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If a commercial jet is in trouble, how does a pilot send a signal indicating distress? Does it take long?

For instance, behind reception desks in many institutions and executive offices, there's a hidden red button that, when pressed, calls security or the police. (I know this since I sit at such a desk.) Do commercial airliners have a similar switch, or do they need to radio in for help? What happens when such a transmission is received?

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3 Answers 3

up vote 23 down vote accepted

There are three common ways to send a distress signal (roughly in order of preference):

  1. Over the radio, to whoever you are currently talking to.
    Key the microphone and announce to the world Mayday, Mayday, Mayday, the flight's callsign, and a description of the emergency. This is generally the way an emergency is declared, particularly for commercial (airline) service, as they're generally always in contact with an ATC facility

  2. Over the radio, to anyone monitoring the "emergency" frequency.
    The VHF frequency 121.5MHz is reserved for aircraft emergency communication. It is monitored at most air traffic control facilities, as well as by many airliners and other pilots with multiple radios in their aircraft. If you're not able to make contact with the facility you're currently talking to in (1) above (or if you're not currently talking to an ATC facility).

  3. Using the aircraft's transponder.
    Three transponder codes are reserved for unusual/emergency situations:
    7700 for general emergencies
    7600 for loss of communication (radio failure)
    7500 for hijacking or other unlawful interference
    All of these transponder codes alter the way the aircraft's radar target is displayed, to alert the controller to the possible problem (exactly how the display is modified depends on the radar equipment the controller has, and can range from additional text in the data block, like RDOF (RaDiO Failure) for code 7600 to changing the color of the displayed block on newer systems.
    Bret detailed some of the procedures around these codes in his answer, and more information is available from the FAA, though you'll have to dig through a few referenced documents to put it all together.


Note that sometimes the order of preference changes - For example if a pilot is being threatened by a hijacker they may quietly set their transponder code to 7500 without saying anything on the radio to alert the hijacker.

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20  
A nice mnenonic for the transponder codes: seven five, man with knife; seven six, radio to fix; seven seven, going to heaven. –  Jelmer Mar 11 at 13:50
    
The transponder is only effective within radar range, right? How does that compare with the range of the radios? –  David Richerby Mar 11 at 17:13
1  
@DavidRicherby Radar coverage depends on where you are - for example on some oceanic routes you're likely to be out of radar range, but still in radio contact with someone (either the oceanic controllers or other aircraft). If you're using any of these transponder codes you're likely to be heading back toward an airport though, and once you're in radar range the code will be detected and handled appropriately. –  voretaq7 Mar 11 at 19:33
    
If you're already in contact with a controlling agency, you aren't required to say Mayday, its much simpler, and often much less chaotic (since everyone cringes when they hear mayday) to simply say "Callsign xxx declaring an emergency". Plus you'll also sound like a huge dork if you go full on mayday after having just said something to center 30 seconds ago. –  SHAF Nov 14 at 1:32
    
What about pan pan. –  CGCampbell Nov 14 at 15:20

It depends on the type of distress, but generally the easiest way to communicate a problem is to use the radio and describe the situation. However, there are some special transponder codes which will get the attention of any controller which has you on their radar screen.

  • 7500 indicates a hijacking
  • 7600 indicates a communications radio failure
  • 7700 indicates an emergency

What happens after switching to one of these beacon codes depends on the scenario. In any of the cases, ATC will attempt to make contact with the aircraft. In most cases, they will also try to vector other traffic away from the aircraft for everyone's safety (especially true in the case of a hijacking).

I assume your question is primarily asking about a hijacking scenario. A recent, and very odd, example of 7500 being used is Ethiopian Airlines’ Flight 702 where the co-pilot actually hijacked the plane and squawked 7500. Unfortunately, pilots don't always have the time to reset the transponder if they are engaged in a physical fight in the cockpit. Also, the first step of the hijackers may be to turn off the transponder (this is what happened on 9/11), but if a controller is able to see the 7500 squawk, even briefly, it may help the controller understand what is happening if communications are lost, and to alert the military, if necessary.

As for the other two "distress" signals:

For 7600 (lost communications) they will probably ask "if you're receiving this, ident" or "if able, reset transponder code XXXX." If you comply with their request, it indicates that you can receive messages, but cannot transmit. A tracon or center controller might ask you to ident to acknowledge further instructions (when you ident, your datablock flashes on their screen), or just watch your or just watch your actions closely. If you're on with a tower controller in visual conditions, rocking your wings or flashing your landing light can be used to acknowledge instructions. At towered airports, light-gun signals are used to give landing clearances when a pilot cannot receive radio communication.

For 7700 (emergency) they will try to inquire as to the state of the emergency and offer any assistance possible, such as providing you suggested headings or information about nearby airports.

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There is another procedure which has not been mentioned although I doubt it's ever been used.

If you are lost and the radio has failed and the transponder has failed, flying at least 2 triangles is a recognised distress signal.

Since the transponder has failed, this does require that someone sees you on primary radar which is a long shot.

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