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 I'm way out of date, having flown the A320 from 1988 - 2001, but remember the scenario you describe only too well.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.

If that's 21st-century aeronautical syntax, I bow to your superior knowledge, FlightDetent! Or could it simply be the invention of a Francophone technical writer at St. Martin-du-Touch?

If it is standard terminology, is there a new-age syntax guide available somewhere? :rolleyes:

Probably not clear in my post, but I meant to say that you can't call it a locked "GS LOCK" speed. If there is a tailwind on approach, the ground speed will increase above this "locked speed".

Vapp + any tailwind will give a higher than minimum groundspeed.

Yes, as you suggest I think the term GS-mini is hard to beat, as it is indeed a minimum GS - not a target GS. Of course Airbus did not invent the concept; aviators a lot cleverer than I - some of them flight engineers - started doing calculations of their own on previous types as soon as INS provided real-time readings of GS in the 1970s. But, AFAIK, it was Airbus that pioneered the automation of it and introduced it to the airline world with the A320 from 1988.

Hi again Uplinker,
Must admit I had Jersey in mind! I assume improvements may have been made to A/THR response since my time. It seemed seemed marginally worse on V2500-engine a/c than on the early CFM-56 ones.

The GS-mini itself works so well in windsheer conditions, provided the managed IAS is used as a target speed, not a minimum speed. The thrust changes required are smaller and less frequent, of course, than those needed in selected IAS. My own preference was to use manual thrust for manual landings. The A320 manual throttle control is excellent - better than any of the other five jet types I flew. (Since then, in today's litigation-conscious environment, manual thrust has been banned in the airline I used to work for. I think that's a backward step.)

The largest thing I’ve landed at Jersey so far was a 737-300 at about 50 tonnes, so an A319/320 would probably focus the mind!

As you say, GS mini along with the auto-thrust response and gains etc, have probably been refined and improved via software updates since 1988, and I personally have never had any problem with it, (flown it since 2005). A superb and very clever system in my view.

I agree that A320 is perfectly flyable and easy to fly in fully manual. However, the auto-thrust is now so good, (and of course there is no pitch/power couple), that it is not often necessary to take it out. (I know - I’m being lazy !).

Citation

Hello again, healthy 2020! I feel the opposite is true. It is you who used superior aviation knowledge to decipher an unknown code by observing the circumstances and connecting the dots through experience. My hat's off to you. Unless you've been pulling my socks, in which case a bow is included. :ok:

Back on the topic: The bulletin I provided at post #10 is dated 2004 but taken from FCOM version 2008 (Volumes 1, 2, 3, 4). Shortly afterwards, the whole bookset had been heavily restructured and all blue FCOM bulletins cancelled. Their information was only partly embedded into the main text and I believe for a few years the explanation of GSmini internal workings was lost altogether.

Fast forward today, FCOM says what's shown upthread, and that does NOT include the 10 kt "mandatory rwy HW" that I pointed out.

Yet that's what the machine does. Courtesy of a couple of talented and tolerant colleagues, a little controlled observation was done. The GSmini algorithm that adjusts the Vapp really assumes at least 10 kt headwind component at the threshold. Calm wind, tailwind, at all times.

https://cimg7.ibsrv.net/gimg/pprune....eb59427026.jpg



https://cimg8.ibsrv.net/gimg/pprune....68387ca01d.jpg

To observe it live without trying too hard, an airport where HWC is less than 10 kt at threshold would be best. With the landing flap out, check ND for TAS - GS and observe that

• for HWC stronger than 10 kt, any variation will be reflected on the Vapp target;
• for HWS slower than 10 kt, adjustments to Vapp target do not happen.

Done.

Hello FlightDetent. Thank you for that info. I was unaware of that. :ok:

I think the GS mini is based on VLS and not on Vapp. It excludes ATHR addition of 5kt. That's why it doesn't react to less than 10kts. Rightly so because strictly Vapp addition is not required through flare so why have it on GS?

