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Airbus groundspeed mini & unstable approaches

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Airbus groundspeed mini & unstable approaches

Old 4th Jun 2012, 18:53
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Aircraft records a momentary speed fluctuation of up to VAPP +25 before it settles down again.
There's your problem - the wrong parameter is being monitored/recorded. It should be a difference from Managed Speed Target, not Vapp (which is fixed).
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Old 7th Jun 2012, 08:39
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I think many have missed the point. The secret is in the name, ground speed mini. In truth I don't care about maintaining a minimum ground speed, this has absolutely nothing to do with maintaining safe flight. It does however make the life of ATCO easier as it will facilitate stable aircraft separation.

Like many who have flown airbus in very disturbed weather conditions I have seen some ridiculously high commanded air speeds whilst using managed speed and A/T. I'm sure others like me have seen commanded speeds of VLS plus 40 or more. How likely are you to experience, unannounced, 40 knots plus of wind shear which would threaten the aircraft?

On the other hand I and others alike have flown approaches with very strong and blustery crosswinds where g/s mini will add on nothing at all to VAPP.

When conditions are difficult I will decide on my own safe airspeed and fly that irrespective of what g/s mini thinks.

It's worked all the way from L-188 through B727 etc and now airbus.

Don't be a slave to the automation.
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Old 7th Jun 2012, 14:22
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Ground speed mini explanation

Please find below a full and complete explanation of what ground speed mini does and why, as there seems to be much confusing in this thread and in general about what it will do and why. The notes below are from another Captain.

Groundspeed Mini Explanation


I am a Training Captain on the A320 series. In my First Officer days I started asking questions of Training Captains about how GS Mini worked. It became very apparent that none of them really understood it except in the most general terms. Being the sad spotter I am, I decided to really go for it and spent a whole day modelling it on an Excel spreadsheet and puting in the countless permutations the equations create. At last I understood it and it now makes total sense! My subsequent experience tells me this is the least understood feature of the aircraft by the overwhelming majority of Airbus drivers. That is not in any way to be condescending, but that is my personal observation after thousands of hours on type. It is also possible to be flying the Airbus for many years and not really grasp what is going on in this department.

A sure sign of not grasping how it works is when someone thinks that it is 'dangerous' to have a high approach speed generated by the GS mini function on a relatively short runway. If you think that then read on!

Question 1. Why do we have a groundspeed mini function in the first place?

Answer: To enable the aircraft to make an approach at the minimum safe approach speed.

Question 2. What is 'groundspeed mini' anyway?

Answer: If you consider an approach in a conventional aircraft, we will all accept that the aircraft groundspeed is the difference between the TAS and the headwind component of wind. (For all practical purposes, TAS = IAS/CAS at the low levels and speeds associated with nearly every approach). If there is a gust of wind, due to the inertia of the aircraft, the goundspeed stays constant (in an instantaneous sense) but there is an instantaneous drop/rise in IAS. Over a period of several seconds, the groundspeed eventually settles to a lower level (assuming an increase in headwind component) and the IAS settles back to its original level before the gust. If that gust then disappears completely, groundspeed instantaneously becomes IAS (ie TAS) until the same settling process occurs as described previously. If that original gust was substantial (say 25kts+) and the loss of gust is equally substantial, a situation can arise whereby the aircraft is encroaching into the stall regime and at the very least may experience a sifnificant, and potentially dangerous, loss of speed/lift. In a conventional aircraft this potential problem is overcome by adding up to 15kts, typically, onto your approach speed in gusty or crosswind conditions. In an Airbus the problem is handled by working out the minimum groundspeed that is acceptable for a given wind condition and ensuring the aircraft never drops below that value. This ensures that regardless of gusts the aircraft is guaranteed a safe flying speed. This minimum groundspeed is known as 'groundspeed mini' or 'gs mini'. Easy!

Question 3: What do I need to know about 'gusts of wind'?

