Favorite ROTs (Rules of Thumb)

jdwatson

jdwatson

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JDW
On a recent night flight to ILM, I was trying to recall the rule of thumb to calculate my descent point.

I couldn't remember the ROT, so that got me thinking about what other ROTs are out there that I can't remember.

So, what's your favorite ??

P.S. Here's the ROT...
To estimate the point from the field to begin your descent, multiply your altitude (whole numbers)by three and add ten.
To descend from FL 350: 3 x 35 = 105 + 10 = 115 miles out
 
I just use time remaining and select a rate I would like (when ATC is on board).

I want 1500fpm from fl350 to tpa of 1000', at what 'time remaining' should I start descending?
Gives the brain a smidge of arithmetic to do when bored. Not sure how yours works without knowing your speed, JD.
 
Hey, I didn't say I used this ROT just that I couldn't remember there was one. I do it the old fashioned way. I can't seem to remember ROTs so I rely on arithmetic and algebra. We were travelling 120kts at 6500' using our normal 500'/min descent. TPA is 800', so I have 5700' to lose. I need ~12minutes to descent. 2nm/min (120kts) is 24nm to get to TPA. (Altitude-TPA/descent_rate)*GS nm/min. Neither myself nor my pax could remember the short way to do this. LOL.
 
jdwatson said:
To estimate the point from the field to begin your descent, multiply your altitude (whole numbers)by three and add ten.
To descend from FL 350: 3 x 35 = 105 + 10 = 115 miles out
Click to expand...

This one sounds like the one you mentioned, JD.
PS I don't use ROT much, no memory for them. Well, I use ones like that posted on the Osh briefing tent; "Don't do nothing stupid" or "Don't crash, y'idiot".

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1. Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent.
2. Now take half your ground speed. This is your rate of descent in hundreds of feet.
Ex.If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
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Here's some of my favorite Rules of Thumbs.
Aviation related: I travel about three miles forward for every thousand feet I need to lose. Ipsofacto, if I've got to lose 20,000 feet I want to be starting down at around sixty miles out.
Life in general:
I'll always want more than I'll ever get.
Never bet against the house.
Never eat anything bigger than your head.
Fail to plan...plan to fail.
It is what it is.
I hope that helps. Somehow. :D
 
Let'sgoflying! said:
This one sounds like the one you mentioned, JD.
PS I don't use ROT much, no memory for them. Well, I use ones like that posted on the Osh briefing tent; "Don't do nothing stupid" or "Don't crash, y'idiot".

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1. Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent.
2. Now take half your ground speed. This is your rate of descent in hundreds of feet.
Ex.If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
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Click to expand...

Trouble with that is that it has me coming down way too fast especially when there's a tailwind. I generally plan 6-8 NM per thousand.

Now if you have DME or GPS that gives time to dest all you need to do is double the thousands to lose and come down at 500 FPM with a slight adjustment for speed picked up in the descent (generally not much unless you are descending out of a big headwind).
 
jdwatson said:
Hey, I didn't say I used this ROT just that I couldn't remember there was one. I do it the old fashioned way. I can't seem to remember ROTs so I rely on arithmetic and algebra. We were travelling 120kts at 6500' using our normal 500'/min descent. TPA is 800', so I have 5700' to lose. I need ~12minutes to descent. 2nm/min (120kts) is 24nm to get to TPA. (Altitude-TPA/descent_rate)*GS nm/min. Neither myself nor my pax could remember the short way to do this. LOL.
Click to expand...

Adjust your cruising groundspeed by 5-10% to get descent groundspeed unless you reduce power to maintain crusing speed in the descent.

Divide descent GS by three, round to nearest 10Kt.

Drop the zero and multiply by the altitude to lose in thousands. This is the distance to descend that far at this groundspeed.

To get the top of descent distance, add about 10% of the difference between the descent speed and pattern speed. Add another 3-4 miles if you are enterring via final (straight in).

That "Three times altitude in ten thousands + 10 would probably work for jets, but I don't like coming down at 3500 FPM and given my typical cruising altitude, the airport elevation can be a significant factor.
 
5 miles per thousand feet will give me a 400fpm descent.

400fpm=2.5 minutes per thousand feet
2.5 minutes at 120knots=5 miles
 
The Old Man said:
5 miles per thousand feet will give me a 400fpm descent.

400fpm=2.5 minutes per thousand feet
2.5 minutes at 120knots=5 miles
Click to expand...

Yep. Rather than go through the math each time you have to plan a descent, it's much easier to bracket the problem using a couple typical groundspeeds and just fudge it from there. If you descend at 120 KTAS then work the problem for 100 and 140 and you can interpolate based on the winds.
 
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