Class A Aircraft - Take-off — Page 395, Lesson 491

Right, let's pick this up with the take-off mass limitations. We've already worked out the regulated take-off mass, so now we're moving into the next stage of the performance calculation. First, I want to cover the runway strength limitation. The operating mass of the aircraft may be limited by how strong the pavement is. The bearing strength of a pavement is expressed by a PCN, which stands for Pavement Classification Number. This is compared to the ACN, the Aircraft Classification Number. The UK system of classification is the LCN, the Load Classification Number, but this can be converted into the PCN system. So the PCN is compared to the ACN. Operation on the pavement is permissible if the ACN is less than or equal to the PCN. Because the PCN includes a safety factor, a 10% increase of ACN over PCN is generally acceptable for pavements that are in good condition. And for occasional use, aircraft with ACNs up to 50% greater than the PCN may be permitted. In such circumstances, the movement of the aircraft must be very closely monitored for damage to the aeroplane and pavement. Now, let's look at the Maximum Take-off Mass. Consideration of the mass determined by the field length available, the climb requirement, the tyre speed limit, and the brake energy limit will determine the maximum performance mass for take-off. It will be the lowest of the masses given by the above limitations. This mass is called the performance limited mass. The performance limited mass must then be compared to the maximum structural mass, and the lower of the two masses is then selected as the take-off mass. This mass is known as the regulated take-off mass. If there are obstacles to be considered on the take-off flight path, this may determine a further limitation on take-off mass. Analysis of obstacle clearance limited mass is examined in Chapter 15. Now, when the maximum permissible take-off mass, the regulated take-off mass, has been determined, it is necessary to find the corresponding take-off speeds and thrust settings. CAP 698 on pages 17, 18, 19 and 20 of section 4 show the presentation of the take-off speeds V1, VR and V2 and the % N1 for take-off. Having chosen the regulated take-off mass, which for the purpose of an example we shall assume is 57,900 kg, we are now able to select the take-off V speeds of V1, VR and V2. Before we calculate these speeds, the speed band needs to be selected. At the bottom of page 17 of section 4 of CAP 698 is a small table, and this is reproduced in Figure 14.16. This graph is the density correction graph for the take-off V speeds. As an example, let us assume a temperature of 25 degrees Celsius at an aerodrome pressure altitude of 2000 ft. In our example, the speed band to use is speed band B. So to recap: we have the runway strength limitation with PCN and ACN, then the performance limited mass from field length, climb, tyre speed, and brake energy. We compare that to structural mass to get the regulated take-off mass. Then we use that mass to find the V speeds and thrust settings, starting with selecting the correct speed band from the density correction graph.