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

Right, let's pick this up with the obstacle clearance rules for a Multi-engine Class B take-off. This is where we start shaping the actual take-off flight path. First, the lateral clearance criteria. If your flight path doesn't require track changes of more than 15°, you don't need to consider obstacles that are more than 300 m away laterally, but that's only if you're in VMC. For all other conditions, that distance stretches to 600 m. Now, if your flight path does require track changes of more than 15°, the margins grow: obstacles need not be considered if the lateral distance is greater than 600 m in VMC, or 900 m for all other conditions. So remember the pattern — a track change of more than 15° doubles the lateral clearance requirement, and moving out of VMC adds another 300 m on top. Now, the take-off flight path profile. When we construct it, we must account for four things. The mass of the aeroplane at the commencement of the take-off run. The pressure altitude at the aerodrome. The ambient temperature. And the wind — and here's a critical detail: we use not more than 50% of the reported headwind component, and not less than 150% of the reported tailwind component. So a headwind is discounted to half its reported value, while a tailwind is penalised by being multiplied by one and a half. The construction of the flight path depends on whether visual reference is lost before reaching 1500 ft. Let's take the first case: visibility clear to 1500 ft. We determine the take-off distance required, the TOD, for the take-off mass. Then we determine the all engines net gradient, which is the gross gradient multiplied by 0.77. Then we divide the height gain — which is 1450 ft, that's the 1500 ft minus the 50 ft screen height — by that net gradient to find the distance travelled in feet from 50 ft to 1500 ft. The profile can then be plotted and obstacle clearance assessed. There's an alternative method for a single obstacle. Find the TOD required and the gradient as before. 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's height at the obstacle distance. That height must exceed the obstacle height by 50 ft. If it doesn't clear by that 50 ft margin, you assume a lower take-off mass and recalculate the height. Then, by interpolation, you can determine the maximum mass which will just clear the obstacle by 50 ft. Now the second case: cloud base below 1500 ft. If visual reference is lost before 1500 ft, the flight path splits into two segments. Segment 1 runs from 50 ft up to the cloud base. The distance for that segment equals the height gain divided by the all engines net gradient, multiplied by 100. And the height gain here is the cloud base minus 50 ft. Segment 2 runs from the cloud base up to 1500 ft. Its distance equals the height gain divided by the gross gradient with one engine inoperative, multiplied by 100. So note the contrast: below the cloud base we use the all engines net gradient, above the cloud base we switch to the gross gradient with one engine inoperative. The profile is plotted and obstacles assessed. If the required clearance isn't achieved, you assume a reduced take-off mass, calculate a second flight path, and again determine the maximum permissible weight by interpolation. Finally, if your climb data is given as a rate of climb rather than a gradient, you can convert it. Gradient as a percentage equals the rate of climb in feet per minute divided by the aircraft's true ground speed, multiplied by 100. Alternatively, you can work in time. Time in minutes equals the height gain in feet divided by the rate of climb in feet per minute. Then the distance on each segment is the aircraft's true ground speed in feet per minute multiplied by the time in minutes. So the whole picture: lateral margins for obstacles, the four parameters shaping the profile, the two construction cases depending on cloud base, and the conversion between rate of climb and gradient. That's the take-off flight path for a Multi-engine Class B aeroplane.