Performance - Introduction — Page 130, Lesson 151

Let’s start at the very beginning of the Performance section, because every chart and calculation you’ll do later depends on these definitions. I’m going to walk you through them in the order they appear, and I’ll make sure each term is nailed down precisely. First, Altitude. On the performance charts, the altitude shown is pressure altitude. That’s the height in the International Standard Atmosphere at which the prevailing pressure occurs. In plain terms: you take the actual air pressure around you, and you ask, “In the standard atmosphere, at what height would that pressure be found?” That height is your pressure altitude. You get it by setting the subscale of a pressure altimeter to 1013 hPa — that’s the standard sea-level pressure setting. So when your altimeter subscale reads 1013, the altimeter is showing pressure altitude, not the height above sea level you’d get with a local QNH setting. Next, Angle of Attack. This is the angle between the chord line of the wing and the relative airflow. The chord line is the straight line from the leading edge to the trailing edge of the wing. The relative airflow is the air that the wing is moving through — effectively the direction the air comes at the wing. The angle between those two is the angle of attack. It’s a fundamental aerodynamic parameter, and you’ll see it referenced throughout performance work. Then we have Apron. On a land aerodrome, the apron is a defined area provided for the stationing of aircraft — for the embarkation and disembarkation of passengers, the loading and unloading of cargo, and for parking. So it’s the paved area where aircraft sit at the terminal, where people get on and off, and where cargo is handled. Next, the Auxiliary Power Unit, or APU. This is any gas turbine-powered unit that delivers rotating shaft power, compressor air, or both — and it is not intended for direct propulsion of the aircraft. So it’s a small turbine engine that provides electrical power and bleed air on the ground, and sometimes in flight, but it doesn’t produce thrust for moving the aircraft forward. Now, Balanced Field. A runway for which the Accelerate-stop Distance Available is equal to the Take-off Distance Available is considered to have a balanced field length. Let me unpack that. Accelerate-stop distance is the distance needed to accelerate to a critical speed and then stop if you abort the take-off. Take-off distance is the distance needed to accelerate and climb to a specified height. When those two distances are equal, the field is balanced — and that’s a key concept in take-off performance calculations. Next, Baulked Landing. This is a landing manoeuvre that is unexpectedly discontinued. In other words, you’re coming in to land, and for some reason you have to abort — you go around. That’s a baulked landing. Then, Brake Horsepower. This is the power delivered at the main output shaft of an aircraft engine. So it’s the actual mechanical power coming out of the engine’s shaft, as opposed to the power generated inside the cylinders or the turbine. It’s the usable power that drives the propeller or other accessories. Next, Buffet Speed. This is the speed at which the airflow over the wing separates, creating turbulent airflow aft of the separation point, which buffets the aeroplane. So when you reach buffet speed, the airflow breaks away from the wing, and you feel a vibration or buffeting. That’s an important limit in high-speed and low-speed operations. Then, Calibrated Airspeed, or CAS. This is the indicated airspeed, corrected for position and instrument error. So the airspeed indicator shows you a raw value, and you correct it for errors in the instrument and in the placement of the pitot-static system — that gives you calibrated airspeed. And here’s a key relationship: calibrated airspeed is equal to True Airspeed (TAS) at Mean Sea Level (MSL) in a Standard Atmosphere. So at sea level, under standard conditions, CAS equals TAS. At altitude, they diverge. Next, Climb Gradient. This is the ratio, in the same units of measurement, expressed as a percentage, obtained from the formula: gradient = vertical interval ÷ horizontal interval × 100. So if you climb 100 feet while covering 1,000 feet horizontally, your gradient is 10%. It’s a measure of how steeply the aircraft climbs, expressed as a percentage. Then, Clearway. This is an area beyond the runway, not less than 152 m (500 ft) wide, centrally located about the extended centre line of the runway, and under the control of the airport authorities. The clearway is expressed in terms of a clearway plane, extending from the end of the runway with an upward slope not exceeding 1.25%, above which no object or terrain protrudes. However, there’s an exception: threshold lights may protrude above the plane if their height above the end of the runway is 0.66 m (26 inches) or less, and if they are located to each side of the runway. So the clearway is a protected area beyond the runway that allows the aircraft to climb over it without hitting obstacles — within that defined slope. Next, Cloud Ceiling. In relation to an aerodrome, cloud ceiling means the vertical distance from the elevation of the aerodrome to the lowest part of any cloud visible from the aerodrome which is sufficient to obscure more than one half of the sky so visible. So it’s the height above the aerodrome’s elevation to the base of the lowest cloud layer that covers more than half the sky. Then, Contaminated Runway. A runway is considered contaminated when more than 25% of the runway surface area is covered by surface water more than 3 mm deep. So if more than a quarter of the runway has standing water deeper than 3 millimetres, it’s classified as contaminated — and that changes your take-off and landing performance calculations significantly. Finally, two related ratings. Continuous One Engine Inoperative Power Rating — this is the minimum test bed acceptance power, as stated in the engine type certificate data sheet, when running at the specified conditions and within the appropriate acceptance limitations. And the Continuous One Engine Inoperative Thrust Rating — the same definition, but for thrust instead of power. So these are the certified minimum power or thrust levels an engine must produce, on the test bed, under specified conditions, for continuous operation with one engine inoperative. Power applies to propeller-driven engines, thrust to jet engines. So that’s the full set of definitions that open the Performance section. Each one of these will come back in the take-off, climb, and landing performance work you’ll do later. Take a moment to absorb them — especially pressure altitude, calibrated airspeed, balanced field, and the contaminated runway criteria, because those are the ones that drive the calculations.