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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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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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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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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. –  thanby Feb 12 at 16:46
    
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. –  Lnafziger Feb 12 at 18:24
    
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. –  casey Feb 12 at 20:16
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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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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. –  Lnafziger Feb 13 at 18:57
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