Mental math for "cutting corners" during enroute nav?
- Thread starter C150J
- Start date
moxiepilot
Well-Known Member
how much more can you cut off when you're /L ?
lol.
lol.
C150J
Well-Known Member
Between preferred routes, SIDS, and STARS, we rarely get direct. I'm specifically referring to large course changes - say a 45 degree turn - when coming over a VOR.
J.
fish314
Well-Known Member
There are several techniques out there. What kinds of things are you looking for? Here's a few off the top of my head:
WIND Stuff:
X-wind component/Groundspeed in Nautical miles per minute = drift correction in degrees.
Speaking of winds, to figure the crosswind component you can either use the clock method or the 20% method:
Clock method: However far off the nose the wind is in degrees, imagine where that number would be in minutes on a clock, and then use that porportion of the whole clock. So for example if the wind is 15 degrees off of the nose, 15 is one quarter of an hour, so use about 25% of the wind as the crosswind. 30 degrees would be half an hour, so half the wind is the crosswind. 45 degrees corresponds to 75%, and 60 and above is the whole hour, so figure the crosswind is 100% of the total.
20% Method: Add 20 to the degrees off and then use that percent of the total as the crosswind component. Anything between 80 and 90 degrees off, just use 100%. So if the wind is 20 knots at 55 degrees off the nose, 55+20=75, so the crosswind should be about 75% of 20 knots, or about 15 knots.
TURN RADIUS STUFF:
Groundspeed in NM/min squared then divided by 10 = turn radius. So if you are doing 120 ground, that's 2 NM/min. 2*2=4. 4/10= 0.4, so the turn radius is about .4 NM, so you should lead a 90 degree turn by .4 NM.
Another turn radius one works well at faster speeds, but doesn't really work for anything less than about 210 ground. Anyways, you can just use GS in NM/min - 2. So at 240 ground, that's 4NM/min. So it's about 4-2=2NM turn radius. The method above would have given you 1.6 NM, so it's pretty close.
If you have a Mach meter, you can also use (Mach*10)-2. So if you are doing .4 mach, that's also about a 2NM turn radius, because Mach *10 is pretty close to your miles/minute in true airspeed.
DESCENTS:
For descents, if you imagine your distance to go (in NM) on the 10 degree nose down marker on your ADI, then imagine your altitude in thousands of feet on that scale and it will give you your descent angle required. That might require some explanation, so imagine you have 21 miles to go until an altitude restriction, and you have to lose 10,500 feet. Well if you imagine the 10 degree nose low mark as "21" then 10.5 would be halfway down (since 10.5 is half of 21). Well halfway down the scale is 5 degrees nose low, so you need 5 degrees below what is required for level flight to hit your fix at the required altitude exactly.
A guess of your true if you know your indicated is half your altitude in thousands of feet, times ten, plus your indicated. So if you are at 10000, flying 200 knots indicated half of "10" is 5. 5 times 10 is 50. 200 plus 50= 250 knots true. This isn't the most accurate method out there, especially once you get above the mid-teens, but it is easy.
VOR and 60 to 1 rule:
Divide 60 by your current DME and that's the number of radials/mile. So if you are on the 30 DME arc and you want to lead a turn by 2 NM, you would need to use a 4 radial lead point (60/30DME=2 radials per mile).
Another 60 to 1 application comes from the fact that a mile is about 6000 feet long, so for a 1 degree descent or climb, that's basically 100 feet every 6000 feet, or said another way it's 100 feet per NM. So if you want to cross check your visual glideslope is approximately 3 degrees you could check to make sure you are 300 feet above the field elevation at 1 NM from the aimpoint, or 150' when you are 1/2 NM from the aimpoint.
Anyways, like I said there are literally HUNDREDS of these out there!
WIND Stuff:
X-wind component/Groundspeed in Nautical miles per minute = drift correction in degrees.
