What is the Critical Point (Equal Time Point) in aviation?

It is the point along a route where the time to continue to the destination equals the time to return to the departure point.

Why is the ETP important in flight planning?

It helps determine the optimal decision point for emergencies, ensuring the quickest route to safety.

Is the Critical Point the same as the Point of Safe Return (PSR)?

No, the Critical Point is based on time, while the PSR is based on fuel endurance and reserves.

What factors affect the location of the ETP?

Groundspeeds outbound and homeward, total distance, and wind conditions all influence the ETP position.

Critical Point and Equal Time Point

Q: How do you calculate the Equal Time Point (ETP)?

Use the formula: Distance to ETP = (Total Distance × Groundspeed Home) ÷ (Groundspeed Out + Groundspeed Home).

Medium4 min readFlightplanning

Rarely Examined

Why this matters

Knowing how to determine the ETP ensures that, in an emergency, a pilot can make the safest and most time-efficient decision about whether to continue or return. This is vital for flights over areas with limited diversion options, directly impacting safety and operational effectiveness.

Critical Point (CP) and Equal Time Point (ETP) are essential concepts in flight planning, especially for long overwater or remote area flights. The ETP, also known as the Point of Equal Time (PET), is the position along a route where it will take the same amount of time to continue to the destination as to return to the departure point, considering wind and groundspeed differences. Understanding and calculating these points helps pilots make informed decisions in case of emergencies, ensuring the safest and most efficient route is chosen.

Quick Check

Which formula correctly calculates the distance from departure to the Point of Equal Time (PET) in flight planning?

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In depth

Explanation

What Are Critical Point and Equal Time Point?

The Critical Point (CP) and Equal Time Point (ETP) are used in contingency planning during flight operations. The ETP (often called PET in EASA ATPL) is the point along your route where the time to continue to your destination equals the time to return to your departure airfield, factoring in actual groundspeeds (which are affected by wind).

How to Calculate ETP

The standard formula for ETP (PET) is:

Distance to ETP = (Total Distance × Groundspeed Home) / (Groundspeed Out + Groundspeed Home)

Total Distance (D): The distance between departure and destination.
Groundspeed Out (O): Groundspeed from departure to destination (taking wind into account).
Groundspeed Home (H): Groundspeed from destination back to departure (again, with wind).

Once you have the ETP distance, you can find the time to reach the ETP by dividing this distance by the outbound groundspeed. This allows you to determine the ETA at the ETP.

CP vs ETP

While the ETP is based on time, the Critical Point (CP) is often used interchangeably in some contexts but can refer more broadly to the most critical decision point for a given contingency (such as fuel or medical emergencies). For EASA ATPL, focus on the ETP calculation for exam purposes.

Practical Use in Flight Planning

Identify the ETP before flight, especially for oceanic, polar, or desert crossings.
Use the ETP to decide whether to continue or return in case of technical or medical emergencies.
Always recalculate the ETP if actual groundspeeds differ significantly from planned values.

Key Steps for ETP Flight Planning

Calculate outbound and return groundspeeds using wind data.
Apply the ETP formula.
Record the ETP distance and ETA in your flight plan.
Update ETP calculations as conditions change during flight.

The essentials

Key Points

The Equal Time Point (ETP) is where time to continue equals time to return.

ETP calculation uses actual groundspeeds, not just distance.

Formula: ETP distance = (Total Distance × Groundspeed Home) / (Groundspeed Out + Groundspeed Home).

ETP is critical for contingency planning on remote or overwater flights.

Always use wind-corrected groundspeeds for accuracy.

ETP position and ETA should be included in the flight plan.

Recalculate ETP if actual flight conditions change significantly.

Watch out

Exam Traps & Typical Mistakes

Confusing ETP (time-based) with PSR (fuel-based) calculations.

Using airspeed instead of groundspeed (must use wind-corrected values).

Switching outbound and return groundspeeds in the formula.

Assuming ETP is always at the halfway point (it rarely is unless groundspeeds are equal).

Neglecting to update ETP if winds or groundspeeds change during flight.

Test yourself

Example Exam Questions

Question 2Easy

What does the Critical Point (Equal Time Point) represent in flight planning?

Question 3Medium

Given: Distance between A and B is 600 NM, outbound groundspeed is 150 KT, return groundspeed is 120 KT. What is the distance from A to the PET?

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