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I've been asking myself for a while how ATC can maintain air traffic safety with regard to military aircraft - or if they just don't.

As I understand it, all bigger planes (let's for now focus on commercial airliners) have mandatory transponders and warning systems like TCAS and similar. By usage of such systems, ATC knows their position, altitude, heading etc. which is used for planning of take-offs and landings just as well as air traffic safety. Also, systems like TCAS allow pilots to directly respond to unsafe situations even if ATC "didn't see it coming". In short, lots of regulations and techniques to make flying safe.

Now I know even less about military aircraft, but I do know they have transponders, too. I suppose, since enemies would have it a bit easy, those transponders don't operate the same way civil transponders do; or at least they use different frequencies, encryption and what-not. I don't know if they have TCAS but, I suppose, even that could possibly be exploited by enemies.

If I assume correctly, civil ATC cannot necessarily see military aircraft. I've read in another question that FAA doesn't have much power over military but that's just the US anyways. Also, the answer in yet another question suggests that military aircraft do not have to maintain separation.

So, what's the verdict? Do military aircraft assume civil regulations while not in "combat mode"? Or are civil ATC indeed oblivious of military air movements and the military has to provide safety by regulating military and watching civil traffic?

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Military aircraft do have transponders that can reply to civil ATC radar and TCAS interrogations. Normally military aircraft operating in civil airspace are visible to civilian ATC and also will trigger TCAS advisories and alerts if they are getting close to airliners.

During wartime operations, and sometimes during combat practice in dedicated airspace, the transponder is operated in a different mode and will not be replying to civil radar interrogations.

Typically when operating in civil airspace, military aircraft will fly under the civil regulations for that airspace. Training flights are usually conducted in dedicated airspace under military traffic control.

In some cases the military ATC assumes responsibility over military aircraft in otherwise civil ATC controlled airspace. In such a case the military ATC will be responsible for the separation between the military aircraft themselves and between military and civilian aircraft.

In other cases military aircraft may be allowed to maintain their separation from each other on their own, without civil ATC interfering in their operations. This allows for operations where military aircraft flying close to each other like formation flying and aerial refuelling outside the dedicated military airspace. In the USA these operations are called MARSA (Military Assumes Responsibility for Separation of Aircraft). Civil ATC is responsible for keeping Instrument Flight Rules (IFR) aircraft separated from MARSA operations. Visual Flight Rules (VFR) pilots have to maintain separation visually from MARSA flights and vice-versa. If VFR pilots are in contact with ATC they will be advised of ongoing military operations.

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  • $\begingroup$ @DeltaLima Nice, thank you! Am I right assuming that military aircraft, even with deactivated transponders, should still be visible to civil ATC on primary radar? Or can they actually hide completely these days? $\endgroup$ – jhr Jun 2 '15 at 13:26
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    $\begingroup$ If ATC has primary radar, which is often the case, they can see military aircraft. Unless they are 'stealth' aircraft. $\endgroup$ – DeltaLima Jun 2 '15 at 13:29
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    $\begingroup$ Military aircraft do flight plan and even if they switch-off transponder, ATC knows that some flights are operating in their airspace. In that case ATC will not allow civil flights to operate in that area or they will increase the separation in addition to traffic information service . during war time operations the airspace is normally restricted for civil flights. $\endgroup$ – Shahab Mohamed Jun 2 '15 at 15:14
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    $\begingroup$ @ShahabMohamed Indeed. Usually when military aircraft are operating exclusively in an area, temporary flight restrictions are put in place the airspace effectively making it a military training zone. Civil aircraft are not allowed to plan their routes through restricted areas. There is no increased separation, there is just a no-go area. $\endgroup$ – DeltaLima Jun 2 '15 at 15:24
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    $\begingroup$ Another good example of TCAS at work keeping military and civil aircraft apart: avherald.com/h?article=4414b03b&opt=0 $\endgroup$ – UnrecognizedFallingObject Jun 2 '15 at 22:34
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U.S. military aircraft have multi-mode transponders. within U.S. airspace, they "squawk" using Mode 3, which is identical in its form and operation to civilian Mode C (a four-digit code plus altitude, allowing ATC systems to calculate true distance instead of using slant distance which varies with altitude). While in theater, fighters use enhanced transponder modes that allow for digitally-encoded and even encrypted interrogation/response to prevent false positives, as well as additional information sharing from the INS such as tail number, GPS position, bearing and airspeed, reducing the need for radio communications and radar systems between friendlies. Similar information is also available from civilian planes using Mode S transponders, which most commercial airliners possess.