Answer - What we conventionally think of as gusts and what Airbus calls a gust are 2 different things! A 'conventional' pilot thinks about a gust of wind as being an unanticipated and rapid change in speed or direction of a volume of air. (There are no doubt better definitions but I think you get my drift!) The Airbus, being a dull machine, has a different way of assessing a 'gust'. On the PERF App Page, one of the programmable fields is for wind - that wind is known as the 'Tower Wind'. Although you do not see it, a computer takes that wind and resolves it into a headwind component relative to the programmed runway. The Tower Wind is used to provide a datum setting of guaranteed minimum wind that may be safely assumed to always be there (which is why Airbus insists on entering the wind without the gust component). In addition, the IRS's are always calculating a w/v which is displayed to the pilot on the ND. That wind is resolved into a headwind component by one of the computers. Although that value is never formally displayed, it is easily calculated by taking the groundspeed from the TAS on the ND. The aircraft then takes that value and compares it to the headwind component of the Tower Wind (wind in the Perf App page) - known as the ʻTower Head Wind Componentʼ or THWC. The difference between the two values is taken as the 'gust' - ie the 'unanticipated' wind component. The calculation assumes that the THWC is a minimum of 10 kts so if the Tower wind is say 260/6 the calculation will assume it is 260/10.

Question 4: What does the Airbus do with that calculated gust of wind?

Answer: It simply adds the rest of that ʻgustʼ onto the calculated approach speed (VAPP) on the PERF App page. That is then displayed to the pilot as the VAPP TARGET, which is the magenta triangle approach speed we all know and love on the PFD.

Question 5. How many possible approach speeds does the Airbus calculate and what one does it use?

Answer: The Airbus actually calculates 4 possible approach speeds but only displays the highest one to the pilot as the magenta speed triangle (VAPP TARGET). That also becomes the autothrust speed target. Two of those speeds do not consider groundspeed mini and 2 do use it. The first 2 are straightforward and are calculated from the following equation:

VAPP = Max (VLS + 5, VLS + 1/3 THWC [to max of 15kt]) The important thing about VAPP is that it is known beforehand as it appears on the PERF App page. It is the highest of VLS + 5 or VLS +1/3 of the THWC (limited to a max of 15 knots). Say for example VLS is 125 kts, and the reported wind is 260/50 on runway 26 (ie all headwind) then VAPP would be 140 kts as the max value of the tower headwind component would be 15kts. VLS + 5 would only be 130 kts so the higher value would be displayed on both the PERF App page and on the magenta triangle speed bug on the PFD.

In equation terms, groundspeed mini is described as follows:

GSmini = VAPP – THWC or VAPP – 10 [If Tower tailwind or THWC < 10] This leads us to the calculation of the next 2 possible speeds, both of which consider groundspeed mini. The equation is as follows:

VAPP TARGET = Max (VAPP, GSmini + Current HWC)

As an example:

VLS = 120kts Tower Wind 260/27 1/3THWC = 9.0 R/W Dirn 260 VAPP = 129 kts Current Wind 260/35 Current HWC = 35.0kts x-wind = 0 GS Mini 102kts

VAPP-derived speeds:

VLS + 5 = 125kts VLS + 1/3 THWC (max of 15kt) = 129kts

Groundspeed mini-derived speeds:

VLS + 5 -max(THWC,10) + Current HWC = 133kts VLS +min(1/3THWC, 15) - max(THWC, 10) + Current HWC = 137kts

Therefore, VAPP TGT = 137 (which is displayed on the PFD)

Question 6: In general terms then, what is the rough rule of thumb about the expected approach speed?

Answer: The magenta bug speed will always be VAPP from the PERF App page, plus any ʻgustʼ along the runway axis.

Question 7: Does it matter what wind I write in the Perf App page?

Answer: If the wind is 10kts or less you can write anything you like and it will have no effect whatsoever on the final approach speed. So, for example, if landing on runway 26 you can write 080/10 and the approach speed will still be VLS + 5. Once the wind is greater than 10kts what you write does affect VAPP TGT (ie the magenta bug speed).

