First, we have the definition of an Aircraft — Page 130, Lesson 150

Let’s start with the big picture. This passage is a glossary of definitions, and each one is a legal, technical term you’ll be tested on. I’m going to teach you each definition exactly as it stands, because in this subject, the precise wording is the knowledge. First, we have the definition of an Aircraft. It says: A machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the air against the earth’s surface. Let me unpack that. The key phrase is "derive support in the atmosphere from the reactions of the air." That means the machine stays up because the air itself pushes on it — that’s lift from aerodynamic reactions on surfaces that remain fixed under given conditions of flight. The crucial exclusion is "other than the reactions of the air against the earth’s surface." That rules out hovercraft or ground-effect machines that rely on air bouncing off the ground. So an aircraft is supported by the air alone, not by air reacting against the ground. Next, the Aircraft Classification Number, or ACN. This is a value assigned to an aeroplane to show its load force. Think of it as a number that tells you how heavy a load the aircraft imposes on a pavement. Now, this number must be compared to the Pavement Classification Number, or PCN, of an aerodrome — that’s the number telling you how much load the pavement can bear. The rule here is important: the ACN may exceed the PCN by as much as 50%, but only if the manoeuvring of the aeroplane is very carefully monitored. If you exceed that limit without careful monitoring, significant damage may occur to both the aeroplane and the pavement. So the 50% is not a free pass — it’s a conditional allowance tied to careful monitoring. Next, the Airframe. This is a long list, so let me go through it piece by piece. The airframe is the fuselage, booms, nacelles, cowlings, fairings, aerofoil surfaces — including rotors but excluding propellers and rotating aerofoils of engines — and the landing gear of an aircraft, and their accessories and controls. So the fuselage is the main body, booms are the tail support arms, nacelles are the engine housings, cowlings are the removable covers over engines, fairings are the smooth aerodynamic covers, and aerofoil surfaces are the wings and rotors. Note the exclusions: propellers and the rotating aerofoils of engines — like the fan blades inside a jet — are not part of the airframe. And the landing gear, with all its accessories and controls, is included. Now, the Air Minimum Control Speed, abbreviated VMCA. This is a critical performance speed. It’s defined as the minimum speed at which directional control can be demonstrated when airborne with the critical engine inoperative and the remaining engines at take-off thrust. Let me break that down. The "critical engine" is the one whose failure is most adverse to handling — usually an outboard engine on a multi-engine aircraft. So you’re flying with that engine failed, the other engines at full take-off thrust, and you need to maintain directional control — meaning you can keep the nose pointed where you want. To establish this speed, you’re permitted full opposite rudder and not more than 5 degrees of bank away from the inoperative engine. So you can use all the rudder you have, and you can bank slightly away from the dead engine to help counter the yaw, but no more than 5 degrees. There are also two limits on VMCA. It may not exceed 1.2 times VSI, or 1.13 times VSR. Let me define those. VSI is the stall speed in a specific configuration — the "I" typically denotes a particular configuration or condition. VSR is the reference stall speed. So VMCA is capped relative to these stall speeds to ensure you always have a margin above stalling. In plain terms, the minimum control speed can’t be so high that it crowds the stall. Finally, the Alternate Airport. This is simply an airport at which an aircraft may land if a landing at the intended airport becomes inadvisable. So it’s your backup destination — the place you divert to if your planned destination becomes unsafe or unavailable for any reason. Now, let me tie these together. The ACN/PCN relationship is about pavement strength and load. VMCA is about your ability to control the aircraft after an engine failure. The airframe definition tells you exactly what structure is included in that term. And the aircraft definition sets the legal boundary of what counts as an aircraft at all. Each of these will come back in later chapters, so hold onto the exact wording. I want to show you a figure that illustrates the field limit mass — that’s the mass limit set by runway length and obstacle clearance. Let me bring that up for you. That figure shows how field limit mass is determined — it’s the maximum mass at which you can still meet the take-off distance and accelerate-stop distance requirements. And here’s another figure showing the take-off climb limit and climb limit mass. And this one shows the tyre speed limit mass — the mass limit imposed by the maximum speed your tyres can safely handle during take-off. These figures are all part of the same performance framework — each limit mass constrains how heavy you can be for a given runway and conditions. But for now, the definitions we just covered are the foundation. Let me know if you want me to go deeper on any of them.