ILS and crosswind
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The Dash 1 mentiones: “The pilot flies the FPM to the command steering cue to intercept and maintain the localizer course and the glideslope for an ILS approach.” However, when there’s a crosswind the steering cue will not bring you to/keep you on the localizer course (verifiable on HSI and visually). Is this intended behaviour or a bug? It’s clearly visible with higher cross winds.
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Hi @Bowser
Can you post a screenshot or video where you are established on ILS glidepath and the steering cue not pointing to the runway?Here I have set up a simple scenario - landing on Gunsan 36, wind roughly 40 deg, 15 kt - clearly wind correction angle into the wind must be applied. Maybe you have DRIFT C/O switch in DRIFT C/O position, which looks like both localizer and flight path marker not aligned with the runway (screenshots 2. and 4. below), but in fact, the ILS cues take the wind into account. Look at the examples below:
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DRIFT C/O in NORM position
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DRIFT C/O in DRIFT C/O position (both screenshots 1. and 2. taken during the same freeze)
And again, a little bit closer to the runway:
3. DRIFT C/O in NORM position
- DRIFT C/O in DRIFT C/O position (again, aircraft in the same position as in screenshot 3. above, going rather correctly toward the runway)
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See picture, the ILS bars line up OK but the command steering cue is off to the right (while looking at the HSI I should be a tad more to the left even). When following the tadpole it will take you to the touchdown point but not on the localizer course when there’s crosswind. Can any of the devs confirm this is a (known) bug?
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Usually these kind of systems take localizer time rate data into account when providing command steering. I believe BMS is simply comparing heading and localizer deviation to course. E.g. when loc deviation is zero then it’s making command heading equal to localizer course. That’s obviously not the best command when crosswind is present. What you’re getting is a balance of factors where you’re getting a constant deviation solution not at zero deviation. You’re downwind which is providing some upwind steering command but not enough to get back to loc center, just enough to settle in a constant downwind loc position. If you were in a position of more deviation steering would act to reduce deviation but if you were more central steering would be to increase deviation.
In this kind of system normally the steering command is proportional to deviation, integrated deviation over time, and possibly derivative of deviation. That’s your classic PID controller where P is the simplistic “if left go right”, I is the integration “we’re still not getting there, go right harder”, and D is the “we’re getting there too fast, go right weaker.” Practically the command steering cue position should not overlay on the FPM unless two conditions are met: localizer deviation is zero and localizer deviation rate is zero.
Honestly it looks like this is coded based on heading and not on track. I can’t imagine how this would happen in a track-based calculation. A function of deviation and deviation rate spits out a correction. If correcting to the right steering cue moves right of FPM. Pilot flies a new track, values change. One thing that usually goes with these systems is a gain factor based on proximity to the localizer or the sensitivity keeps increasing as you get closer which can be bad. Sometimes it’s a timer after joining glideslope or radar altimeter, neither is flawless but they’re better than nothing.
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We’re looking into it.