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I know that it's possible for military aircraft to "intercept" a civilian or foreign aircraft (and I'm well aware of the US interception procedures), but why would they be doing it in the first place?

What does it actually prevent from occurring?

They can't remotely control the aircraft, so is there anything that they can do once they intercept it?

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    $\begingroup$ Remember KAL 007 ? $\endgroup$
    – mouviciel
    Commented Oct 30, 2014 at 8:18

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Military interception usually happens after failure to make the aircraft comply by other means (such as radio, light signals, flares). So, first and foremost it is one way to do visual communications, which is usually hard for the pilot to miss. The messages for follow (wide level turn), land (low pass, landing gear extended), and continue on course (climbing turn) are well established (possibly even internationally?) and all pilots should know them, and are probably enough to defuse most situations. Not to mention that you should try to establish radio communications on 121.5 if you're being intercepted.

I'd wager a guess that the most common cause for interception is that a pilot has strayed into restricted airspace without talking to anyone on the radio, without any malicious intent.

If the pilot ignores the messages, there's always the option of using force, if the risk to occupants and collateral on the ground is acceptable.

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    $\begingroup$ Furthermore since flying in close formation to an unknown aircraft is far from standard practice, when someone (specifically a military craft) suddenly appears in front of you it's a pretty clear sign of "Hey you're doing something seriously wrong or suspicious," which in most situations is all that needs to be communicated, unless the pilot actually needs help. I'm not an aviation expert but I assume dispatching an escort is not a simple every-day occurrence (like for example a cop tagging cars on the highway) that can be ignored if you're not doing anything wrong. $\endgroup$
    – thanby
    Commented Feb 12, 2014 at 16:46
  • $\begingroup$ They also sometimes intercept aircraft operating in the ADIZ just to verify that the tail number filed in the flight plan matches the actual aircraft that is flying. $\endgroup$
    – Lnafziger
    Commented Feb 12, 2014 at 18:24
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    $\begingroup$ You might also get "intercepted" if you decided you want to fly through an active MOA and the military aircraft decided they need something to do until you clear the airspace. $\endgroup$
    – casey
    Commented Feb 12, 2014 at 20:16
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    $\begingroup$ But why is there a specific frequency to communicate when you are intercepted? I would assume that modern fighter jets can listen/communicate to all channels at once and know which one the pilot is tuned to. $\endgroup$
    – Taher Elhouderi
    Commented Jul 12, 2015 at 3:06
  • $\begingroup$ The AN/ARC-114A is a military airborne radio which covers the VHF band from 30 to 75.95 MHz and provides 920 channels at a spacing of 50 kHz. I think pilots general have more important things to do than monitor hundreds of channels. Imagine trying ot find your find in the field when you both have a walkie talkie with 920 channels and no agreed on convention. @TaherElhouderi $\endgroup$
    – ChatGPT
    Commented Feb 26, 2023 at 4:29
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The most common reason for interception is that an aircraft is not communicating with ATC and is somewhere that they aren't supposed to be. Often times it turns out to be unintentional with the aircraft on a VFR flight (i.e. poor flight planning or deviating for weather) and straying into a restricted or prohibited area.

In these cases, the standard intercept signals (AIM 5-6-4) are designed to attract the pilot's attention and have them proceed out of the area. In addition to the "standard" signals, they can also drop flares in front of the airplane to really get their attention if needed.

In cases where the airplane still doesn't respond and the aircraft appears to be a threat to national security, the decision can be made by a few senior military and civilian officials to actually shoot down the aircraft.

For more detail, in the US section 5-6-2 of the AIM gives the following reasons that a civil aircraft may be intercepted by a military aircraft:

5-6-2. Interception Procedures

a. General.

1. In conjunction with the FAA, Air Defense Sectors monitor air traffic and could order an intercept in the interest of national security or defense. Intercepts during peacetime operations are vastly different than those conducted under increased states of readiness. The interceptors may be fighters or rotary wing aircraft. The reasons for aircraft intercept include, but are not limited to:

(a) Identify an aircraft;

(b) Track an aircraft;

(c) Inspect an aircraft;

(d) Divert an aircraft;

(e) Establish communications with an aircraft.

...

