A320 HDG to NAV “wiggle”
Joined: Oct 2006
Posts: 93
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From: Earth
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.
Joined: Jun 2007
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From: Wanderlust
are you saying when doing direct to airport you don't get TOD pseudo waypoint?
Only half a speed-brake
Joined: Apr 2003
Posts: 4,459
Likes: 136
From: Commuting not home
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.
Joined: Sep 2013
Posts: 49
Likes: 0
From: USA
Joined: Sep 2013
Posts: 49
Likes: 0
From: USA
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.
Only half a speed-brake
Joined: Apr 2003
Posts: 4,459
Likes: 136
From: Commuting not home
@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?
Guest
Joined: Apr 2009
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From: On the Beach
Joined: Oct 2009
Posts: 1,061
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From: UK
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?
Joined: Jul 2006
Posts: 1,100
Likes: 111
From: Somewhere over the rainbow
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 December 2017 at 16:10.
Joined: Aug 2015
Posts: 30
Likes: 0
From: Around 22N 113E
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
