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

We're starting a new topic: Class A additional take-off procedures, specifically for contaminated runways. Let me walk you through this. First, I want to define a key term you'll see here: hydroplaning speed, also called aquaplaning speed. For non-rotating tyres, this is the speed below which aquaplaning will stop. In other words, if your tyres are not rotating—say, during a rejected take-off—there's a critical speed above which the tyre rides on a film of water and loses contact with the runway. Below that speed, the aquaplaning stops and the tyre regains grip. That's the speed this procedure is built around. Now, the procedure itself. You'll use either Figure 15.3 or the tables in CAP 698, section 4, pages 25, 26, and 27. Let me walk you through the steps in order. Step (a): Calculate the normal limiting take-off mass for a dry runway. That means you determine the field length limit, the climb limit, or the obstacle limit—whichever restricts you. This is your starting point, the mass you'd use if the runway were dry. Step (b): Select the table or tables appropriate to the depth of contaminant on the runway. If the contaminant depth falls between two table values, you interpolate. Step (c): Enter the left column of the top table at your 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. Step (d): Here's a conditional. 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 you set V1 equal to VMCG. VMCG is the minimum control speed on the ground—the speed at which you can still maintain directional control if one engine fails. Step (e): Take the lower of the two values from steps (c) and (d). That lower value is your maximum take-off mass for the contaminated runway. Step (f): Calculate all the V speeds for the actual take-off mass you determined in step (e). So V1, VR, V2—all of them—based on that reduced mass. Step (g): Now, if you were NOT in the shaded area in step (c), you 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 go back to the top table with your actual take-off mass and read off how much V1 must be reduced. Step (h): Apply that reduction to V1. Here's the critical limitation: if the adjusted V1 is less than VMCG, the take-off is not permitted. That's a hard stop—you cannot take off under those conditions. Let me tie this together. The whole procedure is about two things: reducing your take-off mass to account for the contaminant, and reducing V1 to give you more margin for a rejected take-off on a slippery surface. The shaded area in the top table tells you when the mass reduction alone isn't enough, and you need to fall back on the TORA-based limit and set V1 to VMCG. And the final check—V1 must not fall below VMCG—is the safety gate that prevents you from attempting a take-off you can't abort safely.