4 hours ago, KenG said:

No old school fancy math needed, let the airplane do what it is designed to do.

That works great unless you're descending into a field without approaches in a plane that doesn't have fancy onboard electronics.   Which is how I spend a huge chunk of my sim time. 🙂

"thousands of feet * 3" and "GS * 5" is hardly complex math, I think most people could do it in their heads pretty readily.

By all means, use the tech if the tech is there, but don't depend on it.

nm

Edited November 6, 20205 yr by snwboardn

nm

Edited November 6, 20205 yr by snwboardn

2 hours ago, kaosfere said:

That works great unless you're descending into a field without approaches in a plane that doesn't have fancy onboard electronics.   Which is how I spend a huge chunk of my sim time. 🙂

"thousands of feet * 3" and "GS * 5" is hardly complex math, I think most people could do it in their heads pretty readily.

By all means, use the tech if the tech is there, but don't depend on it.

Actually it is 3 * the altitude to lose (not your cruising altitude as stated by others) and 5 * you average ground speed not your current speed.

If I am cruising at FL380 and 480 Knots ground speed and I need to cross XYZ at 18,000 feet then first I have to subtract 18,000 from my cruising altitude FL380 which give me (20000/1000)*3 or 60nm, not 114nm out.

If the crossing restriction also has a speed restriction of 280 KIAS then I must then convert 280 knots indicated to TAS adjust for winds and then find the average speed. I hope you saved your whizz wheel (e6b) to calculate that one.

4 minutes ago, KenG said:

Actually it is 3 * the altitude to lose (not your cruising altitude as stated by others) and 5 * you average ground speed not your current speed.

If I am cruising at FL380 and 480 Knots ground speed and I need to cross XYZ at 18,000 feet then first I have to subtract 18,000 from my cruising altitude FL380 which give me (20000/1000)*3 or 60nm, not 114nm out.

If the crossing restriction also has a speed restriction of 280 KIAS then I must then convert 280 knots indicated to TAS adjust for winds and then find the average speed. I hope you saved your whizz wheel (e6b) to calculate that one.

I know the rule well, you don't need to "well actually" me.   (And the original mention of using cruising altitude in this thread was in relation to a destination at sea level, in which case it's more or less the same thing, if you're not shooting for pattern altitude.)

That aside, you seem to be so keen for an argument that you're missing the point of a rule of thumb.   It's not to provide you with a precise answer.   It's to give you a general solution to use in the absence of any other.

They're also useful for cross-checking your more advanced calculations to make sure they're not obviously in error.

What a bizarre thing to get worked up about.

8 hours ago, Glenn Fitzpatrick said:

The 5 in the rate calculation is NOT 5 degrees. it is Tan 3 degrees which is 0.05 or 5%

I missed that.  Thanks. 

Most guys I fly with just know their 3 times table and keep revising it, plus a chunk to slow down.

So start descent, got 40,000ft to lose, so I need 120 miles, plus 15 to slow = 135 miles

Now I'm at 30,000ft, need 105 miles

Now I'm at 10,000ft, need 40 miles

Etc etc. If you've got more miles than you need you're fine. If you've got less you get down quicker. I've never flown with anyone who starts figuring out descent rates. 

Interesting discussion. I've been using this very informal and messy method since Im not into study level stuff & RW procedures. I fly airliners casually. Its worked perfectly for my needs since I got FS9. Taking into account airfield elevation:

Airliners

• FL X 0.33 = TOD + 10nm
• 2500-2700FPM descent 
• Adjust speed & descent rate to end up at 250kt by 10,000ft AGL

Example: From 41,000ft. 
410 X 0.33 = 136nm + 10 = 146nm (TOD)

Light GA at lower alts (I dont add 10nm for these slow piston planes)
Example: from 9,500ft
95 X 0.33 = 31nm (TOD) - descending at 700-800FPM

Edited November 7, 20205 yr by ThrottleUp

As mentioned above, a VS rate of 5xGS (I find10xGS/2 easier when busy) gives you a 3 deg descent (318ft per NM) which is also the slope of a typical ILS glide slope. So if flying an ILS raw data, 5xGS gives you a nice ballpark target VS for holding the glide slope.  Thinking about 300ft/NM is also handing even when flying a typical visual pattern. If I'm 2 NM miles out when I turn final, I should be about 600 AGL.  Just ballpark rule-of thumb stuff that can be handy.

One of the very best aviation sites on the web is Code7700  by “Eddie” (James Albright) who flew jet fighters, KC-135 tankers, and B747s in the US Air Force, and corporate Gulfstream biz jets after.  Besides a wealth of aviation information, there are also great stories from Eddie’s flying career.

https://www.code7700.com/about_eddie.htm     "Eddie"

https://code7700.com/descent.htm   The TOD 3 x altitude to lose rule of thumb

https://code7700.com/rot_descent_vvi.htm  The 5 times GS rate of descent rule of thumb. Here it is presented as Nautical miles per minute times descent angle times 100, which works out to about 5 times your Ground Speed for a 3 degree descent angle.

https://www.code7700.com/stories.htm   Stories

Al

Edited November 7, 20205 yr by ark

22 hours ago, snwboardn said:

nm

Nautical miles

1 hour ago, Bunchy said:

Nautical miles

haha, I answered him thinking everyone was misunderstanding his question, but in the end it is I who misunderstood his question. 

https://descent.now.sh/

The msfs vnav is simplified, it seems it just targets the STAR altitudes regardless of at or above/below suffix, that makes the atc instruct to descent abnormally and the vnav to dive or slow descent sometimes.