View Full Version : Airbus groundspeed mini & unstable approaches
Scenario to ponder ...
Flying into airfield "A" on a gusty day. Steady headwind of 25 knots at 1000', which turns to zero passing through about 400' on final. The G/S mini function soaks up the wind shift, aircraft maintains the magenta speed target and aircraft touches down at VAPP without recording an exceedance.
Flying into airfield "B" also on a gusty day, with a 15 kt tailwind at 1000', which rapidly turns into a 10 kt headwind at 400'. Aircraft records a momentary speed fluctuation of up to VAPP +25 before it settles down again. Still touches down at VAPP.
Company does nothing to pilot "A", and kicks ass of pilot "B" because a speed variation of +25 was observed on short final.
Question is then, is one an unstable approach and not the other? Should pilot "B" go around and not pilot "A"? From a P of F point of view, isn't it more dangerous to be flying into the decreasing headwind than the increasing headwind?
1st Jun 2012, 00:20
in scenario B, you are saying a 180 wind shift, from 15kts tail to 10kts head.....
Are you talking about a FOQA bust?
1st Jun 2012, 01:48
Aircraft records a momentary speed fluctuation of up to VAPP +25
"Momentary" is the key. What's the Stabilised Approach max speed parameter? How long was the actual speed above it?
isn't it more dangerous to be flying into the decreasing headwind than the increasing headwind?
It could be seen that way, but on the other hand, an uncorrected increase in headwind could cause a balloon and a long landing.
The FOQA data in that outfit is obviously not confidential. :cool:
I have had scenario B.
Have a quick look at your IRS groundspeed and if that get you puckering, you know to give it away :)
1st Jun 2012, 06:54
A competent pilot would handle a or b just fine. Why let the non flying desk guys run your cockpit? I know it has happened now but why? It is really a sad situation when pilots can not do what makes sense because the monitors will bust them.
1st Jun 2012, 07:20
I was on final At las one day in a 737 and could see my ground speed was low so increased about 20 knots. It fixed the wind change at low altitude so we landed with no problem. It probably would have set off the new alarms however.
1st Jun 2012, 07:27
It all depends how enlightened and knowledgable the company are with regard to their stabalised approach requirements and their use and understanding of Vapp and Vref.
Exactly, sounds like your company need to learn more about how ground speed-mini works before giving their line guys a hard time.
If they are asking for no speed variations, I guess you can never land in gusty conditions... Where rapid speed changes will occur.
We are not talking about someone maintaining an excess of 25kt, it was a temporary fluctuation, which A, should not of pinged the FQUA and B, when a human looked at the data should of been able to see what triggered the event without calling the crew!
1st Jun 2012, 13:16
Whatever device your company is using to measure your speed, they should also be able to see the FMS calculated target speed, and the wind speed. So whats the problem? Unless they are unaware that Ground Speed Mini even exists, in which case you should show them your FCOM and make them aware of it.
1st Jun 2012, 16:22
In our airline, certainly, FDM is more complex than simply looking at whether an event has been triggered. A "High speed below 500 feet" event would result in an analysis of the whole approach. The analysts would, in this case, note that the aircraft met all stabilised approach parameters, and that what triggered the condition was a short term environmental fluctuation - something specifically covered in the ops manual, which says: Short-term deviations as a result of atmospheric conditions are acceptable if corrected in an appropriate manner. (or words to that effect)
1st Jun 2012, 18:26
wind shear is wind shear, it just happens, they should have been able to get that out of the box...
YP, thats exactly my point.
However, many of the Asian carriers for example don't have the right person looking at the extracted frames.
Thus the whole system turns into a punishment tool, something that does NOT improve flight safety at all.
1st Jun 2012, 19:21
sittingidly: Mm. But the extra energy carried compensates for the reduced thrust, doesn't it? I have little doubt that a pretty careful analysis was carried out before the idea was implemented. And I also have little doubt that if it can be shown that the policy is less safe than alternatives, then the policy would be worked on.
PT6A: Meh. If the company policy is that you should go around (and presumably divert, ultimately) in these circumstances, then do it. Eventually, a manager somewhere will be led to ask why your company's flights don't get in whilst other carriers do, and the policy will change.
2nd Jun 2012, 05:34
Half the steady and all the gust up to + 20 always worked well for me on my simple Boeing :ok:
Yeah maybe I should have been more clear ... I was talking about the fact that groundspeed mini only works because it adds IAS, and then reduces it when the headwind component is reducing on the approach - it's useless when you have a headwind that increases on short final, or a transition from tailwind to headwind.
I do have a regular airfield where this tailwind-to-gusty-headwind situation occurs and we quite often get a big kick up from stabilized VAPP to all of a sudden VAPP+25. I don't think it's a big safety issue as long as the speed is stable at the flare, it's a super long runway anyway. But half the skippers are terrified of it and either go round or write captain's reports on it every time it happens.
I just wish Airbus could have designed something into the mini G/S to cope with this situation.
In order to cope with the variation that you state, GsMini would have to fly an approach below Vls and risk bumping the tail in the flare, obviously something that isn't going to happen.
GsMini is good at coping with a reducing headwind, but physically there is little it can do in the increasing headwind case. In that instance, it isn't any different from any other jet out there...
I suggest that you approach your company and get them to give you definitive guidance on what to do in that situation, if they want you to bin it and try again/divert then do it. When the costs on the route soar, they will analyse it and come up with 'acceptable' risk... Let them define where the line is, don't risk your job finding out!
4th Jun 2012, 11:26
Thus the whole system turns into a punishment tool, something that does NOT improve flight safety at all.
.....and . . there goes yer bonus! (Very cost effective these companies . . ?)
4th Jun 2012, 11:43
in the increasing headwind case, thrust is increased to maintain minimum ground speed thus energy is maintained.
its win win
Exactly Natstrack! Some of these airlines are not as enlightened as we would like.
It is not correct to think that G/S Mini cannot "cope" with the situation.
The aim of the game is to prevent a situation where, due to sudden wind variations, the aircraft ends up at a speed that is too low for safe flight or with thrust fully at idle while speed is temporarily above command speed.
In the usual system, command speed on final is a steady value, and autothrust will chase that value.
If with the usual system, there is a sudden increase in headwind component, airspeed will instantaneously be (very) high and (auto)thrust will decrease rapidly, perhaps even to idle, which is undesired if it happens just before the flare (imagine that the wind decreases rapidly now, you will be on speed, but without thrust, engines will be very slow to spool up).
If with the usual system, there is a sudden drop in headwind component, airspeed will instantaneously be (too) low and even though (auto)thrust may increase rapidly, there is a chance of stalling.
The trick of G/S Mini is that it will VARY the COMMAND SPEED, to cut drastic thrust changes.
In case of a head gust, yes, the airspeed will increase, but so will the command speed, so thrust will not be pulled back. If now the gust dies down, you will be on speed, with the required thrust.
You are concerned about the situation of, say, 20 kts tailwind on final approach, changing to 5 kts headwind just prior to the flare (yes, such situations do exist in real life).
If you want to achieve the ideal Vref + 5 over the threshold, the only way is to "fly" the approach is at Vref - 20 (you will get the 25 kts increase just prior to the flare.
However, at Vref - 20, the aircraft does not fly anymore.
The G/S Mini idea to maintain Vref + 5 (minimum safe approach speed) is the best you can do in that situation. The resulting high airspeed over threshold is unavoidable. Remember though, that your groundspeed is not suddenly 25 kts higher! and groundspeed is what onground stopping is all about. Just don't let the extra airspeed prolong your flare, touch down at the required point.
Just think about the LH A-320 accident in Warsawa, a long time ago: they had 25 kts tailwind during approach, tower reported wind was about 10 kts from right front, exactly your scenario. Windshear was (of course) reported, so they (unfortunately) religiously applied the (then current) SOP: increase Vapp to Vref + 20. You can imagine the result over the threshold, there was a fatal overrun.
If G/S Mini had been available (and used) then the speed increase near the threshold would have been much less detrimental to safety.
If you properly think about this system, it will also be clear why you have to input reported steady state wind into the FMGC. The gusts are then taken care of by the variations in commanded approach speed , whereas in "normal systems" the steady approach command speed has to be increased, to make sure that variations around that command speed will stay above minimum safe approach speed (or stall speed, so you will).
4th Jun 2012, 18:53
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).
7th Jun 2012, 08:39
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.
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
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.
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. :ok: