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Low-bypass
5th Apr 2010, 20:37
Hi there
Can anybody explain me how to apply a Driftdown procedure for a Jet plane?The Oxford JAA ATP just explain Driftdown method for a light multi-engine.Thanks

bucket_and_spade
5th Apr 2010, 21:01
Hi,

What do you mean by 'how to apply'? Do you mean how you'd actually go about doing/flying it?

B&S

flyburg
5th Apr 2010, 21:59
basically, at our company, but procedures may differ by company, several critical routes are identified. In the dispatch fase you have to determine if the weight at the critical part of the route is low enough so that you would be have (on top of my head) a 1.2% climb rate at the route MORA. If the weight does not allow this, you need a driftdown procedure. (You simply go in the chart in the FCOM and enter with the MORA and it gives you a max weight.)

The driftdown charts we use just let you insert a point in the FMS and we draw a circle around that point (usually 75 miles) where that circle intersects your route you have to have a certain altitude (usually 25000ft, not FL250 so you have to apply corrections). Before you come to the circle and you get an engine failure you have to turn back, after the circle you continue but in both cases your driftdown procedure would clear all obstacles. the reason for the circle is that it would also apply to direct routings(grid mora).

The actual driftdown procedure: select the altitude you want to descent to. Select the driftdown speed(at this point derived from the FMS) select LVL CHG and let the A/C decelarate to this speed and it will start a descent.

Very short summary, there is a lot more to it (several pages as a matter of fact) but in a nutshell.

Hope this helps a little

Low-bypass
5th Apr 2010, 23:17
Flyburg just said it all what i needed to be explained.Thank u very much

GlueBall
6th Apr 2010, 10:38
Engine failure and driftdown [setting MCT] is a non event in the B74; we wouldn't need to construct "circles" on the FMS unless we're cruising in proximity of Mount Everest.

rudderrudderrat
6th Apr 2010, 10:44
Hi Glueball,

I bet you'd be more concerned if you lost half your engines though!

fredgrav
6th Apr 2010, 11:35
In brief, here's how a normal driftdown procedure is accomplished:

A gross ceiling (single engine) based on actual weight, ISA temperature deviation and ENG Bleed demand, is calculated, through the table below: the gross ceiling is then transformed into a net ceiling by entering another table with weight and ISA:
http://img682.imageshack.us/img682/6219/driftdownalt.jpg

Fuel and time to alternate calculations are made by mean of other tables, then a descent is started down to the EO CRZ ALT, that is kept till the new calculated TOD for alternate/dest airport is reached:
http://img62.imageshack.us/img62/6700/driftdownprofile.jpg

Here's, in a very brief form, a typical driftdown profile for Airbus ...

Happy Landings,
fredgrav

Low-bypass
6th Apr 2010, 16:47
Rhanx a bunch fredgruv that clarifies me bette with the graph