Question 8: What is the effect of increasing the Tower Wind on VAPP target?

Answer: It is the exact opposite effect many people imagine. If I am approaching runway 31 and the instantaneous wind is 310/35 but the Tower Wind in the PERF APP page is 310/8 the ʻgustʼ is taken as 25kts (the calculation always assumes a minimum headwind of 10kts). That would be added to the VAPP of say 135 knots to give 160kts magenta bug speed. If I now write 310/15 in the Perf App page as the Tower wind that will have the effect of reducing the approach speed because the gust is now only 20kts. That would be added onto VAPP of 135kts to make 160kts. You can try this for yourself and see it instantly work. So in general terms, reducing the Tower Wind increases the approach speed and vice versa. Therefore it is important to put in the steady state wind and not the max gust because by so doing you can erode the protection the function is trying to provide. Putting in a very high wind at the last minute will instantaneously decrease the approach speed bug.

Question 9: When does the groundspeed mini function cause problems and what can I do about it?

Answer: The function causes problems typically at 1500ʼ above the runway on a very windy day when the wind can be enormous compared to the Tower Wind. If for example on RW 08 with a VAPP of 125kts and the Tower Wind is 080/15 but the instantaneous wind is 080/70 (as can happen) then 55 knots can be added to VAPP making VAPP TARGET 180kts. This can be above the flap limiting speed for Config Full (177kts) and give an enormously high approach speed. However as you approach the ground that speed will progressively decrease as the headwind component (and ʻgustʼ) decreases. There are 2 ways to overcome this. One is to enter an artificially high Tower Wind and thereby reduce the ʻgustʼ and subsequent VAPP TARGET or the more common method is to immediately select a speed (say 160kts) and wait for the gust to die down. As soon as it has done so, you manage the speed again and the VAPP TARGET will be sensible. Dead easy!

Question 10. Why do we activate the secondary runway on a circling approach?

Answer: This is because the groundspeed mini calculation will see any wind over 10kts from the reciprocal direction as only 10kts. Therefore it will make the approach speed VLS + 5 which removes all the gust protection that should be there. If you activate the correct runway then the headwind components are resolved in the correct direction and any genuine ʻgustʼ is taken into account during the VAPP TARGET calculation.

Question 11. Is GS Mini not potentially dangerous on short runways?

Answer: No! The whole point of GS Mini is to provide the lowest possible safe approach speed. It assumes that the ʻTower Windʼ is always there and is not a gust. By definition a gust is temporary and therefore if a gust appears it will be added onto the final approach speed but the groundspeed will still be the same as if the gust was not there. Therefore no extra landing distance will be required even if it is a high approach speed. The key thing is that the correct wind should be entered on the PERF App page – as long as you do that then you will not have any snags.
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Old 7th Jun 2012, 16:09
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it ain't a rocket science fellas. Pretty simple really. Above 400 ft GS Mini adds full head wind and below it drops to a 1/3. That's what usually causes so called hanging through till stabilized again. All the aforesaid scenarios qualify as wind-shear therefore 2 way to cope with it. Either divert or select FLAP 3 and add 15 KTS on VLS, in this case there's sufficient energy till touchdown. Having said that it's far more important to know particularities of the airdrome in terms of vulnerability to win-shear. Perfect example is HKG or NRT with excellent low wind shear warning systems in place. If they broadcast gain and loss of 20 Kts on ATIS you bet you'll get it.

Last edited by 9.G; 7th Jun 2012 at 16:12.
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Old 7th Jun 2019, 13:13
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PT6A, I found your very detailed explanation here with regards to GS Mini and it’s certainly reinforces the concept much more clearly.

I’d like to add another question. Does GS mini work if you manually punch in the VAPP in the FMGS, assuming all other entries (especially the wind) are correct? I cannot find any references in FCOM with regards to this.

For example, VAPP is 140, and I manually put 143 in instead and fly the approach with managed speed. GS mini didn’t seem to work today with strong gusts down the runway axis and it did got us wondering a bit on finals. The magenta speed target stayed at 143 throughout.

