Dont sweat it too much BB2002. Its all a question of proportionality and ratio really. If this is to this, then that is to [?]
For example say its presentley 0414Z and Im doing a g.speed of 472 knots (449 true) and burning 2.73T of fuel per hour, 213 miles out from departure airport. By the gages I have 5430kg of fuel on board. Should I have to turn back RIGHT NOW to my departure airport, what will be my ETA and FOB on arrival?
Itll take only 2 minutes tops to calculate!
1. NOW Ill have a 20 head-wind so Ill have a return GS of 430 knots. Thats about 7nm a minute (430/60)-(mentaly 6 x 7 = 42). If I have 213nm to fly I have 30 1/2 minutes (213/7) - (mentaly 3 x 7 = 21). Add in the 180 deg turn of two minutes and I come up with an ETI of around 33 mins. Throw in an extra minute for TAS reduction during the descent and I get 34 minutes. But Im doing 10nm per hour faster (430 knots not the 420 which I used as 7nm a minute) so in 30 odd minutes Ill do about 5nm more. So Ill knock off a minute and call it an ETI of 33 minutes. So ETA is 0414 + 33 = 0447Z.
2. 2.73T an hour is about 45 kg a minute (2.73/60)- (mentaly 6 x 4 = 24. 27-24=3, which is a half of 6). In 33 minutes Ill burn about (30min x 40 =1200kg, + 3min x 45=135kg) 1335kg calling it 1400kg to conservativley allow for the tiney extra of fuel flow over the 45kg a minute rate I used. Maybe Ill knock off 100 kg to allow for the flight idle descent so Im looking at 1300kg if I want to be super-accurate. So 5430kg is by now about 5400, so 5400kg minus 1300kg = 4100kg overhead departure airport.
See its not too hard huh? Just a matter of ratio and proportion wether its a 737 or Cessna 172. It comes easier with experience and exposure. But if in doubt ALWAYS carry your Jep or Kane whizzwheel and know how to use it. Of course you can use any of the latest electronic calculating toys but whizwheels and brains dont get flat batterys!
Other stuff like calculating drift from winds, how far from an NDB to start the turn from a 90 deg intercept for the next leg from overhead etc is generaly picked up by reading and asking on BBs like Tech Log etc.
Hope this helps.