When flying over Continental Europe a PET is not something you immediately consider as you are always close to an alternate airport should you need to land quickly.
Crossing the World’s Oceans when you are hours away from a suitable airport you will want to know where your nearest alternate is. Over the North Atlantic Ocean, you are never more than 137 minutes from an airport. In the early days of twin powered aircraft such as the A300 and B767 ETOPS 120 approval was awarded (approval to fly no more than 120 minutes from a suitable alternate), before 180 minutes became the norm. Today the A350 can be approved to fly 370 minutes from an alternate due to improved engine reliability.
The PET is sometimes called the Critical Point (CP) or Equal Time Point (ETP)
In the diagram below the PET is equidistant between Gander and Shannon and the time to fly from the PET to either is the same. This is the situation in STILL AIR.
If we now introduce a westerly wind which is the way the wind usually flows across the Atlantic, the PET will move towards Gander as we will encounter a headwind flying back to Gander while continuing with the tailwind on to Shannon.
The time will remain the same to fly back to Gander or on to Shannon but because of the wind the distance is shorter to Gander and longer to Shannon.
PET problems are always a TIME issue and have nothing to do with fuel, range or endurance. You will always land with at least your Final Reserve fuel.
PET Formula
D – Total Distance A to B
D – Distance from A to PET
O – Groundspeed OUT from A to PET (used for time calculation)
O – Groundspeed ON from PET to B may be different to GS OUT (in an engine failure case)
H – Groundspeed Home from PET back to A
PET Formula: d = D x H Time = d
— — — ———
O + H O
Example:
Given the following information for a flight from A to B:
Distance A to B 3450 nm
Track 160° T
W/V 220° T/70 kt
Full Power TAS 280 kt
Reduced Power TAS 235 kt
What is the distance and time from A to the engine failure PET?
Use CRP5 to find GS On and Home using Reduced Power TAS and GS Out using Full Power TAS.
Remember to shuffle the drift!
Reduced GS On 192 kt
Reduced GS Home 262 kt
Full GS Out 240 kt
Distance to Engine Out PET using reduced GS:
3450 x 262
d = —————— = 1991 nm
192 + 262
1991
t = —————— = 8.3 hrs = 8 hrs 18 mins
240