I’ve seen the GS mini many times but to be honest, this is the first time I ever manually increased the speed from the calculated VAPP. I didn’t expect GS MINI not to work.

Any ideas, anyone?
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Old 7th Jun 2019, 13:59
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Yes, the GSmini speed increment is added to create a managed speed target on top of the Vapp from MCDU. That is why modifying MCDU is the preferred method over SEL SPD, in cases where the auto-calculated Vapp is not desirable.

Note: You are joining a thread 7 years cold.

Last edited by FlightDetent; 7th Jun 2019 at 14:41.
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Old 7th Jun 2019, 14:26
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Are you flying the NEO?
The increase in VAPP is only 1/3 of the excess wind.
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Old 7th Jun 2019, 19:09
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Yes, the GSmini speed increment is added to create a managed speed target on top of the Vapp from MCDU. That is why modifying MCDU is the preferred method over SEL SPD, in cases where the auto-calculated Vapp is not desirable
FD
Amending Vapp in the MCDU is not a preferred method but the only method because GS mini is simply not available in select speed. It will behave like conventional aircraft. In very gusty winds Vapp can be amended up to 15kt.

Last edited by vilas; 8th Jun 2019 at 17:00.
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Old 8th Jun 2019, 00:46
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What is the rationale of increasing the VAPP up up VLS+15 in the FMGS when the function of GS mini is supposed to take care of that as opposed to conventional aircraft? It’s still an option or recommendation in FCOM.

I punched in an extra 3 knot, and for the whole approach the target speed remained as such, it didn’t increase with gusts along the runway axis as I have been used to seeing all this while, but it’s the first time I manually adjusted the VAPP in the FMGC. Did put me in the unstable criteria for a long while because if this.

I was on the CEO.
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Old 8th Jun 2019, 01:59
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If your assumed pax weights are wrong, your Vapp will also be wrong. If the Vapp is sitting right above Vls, I'll add a few knots, just in case.

3kts shouldn't make you unstable. In our books, the airspeed requirement is within xkt of the TARGET speed. So even if you added 15 to Vapp, and GS mini added another 10, as long as you're within xkt of the target speed, you're technically stable. May have to adjust your flare technique in that case though 😊
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Old 8th Jun 2019, 05:15
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GS mini will be active as long as the aircraft is in APP PHASE and MANAGED SPEED. It can be active even on downwind if the winds drastically change. And the minimum Air Speed threshold is set by the Vapp that is in the MCDU irrespective of whether original or altered with some additive by you. Due high gust or turbulence if you decide to have higher Vapp then you'd do so by increasing the Vapp. The GS mini won't do that. It will not disable GS mini whether in CEO or Neo. It's the design, period! While it's one thing to increase Vapp for added protection, it's quite another to increase the winds speed in the MCDU thinking it will be safer. Increasing wind speed will reduce the ∆wind and decrease GS mini protection.

Last edited by vilas; 8th Jun 2019 at 17:02.
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Old 9th Jun 2019, 13:16
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Forgive me, PT6A, your friend might well be right but their explanation is very long winded and not that clear to me.

I am always confused when pilots go selected speed to kill G/S mini to “stop the A/THR reducing thrust” because I find that G/S mini works well in gusty conditions. This is my understanding of it:

Consider a hypothetical day. A Boeing and an Airbus FBW which are both fully configured at 6 miles, are making simultaneous parallel approaches to parallel runways. Vapp for both is 140kts and the wind is calm, so both have a ground speed of 140kts.


A 20kt headwind gust appears:
The Boeing reduces thrust to keep the IAS at 140kts; its ground speed is now 120kts.
The Airbus increases thrust and airspeed to 160kts; its ground speed is thus kept at 140kts.

The headwind disappears back to calm:
The Boeing now has an IAS of 120kts and its engines are at idle. Its ground speed is 120kts.
The Airbus has an IAS of 140kts and its engines are spooled up. Its ground speed is still 140kts.


