what_next wrote:
Best glide speed and blue line speed (= Vyse = best rate-of-climb speed single engine) are not related to each other. Aircraft might be configured differently to achieve those speeds: For Vyse takeoff configuration is assumed whereas it is clean configuration for best glide speed. Then, Vyse is a best rate speed, whereas glide speed is a best angle speed. Additionally, Vyse is independent of aircraft mass, but best glide speed depends on it as can be seen in the table above.
Of course we know that. We wouldn’t be competent pilots if we didn’t.
However, here we are, 20nm offshore at FL100 with a double engine failure in an unfamiliar twin. Do we start looking for the obscure graph in the POH or do we go immediately to a safe speed?
The question I am asking is whether we think that Blue Line is likely to be the most likely speed to save our lives, under the circumstances?
So, given that you have that graph for the C421, how does it compare to Blue Line?
Timothy wrote:
Of course we know that. We wouldn’t be competent pilots if we didn’t.
Of course? I lost all my faith in what my fellow pilots and students know long ago. Especially since multi-engine flying can be taught by 200-hour ATPL freshmen with an additioal 30 hours of instructor training (on singles) who have not flown on twins for more than 15 hours during their training. So I always assume the worst.
Timothy wrote:
So, given that you have that graph for the C421, how does it compare to Blue Line?
Blue line speed for the C421 is 111kt. So it is inside the range of the best glide speeds and safe to use, but a little inefficient at low and high mass. If I would still fly the 421 regularly, I would probably memorise 115kt for best glide and vary that by +/- 5kt for heavy and light condition. One would never be far away from the optimum then.
It is quite different for the little Citation I fly now. It’s “blue line speed” (V2) is between 115 and 120kt depending on conditions while best glide speed is around 150kt. Trying to fly V2 instead would be problematic because in clean configuration it would fall out of the sky at that speed…
what_next, out of curiosity how close is best glide to Vx? (for the citation)
Pirho wrote:
what_next, out of curiosity how close is best glide to Vx? (for the citation)
There is no Vx in the classic sense. “Recommended climbout speed”, which is based on optimum values for time, fuel and distance to climb is 250kt for “my” C560 at any mass, all the way from sea level to FL450 (Mach .62 at the higher levels when IAS drops off the range, so to say). Best glide is around 150kt. No correlation there!
It’s true that there is no certification requirement for manufacturers to publish a best glide speed, as there is for singles. If a manufacturer does, it’s just best practice on their part.
“Best glide speed” for what purpose of glide? The speed required by certification to be presented as the “best glide speed” for a single must be the speed which results in the greatest distance flown for altitude lost. However, gliding at that speed on short final, with the expectation of a decent landing could leave the pilot disappointed. Gliding at that speed does not store any energy to “spend” to execute a decent flare. Rather, from that best glide speed, you’ll pull to flare, and the plane will instead stall, and simply continue down to a big thump!
It is for this reason that I discourage climbing out slower than Vy, when clearing an obstacle is not required – if you loose power, you might not achieve an adequate speed from which you can flare, before you hit.
Pilot_DAR wrote:
“Best glide speed” for what purpose of glide? The speed required by certification to be presented as the “best glide speed” for a single must be the speed which results in the greatest distance flown for altitude lost. However, gliding at that speed on short final, with the expectation of a decent landing could leave the pilot disappointed. Gliding at that speed does not store any energy to “spend” to execute a decent flare. Rather, from that best glide speed, you’ll pull to flare, and the plane will instead stall, and simply continue down to a big thump!
The best glide speeds for the SEP types I fly are in all cases higher than the recommended speed over threshold with flaps – in some cases considerably higher.
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.
Citations don’t have Blue Lines.
The formula used to estimate the best Glide speed for a PA30/39 is set out here by the International Comanche Society updated Flight Manual for the type as:
1.5xVS1 (Clean Config)
“However, gliding at that speed on short final, with the expectation of a decent landing could leave the pilot disappointed. Gliding at that speed does not store any energy to “spend” to execute a decent flare. Rather, from that best glide speed, you’ll pull to flare, and the plane will instead stall, and simply continue down to a big thump!”
I find this a rather bizarre idea – I can’t believe any certified aircraft are unable to round out for a landing from best glide speed? Have you any examples please?
The Tecnam twin AFM has a specific emergency procedure section on landing with double engine failure and specifies Vy as the best glide speed. I guess the relevance is to the potential complex engine failure on take off scenario when closing both throttles and landing ahead is the best option.
