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A320 Magenta Donut

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A320 Magenta Donut

Old 3rd Nov 2010, 03:12
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A320 Magenta Donut

Hi all

I'm doing the A320 type rating course at the moment and during our Circling approach lessons, we were introduced to the "Magenta Donut" which you follow vertically after activating the SEC FPLN(with opposite runway) when breaking off an instrument approach.

We were taught to follow the donut all the way down to the runway. But... I couldn't find any reference in the FCOMs/FCTM to the actual vertical/horizontal path algorith for the computation of the donut profile.

My understanding is that it's nothing but a normal VDEV indication that represents FMGC vertical path, but is it buiding 3degree path streching up from the threshold assuming a certain turning radii and final leg distance?

Thanks
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Old 3rd Nov 2010, 08:41
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No.

If you are to use FMGC data for the vertical part of an approach, you check and do a few things first and the doughnut becomes a brick.

The descent from circling minimums is a visual manoeuvre done with the landing threshold in sight.

The SEC flight plan is activated so you can put the wind in the performance page for the landing runway - in order to get GS MINI working in the right direction.

The doughnut is for descent and intermediate approach monitoring - using it on an approach will only end in tears, even if some of the time it may well be about right.
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Old 3rd Nov 2010, 09:46
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Thanks HPP,

It's a hypothetical question (of probably no practical use) but IF you had JUST the landing RWY(plus may be CF) in your FMGC FPLN(with NO IAP selected), and say you are on visual downwind leg of that runway, and had APPROACH PHASE activated, what vertical path would the "doughnut VDEV" (or the "yo yo") be following?

My training organisation (quite reputable) is officially teaching this method to guide you vertically from the start of base turn in a circling approach, and if i was to follow it I want to know what exactly I'm following. So far no one has given me any definitive answer with a reference.
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Old 3rd Nov 2010, 10:05
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FCOM 3.03.20 details visual approaches and has a couple of nice diagrams to supplement these along with the associated timings. It is not clear from your posting how your TRTO is teaching you to fly these. In our operation we follow the airbus FCOM procedures and FCTM and fly as per the diagrams and they work out nicely.

The "donut" is just another aid to assist you in the approach. Supplement this with the papis/vasis/dme/prog page distance to RWY/mach 1 eyeball etc to get yourself in the slot.
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Old 3rd Nov 2010, 11:28
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Ellsworth,

The doughnut should be roughly correct, most of the time. When you enter the landing RWY in the box, you will get a 5 mile extended centre line with a 3 path (between the CF and the RWY).

Initially, the machine will think you will want to go via the CF, so it will show you as low. At some point as you turn base, it will auto-sequence and will show you your height above the planned path, assuming you turn straight towards the RWY (not quite what you are doing). So it will be close to correct, but if anything will try to pull you slightly low.

BUT

Here are the issues I can think of, and I bet someone more clever than me will think of more.
  • You may not have checked the nav accuracy.
  • The data in the database for the RWY + 5nm will not be subject to the same QC as a proper NPA.
  • You are breaking the Airbus rule of using the dot on final app (not the brick). If it's a dot, use raw data only.
  • The slope you need to fly may not be 3. Often there is nasty close-in terrain and a steeper approach.
  • The dot encourages eyes-in, when you are flying a purely visual manoeuvre which needs as much eyes-out as possible.
  • What if the dot says one thing and your eyes say the other? Which is correct? Could the dot subtly lead you into trouble?
This may all just be coming from the visual limitations of the sim, where some sim smart arse has come up with a system to help pilots turn base. I would keep smiling, do what they say, and pass the course. But from my experience, I would suggest you fly the last part for real with your eyes out and if it doesn't look perfect at all times then bin it and go somewhere else.
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Old 3rd Nov 2010, 11:33
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PS.

The Airbus is a fantastic tool, with so many aids, safety nets, tools and assistance that you can fly it safely in all sorts of conditions.

But as a pilot you still must decide the difference between good data and bad data. I suggest that using the FMGC profile in this case is bad data.
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Old 3rd Nov 2010, 12:48
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Hi, my understanding is that the dot is a total energy ring rather than a descent path indicator like the brick on a non precision approach, so if you were a bit fast or slow, it would not give a correct path indication. However, assuming FMCG is programmed and sequenced, it does seem to give a good 3 degree path, though obviously for information only, visual references being the primary guidance.
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Old 3rd Nov 2010, 12:58
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Bit off topic but the doughnut and the brick are green in new aircraft. Once the next waypoint is the runway the doughnut always seems to be spot on, have seen it way off at other times though.
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Old 3rd Nov 2010, 17:13
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Ellsworth,

One point to remember is that the donut will bring you over the threshold at the runway elevation. Of course you want to cross the threshold at 50' so it is a good idea to remain slightly above the donut guidance so that you do not take out the threshold lights.
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Old 7th Nov 2010, 08:27
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Donut path is a straight line between two waypoints and their associated altitudes in the RH column of F-PLAN A. So in this case that is from CF at its' altitude, and RW and (normally) RW Elev plus 50' , both inserted by the database.

With GPS PRIMARY it is pretty accurate (see MERIT) but not as accurate as ILS GP.
Without GPS PRIMARY it could be miles out.
Use as a guide but treat with caution, is my advice.
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Old 7th Nov 2010, 17:52
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Yes.

In my experience, even with GPS and managed/managed in a straight in VOR approach I fly very often the airplane is far from an "ILS like" profile.

Circling is a visual approach. Any FMGC data must be used only as cues, never as a command, like the G/S of a glideslope.

The donut sometimes is wrong, specially if the waypoints are not correctly secuenced or you are not in the green track.

Many don't understand well the donut. The donut you see from the TOD is a geometrical path and it is not perfect. And it can be misleading if you are not flying a similar lateral route or you are flying at a different speed. I often see guys extending spoilers believing they are high, and they have just decelerated earlier. Even the airplane in DES mode will chase the donut in a very nasty and untidy way, very often, even with accurate winds. The donut is not an energy level, since it assumes a given airspeed, not your actual speed.

In the case of a CF leg, provided the waypoint TO (in white upper right ND) is the rwy threshold and you are on final, the donut should be a good cue, to use along with the ND map, the progress page NM to threshold, any DME available, altitude minus elevation, PAPI or VASI if any and even "seat of the pants", too.

If you are not aligned with the runway, I don't know if the donut behaves similarly to an ILS G/S but I don't think so.

In the FCTM there is more about the Circling than it is in the FCOM.
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Old 7th Nov 2010, 18:07
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Which is why it's known as the 'Chinese glide-slope'.
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Old 8th Nov 2010, 01:03
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Thanks all for your valuable inputs.

If you are not aligned with the runway, I don't know if the donut behaves similarly to an ILS G/S but I don't think so.
That's one things that bugs me when I'm told to use it as a guidance as you turn base (with CF as FROM WPT and RWY as TO WPT). I don't know whether it's guiding you along the surface of the '3degree geometrical plane' to the runway like ILS GS (ie not much vertical displacement when you are on base) or is it actually giving you 3 degree descent along your actual circuit flight path(ie continuous 3 degree path to the runway threshold from base).

I guess it's nothing to lose sleep on I was just curious.
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