WHEN AND HOW TO DESCEND AN A/C

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Peter Armstrong
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WHEN AND HOW TO DESCEND AN A/C

Post by Peter Armstrong » Wed Oct 18, 2017 10:52 am

Looking at the CTR ATC Material, it gives us a general (simple formula) idea of when to start a typical descent. What I call the “4x rule”.

The example shows a descent from 31000 to 1000, resulting over 120 nm.

Which means, a descent of 30000ft in 120 nm. Which then extrapolates into 250ft/mile. If we assume that the descent is controlled at a speed of 250k (or, to keep it more simple 250mph), then we have another set of parameters.

250 mph = 4 miles per minute (4.2 to be exact)
So, over 4 miles (according to above simple formula 250ft/mile) we have descended 1000ft. Which in turn, confirms our simple formula:-

120nm divide by 4 = 30 add the 000s = 30000ft over 120 nm

So, my point being is that some months ago, I witnessed a controller giving an A/C, entering our TRACON at Seattle, an instruction to commence a descent to 5000 from Raddy at a height of 16000. I wondered in my mind if that was a safe thing to do? Because after Raddy, we have sections (MVAs) of 8000ft then 7000ft followed by 6000ft and 5700 ft. prior to reaching a safe MVA at 5000ft (about 11 nm from Aubrn – ****about 20 nm from Raddy). However, after studying the descent rate of the above simple formula, the instruction seemed safe! And, when considering a possible Jet descent rate of about 1000ft/minute and the 20nm journey from 16000 down to 5000, it was perfectly safe! What do I know? LOL. However, the descent will depend entirely on the rate of descent and the speed of the descent over the mileage to cover. That could get tricky! So, is it safe to instruct a simple descent (lets you do other tasks, without spending too much time on safe step descending, which is my choice). What do you think?

Peter

**** 20nm divide by 4 = 5, add the thousands = 5000ft from 16000ft = 11000ft
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Frank Miller
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Re: WHEN AND HOW TO DESCEND AN A/C

Post by Frank Miller » Wed Oct 18, 2017 7:14 pm

The classic "Rule of 3" for descents has been tried and tested by pilots for a long while and (I think) remains just about standard for IFR descents. 3nm for each 1000 feet of descent.

Frank

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