Control of Aircraft — Page 343, Lesson 460

Let's pick up with the Air Traffic Advisory Service. This is a service provided within advisory airspace, which is class F airspace. Its purpose is to ensure separation between aircraft operating on IFR flight plans — that's Instrument Flight Rules, where the pilot flies by reference to instruments rather than visual cues. Now, here's the key distinction. This service is established in response to a request where a full ATC service hasn't been decided upon yet. So it's a temporary arrangement, pending a decision on whether a full ATC service is required. The objective is to make information on collision hazards more effective than it would be with just a flight information service. It's provided to IFR flights in advisory airspace or on advisory routes. Now, operationally, this service does not afford the same degree of safety, and it does not assume the same responsibilities as a full ATC service. That's crucial. Because of that, clearances are not issued. Instead, only advice or suggestions are offered by the Air Traffic Control Officers — the ATCOs. If an IFR flight elects to use this service, it's expected to comply with the same procedures as would be in force in Controlled Airspace, or CAS. The one exception is that Flight Plan changes are not subject to clearance. All IFR flights flying in class F airspace are required to file Flight Plans. Traffic electing to use the advisory service will receive only an 'acknowledgement' of the submission of the Flight Plans — not a clearance, just an acknowledgement. And here's a specific rule: IFR flights crossing an advisory route are required to do so at 'right angles' to the route direction, at a level appropriate to the semi-circular rule. That semi-circular rule is the standard altitude separation based on track direction — odd thousands for one direction, even thousands for the other. Now let's move to Aircraft Emergencies. The most expeditious way to indicate that an aircraft requires assistance is the use of the reserved SSR codes. SSR is Secondary Surveillance Radar — the transponder-based system. Clearly, the aircraft must be flying in an area where radar is used for ATC, and altitude can be a factor here. Even outside radar control areas, there are military facilities and mobile units that can interrogate and receive SSR information, which can assist the alerting service in providing the necessary assistance. The reserved codes are these. Mode A code 7700 means Emergency. Mode A code 7600 means Radio failure. Mode A code 7500 means Unlawful Interference. Additionally, Mode A code 7000 is the conspicuity code for an aircraft operating in an area where a radar service is available but the aircraft is not in receipt of that service — commonly referred to as VFR conspicuity. And Mode A code 2000 is used when an aircraft is operating in an area where a radar service is not available, but will be entering an area where a radar service is available and will be requesting that service. This is used for eastbound NAT traffic east of 30W that intends to enter the Scottish UIR — that's the Upper Information Region. Now, one important caveat. If a pilot is already in receipt of a radar service and has been identified by the use of a discrete SSR code — that's a unique code assigned to that specific aircraft — then selecting a reserved code to indicate an emergency may actually delay the application of assistance. In that case, the allocated Mode A code should be retained. So the rule is: if you're already identified with a discrete code, keep it rather than switching to a reserved code.