ATCOs to control their respective sectors efficiently, each one will be assigned a particular VHF radiofrequency. For example, 125.475. As aircraft move towards the edge of one sector, the ATCO will hand the pilots over to the next sector by giving them the new frequency.
Loss of communications
The problem comes when there is an error with the handover and the pilots end up on the wrong frequency. This can happen if the controllers give the pilots the wrong frequency for the next sector, if the pilots misunderstand that frequency, if the ATCO forgets to hand the aircraft over and it goes out of radio range or, in rare circumstances, if the radio on the aircraft fails.
That said, most of these situations are easily fixable.
When contacting the new frequency, if we receive no response after a couple of calls, we simply go back to the previous ATCO to check if we got the new frequency correct. More often than not, there will be a digit wrong and the problem is solved. However, if we end up out of radio range, the frequency which we are using will be no good. To cover this eventuality, we always listen out on another frequency at the same time - 121.5, the emergency frequency, also known as 'the guard frequency', or if you're a pilot, simply, 'guard'.
If a controller loses contact with an aircraft, they are able to broadcast on guard. The pilots of that aircraft should be listening out and hear that transmission, regaining contact with ATC in the process. If this doesn't work, ATC may ask other aircraft in the area to try and call the non-communicative aircraft on guard.
However, if the ATCO is ultimately unable to contact the pilots, they have a problem. An aircraft is flying through its airspace, unresponsive.
At this point, it may be unclear what is actually going on in the aircraft. The pilots may have become incapacitated, as in the case of Helios flight 522 which crashed after the aircraft suffered a decompression which the crew did not recognise. Or, with ever-increasing flight duty times, the crew may have fallen asleep. ATC also have to consider the fact that the aircraft may have been hijacked.
The only way to ascertain what exactly is going on is to scramble a military jet to intercept the aircraft and see what is going on.
The rules of an interception
The decision to intercept a civilian airliner is not one that is taken lightly. As a result, there are international rules in place which govern how and why an aircraft should be intercepted.
They state that the interception of a civilian aircraft should be undertaken only as last resort. If an interception must take place, it will be limited to determining the identity of an aircraft unless it is necessary to change the course of that aircraft. This may be done to direct it away from a Restricted or Danger Area, direct it beyond the boundaries of national airspace, or to instruct it to land at a designated airfield.
When intercepting an aircraft, directional guidance and any other information must be passed to the pilots by radio and if the aircraft is required to land in the country over which it is flying, the nominated airport must be suitable for the safe landing of that particular aircraft type.
The interception itself is carried out in 3 phases. In Phase 1, the lead aircraft will approach from behind and then take up a position high out to the left of the intercepted aircraft at a distance of no closer than 300m. Phase 2 involves the military jet slowly closing in towards the civil aircraft until a point where communication is established between the pilots. It's key at this stage that the intercepting jet uses caution to avoid startling any passengers of the civilian aircraft who may be able to see it.
The final phase involves gently breaking away from the intercepted aircraft in a shallow dive once the communication is complete.
In the flight deck
Having never been intercepted myself, I can imagine that the first reaction to this event is one of complete surprise. As mentioned above, it will most likely come after a long period of silence on the radio. This may seem an obvious sign but even in some of the busiest airspace in the world, the radios can legitimately go quiet for a while, particularly at night. Seeing a fighter jet just outside the window would be enough to make anyone sit up and pay attention.
However, like with all potential abnormal scenarios we could encounter such as engine failures and flight control issues, dealing with an interception is something that pilots are trained to deal with. There is a defined set of procedures that we must apply to determine what course of action we must then take.
First and foremost, we must follow the instructions given to us by the intercepting aircraft. Chances are that they have come to investigate us because we have lost contact with ATC on the ground. Therefore, the first thing we must do is try to make contact with ATC to inform them that we have been intercepted. However, chances are this won't be possible so we must make contact with the pilot of the intercepting aircraft and this is done on the guard frequency.
Next, we must change our transponder code to 7700. The transponder is a clever piece of kit that transmits a whole range of data about our flight path, such as altitude, speed and heading to ATC on the ground. Each flight is given its own transponder code before it departs so ATC knows which aircraft on its screen is which. However, in the case of an emergency, we change the code to 7700. This will then alert ATC to the fact that we have a problem.
These communications between the pilots must be made in a common language. However, even though English is the common language used for civil aviation and all airline pilots must be able to speak it to a certain standard, the same rules may not apply to military pilots. As a result, it may prove difficult to communicate effectively and for the military pilot to convey their instructions. If this is the case, there is a backup system - the use of signals.
Signals used in an interception
If the pilots of the two aircraft are unable to communicate via radio, a number of visual signals may be used instead. Understandably, as this is the last resort, it is imperative that both sets of pilots strictly adhere to these signals.
To make initial contact, the intercepting aircraft will rock its wings and flash its navigation lights at irregular intervals. This means "you have been intercepted, follow me". To respond, the pilots of the intercepted aircraft must also rock their wings, flash their lights and then proceed to follow the military jet.
Once following, if the military jet makes an abrupt break-away turn, consisting of a climbing turn without crossing the intercepted aircraft flight path, it means that the civilian aircraft may proceed with its flight. To acknowledge this, the intercepted aircraft rocks its wings.
If the military jet lowers its landing gear, turns on its landing lights, and overflies a runway, it means that the intercepted aircraft should land at that airfield. To acknowledge this, the intercepted aircraft will lower its own gear, turn on its landing lights and after overflying the runway, and the pilots consider the landing safe, proceed to land.
This is all well and good if the intercepted pilots can comply with the instructions, but what if they are unable? For this scenario, there are a set of signals which the intercepted pilots can send back.
If the crew are unable to comply with an instruction, they must switch all available lights on and off in a regular manner, but in a way that is different from normal flashing. If they are in distress, they must flash all lights in an irregular manner.
If they deem that the designated landing airfield is inadequate, be it that the runway isn't long enough or the winds are too strong, they must raise the landing gear and flash the landing lights, all whilst passing over the runway between 1,000 - 2,000ft and continuing to circle that runway until new instructions are given.
Related: How do pilots handle diversions at one of the world’s busiest airports?
A commercial airliner being intercepted by a military aircraft is incredibly rare. However, like for all possible scenarios, no matter how unlikely, pilots are always trained to deal with it. An interception is most likely for a loss of communication between the pilots and ATC, as happened over southeast England in December 2019.
Once intercepted, it's imperative that communication is established between the two crews to ensure that the intercepted aircraft can comply with all instructions it is issued. If it is not possible to establish effective communication via voice, a series of physical signals may be exchanged.
Featured Image by Petras Malukas/AFP via Getty Images