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Rate of descent restriction while descending to MDA

 
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aviator
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PostPosted: Mon Mar 06, 2006 5:37 am    Post subject: Rate of descent restriction while descending to MDA Reply with quote

hi, the issue is about descending to MDA or a stepdown fix from initial approach altitude in an instrument approach procedure e.g. if the initial approach altitude overhead a VOR is 5000 feet and the next step down fix on the outbound leg is 3000 feet, then is there any restriction in your rate of descent while coming down to 3000 feet from 5000 feet? I mean restriction regarding descending at a rate greater than 1500 feet per minute or some other value? thanx
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K.Haroon
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PostPosted: Tue Mar 07, 2006 5:16 pm    Post subject: Descending to MDA Reply with quote

Hello. A stabilized approach technique for non-precision approaches is generally preferred over immediate descends. This technique requires a continous descent with a rate of descent adjusted to achieve a constant descent gradient to a point 50 feet above threshold. And ofcourse you have to cater for any stepdown fix or minimum crossing height for the FAF etc.

However regarding your question i.e descending from 5000 feet initial approach altitude to 3000 feet step down fix, the restriction which I know is of the descent gradient and not a particular rate of descent i.e. you cannot descend with a gradient of more than 15%. This means for a fixed gradient, your rate of descent will vary at different ground speeds e.g. flying a descent gradient of 13% at 140 knots requires a rate of decent of 1800 fpm whereas flying the same 13% descent gradient at 120 knots requires a rate of descent of 1600 fpm.

So the bottom line is the descent gradient i.e. you cannot descend with a gradient of more than 15%. This is according to PANS-OPS Doc 8168 as referred by Jeppesen in the ATC section of the route manual, under the topic of Approach Procedures (page 213).

N.B. For calculating your rate of descent for a particular gradient you can refer to Jeppesen's route manual "Terminal Section" for the Gradient to Rate table.


Last edited by K.Haroon on Mon Oct 22, 2012 12:33 pm; edited 1 time in total
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aviator
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PostPosted: Thu Mar 16, 2006 6:03 pm    Post subject: descent restrictions in final approach Reply with quote

thanx for the references and what about the restrictions in the final approach phase?
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K.Haroon
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PostPosted: Fri Mar 17, 2006 1:59 pm    Post subject: Descent restrictions in the final approach Reply with quote

Descent restriction in the final approach of a non-precision procedure with FAF is mentioned in terms of descent gradient or angle and without FAF it is in terms of rates of descent i.e.

The minimum descent gradient in the final approach of a non-precision procedure with FAF is 4.3% (2.5 deg) whereas the maximum is 6.5% (3.7 deg) for Cat A and B aircraft and 6.1% (3.5 deg) for Cat C, D and E aircraft. The optimum is 5.2% (3 deg).

For non-precision procedures like VOR, NDB and no FAF, rates of descent are applicable in the final approach phase. The limitations for Cat A and B aircraft are min 400 feet/min and max 650 feet/min and for Cat C, D and E the min is 600 feet/min and max 1000 feet/min.

For all practical purposes I have rounded off these values. To see the exact values please refer to Jeppesen's route manual, ATC section, page 215, part 2.4.2

Also check out the Jepp's Terminal section for the vertical descent angle reference table. It provides a method to determine a vertical descent angle with respect to a defined height at FAF and distance to runway threshold. You can also determine your desired rate of descent for flying a specific gradient. It will be useful to fly a Constant Descent Final Approach (CDFA).
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