Critical Points (CP)

Critical points also go by the names “Equi-time Point” (ETP), and “Point of Equal Time” (PET).

Definition: A Critical Point (CP) is the position from which it takes the same time to GO ON to an airport ahead of the aircraft, as it does to GO HOME to an airport behind the aircraft.

When preparing a flight plan, we must consider the amount of fuel required to cover three possible occurrences. These are:

Normal Operations from departure airport, to destination airport.
Normal operations out to the Depressurised critical point, then depressurised operation ON to the airport ahead, or HOME to the airport behind.
Normal operations out to the 1 Engine Inoperative critical point, then flight with 1 Engine Inoperative ON to the airport ahead, or HOME to the airport behind.

The Critical Point is calculated using the true airspeed (TAS) that relates to the operation. So for the calculation of the Depressurised CP, the depressurised TAS is used in the formula. For the calculation of the 1 Engine Inoperative CP, the 1 Engine Inop TAS is used. When finding the position of the Normal Operations CP, the Normal Operations TAS is used.

Each operation will have it’s own particular TAS unique to it, and it will also have it’s own particular cruise level.

For instance, if a cabin depressurisation occurs, it is essential to get the aircraft down to level at which the passengers can breath normally, without the continuous need for oxygen masks to be worn. Typically this is FL130. If the cabin depressurises, we will be either going ON to the airport ahead at FL130, or going HOME to the airport behind at FL130. The Depressurised CP defines the position where it takes the same time to go ON at the depressurised level (FL130) and TAS, as it does to return HOME to the airport behind you at FL130 at the depressurised TAS. The formula must include the depressurised level winds.

When Nil wind conditions exist , the CP will be exactly half way between the two airports. If there is a headwind ON (tailwind HOME), the CP will be into wind from half way. This is due to the reduced groundspeed when flying into a headwind, and the increased groundspeed with a tail wind.

Factors effecting the CP position

The distance between the two airports.
The aircraft TAS. This must be the TAS for the type of CP under consideration (eg: 1 Engine Inop TAS used in the 1 engine Inop CP calculation).
The average wind component. The CP is always upwind from the half way position.

The way to find the effect of wind on the CP position is by the formula below

Example . Depressurised operations CP

Departure airport “A” Destination airport “B”.
Aircraft Depressurised Ops TAS 250 kt.
Distance between airports 1, 000 nm
Average tail wind component is 30 kt tailwind ON (30 kt head wind HOME)

Which of the answers best represents the position of the Depressurised operations CP (CPDP), expressed as a distance from the departure airport ?

a. 500 nm b. 440 nm c. 560 nm

We know the CP always moves into wind from half way because of the difference in the groundspeed HOME and groundspeed ON. We can apply the formula to find how far into wind the CP is in these conditions ...

Measured as a distance from the departure airport “A”, this is 440 nm.

Answer “B” best.

Let us check that it will take the same time HOME from the calculated CPDP, as it will to go ON.

Distance HOME 440 nm @ GS of 220 kt = 120 minutes.

Distance ON 560 nm @ GS of 280 kt = 120 minutes.

As it will take the same time to fly ON to the airport ahead, as it will to the airport behind, the fuel ON and fuel HOME can be assumed to be the same.

The fuel burn HOME from the CP = the fuel burn ON from the CP.

The flight out to the CP does not enter the equation at all.

The flight out to the CP is always at Normal operations.

This is a small section taken from the ATPL Navigation training texts. These will be available online soon, complete with online assignments marked by the computer. They will also be available in book form and on CD -ROM from the Aviationshop online store..

I hope this helps understand Critical Points a little better.

Best Wishes,

Rob Avery

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Marty says ... "Goodbye to GA".