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Airbus groundspeed mini & unstable approaches

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Airbus groundspeed mini & unstable approaches

Old 11th Jun 2019, 04:09
  #41 (permalink)  
 
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Originally Posted by Ajax View Post
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?
well our Airbus FCOM stated stable as TARGET +10 (magenta bug as positioned by GS mini) and VREF -5
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Old 11th Jun 2019, 06:45
  #42 (permalink)  
 
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It's a 7year old thread. It took some resolve to go through the long winded explanation by the training capt only to find not to be wholely true.
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!
The aerobatics suggested by him are based on an erroneous assumption and are not necessary. GS mini cannot exceed speed limits at either end of the scale. It will not drop speed below Vapp nor will it increase beyond VFE-5 and if doing decelerated approach then flap2 or 3 VFE next. The very purpose of GS mini is to cater for gusts throughout the approach and not just near the ground. All you need to do is just leave it alone and let it do it's job.
As far as the OP hypothetical case of being hauled up on account of GS mini speed exceedence where is the exceedence when the Vapp itself has increased? It can be corroborated from other parameters such as ground speed, winds etc.

Last edited by vilas; 11th Jun 2019 at 07:21.
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Old 11th Jun 2019, 10:01
  #43 (permalink)  
 
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Originally Posted by vilas View Post
IAll you need to do is just leave it alone and let it do it's job.
I believe that sums up the whole thread
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Old 11th Jun 2019, 10:03
  #44 (permalink)  
 
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Many aspects of the (excellent) Airbus FBW family are poorly understood or not understood at all. Examples include how to use the sidestick and Fly-by-wire combination, another is G/S mini.

...............what can I do about it?..........causes problems.........enormously high approach speed**........There are 2 ways to overcome this............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
The parts I have highlighted in bold worry me - especially from a training captain ?!? Selecting and pulling speed while flying a turbulent approach, when hands should be on the primary controls is distracting, unnecessary and not a good plan. It will also leave an Airbus in a poor energy state.

One does not need to ‘overcome’ or otherwise ‘fix’ G/S mini. It is there to keep us safe: one simply lets it and the A/THR do their job !


** Look at your groundspeed.





Last edited by Uplinker; 11th Jun 2019 at 10:18.
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Old 12th Jun 2019, 02:05
  #45 (permalink)  
 
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Thanks for the great discussion guys. Didn’t mean to revive a 7 year old thread, I was just being caught out that day when my GS mini didn’t work after I manually tuned in a higher VAPP at about 1200 feet. On another flight, I did the same with the VAPP but at TOD, and everything went as expected. Just wondering if there was any reason why it didn’t work in the first case (it definitely didn’t with my PM colleague concurring).
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Old 12th Jun 2019, 05:48
  #46 (permalink)  

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Hm. Data lock - checked. Dual eyes and brains - checked.

As eerie as it may feel, and all of us live through such moments, bets are that you guys did something different or additional to what you remember.

The GS mini does not react to headwind gusts as such. It reacts to the difference of the present headwind against the entered runway wind. One way to invalidate its calculations is to put a higher or/and gust included runway headwind (component).

QFU 200
W/V 160/11G16 140V200

MCDU 160/11 - correct (rwy hwc 8)
MCDU 200/16 - wrong (rwy hwc 16)
MCDU 140/11 - max GS mini effect (rwy hwc 5)

In the second example, the GS mini would not profess until steady headwind wind component exceeds 16 knots. With 160/22 the GS additive is correctly zero. Into the wind crab angle intensifies the impression something should be happening, but the math is still right. How momentary gusts are treated (=filtered) is not disclosed, but they must be at least a little.

B) I think adding a few extra knots to Vapp is horsesheets. Unless LDS weights are suspect. Stick to your old practice, unless it disqualifies you in the mind of the one-eyed superiors.

C) Adding an extra speed buffer in case of gusty, turbulent conditions is a fair and recognized practice. To avoid dynamic speed fluctuations taking you too slow at the lower extremes. Up to 15 knots ok not to complicate the flare too much. Do not forget to add those (and 5 for athr) in the LD perfo calcs. This is not for the flare-against-approach wind difference (GS mini takes care of that), this is for transient speed drops no matter the wind direction.

