Point of Safe Diversion
Safe end avail x ETI->Desto/ETI->desto + ETI Desto->Alternate.
Can't draw diagrams here but draw on a pice of paper a Point A.
Point B (Desto) draw at 12 o'clock a suitable distance up the page. Point C (Altn), draw a line from B to a Pont C at 4 o'clock and a suitable distance down and across.
Example;
Safe Endurance available is 60 minutes.
ETI A->B is 40 minutes.
ETI B->C is 30 minutes.
X = the distance (in minutes) you can travel from A->B and still make C with reserves intact.
X= 60x40/40+30
X=2400/70
X= 34.2857
minutes from A.
It's been many years since I used the above but it's easily proven by then calculating cruise fuel flows x time intervals from A->X->C to ensure yourself that it works....obviously you want to arrive at C with reserves intact and enough fuel to carry out an approach and landing, or hold first then approach and land, if the weather is such, so be carefull with calculating safe endurance...this is where you'd likely make a mistake that invalidates the formula.
It's merely a ratio type calculation.
Hope that helps
Mind you the type of simple, efficient calculation above is not gauranteed to give the right answer on a CASA flight planning exam...they would require 40 minutes of number crunching to come up with their 'correct' answer...this one would work in the real world though...and you wouldn't be past the PNR by the time you'd worked out where it was
Editted for the playfull smiley I forgot to add after the 4 decimal places...and yes Woomera you'd hope those days were over...but are they?
