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On What is the significance of a squawk code? @DeltaLima gave a fairly concise answer. In it he listed several 'standard' codes used for things like emergencies.

Also in the list was Code 2000, for which he included the description:

Used when entering a Secondary Surveillance Area and no code has yet been assigned

So, if I'm in the US, under VFR (code 1200) and I enter a Secondary Surveillance Area, I automatically change to Code 2000? Why wouldn't I have already been given (a specific) designator?

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The purpose of squawk code 2000 is to prevent aircraft entering a Secondary Surveillance Radar (SSR) area from transmitting a code that is the same as a discrete code assigned by ATC to an individual aircraft.

If you are flying in the USA under Visual Flight Rules (VFR), you will be assigned (implicitly) code 1200. Upon entering a SSR area you might get assigned another code, but often you will keep code 1200 if you are not near class C airspace.

If you are flying Instrument Flight Rules (IFR) outside a SSR area (e.g. entry into the US from oceanic airspace), you will use code 2000. Upon radar contact you will then get assigned a discrete code.

From FAA Advisory circular 91-70A - (Oceanic and International Operations):

2. EXPANDED OCEANIC CHECKLIST.

.......

g. After Oceanic Entry

(1) Squawk 2000. Thirty minutes after oceanic en try, crews should Squawk 2000, if applicable. There may be regional differences such as Squawking 2100 in Bermuda’s airspace or maintaining last assigned Squawk in the West Atlantic Route System (WATRS). Crews transiting Reykjavik’s airspace must maintain last assigned Squawk.

Another use of code 2000 is on the airport: before power up you can select code 2000 to avoid code conflicts that could occur when you would maintain your code from a previous flight. It is standard practice in many airlines to select code 2000 after arrival at the gate.

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    $\begingroup$ The SOP written in to every company that I've worked so far is to automatically change to 2000 30 minutes after entering oceanic airspace. I'm not sure if this is required or not (I don't have a reference) but it is how the international procedures have been written that I have seen, and I actually haven't had the code specifically assigned to me ever. $\endgroup$ – Lnafziger May 5 '15 at 18:59
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    $\begingroup$ @Lnafziger It is usually after 30 minutes indeed, unless the controller assigns you A2000 on handover. See FAA Advisory circular AC 91-70A $\endgroup$ – DeltaLima May 5 '15 at 19:05
  • $\begingroup$ That could be useful information to add to your answer. :) $\endgroup$ – Lnafziger May 7 '15 at 3:06
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You'll often hear ATC (Oakland Center or Los Angeles Center) tell aircraft that are heading out over the Pacific something along the lines of "UAL ###, radar services are terminated, squawk 2000, contact ARINC on HF frequencies" when they hand off control to the oceanic ATC. The 2000 code simply means that the aircraft IS being controlled, but NOT right now by any radar controller.

If they kept whatever squawk they had leaving Oakland or LA center, then there would be the possibility of a conflict as they entered somebody else's radar coverage (Hawaii, Guam, Japan, etc). So instead of sorting out what the aircraft that are no longer in radar contact are squawking, they simply give them all the standard 2000 code, and when the aircraft is back in radar contact with (whoever), then that controller assigns them a code that works with his center.

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The US ATC procedures say that 2000 is to be used for departing IFR aircraft (section 5-2-6):

  1. Code 2000 to an aircraft which will climb to FL 240 or above or to an aircraft which will climb to FL 180 or above where the base of Class A airspace and the base of the operating sector are at FL 180, and for inter-facility handoff the receiving sector is also stratified at FL 180. The en route code must not be assigned until the aircraft is established in the high altitude sector.

It looks like the specific information you quoted from the other answer simply doesn't apply in the US. The term "secondary surveillance radar area" isn't in the P/CG and Wikipedia says that the use of 2000 in relation to SSR areas is for ICAO countries only:

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 (ICAO countries)

It does give this information for the US, though:

Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)

The Order it mentions is the ATC instructions. I have no idea about oceanic operations, so I can't comment on that part.

As for VFR, the AIM simply says that all VFR traffic must squawk 1200 unless otherwise instructed by ATC.

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    $\begingroup$ Are you distinguishing between "the US" and "ICAO countries"? The USA is one of the original signatories to the Chicago Convention and a full member of ICAO. $\endgroup$ – Henning Makholm May 5 '15 at 17:52
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    $\begingroup$ @HenningMakholm Yes, that's true, but there's always been a difference between US and ICAO practices, e.g. this recent, well-known example. It's still common to talk about "US vs ICAO" when discussing regulations, even though the US is indeed a member. To be fair, the FAA is more accurate and usually talks about FAA vs ICAO $\endgroup$ – Pondlife May 5 '15 at 18:01

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