A 20kt tailwind gust appears:
The Boeing increases thrust to keep an IAS of 140kts. Ground speed is now 160kts.
The Airbus reduces thrust but it does not allow its IAS to reduce below 140kts. Ground speed is 160kts.

The tailwind gust disappears back to calm.
The Boeing now has an IAS of 160kts, a ground speed of 160kts and its engines are spooled up.
The Airbus has an IAS of 140kts and its engines are at idle but its ground speed is still 140kts.

So, when the gusts disappear; the conventional aircraft is at either - 20kts with engines at idle or + 20kts with its engines spooled up. The G/S mini aircraft is at either - 0kts with engines spooled up or + 0kts with engines at idle, so it does not deviate as much for the same changing conditions.

I think some “conventional pilots” get confused and nervous about IAS and the speed target changing so much with changeable winds, but G/S mini gives a safer energy state.

Last edited by Uplinker; 10th Jun 2019 at 11:52.
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Old 9th Jun 2019, 13:48
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Originally Posted by stilton
Half the steady and all the gust up to + 20 always worked well for me on my simple Boeing
Exactly, works also on airbus
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Old 9th Jun 2019, 16:54
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Perhaps important: all the proper FOQA/FDM/QAR software suites should challenge the speed against MCDU Vapp + GS mini correction.
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Old 9th Jun 2019, 16:59
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Originally Posted by vilas
Amending Vapp in the MCDU is not a preferred method but the only method because GS mini is simply not available in select speed.
Unless below 700 ft RA, where MCDU has data lock. Any changes will be accepted as input but not taken into account. The other, non-preferred option of SEL SPD becomes the only option and GS mini is lost - if increase on the speed target is desired.
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Old 10th Jun 2019, 11:10
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The Boeing way is to add a fixed amount to Vapp as gust factor based on tower reported wind to ensure at least Vapp close to flare height and that is below the bug. It has nothing to do with winds higher up on the approach. However you carry this baggage during entire approach even when there is no gust. Also during approach when experiencing higher or lower gust factor the speed can drop below the bug or overshoot the target before recovering. This leads to excessive ATHR activity. GS mini concept doesn't target a Vapp but targets the ground speed based on tower reported wind. To achieve this it varies the Vapp when required to keep the GS constant which tends to vary due to instantaneous experienced gust throughout the approach. This reduces the thrust variations. In Boeing method if there's no gust during flare you carry extra speed, in GS mini you will only carry Vapp. Boeing method is gross correction, GS mini is real time correction. Landing distance depends only on the GS.

Last edited by vilas; 10th Jun 2019 at 11:30.
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Old 10th Jun 2019, 11:40
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Yep, all agreed vilas - that is my understanding of it A good system which works well. The ground speed remains impressively constant, and as you say, ground speed is what matters when landing.

The difficulty comes when an Ex Boeing pilot flying an Airbus FBW wants to pull speed to disable G/S mini and stop the speed target changing* But without the added safety factor this could leave the Airbus compromised.

*(Some get really nervous when G/S mini suddenly adds 25kts and the A/THR adds loads of thrust.)
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Old 10th Jun 2019, 11:57
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There are two reasons for that both psychological. First is the security blanket like a child of the old habit of previous aircraft. Second also is the comfort factor of doing something even when it means undoing. They feel comfortable setting and flying extra 20kts when it may not be really required and feel uncomfortable with same extra 20kts added by GS mini when required. They must not be allowed to do so. Boeing guys have habit of flying excess speed. Airbus must be flown Airbus way.

Last edited by vilas; 10th Jun 2019 at 18:22.
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Old 10th Jun 2019, 20:52
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Originally Posted by stilton
Half the steady and all the gust up to + 20 always worked well for me on my simple Boeing
Well Boeing changed that due to high speeds of the 737MAX. Now it is max 15 kts...
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Old 11th Jun 2019, 03:50
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I think Airbus did the same, in the wake of TALPA ARC about the same time when OLD/FLD were implemented. Obviously tied to landing (distance) performance.
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