Speed carret
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Already since 4.34 and the introduction of strong winds I’ve suspected that the behaviour of the speed carret is peculiar. When flying faster than the carret indicates, with constant altitude, velocity and (head)wind you should arrive earlier at the steerpoint than the pre-planned time. But somehow the carret manages to overtake the constantly maintained (faster than required) speed. How is this possible? If at one point you’re faster than the carret and nothing changes in earlier mentioned parameters you should always end up earlier at the steerpoint, right? I understand that the closer you get to a steerpoint, the larger the deviations, but the deviation should always remain on the same side of the carret when everything else remains constant.
Now I’ve found something even more interesting, when changing between CAS and GS the carret is below my current speed with CAS selected and above my current speed when GS is selected! How is this possible?
Related to this, since 4.34 the AI very often takes shortcuts from their original flight plan to arrive in time over the target.
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I’m going to guess that caret is currently being calculated as an airspeed which if flown toward the destination will give the required ground speed to arrive on time. However in a crosswind situation 300 knots with a wind correction angle does not yield 300 knots speed made good because speed made good is reduced by the cosine of the WCA.
First we should do all testing in TAS HUD mode to rule out any TAS-CAS conversion errors. A leg with a planned track speed of 300 knots with a 60 knot crosswind has a WCA of 11.31° and a TAS required of 305.94 knots. So if the HUD caret says “hey dude, fly 300 knots” instead of the actual required 306 you’ll be 98% of the required speed and take 102% of the estimated time.
Another thing you can do is do a known leg and put the HUD into GS mode and check that the caret equals the planned GS.
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Can any of the devs weigh in on this?
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Are you able to respect the planned TOS (+/-10s) by following the caret until about 3Nm remaing distance to waypoint?
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“By following caret” Sure, given this condition anything is possible. No one is questioning that following caret shows up on time.
Some quick learning on the UI. Flight plan schedule does not account for wind. When speed locked it’s “CAS” that is locked not TAS. And TAS is effectively GS rounded to nearest 5 kts (not floor or ceiling function, e.g. 417.3 = 415). Anyway put two waypoints 420.0 apart with times locked one hour apart. TAS shows 420, good sign. Pass waypoint on the hour leg within 1s of desired, next point RQD G/S 420, good sign again. But I’m going 419 and ETA says 1h+2:35s. Hmm. Speed up so DES = ETA, ground speed 434. Ut oh. I know from BMS experience that if you want to have constant speed, constant ETA you must fly ETA number not speed caret. For reference my heading 007, track 002, wind 110/51.
I try again with wind 110/9 (I set 0 but I can’t get 0). Now when my ETA matches DES my GS is 422. What it appears is the case that BMS is calculating “RQD GS” as required TAS.
The issue only happens due to crosswind component. When wind is headwind/tailwind no problem. Error is proportional to the cosine of the WCA. It’s like BMS doesn’t understand that with crosswind your GS is reduced because your TAS vector isn’t pointed at your destination but skewed by WCA. I have high confidence this issue doesn’t manifest in real F-16. If ETA = DES TOS then GS = RQ GS, every time. If anything it should be CAS that is imprecise (when not on caret) since it has to guess what CAS is needed to produce GS that is on schedule.
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Frederf, would you mind posting your experience as a bug report? https://bmsbugs.blu3wolf.com/bug_report_page.php
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Issue is still not resolved
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And maybe will not anytime soon as it is a minor issue (especially compared to some other things).
As long as you follow the carret you will be on TOS anyway. -
And maybe will not anytime soon as it is a minor issue (especially compared to some other things).
As long as you follow the carret you will be on TOS anyway.No, you often won’t depending on crosswind. I missed your earlier question (Are you able to respect the planned TOS (+/-10s) by following the caret until about 3Nm remaing distance to waypoint?)
