Please could someone remind me of the 'easy' formula to calculate when to start a descent from cruise altitude '...to reach a certain point at X feet'Many thanks.Toni.

Take your current altitude and subtract your target altiditude, i.e. the altitude that you want to descent to. Then divide it with your planned descent rate which is typically 500'/minute.For example if your altitude is 10.000' (FL100) and you want to descent to 3000' with 500' per minute, you want to descent 10.000' - 3.000' = 7.000' and if you descent by 500'/min. then 7.000' / 500' = 14 minutes. So calculate 14 minutes to descent.Now take your descent speed, e.g. 120 knots and find out how far you will fly in 14 minutes (= 14 * 2 = 28 nm). So descent 28 miles before the position where you want to be in 3.000'.There are several ways to calculate the distance flown in 14 minutes. I use speed factors or sixty factors. 120 knots is speed factor 2 = 2 * 60 nm. Therefore you fly two miles per minutes and thus you fly 28 miles in 14 minutes.It may sound complicated but when your getting used to it you can use this method to calculate your descent point by head - no calculators needed.Cheers, Kim Pedersen

Thanks Kim....I thought there was a simpler way to calculate the descent start point giving just miles to waypoint?Toni.

Hi Toni,There is a shortcut. As you can figure out you will have to take your airspeed and descent rate into account. But if you assume that you always descent by 500' / min. and you descent by 120 knots = SF 2 (Speed factor) then you can do like this:From 10.000' to 3.000' = descent 7.000'. Now remove the zeroes and multiply by four = 7 * 4 = 28 nm before the target position.If you are flying 180 knots, then you are flying SF 3. Now the calculation is 7 * 6 = 42 nm.This is so because it takes you 2 minutes to descent 1.000' with a descent rate of 500' / min. and therefore I multiply the SF by two. SF 2 * 2 = 4.I hope that this makes it easier./Kim....

Try the 3 to 1 rule of thumb.3nm for every 1000 ft of descent.You want to descend 10,000 ft, divide by 1000 then multiply by 3. 10 X 3 = 30 nm. From 35000 ft... 35 X 3 = 105nmFrom 10,000 ft to 3000 ft10-3 = 7.7 X 3 = 21 nm.Regards.Ernie.

Hi Ernie,The 3 to 1 is quite easy to remember, but this means an airspeed (or groundspeed) of 90 knots, perhaps in a Cessna. So it depends on the aircraft that you are flying. A 4 to 1 would fit most small piston engined aircrafts the most as this is equivalent to 120 knots. When I descent I trim down the nose and more or less keep my power settings. This means that the airspeed increases somewhat.Anyway, you just have to take your airspeed into account when calculating the miles used to descent. But I like the idea about the 3 to 1 or 4 to 1 rule of thumb. It is a very easy way to quickly calculate when to descent./Kim......

You don't need to change the 1000ft/3nm rule of thumb with speed.The rule of thumb states the descent rate is 333ft/nm. The aircraft speed in kts/min is Vkts/60 so the descent rate required is 333*Vkts/60 = 5.55*Vkts in ft/min.V = 90 kt Descent Rate = 500 ft/minV = 150 kt Descent rate = 833 ft/minThese are practicable values.

I do not agree. If you increase your descent rate by higher speed you will end up with high-speed descents which you should only use in emergency descents, e.g. when your cabin pressure drops and you need to get below 10.000' fast. You can of course use descent rates as you please within some limits. But when you use high descent rates then your passengers will fell uncomfortable as the ear pressure increases. So therefore you should keep your descent rate around 500 ft/min.If you are flying with pressurised cabin then you can descent more rapidly as the cabin pressure will align with the sea level pressure gradually.But all-in-all I would say that you should stick to a planned descent rate of 500 ft / min.

I do not agree. If you increase your descent rate by higher speed you will end up with high-speed descents which you should only use in emergency descents, e.g. when your cabin pressure drops and you need to get below 10.000' fast. You can of course use descent rates as you please within some limits. But when you use high descent rates then your passengers will fell uncomfortable as the ear pressure increases. So therefore you should keep your descent rate around 500 ft/min.If you are flying with pressurised cabin then you can descent more rapidly as the cabin pressure will align with the sea level pressure gradually.But all-in-all I would say that you should stick to a planned descent rate of 500 ft / min.

We are discussing normal descent rates here and the rates I quote are perfectly acceptable. On a normal ILS approach at a 3 deg glide slope UK ILS charts quote a range of: V = 80 kt Descent Rate = 420 ft/min toV = 160 kt Descent Rate = 850 ft/minThese are consistent with the figures I gave, after all 1000 ft in 3 nm is a slope of 3.14 deg.

For standard rates of descent - the rule of 3 has been around forever and is used by most RW pilots I know to ballpark the start of descent. Obviously if you are told to descend faster or slow down, you must adjust but, all things being equal3 miles/1000 feet is accurate.Vic

Thank you everybody!Toni.