Driftdown procedure and V-speeds
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Driftdown procedure and V-speeds
As I understand it, the sequence of events in the driftdown procedure is as follows:
Twinjet cruising at 1.32Vmd for max range. Engine failure and therefore loss of thrust means you can't continue at that altitude. Initially you trade airspeed to maintain altitude while slowing to Vmd which gives best range during the subsequent glide down (or does it?). Vmd is then maintained until a certain 'level-off' altitude where thrust is sufficient once again to fly level.
Now, I assume that you would want to get back to 1.32Vmd at your level-off altitude to continue to get max range but if you're drifting down at Vmd at what point do you transition back to 1.32Vmd, or don't you?
Should you drift down at Vmd or some higher speed? I'm missing something but not sure what!
I suppose another way of looking at it, is given an engine failure in the cruise and 30 mins of fuel remaining, which technique would carry you the greatest horizontal distance before hitting the floor?
Twinjet cruising at 1.32Vmd for max range. Engine failure and therefore loss of thrust means you can't continue at that altitude. Initially you trade airspeed to maintain altitude while slowing to Vmd which gives best range during the subsequent glide down (or does it?). Vmd is then maintained until a certain 'level-off' altitude where thrust is sufficient once again to fly level.
Now, I assume that you would want to get back to 1.32Vmd at your level-off altitude to continue to get max range but if you're drifting down at Vmd at what point do you transition back to 1.32Vmd, or don't you?
Should you drift down at Vmd or some higher speed? I'm missing something but not sure what!
I suppose another way of looking at it, is given an engine failure in the cruise and 30 mins of fuel remaining, which technique would carry you the greatest horizontal distance before hitting the floor?
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Do remember that if you drift down you will most likely level off at your single engine ceiling in which case it might not even be possible to fly 1.32Vmd with one engine out. In this case you would be using max continuous thrust and see what speed results from that.
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Typical Drift-Down profiles that I've done the number crunching for utilise Vmd for the dual purposes of (1) Minimising Rate of Descent to reduce/delay traffic conflict, and, more importantly, (2) Achieving the maximum possible ceiling with view to clearance of high en-route terrain.
Cruise at Vmd and MCT might be at a higher level, but the fuel economics stink, but an acceptable stink if high terrain is a concern.
Good policy is to use the Minimum Rate/Maximum Ceiling technique until such time as traffic and terrain are no longer a concern, and to then increase speed to Maximum Range Speed allowing the aircraft to drift down to the (lower) highest level available at Max Range
Cruise at Vmd and MCT might be at a higher level, but the fuel economics stink, but an acceptable stink if high terrain is a concern.
Good policy is to use the Minimum Rate/Maximum Ceiling technique until such time as traffic and terrain are no longer a concern, and to then increase speed to Maximum Range Speed allowing the aircraft to drift down to the (lower) highest level available at Max Range
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Are we sure that "driftdown speed" is scheduled at Vmd and not L/D max? One would assume climb ratio is the overriding concern in a drift down not range.
On the old Tristar (ducks head for incoming) we could choose crit or non crit driftdown depending on if obstacles were a factor. Non crit was for range.
In any case the OEI ceiling alt at "driftdown speed" is higher than OEI ceiling at OEI LRC so on the way down you need to asses obstacle clearance for the level off and ensure 1000ft clearance and then choose to accelerate and descend further if required for range.
On the old Tristar (ducks head for incoming) we could choose crit or non crit driftdown depending on if obstacles were a factor. Non crit was for range.
In any case the OEI ceiling alt at "driftdown speed" is higher than OEI ceiling at OEI LRC so on the way down you need to asses obstacle clearance for the level off and ensure 1000ft clearance and then choose to accelerate and descend further if required for range.
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On the old Tristar (ducks head for incoming) we could choose crit or non crit driftdown depending on if obstacles were a factor. Non crit was for range.
'Tis a fact...circa 1976, or so.
when the first Hamilton Sundstrand FMS units became available.PFM.
Last edited by 411A; 30th Jan 2009 at 22:50.
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Drift down vs single engine service ceiling
Here is my question; is drift down ceiling different then single engine service ceiling? Why I am asking is the King Air manual has a Single engine service ceiling chart and people are using as drift down. They are entering the chart at OAT that is derived from flight level temps and using those figures as opposed the OAT temps on the ground at takeoff, resulted numbers are much different to say the least. I say there are no published drift down charts published for this aircraft and the chart is used incorrectly if using FL OATs. Which is correct please?
