AIRBUS A320 FMGC query
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AIRBUS A320 FMGC query
Couple of nights ago we were doing lazy 4 minute racetrack holds at FL 360 for about 45 mins. Although the FMGC had been set for the hold, the actual flying was done in HDG and SELECTED Mach., with the shape of the hold being used for situational awareness. All normal so far.
Upon setting off for home using a DCT TO a waypoint approx 100 miles away, with NAV engaged and Managed Speed, the Managed Speed Target set itself to 229 kts, 3 knots below Green Dot, but above Vls. This was seen and an appropriate Selected Mach was re-engaged.
We cannot work out why the Perf Page Cruise phase Managed speed was 229 and would not shift with changes of mode, cost index or FMGC Cruise FL.
In the end, we Activated Appr Phase (in Selected Speed) then reselected the Cruise FL in the FMGC and the Managed Cruise Speed corrected itself to .78.
We recently had FMS Rev 1A fitted which already has one anomaly which has been Flight Crew Noticed to us and wonder if this is another.
Upon setting off for home using a DCT TO a waypoint approx 100 miles away, with NAV engaged and Managed Speed, the Managed Speed Target set itself to 229 kts, 3 knots below Green Dot, but above Vls. This was seen and an appropriate Selected Mach was re-engaged.
We cannot work out why the Perf Page Cruise phase Managed speed was 229 and would not shift with changes of mode, cost index or FMGC Cruise FL.
In the end, we Activated Appr Phase (in Selected Speed) then reselected the Cruise FL in the FMGC and the Managed Cruise Speed corrected itself to .78.
We recently had FMS Rev 1A fitted which already has one anomaly which has been Flight Crew Noticed to us and wonder if this is another.
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Hold
Did you EXIT the hold, did you clear the hold? Or did you just put in the Direct To?
Sounds like the FMGS speed was based on the Hold and did not clear properly.
This not being the fault of the system or an anomaly thereof.
Sounds like the FMGS speed was based on the Hold and did not clear properly.
This not being the fault of the system or an anomaly thereof.
Last edited by Natstrackalpha; 23rd Mar 2012 at 12:25. Reason: afterthought
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Well spotted, I deliberately left that info out. We did not use the Imm exit function and did use the DCT TO a waypoint further down route. As you know Airbuses have a long history of the FMGC getting confused with its predictions if Imm Exit is not used, but it is perfectly acceptable to use the DctTo in our SOP if you want to and all the elements of the hold had been removed from the flight plan page. Even if this was the problem, it doesn't explain the speed properly, which was below Green Dot and only just 1kt off VLs and the managed hold speed should have been about 5kts higher than that.
Thanks for the response, though, I suspect this will be another 'whats it doing?', 'its an Airbus' thing.
Thanks for the response, though, I suspect this will be another 'whats it doing?', 'its an Airbus' thing.
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Yeh, I guess you are right. It sounds like you took it so far away from the hold that it got the impression you were going to an approach - that might have been an APP speed, which you may have over-ridden with Selected spd.
Either that or you were in some sort of "loiter" speed.
(When I was on Tornadoes (yeh, right, I wish . . ))
Either that or you were in some sort of "loiter" speed.
(When I was on Tornadoes (yeh, right, I wish . . ))
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Why so slow??
Sounds like the FMGS speed was based on the Hold and did not clear properly
(query posed with zero hours on type)
TSIO.
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Don`t be picky! They said "lazy"
229 knts at FL360 is not slow.. On an ISA JSA day 229kts would give you 410kts TAS and about Mach 0.71 which gives an approximate Still Air speed of 7 miles a minute +/-
229 knts at FL360 is not slow.. On an ISA JSA day 229kts would give you 410kts TAS and about Mach 0.71 which gives an approximate Still Air speed of 7 miles a minute +/-
Last edited by Natstrackalpha; 24th Mar 2012 at 15:31.
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(Why fly a cruise speed 'away' from where you want to go, better to stay at L/D max hold speed until the pointy end is headed in the right direction)
