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RAF Typhoons were scrambled today to intercept an Antonov 26 that lost comms, triggering a sonic boom over London.

The flight was escorted to Stansted airport, and the answer at What's the point in escorting a threatened flight with two fighter jets? says they "guide" the rogue jet to the airport.

How do they actually do this? Once they've established the jet is not a threat, how do they communicate with the other pilots?

I have an image in my head of them writing ← 120° (etc.) on a little whiteboard :P

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This is covered in the FAA's AIM, Chapter 5, Section 6. The UK has similar procedures.

First, the standard guard frequency is 121.5 or 243.0 MHz. If you aren't listening to this frequency and get intercepted, you should start listening.

Methods like LED signs or possibly as whiteboard as you suggested can also be used. Otherwise, they maneuver their aircraft to send standard signals. See the guides above for details on the signals sent by interceptors and how they should be acknowledged. Basically:

  • You have been intercepted, follow me
    • Interceptor: Rocks wings (and flashes lights irregularly at night)
    • Intercepted: Same
  • You may proceed
    • Interceptor: Breakaway, climbing turn away from aircraft
    • Intercepted: Rocking wings
  • Land here
    • Interceptor: Circling airport, lowering landing gear, overflying runway (landing lights on at night)
    • Intercepted: Lower gear, land

The intercepted aircraft can also send signals. The interceptor will respond with one of the signals above.

  • Designated airport is inadequate
    • Intercepted: Raise gear, flash landing lights
  • Cannot comply
    • Intercepted: Switching on and off all lights, but distinct from flashing lights
  • In distress
    • Intercepted: Irregular flashing of all lights

In the case of the Washington, DC Special Flight Rules Area (DC SFRA), there is a Visual Warning System (VWS) of red and green lights that will let the pilot know that they shouldn't be there. The pilot is supposed to turn directly away from the center of the SFRA. If they are in contact with ATC they must advise that they have been illuminated by the system, otherwise they should contact ATC on a guard frequency and identify themselves.

Of course, an interceptor can't force an intercepted aircraft follow instructions. It goes on to explain what will happen if you don't follow those rules, which probably apply in any situation where the risk is too high:

Further noncompliance with interceptor aircraft or ATC may result in the use of force.

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  • $\begingroup$ What happens if the intercepted flight is a neer-do-well with bad intentions, but follows the above signals? At what point will it be shot down? What reasons would a pilot have to not comply? $\endgroup$
    – Scottie
    Commented Oct 29, 2014 at 22:31
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    $\begingroup$ @Scottie how would they find out? $\endgroup$
    – ratchet freak
    Commented Oct 29, 2014 at 22:45
  • $\begingroup$ @Scottie I'd guess there's procedures in place to handle deviations from issued orders (like, target stops following for 20 seconds, doesn't respond to new orders, etc.). $\endgroup$
    – jwenting
    Commented Oct 30, 2014 at 8:47
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    $\begingroup$ There are "rules of engagement" for interceptors in various situations, taking into account the severity of the violation for which the aircraft is being intercepted as well as the potential for harm to others as a result, so the question of when they would take the step of blowing a pilot out of the sky is situation-dependent. Nick the edge of DFW's Class B bubble in a 172 and you'll get a talking to when you land. Fly directly into the DC FRZ at 250 feet ignoring comms, and you're likely to get a very close look at a Raptor or F-16 within seconds, especially if the President's home. $\endgroup$
    – KeithS
    Commented Jun 22, 2015 at 23:35
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    $\begingroup$ Lastly, the only two scenarios I can think of where an aircraft would at least appear to ignore an interceptor's instructions are (1) a non-pilot at the controls with the PIC incapacitated (as is fantasized in so many movies and books) who doesn't understand the meaning of the intercepting aircraft's signals, or (2) a complete electrical system failure rendering the pilot unable to signal with the lights (they can still signal with wing rocks etc). Neither are likely, but they aren't impossible. $\endgroup$
    – KeithS
    Commented Jun 22, 2015 at 23:56
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AIM section 6 describes what the pilot in the intercepted plane should do:

First thing the intercepting craft will do is rock the wings irregularly (and flash the nav lights at night) and initiate a turn (usually on the left). The intercepted aircraft should copy that and follow. That signals that an intercept has taken place and the other plane is compliant.

If time is of the essence then the fighter can do a climbing turn in front of the other aircraft in the direction it should turn.

If the escort is at an end (and allows the craft to fly on) then the fighter will break away with a 90° turn, the response is to wiggle the wings again and continue on.

To signal the intercepted plane to land the fighter will circle the airport and do a flyover over the runway with lowered gear (imitating a go-around). The other plane may do a flyover to make sure it's clear to land and then go land.

If the intercepted pilot believes he can't land on the runway he should stow his gear and flash his landing lights during the flyover.

It's possible the intercepting plane will hand over control to the tower and let it use the light gun to guide it down.

If at any point the pilot cannot comply he should switch on and off his lights at a regular interval. If the intercepted pilot is in distress then he should flash all available lights

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