ATC Descent rate

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1

When ATC tells me to descend to an altitude prior to my planned approach, what descent rate are they looking for? I would like to know so that i can achieve a relatively seemless descent profile between my TOD and the first fix on my planned approach.

Thanks!

2

What aircraft are you flying? IFR or VFR?

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On an ifr flight plan.

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It should be around a 3 degree path so that roughly comes down to descent rate of groundspeed times 5 (x 10 / 2 is easier :upside_down_face:). On a jet that is roughly flight idle descent, depends a little bit on the type. So groundspeed 200 kts = 1000 ft / min required for a 3 degree descent path.

On the plane I fly its pretty simple as in addition to a vertical speed mode we have a Flight Path Angle mode, select -3.0 degrees and the planes flies just that, automatically correcting for head or tailwind to maintain a 3 degree descent path.

In FS2020 it is going up and down a bit, I got descent clearance a few times really close to the actual top of descent, yesterday it was way too early, sometimes its late. You should first of all calculate your own Top of Descent:

The TOD for a jet can normally be calculated as follows:

  • You’ll cover around 3 miles per 1000 ft in still air.
  • Lets say approach starts at 2000 ft, cruising at FL350, means 33 x 3 is 99 nm before FAF or FAP

Then there is two things you need to factor in:

  • Factor in the wind (add 1 nm per 10 kt tailwind).
  • You’ll need a deceleration segment first to reduce speed from around 290 kts on most planes to 250 kts before reaching 10.000 ft and then from 250 kts to 160 kts for approach. You can do those decelerations in level flight or by shallowing the descent to 1000 ft/min. In both cases it adds extra distance. With or without speedbrakes also make a difference altough you normally plan without.

You then have your TOD, if ATC does not come with a descent before this point, I’ll reduce cruising altitude via the ATC window. I made a schematic of this for the Embraer 190/195. It’s a little type specific maybe but it’s probably not that far off from any other jet. Bigger aircraft may be closer to 3 degree descent angle at idle thrust.

During the descent you should continuously cross check distance to go vs altitude and correct descent rate as required. To simplify matters I would say the final approach start anywhere from 6 to 9 nm from the runway, and you’ll need to be level around 10 nm before that for a level deceleration so +15 nm.

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5

Thanks for a really nice summary!

All we need now is for someone to explain this to MSFS ATC
 I don’t often fly IFR, but was enjoying my new toy (Navigraph) last night and heading to the final approach fix only to have ATC instruct me to ‘climb and maintain FL170, expect vectors for the ILS’ Ah well


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I have explained it with a little more complexity than needed I think by calculating towards the FAF / FAP. After the FAF / FAP its a 3 degree path again anyway so to simplify it further you could calculate using total distance to go:

Altitude to lose / 1000 x 3 (3 nm per 1000 ft) + tail wind + deceleration. so with the example above:

FL350 to SL = 35 x 3 = 105 + tail wind (0) + deceleration (around 10 nm in level flight) = 115 nm.

The speed reduction from 290 kts or whatever you are flying above FL100 to 250 kts is not significant. Maybe add a few miles for that also, its not an exact science :joy:. If you want to fly a continous descent all the way down it becomes a little more complex to calculate.

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Thank you very much for your help! I don’t suppose I’ll be able to have a perfect descent down without working vnav (or managed mode on the A320
) but hopefully I can get it a little bit more consistent than just guessing.

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A “trick” which also works pretty well most of the time is to be abeam the airport at 6000 ft if you land in opposite direction. This way you could just tune in to a navaid with DME on the airport and then:

For straight in:

  • Altitude to lose / 1000 x 3 + deceleration + wind.

For downwind:

  • Altitude to lose - 6000 ft / 1000 x 3 + deceleration + wind.

It prevents you having to go into the charts and add up all the individual legs to find out distance to go.