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Point of Equal Time

The Point of Equal Time (PET), also known as the Critical Point (CP), is the location where it will take you the same amount of time to fly to either of two landing sites. The PET helps you determine which of the two landing sites is closer in case of an emergency. These two locations could be the start and end points of your flight, or any other two suitable places.

Point of Equal Time Formula

Point of Equal Time=distance×speed to landing site aspeed to landing site a+speed to landing Site b\text{Point of Equal Time} = \frac{ \text{distance} \times \text{speed to landing site a}}{\text{speed to landing site a} + \text{speed to landing Site b}}

Deriving the Point of Equal Time Formula

The distance between you and the two landing sites is known:

Class D Airspace (Regional Aerodromes)Class D Airspace (Regional Aerodromes)

The PET is somewhere between the two landing sites:

Class D Airspace (Regional Aerodromes)Class D Airspace (Regional Aerodromes)

The PET is the distance from landing site a where it will take you the same amount of time to fly to either of the two landing sites; and time is defined as distance over speed:

xspeed to landing site a=dxspeed to landing site b\frac{x}{\text{speed to landing site a}} = \frac{d-x}{\text{speed to landing site b}}

Solving this equation gives the PET formula:

xa=dxbxba=dxxb=a(dx)xb=daxaxb+xa=dax(a+b)=dax=daa+b\begin{aligned} \frac{x}{a} &= \frac{d - x}{b} \\ \frac{xb}{a} &= d-x \\ xb &= a(d - x) \\ xb &= da - xa \\ xb + xa &= da \\ x(a + b) &= da \\ x &= \frac{da}{a + b} \end{aligned}