MRJT Additional Procedures — Page 120, Lesson 110

I want to walk you through a key ETOPS procedure — calculating the most critical point, also called the CP or ETP. This is the point along your route where, if you lose an engine, you are equally far in flight time from two suitable alternates. Let's start with the big picture. The most critical point is based on the two alternates that are furthest apart along the track. In Figure 8.2, those alternates are labelled C and E. The first step is to find what is called the Critical Line between airfields C and E. You find that line by extending the bisector of the straight line between C and E until it cuts the ETOPS track. The bisector is the line that splits the distance between C and E exactly in half at a right angle. Where that bisector meets the ETOPS track, you have an intersection point. From that intersection, the distances to either C or E are equal. In still air conditions, the flight time at the one-engine-inoperative true airspeed — the TAS — will also be equal from that point. Now let's put numbers to it. Assume your one-engine-inoperative cruise TAS is 400 knots. The distance from that Critical Line intersection with the ETOPS track to airfield C or E is 690 nautical miles. The mean forecast wind velocity for the flight to either C or E, at the planned one-engine-out stabilizing pressure level, is 230 degrees at 85 knots. First, calculate the still air time to C or E. That is 690 nautical miles divided by 400 knots, which gives you 104 minutes. Now, 104 minutes at a wind speed of 85 knots means the wind will displace you by 147 nautical miles. That is the wind effect over that time. Here is the key step: you back-plot a wind vector from the Critical Line intersection with the ETOPS track. You draw that vector in the direction the wind is coming from — 230 degrees — for a length of 147 nautical miles. At the end of that vector, you plot a line called the Equal Line, which runs parallel to the Critical Line. The intersection of this Equal Line with the ETOPS track is your most limiting ETOPS ETP, or critical point. What does this point tell you? Two things. First, at this position, the engine-out flight time to airfields C or E is the same. Second, the flight-planned fuel from this point must be equal to or greater than the Critical Fuel required to reach either C or E. If the fuel on board from that point is less than the Critical Fuel, you must load the extra fuel required before departure. Let me recap the logic. You start with the two furthest alternates, find the bisector between them, extend it to the track, then apply the wind effect to shift that equal-time line along the track. The shifted intersection gives you the actual worst-case point for engine-out diversion. That is the point you use to check your fuel.