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# What is ground speed mini ?

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# What is ground speed mini ?

9th Mar 2010, 11:08

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What is ground speed mini ?

Whats ground speed mini and what good does it do for pilots when they're landing ?
Explain plz
9th Mar 2010, 11:17

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One of the best explanations of GS Mini I think courtesy of Norman Stanley Fletcher.....................

I am a Line 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 made 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 ‘Tower Head Wind Component’ 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 ‘gust’ 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 ‘gust’ 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 ‘gust’ 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 1500’ 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 ‘gust’) decreases. There are 2 ways to overcome this. One is to enter an artificially high Tower Wind and thereby reduce the ‘gust’ 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 ‘gust’ 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 ‘Tower Wind’ 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.

That is it from me – I hope that is helpful. If you got to here then you must be keen! If you are more confused than ever just pretend you never read it!
9th Mar 2010, 13:18

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That's very interesting, where is that extract from?
9th Mar 2010, 17:19

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It's an article written by Norman Stanley Fletcher some years back in response to previous posters asking the same Q that this OP posed. I believe NSF is a training Captain with EZ but I may be mistaken on this point. Hopefully the man himself will pass by here to claim the praise he is due for such a master class in ground speed mini.

Norman...................I salute you Sir
9th Mar 2010, 18:35

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thanks it helped me alot, now i fully understand whats ground speed mini and how's it computed, its a premium explanation far better than the notes i have which just confuse my basics everytime i read them .
Thanks again
9th Mar 2010, 18:37

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Yes, but why would you want to go with the 'lowest possible approach speed' ?

Half the steady and all the gust has always worked well for me
9th Mar 2010, 19:42

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Yes, but why would you want to go with the 'lowest possible approach speed' ?
So that you carry just enough excess IAS to protected you from wind shear on the approach, but still arrive at the flare at the correct speed so that your LDR calculations remain valid.
10th Mar 2010, 11:44

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Stilton - you seemed to have missed the WHOLE POINT! GS mini is not about the minimum approach speed at all - it is simply adjusting the Vapp to preserve the inertial ground speed which in turn protects the aircraft if the wind component should reduce markedly.
10th Mar 2010, 14:58

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I think I'll stay on the Boeing...
11th Mar 2010, 05:49

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Its all about the minimum energy level that an aircraft should have at touch down and this is based on the ground speed that the aircraft will have at Vapp with the expected wind.

lets suppose.

Conventional Aircraft (Head wind 20 kts N1 55% Speed target 130 kts )
AIRBUS (Head wind 20 kts N1 55% IAS target 130 kts )

then the GUST......

On the conventional (Head wind increses to 50kts ,Speed trend is going up
And Thrust will go to idle as Speed target will remain at 130KTS)

On the AIRBUS (Head wind increases to 50 kts ,IAS Target and speed increases to lets say 160Kts Speed trend is going up And Thrust will be increased)

then the SHEAR (From head to calm)........

On the conventional (Head wind decreases to zero Speed trend is going down and bellow the 130 kts target And thrust wich was near idle is know having problems to recover)

On the AIRBUS (Head wind decreases to zero Speed trend is going down from 160 kts to the original 130 kts And thust wich was initially high is reducing with no problems.)

SO.
CONVENTIONAL AIRCRAFT (BOEING) THE ENERGY IS LOW AND CLOSE TO THE GROUND (DANGER), THRUST HAVE TO BE SIGNIFICANTLY INCREASED.

AIRBUS, THE ENERGY IS HIGH, THRUST IS SMOOTHLY REDUCED.

luis
11th Mar 2010, 08:23

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Well luis, coming from a place where winds gusting to 60-70 knots are pretty regular events in the winter, I have a few issues with that.

With gusting winds, I will never let the autothrottles (Boeing, with visual cues and movement and all...) idle the engines on final approach, I will firmly override them in BOTH directions as needed.

I would not like any airplane to increase speed from 130 to 160 kts on its own, especially on a limited (e.g. slippery) runway. Maybe not a real issue on a light 319/320, but on a heavier airplane it is an issue.

Its all about speed management. And in the end its up to the pilot to manage it.

P.S. I do not want to turn this into a Boeing vs. Airbus debate, I have no Airbus experience.

Last edited by oceancrosser; 11th Mar 2010 at 08:43.
11th Mar 2010, 09:47

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While the above descriptions are entirely correct, maybe they overcomplicate things and the simple benefit is hard to see.

