Calculating ET and ETA

[Dutchy]

First time on this forum. Compliments: robust app made for pilots by a pilot.

But....... (there always is one

It seems to be that ET and ETA are being calculated using the absolute ground speed instead of the relative ground speed towards the next way point or end of route. And as a result it looks like that fairly misleading ET's and ETA's are presented.
GS along the track is always positive and rather close to cruise speed. GS along the bearing can be zero (track is 90 degrees plus/min bearing) or even below zero.
At the moment with 50nm DME and flying away instead of flying towards the next way point with 100kts GS 30 minutes ET is indicated. But since you are flying away from the next way point without changing your heading ET is huge and a fuel stop, some food and some sleep might be necessary before reaching your destination.
In case ET is more than a certain value (e.g. 6, 12 or 24 hours) for safety no ET and ETA should be presented.

1. Is it right that the absolute GS is used?
2. In case it is.
Is it an option to make this a configuration item to be changed at users wish: use absolute GS (along the track) or relative GS (along the bearing) for calculating ET and ETA?
3. In case it isn't.
How come that in the above example 30 minutes ET is presented?

[bitbrusher]

Hi Dutchy

Well, I may be wrong but I think the intention here was to indicate that if you are in trouble and need to go to some place to land ASAP, or whatever, if you head to that WPT at current speed, those parameters is what you will have.

I, personaly, prefer this way. But making this user configurable may be a good thing. Let's let Petr comment first.

Cheers

bitbrusher

[marpdiaz]

Hi Dutchy,

Here are a few thoughts from a plain user.

There is something important we should also consider when flying a route and use ETA values:
If you select a wpt in a route (i.e. make it green by tapping it in the active route list) that you already flown
over/by then the ETA to the end of this route will continuously *increase*.
This makes perfect sense to me since you are supposed to fly over/by the select wpt then complete your route.
However, you flying away from the selected wpt, increasing the time to get there and then continue the route.

The point that you raise is that most GPS units use VMG (vel made good) or projected velocity vector along bearing to wpt to calculate ET. A protection is set to avoid ET/ETA overflow in case of VMG close to zero. Negative values are not shown.

ETA is more complicated and there are at least two possible algorithms:

i. assume that after the next waypoint you will follow the route (i.e. VMG = present GS). This is what most of us do.
ii. assume that you will mantain present TRK regardless of forthcoming leg bearings and compute the time to pass abeam
the final wpt (repeating the protections mentioned above).

this is a very interesting discussion indeed.

best,

Marcos

[kitercuda]

Hi,
in the begin (one and half year ago) ET was calculated in a different way. If you do not fly direct to its, ET was longer.
Bigger angel between BRG and TRK longer ET. I disscussed it with some users, looked at some Garmin, and then I change it. Now ET = DME/GS.

[Dutchy]

As life is: it's a matter of choice.
It would be nice if the user can configure ET and ETA to be based on GS or on VMG (= GS * cosine [angle between bearing and track]).
If for the moment way too much effort or even not ever possible for whatever reason to change the app, I'll stick to my good old Garmin Pilot III for ET and ETA since it uses VMG instead of absolute GS (as of most Garmins presumably). I like to be on the safe side, that's why I prefer VMG over GS for ET and ETA. For ET and ETA end-of-route from the next way point on only GS can be used.