Class A - Additional Take-off Procedures — Page 413, Lesson 511

We're starting a new topic now: Class A additional take-off procedures for contaminated runways. This is the procedure you'll use when the runway isn't dry — think snow, slush, or standing water. Let me walk you through it. First, there's a key speed concept you need: hydroplaning speed. This is the speed for non-rotating tyres, or put another way, it's the speed below which aquaplaning will stop. So if you're below that speed, your tyres are gripping; above it, you risk riding on a film of water. That's the physical phenomenon driving everything we're about to do. Now, the procedure itself. You'll use either Figure 15.3 or the tables in CAP 698 — that's the UK CAA's guidance document — specifically pages 25, 26, and 27 of section 4. Those give you the data for contaminated runways. Let me walk you through the steps, letter by letter. Step (a): Calculate the normal limiting take-off mass for a dry runway. That means you work out the field length limit, the climb limit, or the obstacle limit — whichever is the binding constraint. This is your starting point, the mass you'd use if the runway were dry. Step (b): Select the table appropriate to the depth of contaminant. If your contaminant depth falls between two table values, you interpolate. Step (c): Enter the left column of the top table at that normal limiting take-off mass. Travel right to the aerodrome pressure altitude column. Interpolate for both mass and pressure altitude if necessary. Extract the mass reduction. Then calculate the maximum take-off mass for the contaminated runway by subtracting that mass reduction from the normal performance limiting take-off mass. So the contaminant costs you mass — you're reducing your take-off weight to stay safe. Step (d): Now, if you find yourself in the shaded area of that top table, you proceed to the bottom table. Enter the left column with the take-off run available — that's TORA. Move right to the appropriate aerodrome pressure altitude column, interpolate as necessary, and extract the maximum permissible take-off mass. Then — and this is important — you set V1 equal to VMCG. VMCG is the minimum control speed on the ground, the speed below which you can't maintain directional control if an engine fails. Setting V1 to VMCG means you're committing to a take-off decision no earlier than that minimum control speed. Step (e): Take the lower of the two values from steps (c) and (d). That's your maximum take-off mass for the contaminated runway. The lower one wins because it's the more restrictive. Step (f): Calculate all the V speeds for the actual take-off mass you determined in step (e). So V1, V2, VR — all of them — based on that reduced mass. Step (g): If you were NOT in the shaded area back in step (c), then re-enter the top table at the actual mass to determine the V1 reduction to be made. So instead of using the bottom table, you're adjusting V1 directly. Step (h): Apply that reduction to V1. And here's the critical check: if the adjusted V1 is less than VMCG, then take-off is not permitted. Full stop. You cannot go. That's your hard limit. So the whole logic is: contaminated runway means you either reduce mass, or you reduce V1, or both — and VMCG is the floor below which you simply don't operate. Let me show you the procedure visually. And here's a sample data table for a runway with 2 mm contamination. That's the full procedure. The key takeaways: hydroplaning speed is your physical limit, the tables give you mass reductions and V1 adjustments, and VMCG is your absolute floor. If you ever find adjusted V1 below VMCG, you don't take off — that's non-negotiable.