There were more pictures and human observations. Behaviour was consistent with the GSmini function adding a margin on top the Vapp from MCDU. It is possible to modify MCDU Vapp to VLS, VLS+5 (auto), VLS+10 (15?) and observe.

The margin (GSmini adjustment) was observed to be the difference between actual HWC and HWC assumed at the runway threshold.

The assumed RWY HWC was the greater of
- expected HWC component calculated based on the MCDU W/V entry
- the low limiting value of at least 10 knots.
= Max ( HWC.rwy ; 10kt )

Your suggestion does not fit the pictures already provided, the cut-off would be 5 knots. Check the first one, 1590 RA.

VLS = 128 Vapp = 133
HWC.rwy = 22
HWC.thr = 0

present day FCOM
delta HWC = 22
Vapp + 22 = 155 ===>>> not observed

Vls anchor option
delta HWC = 22
Vls + 22 = 148 kt ====>>> not observed

Bulletin 10 kt cut-off
assumed HWC.rwy = 10 kt
delta HWC = 12 kt
Vapp + 12 = 145 ====> visible on the picture

Followed by the other picture RA1510, with manually inserted 10kt HWC at THR into the MCDU. The magenta triangle on speed-tape blinked for recalculation but came back to the same position 145 kt. ,

Changing MCDU HWC.rwy from 0 to 10 kt caused a new computation with an identical result.

Only with MCDU HWC.rwy greater than 10 knots (12 on the following picture) was the Vapp additive reduced, delta HWC.act and HWC.rwy getting smaller by those 2 knots.

https://cimg1.ibsrv.net/gimg/pprune....b155426de5.jpg

HWC.rwy = 12 kt
HWC.act = 17 kt
delta(HWC) = 5 kt = ADD.gsmini
Vapp + ADD.gsmini = 133 + 5 = 138 = managed target

Why on earth the FCOM would remove that critical information about that 10 kt threshold? Now I understand the reason I could not figure out why the VAPP with a strong HWC was slower than expected...:}

What an ugly PFD.

as the saying goes, we showed you ours?

Good question, though. The above is design date around 1985. The technology has progressed, never mind the dead ends, and we're 1/3 of a century ahead.

Carrying a speed additive to cover for the headwind loss is hardly a novel idea, it was a common knowledge that Airbus engineers decided to adopt in the EIS.

Are there any other aeroplanes that would do the same or similar? Or better? There must be a ton of fancy clever suff that can be coded, now that we are well in the iPhone era.

According to a Safety first article the MCDU Vapp calculation has undergone a change from older aircraft.
"AFS Computation of VAPPWhen computed by the AFS, the APPRoach CORrection (APPR COR) used by the AFS isAPPR COR = 1/3 Headwind with 5kt ≤ APPR COR ≤15 kt Excepted on some older A320 aircraft where the APPR COR used by the AFS is 1/3 Headwind + 5kt, limited at 15kt."

Boeing or other aircraft use a Vapp with a fixed addition for wind and gust at the surface. But that doesn't cater for gust above the surface. GSmini is a better system in that sense. Flying Airbus approach in select speed will be similar to Boeing.

Hi Unhooked,
As a retired old fart, I hesitate to argue with a current operator, but I experienced the first 14 years of A320 ops and your scenario remains all too familiar 18 years later.

In those conditions, my airline recommended Config 3 for a better performance margin in wind sheer, but I found the 320 a bit skittish with it and the A/THR sometimes commanding almost idle thrust. So I preferred Config Full. The GS-Mini system allegedly gives a 5 kt margin below Vfe (the subject of this thread), but as you suggest that may not be enough to avoid a short-term exceedance if there's a big gust. Using selected IAS solves that problem, but obviously you lose the safety and handling benefits of GS-Mini.

Can I respectfully suggest you consider the compromise technique described in the second half of my post above, which addresses that scenario?
https://www.pprune.org/tech-log/628054-gs-mini-versus-vfe-3.html#post10642917

A320 in gusty conditions handling wise flaps full is recommended configuration unless performance is an issue then flap3.