Let's pick this up right where the flight plan gets put into action — Page 115, Lesson 182

Let's pick this up right where the flight plan gets put into action. We've filed the plan, we've got our clearance, and now we're looking at the two phrases you'll hear from ATC that tell you exactly which published route to fly. First, "Cleared via (designation) departure." That means you're cleared to fly a Standard Instrument Departure route — we call it a SID — and that route is published in the AIP, the Aeronautical Information Publication. So the word "designation" is just the published name or code of that specific departure procedure. Second, "Cleared via (designation) arrival" — that's a Standard Instrument Arrival route, a STAR, also published in the AIP. So a SID gets you from the airport up onto the en-route structure, and a STAR brings you down from the en-route structure into the terminal area. Both are pre-published, and your clearance names which one you're flying. Now, once you're flying that plan, we get to paragraph 6.38, Adherence to Flight Plan. The rule is simple: you adhere to the flight plan. The only exception is an emergency situation that necessitates immediate action by the aircraft. And even then, you must inform the ATCU — that's the Air Traffic Control Unit — as soon as possible. So the emergency justifies the deviation, but it does not excuse you from telling ATC. There are two specific reporting triggers you need to know. First, if your average true airspeed — your average TAS — at cruising level between reporting points varies, or is expected to vary, by plus or minus 5% of the TAS given in field 7 of the flight plan, you must inform the ATCU. Field 7 is the speed block on the flight plan form. So the moment you know you'll be more than 5% off that filed speed, you call. Second, if your estimate for the next applicable reporting point, or the FIR boundary — that's a Flight Information Region boundary — or the destination aerodrome changes by more than 3 minutes from what you've already notified, you must communicate the revised time to the appropriate ATCU. So a 3-minute change in your ETA triggers a call. And there's a note here that's important for North Atlantic operations: in Operational Procedures for operations in the MNPSA — that's the Minimum Navigation Performance Specification Airspace — the requirement is 3 minutes or more. So in MNPSA, the same 3-minute threshold applies, and the note is emphasising that this is the standard you hold to there. Now paragraph 6.39, Inadvertent Changes. Controlled flights — that's flights operating under air traffic control — are required to operate along the centre line of an airway, or directly between beacons if that's how the route is delineated. So if the route is defined as a straight line between two beacons, you fly that line. When you're navigating with VOR beacons — VHF Omnidirectional Range — and a changeover is specified, you change from one VOR to the next at the specified changeover point, unless otherwise directed. These changeover points are normally every 60 nautical miles. So you don't switch early and you don't switch late — you switch at the published point. Now, if you deviate from any of these requirements, you report it to ATC. And here's the key phrase: if a controlled flight inadvertently deviates from its current flight plan, action is to be taken immediately to regain the track as soon as practicable. So the moment you realise you've drifted off the centre line or off the published route, you don't wait — you take action to get back on track, and you tell ATC. So the whole picture is: your clearance names the SID or STAR, you fly the centre line or the beacon-to-beacon line, you switch VORs at the changeover point, you report any deviation, and you watch your speed and your estimates for the 5% and 3-minute triggers. That's how adherence to the flight plan works in practice.