It might be useful to briefly state here the formulae and Dalton flight computer solutions to the questions: What is the Critical Point? And what is the Point of No Return? The PNR will be discussed on the next blog page.
If, for example, engine trouble occurs during flight it is important for the pilot to know whether the airport of departure or the airport of destination can be reached sooner. For this the Point of Equal Time (P.E.T.) or Critical Point (C.P.) is determined. This is the point from which the continuation of the flight to the destination would require the same time as the return flight to the point of departure (or between any two points on your flight plan A & B).
The formula is T (P.E.T.) = TF X GS Home/[GS Out + GS Home] (Time formula)
T (P.E.T.) = Flying time to Point of Equal Time (or Critical Point).
TF = Time to fly from base (or A) to destination (or B) [Flight plan time or time A-B].
GS Out = Ground speed outbound.
GS Home = Ground speed inbound.
The distance from the point of departure (or A) to the point of Equal Time (Critical Point) is calculated by means of the following formula:
D (P.E.T.) = DZ X GS Home/[GS Out + GS Home] (Distance formula)
D (P.E.T.) = Distance from base (or A) to P.E.T.
DZ = Distance from base to destination (or A to B).
These equations may be transposed to the more convenient proportion form, as follows:
GS Home/[GS Out + GS Home] = T (P.E.T.)/TF or D (P.E.T.)/DZ
On your flight computer simply align the sum of GS Out + GS Home on the inner scale with GS Home on the outer scale. Now read CP or Distance to CP on outer scale opposite Flight Plan time (or A-B time) and Distance departure to destination (or A-B distance) resp.
Distance A to B (DZ) = 920nm. GS Out = 240kt. GS Home = 210kt. Flight time A to B = 3h50min.
Required: Flying time to P.E.T. (C.P). & Distance to P.E.T. (C.P).
Intermediate calculation: GS Out + GS Home = 450kt.
On Flight Computer:
210 outer scale aligned with 450 inner scale. Read 107.5min outer scale opposite 230min (3h50min) innner scale & 430nm outer scale opposite 920nm inner scale.
Result: The P.E.T. (or C.P.) will be reached after a flying time of 107.5min. The distance flown will be 430nm.