Confusion about when it's my navigation, and when it's ATC

In a previous article, Mxsmanic said:
Paul Tomblin writes:
I fly with a Garmin 296 handheld GPS. In my experience, nearly always
just around the same time it says I need to start my descent if I want to
descend at 500fpm to my destination, ATC clears me down to a lower
altitude without being asked.

I have noticed this as well. I suppose if they know the route well,
they know when the descent usually starts.

Except they know where to start my descent whether I'm flying a 100 knot
Archer or a 140 knot Lance, or on one occasion, a Piper Dakota with a 70
knot tail wind.

I suspect there is software they use to handle this.

--
Paul Tomblin http://blog.xcski.com/
#define sizeof(x) ((int)rand()*1024)

Paul Tomblin wrote:

In a previous article, Mxsmanic said:


Paul Tomblin writes:


I fly with a Garmin 296 handheld GPS. In my experience, nearly always
just around the same time it says I need to start my descent if I want to
descend at 500fpm to my destination, ATC clears me down to a lower
altitude without being asked.


I have noticed this as well. I suppose if they know the route well,
they know when the descent usually starts.



Except they know where to start my descent whether I'm flying a 100 knot
Archer or a 140 knot Lance, or on one occasion, a Piper Dakota with a 70
knot tail wind.

I suspect there is software they use to handle this.



It believe it is based on the instrument requirement (?) of 500 fpm rate
of descent.

At a given airspeed and altitude, at 500 fpm an aircraft should commence
its descent at the calculated distance.

This will vary depending on the facility, traffic and procedures.

I calculate the distance in may head for my given cruise altitude and
wait to see if ATC calls me at the appropriate time. They are usually
early on the call to assign lower.

In article ,
john smith wrote:


It believe it is based on the instrument requirement (?) of 500 fpm rate
of descent.

That's the absolute minimum descent rate; more typical is a 3-degree
(roughly 300 feet per nautical mile) descent. ATC is expecting a normal
rate of descent for your particular type of airplane to achieve that.

In a spamcan doing 90 knots groundspeed, 500fpm is about right. In a
typical jet doing 450 knots over the ground, that's going to be more
like 2300 feet per minute. (Groundspeed in knots * 5 will give you a
pretty good target to achieve a 3 degree descent.)