The digital is much closer to one than the other. It doesn’t really cause enough bother to investigate, to be honest.
Who knows, if I have a double engine failure and follow the Blue Line, there is a 50/50 chance it will work in my favour 
Hmmm, I would expect the digital to agree with one of the mechanical.
The easy way to check if it is a pitot issue is to just swap the ASIs, or equivalent.
Actually, Dave, I wonder if it’s because we fly (pretty much) the same type and that it is the two pitots that are to blame?
Quite. I have three ASIs – two mechanical and one glass, all calibrated in the last eight weeks, and all telling markedly different stories 
Didn’t I say this earlier?
Dave_Phillips wrote:
For anything that doesn’t have a prescribed speed, I would use Vy for range and Vx for maximum time in the air. The numbers aren’t quite the same as best glide but good enough for the quoted scenario.
PS, the steam gauge twins I fly ordinarily have a 5kt discrepancy between ASIs. That sort of adds more fuel to the approximation argument.
I gave you my advice Timothy and I thought you rejected it when you wrote:
All this theory trying to reach an exact magic number when there is a lovely line already painted on every twin’s ASI which, everyone is agreed, is close enough and will give a very good approximation to best glide range.
The exact answer is that best glide speed depends on the aircraft, and is not related simply to blue line speed, as the cases we have identified show. Empirically, a bit faster than blue line or Vy would seem to fit better. The operational answer is that it doesn’t matter as the curve is pretty flat around the optimum, so if you want to pick blue line because it’s on the ASI that’s fine, you won’t lose much, but it’s not “a good approximation”.
Bookie, as ever, I bow to your superior opinion.
Some time you must give us the benefit of your advice of what we should do if flying a twin in which we don’t know the best glide speed.
Bookworm
I am not sure that was Timothy’s point unless i have misunderstood.
I thought his point was that in the absence of anything better blue line speed was a good starting point. I wonder for example how many Baron pilots new to type would know the difference for a Baron never mind those less new to type. Fact is i doubt many twin pilots contemplate a double engine failure.
It seems as with any rule of thumb to meet the criteria.
BTW, we started with the ATR72 accident. Best glide for that was supposed to be 137 kt. The crew kept the speed at 170 +/- 10 knots for about 4 minutes after the double flameout and then reduced to 145 +/- 10 knots for about another 4. I suspect the failure to feather the props was a much bigger factor in the glide range.
We started with two data points: the C421B, which has a best glide speed of 123 kt and a blue line of 111 kt, and the Citation, which has a best glide of 150 kt and a “blue line” (V2) of between 115 and 120 kt.
In Post #17 Timothy wrote:
I think that, having read all the discussion, Blue Line in an MEP is a good, safe place to go, where you are not going to be far wrong.
The DA42 at 82 kt for both best glide and blue line was subsequently added, which fed your confirmation bias very nicely.
Now NCYankee supplied a 4th data point, Baron BE55 best glide 120 Kts, blue line is 100 Kts.
That is a significant difference. I wonder what makes the Baron (a very “ordinary” “conventional” twin) so different?
Different from what, exactly?
