military escort (in the US)
mode C or other SSR failure (in the UK).
shall not be used – is a non-discrete mode A code (Europe)
0001: Military code for high speed uncontrolled (non-ATC directed) flight (US)
0033: Parachute dropping in progress (UK)
Instrument Flight Rules (IFR) flight below 18,000' when no other code has been assigned (Canada)
non-discrete mode A code reserved for future use in Mode S radar environment where the aircraft identification will be used to correlate the flight plan instead of the mode A code
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1200: Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace (FAA) when no other has been assigned.
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1400: VFR flight above 12,500' when no other code has been assigned (Canada) .
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2000: The code to be squawked when entering a secondary surveillance radar (SSR) area from a non-SSR area (used as Uncontrolled IFR flight squawk code in ICAO countries , and in Canada for uncontrolled IFR at or above 18,000')
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4000: Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (US)
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4400 to 4477: Reserved for use by SR-71, YF-12, U-2 and B-57, pressure suit flights, and aircraft operations above FL600 (USA only).
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VFR standard squawk code when no other code has been assigned (ICAO) .
UK: this code does not imply VFR; 7000 is used as a general conspicuity squawk.)
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Sudden military climb out from low-level operations (UK)
Used in some countries to identify VFR traffic (France, ...)
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7004: Aerobatic and display code in some countries.
7010: VFR circuit traffic code in the UK
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707X: Paradrop activities in France (7070, 7071, 7072...)
military interception (US) ("Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.")
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non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE). (Germany, Netherlands, Belgium, ...)
In Belgium following codes are assigned for VFR traffic under Flight Information Services (BXL FIC)
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series from 0041 till 0057
In Germany the following codes have been used:
0021: VFR squawk code for German airspace (5000 feet and below)
0022: VFR squawk code for German airspace (above 5000 feet)
As from 15 March 2007 these have been replaced by the international 7000 code for VFR traffic.
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7700: General Emergency
7600: Lost Communications
7500: Unlawful Interference (Hijack)
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Codes assigned by ATC
Most codes above can be selected by aircraft if and when the situation requires or allows it, without permission from ATC. Other codes are generally assigned by ATC units.
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For IFR flights, the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight. VFR flights, when in uncontrolled airspace, will "squawk VFR" (1200 in the US, 7000 in Europe).
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Upon contact with an ATC unit, they will be told to squawk a certain unique code. When changing frequency, for instance because the VFR flight leaves controlled airspace or changes to another ATC unit, the VFR flight will be told to "squawk VFR" again.
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In order to avoid confusion over assigned squawk codes, ATC units will typically be allocated blocks of squawk codes, not overlapping with the blocks of nearby ATC units, to assign at their discretion.
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Not all ATC units will use radar to identify aircraft, but they assign squawk codes nevertheless.
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As an example, London Information – the Flight Information Service station that covers the lower half of the UK
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– does not have access to radar images, but does assign squawk code 0027 to all aircraft that receive an FIS from them. This tells other radar equipped ATC units that that specific aircraft is listening on the London Information radio frequency, in case they need to get hold of that aircraft.