Global "maximum diversion" value and/or alternate airport selection logic

SimBrief Feature Requests
1

Currently, this can be set on a per-airframe basis, but the default value of 400 often results in simBrief selecting alternates that seem “atypical” when e.g. comparing to flight plans on edi-gla or some of the real-world OFPs that can sometimes be found online.

For example, for EGLL-KJFK, I get KIAD KBOS KBWI KPHL, whereas most plans I can find online would use KEWR when practical, with another “typical” option being KSWF, and only occasionally KABE, KPHL or KBOS.

Flight to KMIA, I’m offered KSFB KTPA KMCO KPBI, where a more “typical” alternate would seemingly be KFLL.

Or to LSZH, LFLL EDDF EDDM LSGG where LFSB seems more common, etc.

Or perhaps more generally, the simBrief algorithm is possibly looking a bit too far for alternates, where, weather permitting, picking much closer alternates would generally make more sense from an airline perspective (less fuel, less weight, lower costs)?

So maybe even when the “maximum” diversion is ~400 nautical miles, the algorithm should prioritize as many suitable alternates within ~100nm as possible and only extend the search if not enough of those can be found?

Another thing I’ve noticed is, simBrief is ordering alternates so as to make the primary alternate the farthest one, which makes sense to easily ensure the alternate fuel is sufficient to reach any of the 4 when selected.

But when reducing the alternate list to just 1 from a higher number and re-running “Find”, the behavior is not predictable in an obvious manner, for example, for an LSZH destination, it goes from LFLL EDDF EDDM LSGG to LSGG (the nearest) but for a KJFK destination, it goes from KIAD KBOS KBWI KPHL to KIAD (the furthest). About this it’s just more curiosity than anything.

Regards,

Tim

2

Hi Tim,

For the most part this is all working as intended. SimBrief is by default much more conservative than a real world airline would be when it comes to alternate suggestions, the logic being that since it’s an automated system, it might not be able to consider all the factors (particularly airport or airspace NOTAMs). And the more well-equipped the alternate is, the less likely it is to be closed or otherwise unavailable.

The other reason being, in general, more fuel is better when it comes to flight sims and the wide range of fuel burns that different add-ons might have.

In case you haven’t seen it, there is a brief description of the alternate logic here: SimBrief - User Guide. But to go a bit more into detail than that:

Taking your example of KMIA → KFLL, it kind of depends IMO. KFLL might be perfectly legal weather-wise, but whether a dispatcher would choose it depends on more than just the weather. For a quick ~1 hour flight from KATL, it probably makes sense. For a trans-atlantic flight from EGLL, even if KFLL shows sky-clear for the entire TAF, 10 hours is a really long time and a lot can happen. It’ll really depend on the airline and how tight they want to run their fuel. Airlines will also prefer airports where they have a company presence, for example, amongst a bunch of other things.

SimBrief can’t adjust its logic based on any of that, so I just settled on choosing the best-equipped airport(s) within a target area. Normally, it’ll look at all airports farther than 1/10th the max diversion distance (~35nm by default) and closer than 1/2 the diversion distance (~200nm by default), and choose the best-equipped ones. Subject to the considerations described in the user guide (legal weather, same-country, avoid military, etc). Only if no suitable airports are found within that area will it look closer/farther to the maximum search distance, and it’ll even look a bit farther if it has to before throwing an error.

This is why when you switch from 4 alternates to 1, it won’t consistently choose the closest or farthest one. Rather it’ll choose the best equipped one (best-equipped in this case is mostly based on the number of runways and ILS approaches).

With all that said, I’ll investigate a possible global setting for this.

Best regards,

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