GS mini versus VFE
 

Hi bin31,

Odd that no one has taken up your enquiry. I'm way out of date, having flown the A320 from 1988 - 2001, but remember the scenario you describe only too well.

GS-MINI is a great tool but, as far as I know, there was/is no VFE protection. I'd be delighted to describe the technique we used on windy days to deal with the problem, if you haven't already found one.

Try DSC-22_30-90 P 15/16. Speed target is limited to VFE -5 in Conf Full.

GS-mini is limited to VFE (VFE - 5kts in CONF FULL). Sorry, don't have an FCOM reference but it is in the Airbus PDPs.

FCOM DSC-22_30-90 P 13/16

Another buffer is that the first 10kt of windspeed difference actual VS. tower-reported are ignored. GSmini adjusts the Vapp target only if higher than that.

May have changed from Chris Scott's time (happy to see you here, sir :)). What was that method? So far SPD SEL 160(max) worked for me, and push for managed below 500 ft or so.

Thanks vilas! :O
I wonder if that represents a memory failure on my part, or if that protection has been added since 2001. I'll have to dig out my old FCOM...

A 5 kt buffer is a fairly small margin on a gusty day, however.

Hi FlightDetent,
Am a little confused by the other "buffer" you describe. Perhaps you could re-transmit?

The method I had in mind involves, as in yours, using selected speed until the latter part of the approach. (That's often demanded by ATC, anyway.) The trick, of course, is knowing the right moment to switch back to managed speed, thereby reintroducing the GS-mini protection, without (1) risking a temporary VFE-exceedance and/or (2) provoking a sudden and large change of thrust.

Let's take a simple case of sea-level/ISA (so IAS = TAS), Flaps Full, Vapp = 140, Rwy W/C M20. Therefore minimum GS = 120.

From memory, VFE (flaps full) is 177 on an A320. Let's fly at a selected IAS of 170. At 4 nm finals, the W/C is M60, giving a GS of 110 kt. As we continue and the headwind gradually diminishes, we can watch the GS gradually rising. When it reaches 120, we push the SPD knob to select managed IAS, which at that point will be about 170. Thus no significant change in the speed target or thrust.

Any reference for this? If Vls is 135 HW nil, Vapp is minimum Vls+5 so it will be 140 and GS mini will also be 140kts. If in approach you get wind of HW10kt. GS will drop to 130 so the Vapp will go up to 150kt. to maintain GS mini of 140. Won't it?

Hi. My reference is from 2008, FCOM 3 Blue Bulletin 820/1.A. 4 notes first:
a) my recollection of the feature in own post #5 above is not what the Bulletin actually says;
b) The present-day FCOM reference FCOM DSC-22_30-90 "I": SPEED MODE IN APPROACH PHASE - Managed Speed Target Computation contains no description of any such logic;
c) I think it is fair to assume that the GS mini logic may have evolved over time, while almost certainly the description available from the pilot's books is lagging both in time and details of varied importance.
d) The Bulletin 820/1.A (2008) says there is no upper limit for adjusted Vapp target, however present-day FCOM shows a hard restriction at Vfe-5.

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The bulletin explains that delta(Vapp) = hwc.act - max(10 kt; hwc.rwy); negative results are disregarded.

If the calculation was still the same today,
Chris Scott's example is not affected as it assumes hwc.rwy = 20 kt. By coincidence, this very figure is used in the Bulletin itself. Case 2) in the following table where an increment of +30kt is commanded.
https://cimg6.ibsrv.net/gimg/pprune....5d5d50528f.png

Whereas in vilas's situation, the result would be different than proposed. Assuming airborne HWC 10 kt and runway HWC 0 kt, there would be no increase on the calculated Vapp = Vref 135 + 5 = 140. And then as well:
- For airborne HWC = 20 and runway HWC = 0: only +10 is added as the calculation assumes always at least 10 kt rwy.hwc.
- For airborne HWC = 20 and runway HWC = 10: yes, +10 is added

https://cimg4.ibsrv.net/gimg/pprune....2fe6744dc7.png

So the 10kt correction is not a buffer which would reduce the GS mini additive by default, rather it is a cut-off at the low end of the calculation.

With access to the updated FSTDs, this won't be hard to simulate too much, would it?

In simulator if you have access to lesson plan then there is demo. Otherwise it is not possible to vary the wind on approach. You can only vary the surface wind.

Hi vilas,
Am surprised that, as time approaches the 32nd anniversary of my first go in an A320 sim (an early, highly-unreliable Thomson-CSF model at Blagnac), yours is incapable of simulating the conditions of an everyday, windy approach!

Hi FlightDetent,
Thanks for providing that PDF of the FCOM Bulletin 820/1, dated June 2004. Is it still current, I wonder? it could have been better written, I think, some of it even liable to cause confusion.

It seems the expression "MAX" has been used when "the greater of" would have been more appropriate.

