Simviation Forums

Hi...

I've completely forgotten the formula for calculating descent....

Can anyone remember?

I'd like the precise formula, not the 'multiply GS by five or six'

Thanks

With big jets I seem to remember it's the 1 in 3 rule ... 30,000 feet start descent at 100 miles

If you're flying jets above 18,000 feet, you should be on an IFR flightplan, so just let ATC tell you when to start descending...

I know all that, I need the precise formula for calculating the descent profile.

Thanks though.

Not quite sure where you want to start and end, but here is an example:

Starting with Vertical Speed (VS) and Ground Speed (GS), find Time (T) and Distance (D) to descent a certain number of feet (H)

H [ft] / VS [ft/min]  = T [min]
or
H [ft] / VS [ft/min] / 60 [min/hr] = T

---

GS [NM/hr] * T

---

= D [NM]
or
GS [NM/hr] * H [ft] / VS [ft/min] / 60 [min/hr] = D [NM]

In practice the 3x rule for jets is straightfoward, if you haven't used VNAV to calculate the descent point.

As well below 1000 ft it is still easy to use a set figure, ie at 1000ft a minute descent rate then a minute per thousand altitude loss required is straightfoward.  On most small GA this is the best figure anyway.  

If your aircraft is fine at 1500 ft per minute then it is easy to build a time to go /descent table.

Most real world flying involves these guesstimates, as in the real thing even the  best worked out time/descent rate is thrown out by external circumstances, usually in the form of ATC.

However the airlines do have a descent rate calculation for non-precision approaches known as CANPA which is still not that more accurate than multiplying the groundspeed by 5!

The one in three rule, as mentioned, is the most common. Also, it is worth factoring other considerations, such as decellerations at FL100/10,000ft (to below 250kts) and at the bottom of the decent (to configure the aircraft fo the approach), which will slow the descent rate down.

So as Pete says, if you're at FL300, 100 miles is about right once you factored in the decellerations etc.

Picking up on Andrew's point, it's always best to let ATC know when you want to descend. Good ATC in reasonably quiet airspace will clear you to descend at your discretion, allowing you to start the descent when you want, in other places, such as into London, Europe or some of the busier US airports, ATC will be more controlling of you descent, for example approaching the London in the UK you'll be told to descend to FL200 crossing the French coast.

Yeah, but what I wanted was the formula for the profile.

But I figured it out myself...

Thanks for the effort though guys  :)