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HPSOV
7th Jul 2001, 08:41
Question is specific to 767, but I imagine it is the same in any airliner.
What do each of the above speeds represent?
I have been told by different people that each is the minimum drag speed, so which one is it?
I would have thought the engine out speed was minimum drag (to minimise altitude loss after engine failure). In which case why is the holding speed less than the min drag speed? If this is the case and you were required to hold en-route at altitude, would you reduce below min drag speed and risk having an engine failure on the wrong side of the drag curve?
Thoughts?

VH_KAM
7th Jul 2001, 10:29
HPSOV mate, just stick with it in both cases.

scanscanscan
7th Jul 2001, 13:32
Hpsov.
Vh-kam advice is correct.

If I may add my two cents worth....
Imho it is better for you to try and deal in "facts", and with todays communications you can.

If you need 767 information may I suggest you could e.mail Boeing and then you will know,and nolonger be confused.

If you could read in the 767 manual...
The definitions of Holding speed.
The definition of engine out speed.
The definition of Enroute hold speed
I think this will help you.

Your question covers quite a lot of ground in various areas of 767 operation and I will email you my thoughts, but leave others such as BA trainers on the 767 and more current than me to answer you here.


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We will do the drill according to the amendments to the amendments I er think?

MasterGreen
7th Jul 2001, 19:57
I think part of the problem here is a bad question and an equally bad assumption - viz : "I would have thought the engine out speed was minimum drag (to minimise altitude loss after engine failure)"

This is just not the case. EO speeds are a compromise between acceptable single engine cruising levels and obtainable TAS's. The TAS thingy is of course a function of your single engine 60 minute range from which your ETOPS certification is derived. There are quite significant trade offs in each direction (altitude/fuel and range) but most operators seem to hold the 767 around the 410 TAS mark (local mileage may vary).

Pure driftdown performance to optimise altitude is another beastie altogether. It is rarely taught and even less often understood and it is a big topic. However you are right in that the en route holding speed and pure (max altitude - min ROD) driftdown speeds are closely linked - in that they both have the requirement for min power (and thus FF in the holding case).

Min Drag Speed is another value altogether and is below the holding speed. The problem is that with all but a small number of airframes Vimd is a speed unstable target. In that it is on the lower cusp of the prevailing drag curve and any speed deviation tends to make the drag component vary in the wrong (unstable) direction. So Holding Speeds are elevated away from this point - usually around 1.1 Vimd. Since the bottom cusp of the drag curve is fairly flat in this region this small speed increase has little effect on power (FF) required and has significant benefits in speed stability.

So the summary would be. For range and ETOPS considerations, use your published drift down speeds to achieve published TAS's and ETOPs ranges. If you are worried about Single Engine driftdown altitudes, look at the en route holding speed for your present altitude and come down on that. Once your driftdown problem (in the altitude sense) is complete - and they rarely last long - convert to SOP speeds to fulfill your SE requirements of speed and range.

I may have grasped the wrong end of the stick here, but, hopefully I have brought a little illumination.

MG

Checkboard
9th Jul 2001, 10:07
This is just not the case. EO speeds are a compromise between acceptable single engine cruising levels and obtainable TAS's. The TAS thingy is of course a function of your single engine 60 minute range from which your ETOPS certification is derived. There are quite significant trade offs in each direction (altitude/fuel and range)

Just thought I might add a bit here, from the 737 point of view. While published speeds for single operation are a trade off in altitude/TAS performance, the aircraft commander is not obliged to operate the aircraft at these speeds, and may select the speed shedule to achieve a particular goal. An example:

Flying along at F370 a 737 crew suffers a non-catasrophic engine failure. The following steps indicate one thought process:[list=1] The aircraft obviously cannot sustain its two engine cruise speed (or indeed any speed) at this altitude. As it begins to slow down you have time for a PAN call to indicate to ATC that you will begin descending. You must decide whether or not you will turn off the airway for the descent - this depends on the traffic on the route and the distance between adjactent airways. Lets say in this instance you turn off the airway, in order to parallel it at, say, 10 miles separation. ATC need to know your level off altitude. The ENG OUT page on the FMS gives the current maximum altitude for your weight and list the target speed for that altitude, which is the minimum drag speed indicated on the EFIS by a green dot. At our typical operating weights you have a 50% chance that this will be above F200, keeping you in controlled airspace, or above terrain in very high mountainous areas - in Australia staying within CTA is of interest so you look at this altitude, pick the next lowest hemispherical level in CTA and pass this to ATC as your level off altitude. You now (still on descent) complete the ENGINE FAILURE AND SHUTDOWN CHECKLIST, turn to parallel the airway at 10 miles and level off at or just above green dot speed. Decide where you are going to divert, accounting for weather, runway, services etc. In our company you now have three easily available speed schedules to choose from. In the QRH are altiude capability tables for Green Dot (the same as accessed through the FMS, One eng. inop. long range cruise (LRC) and 280kt One eng. inop cruise.

280 kts is the speed selected for the 737 ETOPS operations for our operation.

LRC is close to green dot, and will keep you (hopefully) within CTA, whilst 280kt will get you home faster, but will require leaving CTA (the CTA base over most of Australia is F200). Both green dot and LRC are below the higher manoeuvering speeds introduced for the 737 rudder controller. Make your speed schedule decision based on fuel, traffic, distance to divert and increased manoeuvering speed, select your new max altitude, advise ATC of any required descent, get traffic, change frequency and begin the diversion.[/list=a]

Simple really! :)

Slasher
10th Jul 2001, 07:57
Checkers assuming 737-300 you also have to consider TAT limits at SE green dot speed as well as LRC. Green dot equates to SE ceiling primarily for drift-down and terrain clearance considerations but around 250kts IAS is a closer rough figure to SE LRC. Enroute (if terrain isnt a problem) I usualy keep in mind my approxamate LRC level, which in ISA+15 to ISA+20 is about 3000 ft below the current SE green dot level. If terrain IS a problem then of course green dot level is paramount. So if you were on a sector with no terrain problems and lost one, you could just set MCT and go straight to your LRC altitude while descending at your LRC speed expected at that level (approx 250kt). This would save you fuel.

Also, if you were going say BNE-CNS in the Wet on a dark and stormey night would you realy consider coming all the way back to 210/220kt?

I dont think going below FL200 would be a problem in Oz as ATC I think would continue to provide operational control for your flight wouldnt it?