Speaking of winds, to figure the crosswind component you can either use the clock method or the 20% method:
Clock method: However far off the nose the wind is in degrees, imagine where that number would be in minutes on a clock, and then use that porportion of the whole clock. So for example if the wind is 15 degrees off of the nose, 15 is one quarter of an hour, so use about 25% of the wind as the crosswind. 30 degrees would be half an hour, so half the wind is the crosswind. 45 degrees corresponds to 75%, and 60 and above is the whole hour, so figure the crosswind is 100% of the total.
20% Method: Add 20 to the degrees off and then use that percent of the total as the crosswind component. Anything between 80 and 90 degrees off, just use 100%. So if the wind is 20 knots at 55 degrees off the nose, 55+20=75, so the crosswind should be about 75% of 20 knots, or about 15 knots.
TURN RADIUS STUFF:
Groundspeed in NM/min squared then divided by 10 = turn radius. So if you are doing 120 ground, that's 2 NM/min. 2*2=4. 4/10= 0.4, so the turn radius is about .4 NM, so you should lead a 90 degree turn by .4 NM.
Another turn radius one works well at faster speeds, but doesn't really work for anything less than about 210 ground. Anyways, you can just use GS in NM/min - 2. So at 240 ground, that's 4NM/min. So it's about 4-2=2NM turn radius. The method above would have given you 1.6 NM, so it's pretty close.
If you have a Mach meter, you can also use (Mach*10)-2. So if you are doing .4 mach, that's also about a 2NM turn radius, because Mach *10 is pretty close to your miles/minute in true airspeed.
DESCENTS:
For descents, if you imagine your distance to go (in NM) on the 10 degree nose down marker on your ADI, then imagine your altitude in thousands of feet on that scale and it will give you your descent angle required. That might require some explanation, so imagine you have 21 miles to go until an altitude restriction, and you have to lose 10,500 feet. Well if you imagine the 10 degree nose low mark as "21" then 10.5 would be halfway down (since 10.5 is half of 21). Well halfway down the scale is 5 degrees nose low, so you need 5 degrees below what is required for level flight to hit your fix at the required altitude exactly.
A guess of your true if you know your indicated is half your altitude in thousands of feet, times ten, plus your indicated. So if you are at 10000, flying 200 knots indicated half of "10" is 5. 5 times 10 is 50. 200 plus 50= 250 knots true. This isn't the most accurate method out there, especially once you get above the mid-teens, but it is easy.
VOR and 60 to 1 rule:
Divide 60 by your current DME and that's the number of radials/mile. So if you are on the 30 DME arc and you want to lead a turn by 2 NM, you would need to use a 4 radial lead point (60/30DME=2 radials per mile).
Another 60 to 1 application comes from the fact that a mile is about 6000 feet long, so for a 1 degree descent or climb, that's basically 100 feet every 6000 feet, or said another way it's 100 feet per NM. So if you want to cross check your visual glideslope is approximately 3 degrees you could check to make sure you are 300 feet above the field elevation at 1 NM from the aimpoint, or 150' when you are 1/2 NM from the aimpoint.
Anyways, like I said there are literally HUNDREDS of these out there!
fish314
Well-Known Member
Oh, I just thought of another one. This one says that the cone of confusion is about the same diameter as the altitude in miles. So if you are at 6000 feet AGL, that's about 1 mile UP. So the cone of confusion is about 1 mile in diameter, or about 1/2 mile in radius. At FL350, you are almost 6 miles UP, so that's a 3 mile radius for the cone of confusion on a VOR.
C150J
Well-Known Member
Thanks Fish - youdaman!
I'm mainly curious as to how pilots determined when to lead turns on airways. The AIM specifically allows for it (otherwise you'd blow right through the next course and have to re-intercept), but I'm wondering how much of a lead people gave it prior to FMS calculations. I guess I'll pay more attention on Saturday when I'm on a trip!
I'm mainly curious as to how pilots determined when to lead turns on airways. The AIM specifically allows for it (otherwise you'd blow right through the next course and have to re-intercept), but I'm wondering how much of a lead people gave it prior to FMS calculations. I guess I'll pay more attention on Saturday when I'm on a trip!