Performance - Introduction — Page 136, Lesson 155

We're starting the Performance section of your Mass and Balance and Performance book, and the very first thing we encounter is a glossary of terms. These definitions are the foundation for everything we'll do in performance calculations, so I want to walk you through each one carefully. We'll take them in order. First, Fixed Pitch Propeller. This is simply a propeller whose blade angle, or pitch, cannot be changed in flight. The blades are set at a fixed angle, and that's it. You'll contrast this later with variable-pitch or constant-speed propellers, but for now, just know that a fixed pitch propeller has a non-adjustable blade angle. Next, Flap Extended Speed. This is the highest speed permissible with the wing flaps in a prescribed extended position. In other words, it's the maximum airspeed you're allowed to fly at while the flaps are deployed to a specific setting. Exceeding this speed could cause structural damage to the flaps, so it's a critical limit. Now, Flight Level. This is a surface of constant atmospheric pressure that is related to 1013.25 hPa. It's conventionally expressed as the pressure altitude to the nearest 1000 ft, in units of 100 ft. So, for example, flight level 250 represents a pressure altitude of 25,000 feet. The key here is that it's based on a standard pressure setting of 1013.25 hectopascals, not on the actual local pressure. Next, Frangibility. This is about objects near runways, like approach lights or masts. Frangibility is the ability of an object to retain its structural integrity and stiffness up to a specified maximum load, but when subjected to a load greater than that, or if struck by an aircraft, it will break, distort, or yield in such a manner as to present minimum hazard to the aircraft. So, it's designed to fail safely if an aircraft hits it, reducing the damage to the aircraft. Then we have Go-around. This is a procedure involving a decision to abort the landing and climb straight ahead to rejoin the circuit. Such a decision might be taken at any time during the final approach, the transition phase, or even after initial touchdown. So, it's the maneuver you execute when you decide not to land, and you climb away to try again. Next, Gross Height. This is the true height attained at any point in the take-off flight path, using gross climb performance. Gross height is used for calculating pressure altitudes for purposes of obstacle clearance and the height at which wing flap retraction is initiated. So, it's the actual height you'd achieve based on the average performance data, and it's used for planning obstacle clearance and flap retraction. Related to that is Gross Performance. This is the average performance that a fleet of aeroplanes should achieve if satisfactorily maintained and flown in accordance with the techniques described in the manual. So, it's the baseline, average performance data used in calculations, not the best-case or worst-case scenario. Now, a very important one: Ground Minimum Control Speed. This is the minimum speed at which the aeroplane can be demonstrated to be controlled on the ground using only the primary flight controls when the most critical engine is suddenly made inoperative and the remaining engines are at take-off thrust. Throttling an opposite engine is not allowed in this demonstration. Forward pressure from the elevators is allowed to hold the nose wheel on the runway, however, nose wheel steering is not allowed. So, this is the speed below which you couldn't keep the aircraft straight on the ground if an engine fails during take-off, using only rudder and elevator inputs. Next, Height. This is the vertical distance between the lowest part of the aeroplane and the relevant datum. So, it's the distance from the lowest point of the aircraft to a reference point, like the ground or a specific altitude reference. Then, Hydroplaning Speed. This is the speed at which the wheel is held off the runway by a depth of water, and directional control through the wheel is impossible. So, when you're going fast enough on a wet runway, the tires can ride on a film of water and lose contact with the pavement, making steering ineffective. Next, the ICAO Standard Atmosphere. This is the atmosphere defined in ICAO Document 7488/2. For the purposes of Certification Specifications, the following are acceptable: the air is a perfect dry gas; the temperature at sea level is 15°C; the pressure at sea level is 1013.2 hPa, which is 29.92 inches of mercury; the temperature gradient from sea level to the altitude at which the temperature becomes –56.5°C is 0.65°C per 100 meters, or 1.98°C per 1000 feet; and the density at sea level under the above conditions is 1.2250 kilograms per cubic meter. This is the standard model of the atmosphere we use for all performance calculations, so it's crucial to know these values. Next, IFR Conditions. This stands for Instrument Flight Rules. IFR conditions are weather conditions below the minimum for flight under visual flight rules. So, when the weather is too poor to fly visually, you're in IFR conditions and must fly using instruments. Finally, Indicated Airspeed. This is the speed as shown by the pitot/static airspeed indicator, calibrated to reflect Standard Atmosphere adiabatic compressible flow at mean sea level, and uncorrected for airspeed system errors. So, it's the raw reading you see on the airspeed indicator, before any corrections for instrument or position errors. Now, let's look at the figures that illustrate some of these concepts. shows clearways, which extend from the end of the runway with an upward slope not exceeding 1.25%. shows stopways, which are able to support the aeroplane during a rejected take-off and are marked by large yellow markings. And illustrates both stopways and clearways that can be found at aerodromes. These definitions are the building blocks for the performance calculations we'll be doing. Make sure you understand each one, because we'll be referring back to them constantly.