Answer is no when using realistic weather maps with 70 - 100 knots of crosswind at altitude. The CAS carret goes up to non-achievable values 10’s of nm’s from the selected waypoint (on 100 - 200 nm legs).I suspect this is also the reason why AI flights often skip half of their waypoints when crosswind is present.
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Please provide a video.
Even with a 80kts xwind I had no issue on my side . -
If you follow caret you’ll have caret slowly creep up over time. It’s asymptomatic passing the point. The math is a little complex but at some point it’ll run away to unachievable. When you notice it (say +5 knots) you’d have to exceed caret by a bit to maintain constant schedule or push it back down.
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Please provide a video.
Even with a 80kts xwind I had no issue on my side .Don’t have video. It’s actually with head/tail wind, not so much with cross wind. Try with 85 kts head wind on a 100 nm leg with 410 kts planned ground speed and follow the CAS carret precisely. The CAS carret starts around 300 kts initially and keeps increasing until it becomes too high to follow around 20 nm from the next waypoint. Doesn’t make a very economical flight.
With tailwind the CAS carret behaviour is the other way around, it starts way too high and decreases throughout the leg.
Current workaround is to switch to GS and follow that carret but this isn’t practical.
Anyway, hope it will be solved sometime.
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Grammar / spelling **** alert: I think the word you’re looking for is spelled “caret”. We now return to our regularly scheduled programme
All the best,
Uwe
EDIT: On 2nd thought maybe it should be renamed to “carrot” as it’s much like a carrot dangling from a stick in front of your nose when you try to follow it
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Don’t have video. It’s actually with head/tail wind, not so much with cross wind. Try with 85 kts head wind on a 100 nm leg with 410 kts planned ground speed and follow the CAS carret precisely. The CAS carret starts around 300 kts initially and keeps increasing until it becomes too high to follow around 20 nm from the next waypoint. Doesn’t make a very economical flight.
With tailwind the CAS carret behaviour is the other way around, it starts way too high and decreases throughout the leg.
Current workaround is to switch to GS and follow that carret but this isn’t practical.
Anyway, hope it will be solved sometime.
In other words: The Speed Caret which is the symbol indicating the DED TOS Req Speed on the HUS’s speed scale is dependent to the HUD speed selector switch and should ALWAYS be calculated using the current total headwind component (which would make sense).
Please check if it is only the “Speed Caret” on HUD, or if the “Req Speed” in DED TOS page is also wrong/changing depending on HUD speed selector (?)
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In other words: The Speed Caret which is the symbol indicating the DED TOS Req Speed on the HUS’s speed scale is dependent to the HUD speed selector switch and should ALWAYS be calculated using the current total headwind component (which would make sense).
Please check if it is only the “Speed Caret” on HUD, or if the “Req Speed” in DED TOS page is also wrong/changing depending on HUD speed selector (?)
The RQD GS indication on the DED is always correct. It’s only the carrets for the CAS and TAS that are wrong.
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Already since 4.34 and the introduction of strong winds I’ve suspected that the behaviour of the speed carret is peculiar. When flying faster than the carret indicates, with constant altitude, velocity and (head)wind you should arrive earlier at the steerpoint than the pre-planned time. But somehow the carret manages to overtake the constantly maintained (faster than required) speed. How is this possible? If at one point you’re faster than the carret and nothing changes in earlier mentioned parameters you should always end up earlier at the steerpoint, right? I understand that the closer you get to a steerpoint, the larger the deviations, but the deviation should always remain on the same side of the carret when everything else remains constant.
Now I’ve found something even more interesting, when changing between CAS and GS the carret is below my current speed with CAS selected and above my current speed when GS is selected! How is this possible?
Related to this, since 4.34 the AI very often takes shortcuts from their original flight plan to arrive in time over the target.
thank you,
the bug has been identified and will be fix in 3 to 4 weeks
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The RQD GS indication on the DED is always correct. It’s only the carrets for the CAS and TAS that are wrong.
actually it was also wrong by a little
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