OCA stands for Oceanic Control Area — Page 141, Lesson 218

I want to walk you through two important procedural areas for flying in the North Atlantic region: how to handle your SSR transponder code, and what's changed with meteorological reporting. Let's start with SSR. When you're flying in the NAT region — that's the North Atlantic — the last SSR code that was assigned to you by a domestic Air Traffic Control Unit is to be maintained for a period of 30 minutes after you enter the OCA. OCA stands for Oceanic Control Area. So for the first half hour after you leave domestic radar coverage and enter oceanic airspace, you keep the code they gave you. After those 30 minutes have passed, you set and maintain Mode A 2000 plus Mode C. Mode A 2000 is the standard conspicuity code for aircraft operating in oceanic airspace when no specific code has been assigned. Mode C gives altitude information. So after the 30-minute grace period, you squawk 2000 with altitude reporting. Now, here's an important point about emergencies. Even if your aircraft is well out of range of any ground radar station — which is very common over the middle of the Atlantic — you should still squawk the appropriate emergency code if you have an emergency, a radio failure, or unlawful interference. The standard codes are 7700 for emergency, 7600 for radio failure, and 7500 for unlawful interference. The point is: you don't stop squawking just because nobody can see you on radar. It's still the correct procedure. Now let's move to meteorological reports. As of 18 November 2010, there was a significant change. Aircraft are no longer required to provide voice reports of MET observations — that's meteorological observations — specifically wind speed and direction, nor outside air temperature. So you don't have to read those out over the radio anymore. However, you are still required to report any turbulence or other significant meteorological conditions you encounter. That should be reported to ATC. And the format for reporting these observations should, where appropriate, be by reference to geographical coordinates — so latitude and longitude, not just a rough position. Now, here's where data link comes in. When a ground unit establishes an event contract with an aircraft to provide ADS position reports, it may also establish an additional periodic report contract — for example, with a 30-minute interval. ADS-C stands for Automatic Dependent Surveillance — Contract. An event contract triggers a report when something specific happens, like a change in altitude or heading. A periodic contract sends reports at regular intervals. These ADS-C periodic reports, unlike event reports, contain wind and temperature data. That means they satisfy the MET authorities' requirements for the provision of meteorological data. Similarly, "FMC waypoint position reports" sent via data link — FMC is the Flight Management Computer — also include wind and temperature data. Aircraft participating in such a data link programme are deemed to meet the MET authorities' requirements. So the system automatically handles the wind and temperature reporting that used to be done by voice. But here's the critical caveat: any such automated MET reports do not include information on any turbulence or any other unusual meteorological phenomena. The data link message doesn't know if you're bouncing around up there. So if you are providing position reports via data link and you encounter turbulence or anything unusual, you must report that information separately — either by voice, or if appropriate, via a CPDLC free text downlink message. CPDLC is Controller-Pilot Data Link Communications. The free text function lets you type a custom message. So to summarise: the automated system handles wind and temperature for you, but turbulence and unusual weather still need a human-initiated report, either by voice or by CPDLC free text.