(Fun) test

[rchcfi]

OK, I'll start by saying I've had a few of those wonderful beers everyone always talks about. Which is probably why I'm spending my time writing this question.

A friend of mine is going for his first jet interview tomorrow, so I decided to pop a question at him. I asked:

"So you're cruisin' along in your Falcon 20 at FL310 running late. ATC says to you 'start your descent at 100NM out. No speed restrictions except for the 250 below 10000'........

I gave him a hypothetical cruise speed of 420 TAS with no wind. What is your descent rate gonna be to get there?

The stupid thing is, both of us couldn't figure it out without a pen and paper to write it down. Anyone care to answer, and anyone else have this strange mental math block?

 

[rchcfi]

OOOPS,

Thank the barley malt...I forgot to mention the airport is at sea level.

 

[350DRIVER]

Plug it into the FMS.... pretty easy that way


3 5 0

 

[wms]

http://www.flightinfo.com/rulesofthumb.asp

 

[falcon20driver]

I only have a few hundred hours in the Falcon 20, but if ATC asked me to start my descent 100 out, I would calculate the altitude that I need to lose and see how close 100 miles is to the standard 3:1 descent, from 310 to yip this is about 30000 feet, multiply 30000 by 3 and gives you 90 miles out and start down at 2500 FPM unless your trying to save fuel and want to use a 2:1 ration than double the altitude and descend at 3500 FPM, thats how we do it in the Learjet.

So 100 out is a little early for a 3:1 going into yip, so I would ask to start down 90 out, for a 2:1 ask to start down at 60 out, if you have a headwind than subtract 10 miles, a tailwind than add 10 miles.
Keep an eye on the distance to the destination and modify your rate of descent as needed, that's how I was taught and it works well.

 

[bart]

100 miles out
31,000 feet
420 TAS = 7 miles a minute
Below 10, 250 about 4 miles a minute

You would want to take your time getting to 10,000 feet and then dive at the airport, to minimize your time.

So, 92 miles over 7 miles a minute equals 13.14 minutes

divide 21,000 feet by that is a 1,598 fpm descent

Then at 8 miles, idle power, haul the nose up, spoilers out, and 5,000 fpm at 250 kias to the airport for a time of 15.14 minutes.