Inflight mental arithmetics
Hey Chappies
There's a little booklet i use as a reference for mental calculations in flight,french book,but most of the time i find them very long to perform while flying and i will give u an example down
An aircraft flies at FL140.the pilot wants to cross overhead next VOR station at FL40 this station is 30 NM from its present position.What the rate of descent is if maintaining a speed of 250 kts from this position?
Very straight forward question in a normal way i'd have done this:since i maintain 250 knots i do 4 miles per minutes since the station is 30 NM from present position it'll take me 7 mins to get there then to loose 10000 ft(from FL140 down to FL40) it'll be 1369 ft/min but i round it up to 1400 ft/min.
But the other book goes this way:
They first of all try to determine Gradient of descent which is(°)
y(°)=Diff in Fl/D(NM),Y(°)=Glide path selected in order to calibrate the descent (in degrees) and D(NM)=distance from top of descent in nautical miles;
Using the same example above on the very same aircraft they do:
y(°)=100/30 which gives them a slope of 3,3° then they go on to get the destcent gradient in % which the formula is:y(%)=y(°)/0.6 which is 3.3/0.6=5.5%,finally they do rate of descent,vz(rod ft/mn)
vz(ft/mn)= y(%).V(kt) 5.5% . 250=1440ft/mn
Don't u guys agree that the last method is kinda long to perform inflight calculation while busy flying?the reason i'm asking this is because i had a discussion with a friend of mine who prefers and says the last method is the best.The bottom line is we get to the same result using different methods.If you've got much easier and faster tips than that please don't hesitate to share.Thank u