For example, in the table near the bottom of Page 3 dealing with VAPP computation for "A320 with Mod 25225 / A319 / A321" the text should - I think - continue more like this:
"VAPP = VLS + the greater of [5] and [1/3 of the Tower headwind-component]"

And the statement in the middle of Page 4 should read more like this:
"IAS target = the greater of [VAPP] and [GS-mini + current wind-component]"

The biggest change to the speed computations since my days, as previously mentioned, is the reduction of GS-mini to VAPP -10 when the headwind component of the W/V pre-inserted in the PERF page is 10 kt or less, or there's a tailwind. It does seem a bit arbitrary, considering that a VAPP of 130, for example, will translate to a GS-mini of 120 whether the pre-inserted W/V amounts to a 10 kt headwind-component or none at all. Taking the classic nocturnal windsheer case, a headwind of 10 kt at, say, 50 ft agl would produce a speed target of 130: equal to VAPP, or VLS +5. If the wind on the runway is a slight tailwind, the IAS could drop below VLS as the flare is initiated.

This slight change of policy on GS-mini may in part have been in response to something i saw all-too-often on windy days. As the target IAS rapidly reduced in the latter stages of the approach, the A/THR seemed unable to keep up, resulting in the IAS at 50 ft agl being significantly above target. This could be potentially compromising on a short runway when the surface wind was calm.

Chris you misunderstood what I stated. You can put any surface wind you want there is no problem but for GS mini demo you need to change the wind say every few hundred feet upwards from ground level as given in the explanation which is not possible. You can do it somewhat by changing surface wind again and again but it's not 100% correct.

Hi vilas,

I was referring to the wind-changes throughout a final approach in gusty weather. The expedient you describe sounds familiar, and perhaps not very realistic, so I'll stick with my comment!

Seasonal greetings,
Chris

Hi Chris, I may be speaking at a tangent to your argument, but I’ve never had a problem with GS mini. For many years’ I flew regularly into Luton in all weathers*, in A320/321s, and the ground speed was always nailed at, say 129kts, from about 6 miles out on the glide - it was most impressive. The groundspeed is what really counts in relation to the LDA of course.

On the truly challenging days, we seemed to approach the limits of roll authority (and went around) before GS mini became an issue.

I think GS mini can be an ambiguous label and might have been better named: “GS Lock” or similar, since it seems to do that very well, i.e. lock the groundspeed, no matter what the wind change.

* Not the longest LDA, and a steep drop just beyond the end of 26 !

Chris Scott I am rather certain that MAX(A;B) is the syntax for exactly the same meaning you explain: the greater of A and B.

A proper SIM should be able to replicate real-life wind profile with changing velocities and directions, that was his point. Noted in agreement with you, the surface wind alone / uniform in a column of air does no cut it. Though I believed the SIMs we use have the option to modify winds in different layers independently.

I agree that GS MINI does a great job.
It is however a minimum. As Vapp is the minimum airspeed we have to fly, the ground speed increases above the minimum if there is a tailwind on the approach.

If Vapp is too close to the barbers pole on approach, just use selected speed until closer to the ground and then push for managed speed again.

FD
I have dealt with latest CAE and True Sim even they don't have options to set winds every few hundred feet on approach nor CAEs earlier five versions had. The oldest I have used I am sure Chris has used, it didn't have Airbus airfield but had Toulose. The wind aloft options are way above platform altitudes. However when you enter surface wind it creates a pattern of winds away from ground which differ in direction and speed that's all. But no way 50kts for 20kts surface winds.
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Just to clarify a common error that I have seen in the GS mini story.
What you see on your PFD is not the GS mini but the IAS Target . The GS mini will never appear anywhere and is internally calculated and never shown to the pilot .

In the example you mentioned above with HW = 0 , the GS mini value stored in the system will be 130 , the minimum value from which the IAS Target will start to kick in . ( This value will never be shown to the pilot)
Any "Actual Ground Speed" lower than 130 will lead to a higher IAS Target . So in that particular case a 10 Kts HW component will bring the "Actual Ground Speed" being equal to "Intenal GS Mini Previously Stored" therefore no adjustment to IAS Target.

A 15 Kts HW component will make the Actual Ground Speed to be 125 therefore 5 Kts less than the "GS Mini Stored Value" and Vapp will be readjusted to whats called a IAS Target to be at 145 Kts . At 145 Kts IAS Target Speed the "Actual Ground Speed" = "The GS Mini Stored Values" =130 Kts. And 145 Kts is not the ground speed mini.

Have to disagree with that, having used GS-mini from its 1988 inception in airline ops. Remember that the system will never produce a managed IAS target below Vapp. Note also that managed IAS is a target speed, not a minimum speed. The only minimum speed is VLS. As for the speed going "near the barber's pole", read my second post above for a strategy that worked well for me during 14 years on type.