Class A Aircraft - Take-off — Page 406, Lesson 494

Right, so we’ve finished with V1, VR and V2. Now there are two more speeds we have to check before we can call the take-off speeds complete. The first is VMCG. VMCG stands for the minimum control speed on the ground. It’s the speed at which, if the critical engine fails, you can still maintain directional control on the ground using rudder alone, without the nosewheel steering being effective. In our example, using 25°C and 2000 ft pressure altitude, the VMCG table gives us 112 knots. And remember from our theory: V1 must never be less than VMCG. In our example it isn’t — V1 is well above 112 knots, so we’re fine. The last speed to calculate is VMBE — the maximum brake energy speed. The graph for this is on page 15 of section 4 of CAP 698, and we’ve already described how to use it. If we use our example airfield conditions — 2000 ft pressure altitude, 25°C, and our regulated take-off mass of 57 900 kg — the graph is not applicable, because we fall in the shaded area. But as an example, VMBE comes out to 175 knots. However, there are corrections. We had a 2% upslope, and that means we need to increase VMBE by 4 knots. We also had a 20 knot headwind, and that means VMBE must increase by 6 knots. So the total correction increases VMBE to 185 knots. Now, all the relevant take-off V speeds have been calculated based on our regulated take-off mass of 57 900 kg. But there’s an important caveat. The speeds shown in the tables we’ve just used are based on a balanced field length — that means TORA equals TODA equals ASDA. Those speeds are not valid if the take-off mass has been derived using stopway or clearway. Where that is the case, V1 may be adjusted for the effects of stopway or clearway, using the table in Figure 14.20. Then we move on to the thrust setting. The thrust setting values are shown in CAP 698 on pages 20, 21, 22 and 23 of section 4. You use the tables on those pages to select the appropriate thrust setting for take-off and for the climb, using the conditions at the airfield. Finally, the stabilizer trim setting. The stabilizer trim setting appropriate to the CG position and take-off mass can be read from the table in Figure 14.21, and it’s shown in CAP 698 at the bottom of pages 18 and 19 of section 4. So to recap the whole take-off speed picture: we calculate V1, VR and V2 from the density band and speed tables, then we apply slope and wind corrections to V1 — in our example, a 2% upslope added 1.79 knots and a 20 knot headwind added 0.5 knots, giving a total correction of 2.29 knots, taking V1 from 142.06 to 144.35 knots, rounded to 144 knots. Then we check VMCG and VMBE, and finally we adjust for stopway or clearway if needed, set the thrust, and set the stabilizer trim. That completes the take-off speed calculation for a Class A aircraft.