# Critical Point (CP)

## Definition

The Critical Point (CP), or Equal Time Point (ETP), is when an aircraft is the same flying time from 2 potential en-route diversions. Calculation of appropriate CPs aids decision making when deciding courses of action following a significant event such as an engine failure or on-board medical emergency.

Plotting of the aircraft’s progress on a chart will help identify Gross Navigation Errors and aid contingency planning. An Equal Time Point (ETP), also known as a Critical Point (CP) should be calculated for Performance Class A aircraft (multi-engined aeroplanes powered by turbo-propeller engines with a Maximum Operational Passenger Seating Capacity (MOPSC) of more than nine or a maximum take-off mass exceeding 5700 kg, and all multi-engined turbo-jet powered aeroplanes) on all sectors when the aircraft is more than 90 mins (4 engine aircraft) still air time at the one-engine-inoperative cruising speed from a suitable alternate airfield.

## Algebraic Solution

Identification of the ETP (C) for an aircraft flying between 2 airfields, A & B, which are on, or close to, track can be achieved by using the following formula:

Algebraic method for calculating Critical Point/Equal Tim Point

## Graphical Solution

The graphical solution to an ETP problem where a third (en-route diversion) airfield is involved is shown below. The aircraft is flying from A to B and the diversion C lies to one side of track. The requirement is to establish ETPs between A and C, and also C and B.

Graphical method for determining Critical Point/Equal Time Point

Step 1: Construct the perpendicular bisector MX of line AC. This cuts the track AB at the still air ETP (as AX=CX).

Step 2: Measure the distance AX and divide by the One-Engine Inoperative (OEI) TAS to give the still air time from X to A.

Step 3: Construct a wind vector YX of length in nm equal to Wind speed (kts) x the flight time from X to A (hrs)

Step 4: From Y draw a line parallel to XM. The point at which this crosses track AB is the wind corrected ETP between A and C.

Repeat the process for ETP for C to B

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