The Critical Point (CP), also referred to as the Equal Time Point (ETP), is a crucial concept in aviation. It marks the moment when an aircraft is equidistant in flying time from two potential diversion airports. Knowing how to calculate CPs is vital for making informed decisions during significant events, such as engine failures or medical emergencies on board. By plotting the aircraft’s progress on a chart, pilots can spot Gross Navigation Errors and enhance their contingency plans.
For Performance Class A aircraft, which include multi-engine planes with a seating capacity exceeding nine or a maximum take-off mass over 5,700 kg, calculating an ETP is essential. This is especially true when the aircraft is more than 90 minutes away from a suitable alternate airfield at one-engine-inoperative cruising speed.
To find the ETP (C) for an aircraft flying between two airfields, A and B, you can use a straightforward algebraic formula. This method helps determine the critical points effectively. Additionally, if a third diversion airfield (C) is involved, a graphical solution can be employed. In this scenario, the aircraft is traveling from A to B, with diversion C located off the direct track.
To establish the ETPs between A and C, and then C and B, follow these steps: First, draw the perpendicular bisector MX of line AC, which intersects track AB at the still air ETP (where AX equals CX). Next, measure the distance AX and divide it by the One-Engine Inoperative (OEI) True Airspeed (TAS) to find the still air time from X to A. Then, create a wind vector YX that corresponds to the wind speed in knots multiplied by the flight time from X to A in hours. Finally, from point Y, draw a line parallel to XM. The intersection of this line with track AB will give you the wind-corrected ETP between A and C. Repeat this process to find the ETP from C to B.
