europaxs wrote:
As you said, it also takes into account the (relatively current) wind direction and velocity and terrain.
How can it know the actual wind vector?
From the last time you were online. That’s why I wrote “relatively”.
LeSving wrote:
or you can use the glide distance feature on SD.
In a real situation, with SD always running on my kneeboard, I’d certainly take it into account.
It doesn’t hurt to play situations like this out in your mind every now and then. SkyDemon may not always be there to safe you.
How can it know the actual wind vector?
It can’t – there is no airdata feed into any tablet app. So the glide circle is not accurate.
From the last time you were online.
The wind forecast would be way off and in this case you are looking at a glide right on the margin of a typical SEP.
Peter wrote:
The wind forecast would be way off
If it is actually way off – you’ll probably know that with a thorough flight preperation – and take it into account. Also with SD one shouldn’t switch the brain off – of course.
Patrick wrote:
It doesn’t hurt to play situations like this out in your mind every now and then. SkyDemon may not always be there to safe you.
Thinking about these things is always a useful exercise. But 90% of the time I’m not in gliding distance to an airport anyway, and with this feature ON at all times on SD, it really helps in the situational awareness, just like the moving map itself. If the engine should quit, and SD shows I’m within gliding distance to an airport, I would go for it without offering it a second thought.
LeSving wrote:
Thinking about these things is always a useful exercise. But 90% of the time I’m not in gliding distance to an airport anyway, and with this feature ON at all times on SD, it really helps in the situational awareness, just like the moving map itself. If the engine should quit, and SD shows I’m within gliding distance to an airport, I would go for it without offering it a second thought.
That is definitely true and a case for activating the glide safe feature at all times. The default setting after installation only shows the glide range when flying over water.
Well, do offer a second thought re: the currency of the wind data and keep a save margin. I’d probably rather land on a decent field or similar that’s well within the glide range visualized by SkyDemon than gamble for an airfield that is at the outskirts of the tailwind portion of the glide save radius.
Alexis wrote:
Left, because you will sooner have a tailwind than if you do a right turn …. (?)Yes, but that tailwind will blow you off course. Anyway, it is easy to calculate. I’ve used the speed and glide assumptions of Buster1 in the situation of JasonC and further simplified by using a constant TAS of 113 kt. (113 kt is the TAS at half the altitude — 5000 feet — for a CAS of 105 kt). I also assume that you make standard turns and are flying with the proper wind correction angle (4°) for a northerly track when the engine stops.
With a right turn, you have to turn 177° (to a heading of 181°) and will reach the runway after 8.4 minutes.
With a left turn, you have to turn 193° (to a heading of 171°) and will reach the runway after 8.6 minutes.
Continuing straight ahead, you will reach the runway after 8.5 minutes.
About half of the time penalty for a left turn compared to a right turn is because the crosswind component blows you off course. The rest is because you have to turn further. Also, you waste a lot of time by turning at all so continuing with a headwind may be preferable. In this particular case it obviously doesn’t matter in practise what you do, but it does show the principles.
(Note: I have software to compute flight tracks, so I didn’t spend as much time on this as you may think.)
Also the 10kt wind (at 10k feet) is virtually nothing and will become even more virtually nothing as you descend. It is more significant relative to Vbg however – around 90kt for most SEPs.
Consequently I reckon most people in this situation will glide straight ahead, despite the slight headwind component, especially by the time they have wasted time sorting out the engine.
One would need to change the debate to whether, upon any engine problem, you should immediately make a turn to the nearest airport. I don’t think that is going to work unless you have a constant airdata-assisted “glide-to-here” computation. Most GA pilots don’t have that (modern airliners do have it) and anyway during most VFR flight (which in Europe is way below 10000ft anyway, due to CAS access practices by airspace class or ATC policy) will not give you an airport in glide range, anyway…
Re: “wasted time sorting out the engine” – one of the things I took away from “Sully”, where the 30 seconds or so they (understandly) took to think hints through cost them an airport landing, is to turn towards the airport BEFORE trying to sort things out. So it would be (assuming engine failure at altitude):
- gain altitude until speed decays to Vbg while turning towards the airport
- establish best glide
- troubleshoot from memory items
- communicate to pax
- communicate to ATC
- reduce electrical load (if you’re at 25k feet you have ample time to empty the battery before you land)
- troubleshoot from checklist
One aspect to have in mind when using the Vertical Speed Required on a GPS navigator is that it doesn’t take into account the reduction in TAS that comes when you get lower.
