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

All right, let's get into the take-off requirements for a multi-engine Class B aeroplane. This is a performance topic, so we're talking about the rules that govern whether an aeroplane can legally and safely get off the ground. First, the big picture. The take-off requirements for a multi-engine Class B aircraft — and I want to be clear, this excludes those in the commuter category — are essentially the same as for a single-engine aeroplane. But there's a critical addition. Because you have more than one engine, the regulations demand that the aeroplane demonstrate two extra things: a minimum climb gradient performance, and an obstacle clearance capability. A single-engine aeroplane doesn't have to prove these in the same way, because if its one engine fails, the situation is fundamentally different. With two engines, you're expected to be able to keep climbing even after losing one. Now, the heart of this is the gradient requirement, which comes from EU-OPS 1.530. There are three climb gradient requirements you must consider, and here's the key operational point: the most limiting of the three will determine the maximum permissible mass for your take-off. So you don't just pick one — you check all three, and the one that restricts your weight the most is the one that governs. You'll find these in CAP 698, under paragraph 3.1.2, on page 9 of section 3. That's your reference document for the actual numbers and charts. Let's break down the three requirements one by one. The first is with ALL ENGINES OPERATING. Here, you need a minimum climb gradient of 4%. That means for every 100 metres you travel forward, you must be able to climb at least 4 metres. The conditions for this are specific. You have take-off power on each engine. Your landing gear is extended — but there's an important exception: if the landing gear can be retracted in not more than 7 seconds, you may assume it is retracted for this calculation. So a fast-retracting gear gives you a performance credit. The wing flaps are in the take-off position. And your 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 slowest speed at which you can still maintain directional control with the critical 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. The second requirement is with ONE ENGINE INOPERATIVE. This is where the multi-engine performance really gets tested. At an altitude of 400 feet above the take-off surface, the climb gradient must be measurably positive. That means you must be climbing, even if only slightly — you cannot be descending or even level. The conditions here are: the critical engine is inoperative, and its propeller is in the minimum drag position. That's the feathered position, which reduces drag from the dead engine. The remaining engine is at take-off power. The landing gear is retracted. The wing flaps are in the take-off position. And your climb speed is equal to that achieved at 50 feet — that's the speed you were doing when you passed the 50-foot obstacle height during the take-off. The third requirement is also with one engine inoperative, but at a higher altitude and with different conditions. At an altitude of 1500 feet above the take-off surface, the climb gradient must not be less than 0.75%. So here you need a specific, measurable climb — not just positive, but at least three-quarters of a percent. 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 — that's a lower power setting than take-off power, because you're in a sustained climb. The landing gear is retracted, and now the wing flaps are also retracted. Your climb speed must be not less than 1.2 times VS1. So you can see the pattern here. As you climb away from the runway, the configuration cleans up — gear up, flaps up — and the power reduces from take-off to maximum continuous. But the climb gradient requirement changes too, from measurably positive at 400 feet, to 0.75% at 1500 feet. And remember, all three of these — the 4% all-engines, the positive gradient at 400 feet, and the 0.75% at 1500 feet — are checked, and the most limiting one sets your maximum take-off mass. Let me also make sure you understand the distinction between the two single-engine cases. At 400 feet, you're still in the take-off configuration with flaps in take-off position, and you just need to be climbing. At 1500 feet, you've cleaned up the aeroplane, reduced power, and you need a specific minimum gradient. Both are critical for obstacle clearance — the ability to get over terrain and obstacles in your departure path. That's the core of the take-off gradient requirements for this class of aeroplane. The key takeaway is that multi-engine performance isn't just about getting airborne — it's about proving you can climb away safely even with a failed engine, and that proof comes in these three distinct gradient checks.