Flightgear.org also has a good wiki on interception procedures that covers some of the common reasons that they are used (and includes all of the ones that I was going to include):

Reasons for interception in real life

Civilian aircraft

While interception of civilian aircraft is a last resort, interception is often the only means available to identify an aircraft that have not filed a flight plan and/or have no transponder and can not be contacted. Apart from identification interception is as well often the only means to redirect an aircraft that is straying into limited airspace or is believed to be involved in illegal activities.

Identification
Visual identification of aircraft that can not otherwise be identified.

Redirection
An aircraft may be intercepted and through visual signals or radio communication on emergency channels be requested to change route and possibly to land at an specific airport if an aircraft

  • is straying away from a route,
  • are entering a danger, restricted or prohibited area,
  • are suspected to fly illegally or is smuggling goods or persons,
  • enters a countries airspace without permit an fails to follow instructions to leave the airspace or land at a specific airport,
  • enters a countries airspace at different positions or routes than permitted, or
  • is a hazard to other aircraft
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  • $\begingroup$ If you've been reading the news in the past week, the RAF and German Air Force have been intercepting quite a large number of Russian jets over international water. Why are they intercepting in this case? Isn't it legal to fly over international water as long as you don't harm any other aircraft? $\endgroup$
    – Taher Elhouderi
    Commented Nov 1, 2014 at 23:02
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    $\begingroup$ @TaherElhouderi It is legal to fly in international airspace. However, if you're flying a bomber near the territory of a not-that-friendly country, they can send interceptors to remind you that you're being watched, and that you can't just slip in to national airspace without being noticed and possibly shot down. On the flip side, one of the points of military exercises near an unfriendly country is to evaluate their air defense based on their response to you, and remind them that you can send aircraft over to their territory. $\endgroup$
    – cpast
    Commented Nov 20, 2014 at 23:56
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    $\begingroup$ Note that if the AIM-5-6-4 intercept signal becomes an AIM-7 Sidewinder, you've not responded properly to the earlier signals. $\endgroup$
    – Jon Story
    Commented Feb 16, 2015 at 16:14
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    $\begingroup$ @TaherElhouderi - International waters and international airspace are different concepts. International maritime law establishes territorial versus international waters, but there are no international laws regulating the establishment of Air Defense Identification Zones or ADIZ. Aircraft entering an ADIZ without following proper procedure for the country establishing that zone will be intercepted; it's the purpose of having the zone in the first place. The only implicit agreement is that an ADIZ cannot overlap another country's ADIZ or its political land borders. $\endgroup$
    – KeithS
    Commented Jul 6, 2015 at 18:06
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    $\begingroup$ @TaherElhouderi - upload.wikimedia.org/wikipedia/en/thumb/d/d8/Alaskan_ADIZ.jpg/… - That is almost exactly what has happened, and Russia tests this zone almost every day. There are some practical limits to the outer borders of an ADIZ as well as legal; the ADIZ is of no use if your radar can't see past the ADIZ borders (unless you spend considerably more money and man-hours using military aircraft patrols). $\endgroup$
    – KeithS
    Commented Jul 9, 2015 at 15:35
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As far as I know, interception of civilian aircraft is done to assess a situation where the safe status of a flight is uncertain.

A Vueling A320 got intercepted some time ago when the French ATC weren't able to communicate with the pilot. The same airline some time later got another interception by the Dutch.

Intercepting the aircraft and watching the situation inside the cockpit and/or the cabin can help to assess whether there is a serious situation developing (and further actions can be taken, such as alerting emergency services on the ground, monitoring the flight path, continuing the interception procedure to try to improve the situation).

It's not about piloting the intercepted aircraft, but about assessing the status of the flight. I think it's just a matter of gaining some time if a hijacking occurs.

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One large part of the reason to intercept any aircraft is to visually confirm the identity of the aircraft. While every other method of communication is valid in establishing the identity, only "eyes on" can unequivocally confirm the tail number and registration of the aircraft.

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    $\begingroup$ Well, they can confirm the tail number that is displayed on the aircraft. Unfortunately, there have been cases (in drug smuggling airplanes for instance) where the tail number has been changed illegally. $\endgroup$
    – Lnafziger
    Commented Feb 13, 2014 at 18:57

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