Multi-engine Class B - Take-off — Page 341, Lesson 417

Right, let's get into the take-off requirements for a multi-engine Class B aeroplane. This is a key part of the performance syllabus, so let's build it up properly. First, the headline. The take-off requirements for a multi-engine Class B aircraft — and I'm talking about those other than the commuter category — are essentially the same as for a single-engine aircraft. But there's a crucial difference. Because you have more than one engine, you must additionally demonstrate two things: a minimum climb gradient performance, and an obstacle clearance capability. That's the extra layer of safety we're dealing with. Now, the heart of this is the gradient requirement, which comes from EU-OPS 1.530. There are three climb gradient requirements you have to consider, and the most limiting one — the one that demands the most performance — will determine your maximum permissible take-off mass. You'll find these in CAP 698, under paragraph 3.1.2, on page 9 of section 3. Let's walk through each of the three. First: All Engines Operating. Here you need a minimum climb gradient of 4%. And that's with a specific configuration. You've got take-off power on each engine. The landing gear is extended — except that if the gear can be retracted in not more than 7 seconds, you may assume it's retracted. So there's a time limit there. The wing flaps are in the take-off position. And the climb speed must be not less than the greater of two values: 1.1 times VMC, and 1.2 times VS1. Let me unpack those. VMC is the minimum control speed — the speed below which you can't maintain directional control with one engine failed. VS1 is the stalling speed in a specific configuration. So you take whichever of those two calculated speeds is higher, and that's your minimum climb speed. Second: One Engine Inoperative. This is where it gets interesting. At an altitude of 400 feet above the take-off surface, the climb gradient must be measurably positive. Not a specific number — just positive, meaning you're actually climbing. The conditions here are: the critical engine is inoperative, and its propeller is in the minimum drag position — that's the feathered position, to reduce drag. The remaining engine is at take-off power. The landing gear is retracted. The wing flaps are in the take-off position. And the climb speed is equal to that achieved at 50 feet — so the speed you had when you passed the 50-foot screen height. Third: The 1500-foot case. At an altitude of 1500 feet above the take-off surface, the climb gradient must not be less than 0.75%. Now the conditions change. The critical engine is still inoperative with its propeller in minimum drag. But the remaining engine is now at not more than maximum continuous power — so you've throttled back from take-off power. The landing gear is retracted, and now the wing flaps are also retracted. And the climb speed must not be less than 1.2 times VS1. So you see the pattern. Three distinct cases, each with its own gradient, its own power setting, its own configuration, and its own speed. The most limiting of these three will set your maximum take-off mass. That's the core of the requirement. Now, let me show you what this looks like in practice. That figure shows the obstacle clearance climb profile when there's no cloud. It ties the whole thing together — the climb path you must achieve to clear obstacles, which is exactly what these gradient requirements are protecting.