Drift down procedure, what is it?
Drift down procedure, what is it?
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,448
Likes: 310
From: various places .....
If an engine failure occurs above the OEI ceiling, the aircraft has to descend ... in which case we need to be concerned with obstacles.
Generally, the two options are
(a) OEI net cruise capability will give at least 1000ft obstacle clearance .. ie we can clear the critical obstacle without too much angst.
(b) OEI ceiling is below the minimum level necessary for cruise obstacle clearance ... in which case the descent to lower levels has to account for obstacle clearance. The usual requirement is that OEI descent ("driftdown") has to clear all obstacles by 2000ft until the OEI net ceiling is reached .. when (a) becomes applicable.
If terrain considerations are critical the driftdown descent will be at max thrust and a speed appropriate to the particular aircraft model.
As to why the term "driftdown" is used, I have no idea .. perhaps one of the ancient ones may be able to tell us ... however, the profile at the start of the OEI descent will be comparatively steep and then flatten out progressively ... the final few thousand feet will be a very shallow descent .. so the term may just be descriptive of the final descent profile ...
Generally, the two options are
(a) OEI net cruise capability will give at least 1000ft obstacle clearance .. ie we can clear the critical obstacle without too much angst.
(b) OEI ceiling is below the minimum level necessary for cruise obstacle clearance ... in which case the descent to lower levels has to account for obstacle clearance. The usual requirement is that OEI descent ("driftdown") has to clear all obstacles by 2000ft until the OEI net ceiling is reached .. when (a) becomes applicable.
If terrain considerations are critical the driftdown descent will be at max thrust and a speed appropriate to the particular aircraft model.
As to why the term "driftdown" is used, I have no idea .. perhaps one of the ancient ones may be able to tell us ... however, the profile at the start of the OEI descent will be comparatively steep and then flatten out progressively ... the final few thousand feet will be a very shallow descent .. so the term may just be descriptive of the final descent profile ...
Joined: May 1999
Posts: 1,846
Likes: 4
From: Bristol, England
The thrust is usually set to Max Continuous Thrust (MCT) on the remaining engine(s) and the speed is usually the speed for best climb/shallowest descent angle which approximates to min drag speed (VMD) on a jet. For a clean jet, most references indicate VMD is about 1.6Vs, perhaps around 210 to 240 kts indicated.
Joined: Jun 2002
Posts: 17
Likes: 0
From: UK
In my aircraft it works like this. Following an engine failure above the single engine ceiling, the aircraft will stay in its automatic mode and maintain level flight until the best l/d speed is reached, and then descend maintaining MCT until level or you hit something. Tables are published in the flight manual giving the distance the aircraft will cover to reach various levels. This gives the brave ones the option to continue towards the alps knowing that you should clear them.
Joined: Dec 1999
Posts: 259
Likes: 0
From: delta.bc.canada
The 'Drift down'aspect applies with Terrain concerns.
In the 320,one can lose the engine,apply MCT,slow to best L/D speed,AND TAKE AN HOUR to drift down to the engine-out alt.
Over the prairies one can descend IMMEDIATELY to the egine out alt at a higher speed..
The driftdown rate is in the 2/300' feet per minute range-lots of time to decide where to go,and how fast
cheers
In the 320,one can lose the engine,apply MCT,slow to best L/D speed,AND TAKE AN HOUR to drift down to the engine-out alt.
Over the prairies one can descend IMMEDIATELY to the egine out alt at a higher speed..
The driftdown rate is in the 2/300' feet per minute range-lots of time to decide where to go,and how fast
cheers
