Best L/D ratio speed or Vfcl

[ooizcalling]

With an engine failure en route in a twin engine turbo-prop (dash8/F50/ATR etc) and high terrain beneath you. In considering your drift down and wishing to maintain the greatest clearance above that inhospitable terrain would you chose the final climb speed as in the after V1 engine failure case or opt for the best Lift/drag ratio speed. ?

[john_tullamarine]

... or, why not just use the OEM's speeds as scheduled in the AFM ?

[ooizcalling]

"... or, why not just use the OEM's speeds as scheduled in the AFM ? "
Well, the AOM in it's 'Drift Down" data just specifies a speed known as 'Optimum Speed' with no amplification of what criteria produces this speed, and it just so happens that it is the same as Vfto for a given weight.

So what is more important when trying to maintain best clearance from high ground on the drift down ? Is it 'Min rate of descent' which Vfto would produce, OR, would it be best lift/drag ratio which, to my thinking, should produce min gradient descent.

I'm inclined to think that best lift/drag ratio would be the better option.

Any glider pilots out there who can give some feedback on this ?

[john_tullamarine]

.. first I'd do a big more research in the AFM .. couldn't imagine that getting past the certification folk.

[+TSRA]

This is specific to the Dash-8 only and, may I add, THIS POST IS NOT FOR OPERATIONAL USE.

Driftdown information is given to pilots in the Operating Data Manual (ODM). There are two charts that must be used to correctly calculate driftdown:

1. "Driftdown Speed One-Engine Inoperative," and
2. "Time, Distance, and Fuel in Driftdown (ISA xx*C)."

Doing a quick calculation for the first chart shows that the Driftdown Speed for OEI operations exactly the same as your VClimb/VEnr speed (OEI Clean speed, Single Engine Climb - Clean Config, Single Engine Climb - Enroute...take your pick). For example, a Dash-8-300 at 43,000 pounds the driftdown speed is 131 knots and the the Vclimb is 131 knots...at 29,000 pounds your driftdown speed is 107 - Vclimb is 107. There are a couple of weights in the middle where the difference is one or two knots, but not enough for the average pilot to maintain while hand flying or for the autopilot to maintain with the servos.

The second chart (Time, Distance, and Fuel in Driftdown) will show you how many miles, feet, and fuel you will use to descend down to your driftdown altitude. I could go on a long ground school spiel about these charts, but I wont.

I hope this helps by giving some real-world figures.

Now, may I add again (CYA here) - THIS POST IS NOT FOR OPERATIONAL USE.

[ooizcalling]

Thanks +TSRA. I've got similar data in my books. But it still doesn't get me any closer to answering my question Vfcl or best L/D ratio speed. :ugh:

Anyone know where those "Certification Folk" hang out ?

[+TSRA]

But it does; from how you asked the original question:

In considering your drift down and wishing to maintain the greatest clearance above that inhospitable terrain would you chose the final climb speed as in the after V1 engine failure case or opt for the best Lift/drag ratio speed. ?
Note: Bolded for emphasis on my part.

With One Engine Inoperative you fly at the One Engine Inoperative Speed when terrain is at issue (driftdown, climbing up to your 4th climb segment, etc). That is what the manufacturer requires you to fly at.

De Havilland/Bombardier define it as VClimb, Raytheon/Beechcraft as Venr...In any case, Vclimb is 1.3 times your Vs or Vso speed (depending on the configuration they want you at). I bet if I charted that on an L/D graph, that 1.3x would be right near L/Dmax. But then, I would need to know the Coefficient of Lift and Drag for the specific wing...not exactly data presented to us in an AFM. Then I could plug it into the formulas and solve for V squared...but wow, that's a lot more work than I've ever presented any of my ground school students!

Now, if what you really want to know is what you would fly at once the terrain was no longer an issue (i.e., you're above the 4th climb segment or you're level at your driftdown altitude) then you can fly at whatever airspeed you can get out of the airplane taking into consideration fuel, etc.

Alternatively, if you are wondering why they chose those airspeeds for us to fly at, I would suggest you start with reviewing TERPS and go from there.

I hope this helps!