Performance - Introduction — Page 130, Lesson 151

Let’s start at the very beginning of the Performance section, because these definitions are the foundation for every chart and calculation you’ll do later. I’m going to walk you through each one, in the order they appear, and I want you to hear these as the exact professional terms you’ll be tested on. First, Altitude. On all 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 your altimeter, set its subscale to 1013 hPa — that’s the standard sea-level pressure — and the reading you get is pressure altitude. It’s not your true height above the ground; it’s the height in the standard atmosphere that corresponds to the pressure you’re experiencing. 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 moving past the wing. So angle of attack is simply the angle between those two. Then we have Apron. This is a defined area on a land aerodrome 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 the aircraft sits at the terminal. Auxiliary Power Unit — the APU. This is any gas turbine-powered unit delivering rotating shaft power, compressor air, or both, which 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, but it doesn’t produce thrust for flight. 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. I’ll unpack that: accelerate-stop distance is the distance you need to accelerate and then stop if you abort the take-off; take-off distance is what you need to get airborne and clear an obstacle. When those two are equal, you have a balanced field. Baulked Landing. This is a landing manoeuvre that is unexpectedly discontinued. So you’re coming in to land, and for some reason you have to go around — that’s a baulked landing. Brake Horsepower. This is the power delivered at the main output shaft of an aircraft engine. It’s the actual mechanical power coming out of the engine, measured at the shaft. 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 as you slow down or speed up into certain conditions, the airflow breaks away from the wing, and you feel that buffeting. Calibrated Airspeed — CAS. This is the indicated airspeed, corrected for position and instrument error. And importantly, it is equal to True Airspeed, TAS, at Mean Sea Level in a Standard Atmosphere. So at sea level, standard conditions, CAS equals TAS. Climb Gradient. This is the ratio, in the same units of measurement, expressed as a percentage, obtained from the formula: gradient equals vertical interval divided by horizontal interval, times 100. So if you climb 1000 feet over a horizontal distance of 10,000 feet, that’s 10%. Clearway. This is an area beyond the runway, not less than 152 metres — that’s 500 feet — 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 that plane, no object or terrain may protrude. However, threshold lights may protrude above the plane if their height above the end of the runway is 0.66 metres — 26 inches — or less, and if they are located to each side of the runway. 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 of the lowest cloud layer that covers more than half the sky. Contaminated Runway. A runway is considered contaminated when more than 25% of the runway surface area is covered by surface water more than 3 millimetres deep. Finally, two ratings that go together. 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 Continuous One Engine Inoperative Thrust Rating — the same definition, but for thrust instead of power. So one is power, one is thrust, and both are the minimum certified values for continuous operation with one engine out. That’s the full set of definitions for this opening section. Every one of these terms will come back when we start working with the actual performance charts, so it’s worth getting them solid now.