A DA42 will get a dual engine failure in icing conditions, specifically -15C, if the alternate air is not being used.
It’s funny… the IO540-C4 does exactly the same 
I’m not sure many of us would be focussing on a few knots if we had a dual engine failure at about 200ft on climb out. 
No, but your point was that engines will never fail together. Immediately someone came back with a scenario on a type with which you are hugely familiar.
The thing is, before that double FADEC fail people would have said “there is no way…” . They fixed that particular one and now people are again saying “there is no way…”.
The learning point is that Murphy will find a way. Think Hudson River.
Approaching any faster will put you higher up on the total drag curve and compromise your glide performance so I can’t see how it can be recommended.Quote
Yes. If you’re trying to make the coast, or over the mountains to the green pastures, “best glide speed” for sure. However, if “making the spot” is less a concern, the objective can become “making it into the spot safely”. I have ridden through many forced approaches (which I usually initiate 800 – 1000 feet directly overhead the runway) where the pilot simply did not make it in. Too much focus on getting the speed right, and flying a defined circuit with defined “points” and turns. By the time they did all of that, they were usually low, which leads to stretched glides at the last, and a crash – when the field could have been made from the start.
Inspired by John Farley’s writing (A View From the Hover), and several successful forced landings, I have learned, and now teach, to pick a spot close, even underneath, and do whatever you need to to get in there with certainty. That usually means a faster than “best glide speed” glide, full flap, and even some slipping. Get yourself set up on a perfect final, just high, and a bit fast. If you’re too fast, you can slip off the excess speed easily. In the very worst, you misjudged, and crossed the fence too fast. So you’re going to run off the end into the hedge. Much better take out the far hedge at 15 MPH on the wheels, than to wipe the wheels off on the stone fence at the near end at 65 MPH!
I agree that different types handle this differently. I have next to zero sailplane experience – but I have enough twin experience to know that they glide nothing like a sailplane! It’s just a matter of broadening your thinking, and then practicing variations on the technique, so you can optimize your technique for the conditions with understanding as to why.
In the absence of power, there are two important points on the drag curve. One is at minimum sink rate Vms, and the other is at minimum drag (which is also “best glide”) Vbg. There’s a mathematical relationship between the two derived from the shape of the drag curve: Vbg = 1.32 * Vms. (1.32 = 3^(1/4))
If we had engines that delivered perfectly constant power with respect to speed, best rate of climb Vy would occur at Vms. But piston engines deliver more power at higher speed. So by flying faster than Vms, we get a bit more power, which means that it’s worth flying faster than Vms, and Vy is typically rather higher than Vms. Blue line speed is somewhere between the two, because on one engine the increase in power with speed is less. The ratio by which Vy exceeds Vms depends on how over- or under-powered the aircraft is. At the service ceiling, Vy = Vms, i.e. the ratio is 1. Typically, at sea level, it might be something like 1.3 or perhaps 1.2 or 1.15. Hence it’s quite coincidental that Vbg is close to Vy or blue line speed, but it’s not bad as a rule of thumb if you don’t know better.
A DA42 will get a dual engine failure in icing conditions, specifically -15C, if the alternate air is not being used.
Not it won’t – it may. Been there done it.
The dual failure of FADEC was due to procedures being adopted outside of those specified in the AFM at the time. Diamond still felt obliged to close that gap. Sure, if people operate aircraft outside those procedures defined in the manuals things may go wrong. Flip, I can create a simultaneous dual engine failure by pulling both red levers simultaneously or switching-off both masters in the DA42. Let’s be sensible here.
Dave_Phillips wrote:
Let’s be sensible here.
Just to remind everyone: Almost ten years after that dual Da42 FADEC failure, an Airbus A400M suffered a quadruple FADEC failure. What can happen will happen.
bookworm wrote:
In the absence of power, there are two important points on the drag curve. One is at minimum sink rate Vms, and the other is at minimum drag (which is also “best glide”) Vbg. There’s a mathematical relationship between the two derived from the shape of the drag curve: Vbg = 1.32 * Vms. (1.32 = 3^(1/4))
The minimum sink rate and minimum drag are the same point (as in the absence of power you need sink to overcome drag). Best glide is at the point of maximum L/D, which is always at a higher speed than minimum sink/drag.
Your mathematical relationship may be a good rule of thumb, but in practise it depends on the airfoil and the parasitic drag of the airframe.
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.
Presumably we have all suffered emergencies at some time in our flying careers? Does our memory of those times, and what it felt like, not lead us to want to focus on a neat, simple, memorable answer? Like “in the event of a double engine failure, feather immediately and fly at Blue Line.”
One thing that would interest me from the gurus is whether it is a better strategy to pull back and gain a little altitude as you come back to Blue Line, or if it better to maintain altitude and gain some distance as you slow down?