You're at 200 ft with 30 kt headwind, however THR reported component is only 6 kt. At a certain time, you are about to lose 24 kt and it may come quite suddenly. So you are facing 24 kt drop of IAS unless you compensate with thrust, which of course is a second nature (no pun) and normal, required piloting skill. You arrive to 15 ft with Vapp 130, albeit with little more N1 added than was previously required to maintain Vapp on final.

With the same scenario, on AB the Vapp is 130 kt but as you decelerate in managed spd (VNAV SPD was the B term if I remember) the SPD target stucks at 130+24=154 kt and so you fly, N1 stable. As you cross the shear layer, headwind diminishes and your IAS drops by 24 knots. Voila, here's your Vapp exactly as you wanted and exactly where you needed.

If you do not like the behaviour, just go to selected speed (FCU) but be prepared to increase thrust to compensate for the shear.

Frankly, it is a matter of technique not technology. Either you fly speed and add thrust to flare to maintain Vapp, or you fly energy and watch ASI unwind to target. It works quite well and is no groundbreaking feat, AB had been around the block for 22 or so years.

Yours,
FD (the un-real)
11th Mar 2010, 16:07

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You got it Ocean, Airbus only tries to release workload, you can always override the autothrust, but the protection gives you a steady as posible ground speed at flare, it doesnt matter yor IAS target at approach is 160 or 170 your Ground speed will be as steady as possible (VAPP).

Dont want to make a debate either, i dont have Heavy Boeing experience, soon i will, and i ll be able to compare them and have a flight criteria which is the important thing to have.

cheers
11th Mar 2010, 19:28

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Thanks everyone for all the valuable responses.
regards
11th Mar 2010, 22:16

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GS mini--great aid, like so many protections on the 'bus'.
12th Mar 2010, 17:35

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FD,

Be very wary of intervening with selected speed - it tends to de stabilise everything and I have seen some low energy arrivals with guys not trusting GS Mini.

The other thing is some of our 'other' colleagues seem intent on using manual thrust on the Airbus........ Boeing throwback I fear.

Then they try and chase a moving target - fail and it is a real buggers muddle.

GS Mini works well, on 320, 321 and 330. The only thing is, you need a working brain to realise what is happening and on the 330 you need to anticipate the thrust couple effect when flying manually.
13th Mar 2010, 11:44

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"Other" colleagues Javlin? Enlighten me! I use gs mini with auto-thrust on 9 out of 10 approaches but into CUN with huge thermal up drafts from them there jungles I tend to use manual thrust, otherwise the power is up and down more often than a lady of the nights under garments..
13th Mar 2010, 20:48

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Well...........

If the AP is in, there will be speed variations, 90% of the time, they are transient and if the Vapp is adjusted correctly after Config Full, all should be well, even in thermic conditions, CUN, SFB, LAS etc.

GS mini is busily doing it's thing while all this gusty thermal stuff is going on as well - the target Vapp will usually be dancing around.

Now, if said AP is out, and it is thermic, you need to be aware of the shortcomings of the FBW system that will tend to exaggerate PIO on final. The onset of power by the autothrust should be countered by a sympathetic amount of forward stick as the operative cannot think/act as fast a the AP. Vice Versa for reduction of thrust.

I was fortunate to have had full factory courses for 320 and 330 and personally would not fly manual thrust unless exceptional circumstances prevail or in the sim. Let's face it other aeroplanes are autothrust all the time as well and that is the way Airbus designed their aeroplane

Boeing went another way, which is fine for Boeing.............. Not for Airbus
13th Mar 2010, 22:30

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Now, if said AP is out, and it is thermic, you need to be aware of the shortcomings of the FBW system that will tend to exaggerate PIO on final. The onset of power by the autothrust should be countered by a sympathetic amount of forward stick as the operative cannot think/act as fast a the AP. Vice Versa for reduction of thrust
Mate, you need to go back on the B757. Get used to using that right hand for what it was designed for.

I'm back, by the way. Squinting Cat before summer starts?
14th Mar 2010, 00:50

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"was fortunate to have had full factory courses for 320 and 330 and personally would not fly manual thrust unless exceptional circumstances prevail or in the sim. Let's face it other aeroplanes are autothrust all the time as well and that is the way Airbus designed their aeroplane''

If I'm hand flying, I tend to use manual thrust as well. It just seems more natural to me and there are no ''out of phase'' moments. And if you're in a heavy A321 with V2500s in gusty conditions, it make sense. The complete range of 320 aircraft seem to have the same AT response from the FMGC and in this case, it just doesn't seem to have the required response.

And as for Norman's GS Mini piece . It's the best expaliantion of anything ever in the history of PPruNe. I cut and pasted it and it now featured in my notes.

Thanks Norman.