A320 - GS MINI Question

[Wireflyer]

Hi everybody!

One question about the GS mini concept:

According my memory, the increased IAS target (if GS mini + current HW component > reported tower HW) reduces at 400ft height above threshold to reach VAPP over threshold.

My problem: I cannot find anything about the 400ft gate in the official airbus documents (FCOM/FCTM/OEBs/).
Can anyone please give me a reference to look this up or am I mixing up something?

Thank you very much! :ok:

W.

[Iceman49]

Not sure if there is a reduction...did a app into AMS last year. Indicated was 160 at touchdown, GS was 95.

[kijangnim]

Greetings,
The FMS views the 400 ft wind as the ground (surface) wind

[john_tullamarine]

The FMS views the 400 ft wind as the ground (surface) wind

I'm not an Airbus man but I would hope that the statement is not correct. Can you cite appropriate references to support the statement ? 400 ft wind is generally not a good predictor of surface wind ....

[mcdhu]

I've been on the 320 fam quite some time now and this is the first time I've heard of this 400' thing. I my experience, GS Mini works perfectly as advertised and those who bad-mouth it either won't or can't try to understand what it is doing for them.

But the key is to have an accurate Tower wind in the box. Quite a few folk put the Atis/Volmet wind in the mcdu and wonder why strange things happen (perhaps as described above) when actually the tower wind has changed markedly since they entered it. So why not update the wind when checking in with the tower or when the guy in front receives an update as he is cleared to land.

Cheers
mcdhu

[kijangnim]

Greetings John

Entering the wind in the descent page:

Entering 400 ft will be repleaced by GRND, likewise you can use GRND in lieu of height.

Regards

[avionneta]

that 400 ft threshold simply doesn't exist and if real wind at runway threshold is higher than the tower wind, then your target will be VApp + the delta Hw

[K.Whyjelly]

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!

[CJ1234]

:D

finally I understand GS Mini!

Superb explanation

[kit330]

:DHey Kyjelly I know its an old post of yours but heck, its one of best I've read yet about gs mini:ok: