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

Right, let's get into the take-off performance for a Class A aircraft. We've already worked out the regulated take-off mass, so now we need to look at what limits that mass in the first place, and then how we turn that mass into actual speeds and thrust settings. First, let's talk about the runway itself. The operating mass of the aircraft may be limited by runway strength. 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. So the pavement has a PCN rating, and your specific aircraft has an ACN rating. There's also the UK system, which uses the LCN, the Load Classification Number, but that can be converted into the PCN system. The rule is simple: operation on the pavement is permissible if the ACN is less than or equal to the PCN. Now, 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. But in those circumstances, the movement of the aircraft must be very closely monitored for damage to both the aeroplane and the pavement. Now, let's move to the maximum take-off mass. This is where we bring together several limits. 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 these limitations. That lowest value 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 is selected as the take-off mass. This final value is known as the regulated take-off mass. And if there are obstacles on the take-off flight path, that may impose a further limitation on take-off mass, but the analysis of obstacle clearance limited mass is examined in Chapter 15. Once we have that regulated take-off mass, we need to find the corresponding take-off speeds and thrust settings. CAP 698, on pages 17, 18, 19 and 20 of section 4, shows the presentation of the take-off speeds V1, VR and V2, and the % N1 for take-off. So V1, VR and V2 are our take-off V speeds, and N1 is the fan speed, expressed as a percentage. Let's work through an example. We've chosen a regulated take-off mass of 57,900 kg. Before we calculate the V speeds, we need to select the speed band. At the bottom of page 17 of section 4 of CAP 698 is a small table, reproduced in Figure 14.16. This is the density correction graph for the take-off V speeds. Let's 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 the process is: determine the regulated take-off mass from the lowest of the performance limits and the structural limit, then use that mass with the density correction graph to select the correct speed band, and from there we can read off V1, VR, V2 and the % N1.