Multi-engine Class B - Take-off — Page 349, Lesson 425

Let’s start with the obstacle-clearance rules, because they set the scene for everything else in this take-off flight path work. If your flight path does not require track changes of more than 15°, then obstacles do not need to be considered if the lateral distance is greater than 300 m in VMC, or 600 m for all other conditions. If the flight path does require track changes of more than 15°, then obstacles need not be considered if the lateral distance is greater than 600 m in VMC, or 900 m for all other conditions. So the rule is: bigger track changes, bigger lateral clearance margins; and VMC gets a smaller margin than instrument or night conditions. Now the take-off flight path profile performance should take account of four things. First, the mass of the aeroplane at the commencement of the take-off run. Second, the pressure altitude at the aerodrome. Third, the ambient temperature. And fourth, a wind correction: not more than 50% of the reported headwind component, and not less than 150% of the reported tailwind component. So you never credit the full headwind, and you penalise yourself with a tailwind that is 1.5 times what is reported. The construction of the flight path depends on whether or not visual reference is lost before reaching 1500 ft. Let’s take the case where visibility is clear to 1500 ft. First, determine the take-off distance required, the TOD, for the take-off mass. Then determine the all-engines net gradient, which is the gross gradient multiplied by 0.77. Then divide the height gain, which is 1450 ft, by the gradient to determine the distance travelled in feet from 50 ft to 1500 ft. The profile may be plotted as shown in Figure 11.2 and clearance of obstacles assessed. Alternatively, for a single obstacle, find the TOD required and the gradient as above, then multiply the distance from reference zero to the obstacle by the gradient to find the height gain, and add 50 ft to find the aeroplane height at the obstacle distance. This must exceed the obstacle height by 50 ft. If the obstacle is not cleared by 50 ft, a lower take-off mass must be assumed and a revised height calculated. The maximum mass which will just clear the obstacle by 50 ft can then be determined by interpolation. Now the case where the cloud base is below 1500 ft. If visual reference is lost before 1500 ft, the flight path will consist of two segments. Segment 1 runs from 50 ft to cloud base. The distance from 50 ft to cloud base equals the height gain divided by the all-engines net gradient, multiplied by 100. The height gain here is cloud base minus 50 ft. Segment 2 runs from cloud base to 1500 ft. The distance equals the height gain divided by the gross gradient with one engine inoperative, multiplied by 100. The profile may be plotted as shown in Figure 11.3 and clearance of obstacles assessed. If the required clearance is not achieved, a reduced take-off mass must be assumed and a second flight path calculated. As before, the maximum permissible weight may be determined by interpolation. Finally, if the climb data is given in terms of rate of climb, this can be converted to gradient. Gradient as a percentage equals the rate of climb in feet per minute divided by the aircraft true ground speed, multiplied by 100. Alternatively, the time on each segment can be calculated. Time in minutes equals height gain in feet divided by rate of climb in feet per minute. And the distance on each segment is obtained from distance in feet equals aircraft true ground speed in feet per minute multiplied by time in minutes. So the whole picture is: you build the flight path, you check obstacle clearance, and if you don’t clear by the required margin, you reduce mass and interpolate to find the maximum permissible weight.