A320 HDG to NAV “wiggle”
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I have noticed that "wiggle" on my aircraft sometimes (ATR76) and I think it's one of two things. 1. Wind correction 2. Aileron trim. Sometimes the ATR needs a bit of trim aileron wise to keep straight. If that is not there, the autopilot will keep a minute but constant wing down to keep level. The quick changeover from LNAV to HDG and vice versa causes a little wiggle.
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are you saying when doing direct to airport you don't get TOD pseudo waypoint?
Only half a speed-brake
The TOD and DECEL waypoints are surely there. But Altcrznav wants a fix at 30 NM distance, I suppose in order to put 250 kt and 10000 ft restriction on it. Fair enough.
My experience suggests that the automatic pseudo WPT DECEL of 250 kt at 10000 VNAV altitude is actually further away from the landing than 30 straight miles. Thus the above will force a VNAV alt increase at newly inserted 30 NM fix, which FMS will counter by slowing down way below 250 to keep the energy in check. Not exactly the desired result.
My experience suggests that the automatic pseudo WPT DECEL of 250 kt at 10000 VNAV altitude is actually further away from the landing than 30 straight miles. Thus the above will force a VNAV alt increase at newly inserted 30 NM fix, which FMS will counter by slowing down way below 250 to keep the energy in check. Not exactly the desired result.
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The TOD and DECEL waypoints are surely there. But Altcrznav wants a fix at 30 NM distance, I suppose in order to put 250 kt and 10000 ft restriction on it. Fair enough.
My experience suggests that the automatic pseudo WPT DECEL of 250 kt at 10000 VNAV altitude is actually further away from the landing than 30 straight miles. Thus the above will force a VNAV alt increase at newly inserted 30 NM fix, which FMS will counter by slowing down way below 250 to keep the energy in check. Not exactly the desired result.
My experience suggests that the automatic pseudo WPT DECEL of 250 kt at 10000 VNAV altitude is actually further away from the landing than 30 straight miles. Thus the above will force a VNAV alt increase at newly inserted 30 NM fix, which FMS will counter by slowing down way below 250 to keep the energy in check. Not exactly the desired result.
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Could it be that, when you pull HDG eg. 300, but the track to a waypoint is 300 and a half, the wiggle is because he has to turn a half of a degree when you insert the direct to the said waypoint?
Only half a speed-brake
@Altcrznav: cool, use the tools. This discussion should never turned into WHY we create the wpt ...
@FE Hoppy: Your point that FMS leg is not a constant heading line is well noted, but it really is 2 seconds or even less.
@CM: any update on the FMS versions?
@FE Hoppy: Your point that FMS leg is not a constant heading line is well noted, but it really is 2 seconds or even less.
@CM: any update on the FMS versions?
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Could it be that the track is calculated so precisely that it will read something like 332.4deg and when you select heading, it goes to 332.0 as it doesn't allow the precision of decimals. Pushing NAV it will try to recover the track again which is probably far preciser. Perhaps you can look at the IRS readout in the box and see if that gives you any clues?
I guess you could find a NAV track that is precisely aligned with a degree without a decimal and try pulling HDG for a few seconds and then pushing NAV again?
I guess you could find a NAV track that is precisely aligned with a degree without a decimal and try pulling HDG for a few seconds and then pushing NAV again?
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I think the reason is that the roll limit is different whenever you are on hdg or nav mode thus making that noticeable wiggle when switching between them.
They talk about it here:
A320/321 AP bank angle limits
Airbus Bank angle
They talk about it here:
A320/321 AP bank angle limits
Airbus Bank angle
Last edited by pineteam; 25th Dec 2017 at 16:10.
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Pardon me, but I am not satisfied with the responses.
I am proceeding direct to waypoint ABC which is 300 NM in front of me. I decide to create a new waypoint which will be ABC -100NM. I open the FIX INFO page. I type in ABC and 100 in radius. I select 4L which drops the radius into FPLN. That radius fix remains dynamic until I select heading.(does it not?) I select heading, insert, then select NAV. I can do that in one second, yet I still get a wiggle.
I am proceeding direct to waypoint ABC which is 300 NM in front of me. I decide to create a new waypoint which will be ABC -100NM. I open the FIX INFO page. I type in ABC and 100 in radius. I select 4L which drops the radius into FPLN. That radius fix remains dynamic until I select heading.(does it not?) I select heading, insert, then select NAV. I can do that in one second, yet I still get a wiggle.
These always a mismatch of a Nav database geometric convention versus a calculated or assumed point on space. Older FMS standards have these specifics. Perhaps take a look at the FCOM in system related (21 or 34) for these anomalies