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They are entering the chart at OAT that is derived from flight level temps and using those figures as opposed the OAT temps on the ground at takeoff, resulted numbers are much different to say the least.
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Do you are saying it is correct to use the single engine service ceiling chart ad a drift down chart? My argument is the single engine service ceiling chart says being able to still climb at 50fpm. Not drift down to it then climb 50fpm.
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B4LandingChx.... its pretty much semantics.....you don't want to be anywhere near coffin corner on single engine. Your highly unlikely going to be climbing anyway. Big picture stuff, keep clear of the terrain, look for somewhere suitable to land....unless we are studying for ATPL's then the thickness of a pencil line is the most critical thing 
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This issue is about practicalities. I was always advised that SE ceiling was a theoretical 'climb to' FL. this would suggest guidance following an engine failure during departure. The drift down advise is what FL you can sustain after an engine failure in CRZ. In both cases I was always advised to accept a FL 2-4000' lower than the max, in both cases, to ensure you had enough power and speed control and allow the engine to operate at less than MCT. MSA would dictate if this was possible.
It was always a discussion point to operate at less than limits if possible. Why burn the edges?
It was always a discussion point to operate at less than limits if possible. Why burn the edges?
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So its 4000ft ...plan on the lower one and your safe....as I said I hope your not trying to keep the thing airborne for long anyway.....pretty much planning on landing nearest suitable.
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Thanks but I am asking about the validity of using a single engine service ceiling chart as a drift down chart....i say no. Not valid. Drift down charts are a different animal. Least in my opinion.
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before landing check list,
Not exactly no. I have no experience in the King Air and don't know what charts you have available for en-route performance in the AFM. What you said was:
...and what I am saying is:
What I mean is that the en-route OEI performance should be based upon the conditions you would expect to encounter at altitude along the route rather than extrapolating some surface observation using a standard lapse rate.
You say there are no drfit down charts published. But for a commuter category aircraft there must be a chart that allows you to determine the OEI en-route climb performance. It is not specified exactly what format the chart must take, only that you must be able to determine the climb performance given the weight, altitude and ambient temperature. That is neither the "service ceiling" nor the absolute ceiling, but it may be that all those things can be determined from the same chart.
The Pacific and General Aviation section of the forum seems to have a few people experienced with the King Air who could probably tell you exactly what chart to use for drift down if you post this question there.
Do you are saying it is correct to use the single engine service ceiling chart ad a drift down chart?
the King Air manual has a Single engine service ceiling chart and people are using as drift down.They are entering the chart at OAT that is derived from flight level temps and using those figures as opposed the OAT temps on the ground at takeoff, resulted numbers are much different to say the least. I say there are no published drift down charts published for this aircraft and the chart is used incorrectly if using FL OATs. Which is correct please?
They are correct to use the temps aloft.
You say there are no drfit down charts published. But for a commuter category aircraft there must be a chart that allows you to determine the OEI en-route climb performance. It is not specified exactly what format the chart must take, only that you must be able to determine the climb performance given the weight, altitude and ambient temperature. That is neither the "service ceiling" nor the absolute ceiling, but it may be that all those things can be determined from the same chart.
The Pacific and General Aviation section of the forum seems to have a few people experienced with the King Air who could probably tell you exactly what chart to use for drift down if you post this question there.
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Considerations, in general.
(a) what the OEM documents might say
(b) drift down generally will be to a higher level than climb to, depending on turbulence
(c) a bit of turbulence and you can probably say goodbye to the altitude pad you might have started out with
(d) subject to (a), I can't see very much wrong with using the OEI climb to data as the basis for terrain clearance planning, again subject to turbulence ... significant turbulence and the bets become increasingly airy fairy ... ie better plan is to get away from seriously high terrain as an early immediate action post failure ..
(a) what the OEM documents might say
(b) drift down generally will be to a higher level than climb to, depending on turbulence
(c) a bit of turbulence and you can probably say goodbye to the altitude pad you might have started out with
(d) subject to (a), I can't see very much wrong with using the OEI climb to data as the basis for terrain clearance planning, again subject to turbulence ... significant turbulence and the bets become increasingly airy fairy ... ie better plan is to get away from seriously high terrain as an early immediate action post failure ..
