Multi-engine Class B - Take-off — Page 341, Lesson 423

Right, let’s get into the take-off performance rules for a Multi-engine Class B aeroplane. This is a dense, memory-heavy section, so I’ll walk you through it piece by piece. First, the speeds. We have three key speeds to define: VR, the rotation speed; the take-off safety speed at 50 ft; and VMC. VR, the rotation speed — that’s the speed at which you rotate the aeroplane to lift the nose off the runway — must not be less than two values. It must not be less than 1.05 times VMC, and it must not be less than 1.1 times VS1. Now, VMC is the minimum control speed — the minimum speed at which you can still maintain directional control of the aeroplane with the critical engine failed. VS1 is the stalling speed in a specified configuration — the speed at which the wing stalls. So VR has to be at least 5% above VMC and at least 10% above VS1. Next, the take-off safety speed at 50 ft. This is the speed you must have reached by the time you’re 50 ft above the surface. It must not be less than three things: a speed that is safe under all reasonably expected conditions; 1.1 times VMC; and 1.2 times VS1. So at 50 ft you need a bigger margin — 10% above VMC and 20% above VS1. And there’s a constraint on VMC itself for take-off: VMC for take-off must not exceed 1.2 times VS1. So VMC is capped at 20% above the stall speed. Now, an important practical point: these speeds are not found in CAP 698 — that’s the UK CAA performance document — so you must commit them to memory. The obstacle clearance requirements, however, are the opposite: they are all in CAP 698, on page 9 of section 3, so you don’t need to memorise them — you just need to know where to find them. Let’s move to the obstacle clearance requirements, which come from EU-OPS 1.535. A Multi-engine Class B aeroplane must demonstrate clearance of obstacles after take-off up to a height of 1500 ft. Obstacles must be cleared by one of two margins. Either a vertical margin of at least 50 ft, or a horizontal distance of at least 90 m plus 0.125D, where D is the distance from the end of the TODA — that’s take-off distance available — or the end of the TOD, the take-off distance, if a turn is scheduled before the end of the TODA. There’s a special case: for aeroplanes with a wingspan of less than 60 m, the horizontal distance may be taken as 60 m plus half the wingspan plus 0.125D. So the 90 m figure is replaced by a smaller value that depends on your actual wingspan. Now, the conditions that must be assumed when demonstrating this obstacle clearance. There are several, and they define the flight path you assume. First, the flight path begins at a height of 50 ft above the surface at the end of the TODR — that’s take-off distance required — and ends at a height of 1500 ft above the surface. Second, the aeroplane is not banked before it has reached 50 ft, and thereafter the angle of bank does not exceed 15°. Third, failure of the critical engine occurs at the point on the all-engine take-off flight path where visual reference for the purpose of avoiding obstacles is expected to be lost. So you assume the engine fails at the worst realistic point — where you lose visual reference. Fourth, the gradient to be assumed from 50 ft to the point of engine failure is equal to the average all-engine gradient during climb and transition to the en route configuration, multiplied by a factor of 0.77. So before the engine fails, you assume a degraded all-engine climb gradient — only 77% of the average. And fifth, the gradient from the point of engine failure to 1500 ft is equal to the one-engine-inoperative en route gradient. So after the failure, you assume the climb gradient you’d get with one engine out in the en route configuration. Let me just tie that together. The whole obstacle clearance picture is: you start at 50 ft at the end of the take-off distance required, you climb with a reduced all-engine gradient — 0.77 of average — until the assumed engine failure point, then you climb with the one-engine-inoperative gradient up to 1500 ft, and every obstacle in that path must be cleared by either 50 ft vertically or the horizontal margin we discussed. One thing to note: the excerpt cuts off mid-sentence on that first gradient condition — it repeats the same text — but the meaning is clear: the 0.77 factor applies to the all-engine gradient segment. So, to summarise the memory burden: the speeds — VR, the 50 ft safety speed, and the VMC cap — those you must memorise. The obstacle clearance margins and assumptions — those you look up in CAP 698, page 9 of section 3, whenever you need them.