Military jets would disable their transponders in the following situations, and very rarely or never in any other:

  • By explicit instruction of ATC. A malfunction or an incompatible squawk mode can cause conflicting data on ATC displays, and in certain circumstances even if it is operating properly ATC will tell the pilot to "stop squawk" and track the plane by other means.

  • When intercepting a plane with TCAS. Traffic Collision Avoidance Systems interrogate nearby aircraft to obtain direction, distance (based on round-trip time) and altitude data from their transponders which it can use to alert the pilot when an aircraft is within minimum separation distances or is on a potential collision course. A military fighter intercepting a civilian aircraft will look like it's on a collision course to the civilian plane's TCAS (it pretty much will be until the interceptor forms up off the beam of the interceptee). This can lead to the pilot of the intercepted aircraft taking evasive maneuvers recommended by TCAS to avoid the potential collision, causing a "dogfight" as the interceptor tries to form up while the interceptee constantly evades. There is a standard squawk code in the U.S. for an intercepting aircraft (0000, 7777 for military) which some TCAS systems will treat differently, but in some cases the interceptor has to at least partially disable their transponder in order to avoid triggering TCAS.

  • When flying in formation. In close formations such as those used during timed flybys at sporting events and airshows, only the flight leader will squawk. This is for two reasons; first, it's redundant for anyone else in the formation to do so, as the composition and layout of the formation can be established by radio contact and nearby traffic advised. Second, ATC radar resolution can be limited especially at longer ranges, so it can be confusing for the ATC's IFF system to interrogate along an azimuth and receive several simultaneous signals.

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    $\begingroup$ "In close formations such as those used during timed flybys at sporting events and airshows, only the flight leader will squawk." This isn't always necessarily true. The transponder requirements vary by facility. Most civilian ATC will require only the flight lead to remain mode C; however, we fly through several facilities that assign discrete codes to each member of the flight, regardless of how close they are to lead. $\endgroup$ – Rhino Driver Jul 29 '15 at 3:59
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Sometimes they just can't. Basically you can turn off transponder on any airplane. In this case all ATC can rely on is the primary radar if it's good enough and they can see objects like military aircraft. The good example will be Russians flying bombers around Europe. There have been several near collisions already. Best options is to send military jet to intercept bomber and follow it. If intercepting jet has transponder on then ATC can know approximate position of the bomber.

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    $\begingroup$ The jet isn't mostly to show where the bomber is. It's to tell the bomber crew "Hi. We see you." $\endgroup$ – cpast Jun 2 '15 at 18:10
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    $\begingroup$ It's common for Western aircraft to do the same thing around Russia, it's just not an issue in the media at the moment. $\endgroup$ – user7241 Aug 2 '15 at 9:06
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First the size of aircraft have nothing to do with transponders.

Certain airspace mandate having transponder. You cannot operate there without one. It follows that big aircraft need to fly through any airspace so they must have transponders anyways.

Military aircraft are not exempt from this rule. They at least must have the transponder.

Military aircraft have this "due regard" regulation, meaning depending on what they're doing they can either keep their transponder on and operate under ATC control or if they have their own radar and have a need to, they could turn off their transponders and do their own separation from other aircraft. This mostly happen only in time of war, when they need to intercept potentially unfriendly aircraft or under certain training ops where they need to hide themselves from other aircraft to simulate air combat.

When aircraft is just going from A to B in friendly airspace, their transponders will be turned on (I guess unless bordering unfriendly airspace). They have no justification to turn it off.

But then we have the fun stuff, like an SR71 or F22 flying above 60000ft, which is class E airspace, which does allow them to turn off their transponders and use their own radar to separate. But nowadays there are no civilian aircraft left that can go there (last one was the Concorde). The SR71 fly so high they really didn't need to worry about transponder or radar at FL800 !

Technically there are some exceptions to the must have transponder rule though, but only for uncritical airspace. You cannot go without a transponder into Class A or B airspace, period.

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