Last edited by FlightDetent; 12th Jun 2019 at 06:04.
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Old 12th Jun 2019, 09:10
  #47 (permalink)  
 
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..........I don't care about maintaining a minimum ground speed, this has absolutely nothing to do with maintaining safe flight..................... I have seen some ridiculously high commanded air speeds whilst using managed speed and A/T...........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.
....comes from the traditional philosophy, which flies bugged airspeed at all times, (and adds a fixed increment to Vapp on a gusty day). This is how it was done for years, but then somebody came up with the minimum groundspeed concept. And when landing onto a runway fixed to the ground (!!!), the speed over the runway is what actually counts.

So, as above, the traditional pilot who does not understand the minimum groundspeed concept will be nervous to see such changes in instantaneously commanded airspeed, (NOTE, Vapp is not changing), because they are used to absolutely nailing their airspeed within +10/-5 kts. Any deviation larger than that is too fast or too slow for them, and their reflex reaction is to take action to contain the airspeed within those limits.

Which is fair enough, BUT minimum groundspeed is a completely different concept. It maintains energy and reacts to head or tailwind gusts, (which may or may not be expected). NOTE: it reacts the opposite way round to the traditional method, and does so to maintain energy. It has been developed, and tested and refined by test pilots. It works.

The example quoted above ‘adds maybe 40kts’. Well, Yes - I HAVE flown approaches with sudden 40kt gusts a few miles out. ‘But it adds nothing for strong and blustery crosswinds’, no, it probably won’t unless they affect the groundspeed.

By pulling speed, one disables G/S mini, which could leave the Airbus in a low energy state.

Last edited by Uplinker; 12th Jun 2019 at 09:30.
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Old 12th Jun 2019, 10:43
  #48 (permalink)  
 
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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.
It doesn't make sense replying to 7year old thread but since Uplinker brought it up I want to.
Actually this person has flown only one aircraft under different certifications and he is doing the same with A320. The statement is ignorance hidden behind pride. It's not Boeing's but even NONFBW Airbuses are also flown his way. FBW made it possible to handle it in a better fashion. Since flight path stability takes care of changes in speed and thrust it became possible to create GS mini.
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Old 12th Jun 2019, 11:37
  #49 (permalink)  
 
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Originally Posted by gusting_45 View Post

Don't be a slave to the automation.
Don't be slave of ignorance, it's as bad.
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Old 12th Jun 2019, 12:50
  #50 (permalink)  

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One last chain link: If the FOQA/FDM is flagging you not-stable due to the effects of GS mini, either the software or liveware algorithms are misconfigured.

That's not much of an opinion really, more of a computer engineering fact. Having seen a little bit of inside of such system or two, OEM AirFase included.
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Old 13th Jun 2019, 09:17
  #51 (permalink)  
 
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The GS mini function is a great Airbus characteristic. As always, the pilot should not be slave to it and a little bit of common sense should always be allowed.
For instance when there's one of those autumn Atlantic storms passing over Western Europe. Strong winds around 45-50 kts at airport level and even 65-70 around 3000' agl are not uncommon.
Big and sudden wind shifts are not be expected, but below 3000' ft and increasing to airport level it will be turbulent and gusty due to the wind blowing over ground obstacles.

In such a case, if you fly managed speed with autothrust on, the GS mini speed will be very close to Vne and you risk exceeding the max flaps speed when selecting the next flaps setting and gusts do occur. (I don't fly NEO's) I have seen GS mini speeds of 185 kts in conf 2 and 170 in conf 3.
I'm not talking about tornado or microbust conditions here, but gusty, RELATIVELY stable storm wind. So what I do when I see extremely high Vapp's at a relatively high altitude is indeed go selected speed (Vfe next - 10 or 15 kts) for a short while and selecting next flaps setting when reducing through Vfe next, and then lower down go to managed speed again. You will not fall out of the sky in the conditions I described, but it will will keep you away from a flaps overspeed. And no, this is not aerobatics as Vilas suggested in reply #42, but common sense. I read here that on the NEO Airbus reduced the Vapp increment due to wind at higher altitudes. If that's true, that's a good thing and it proves the logic of going selected speed temporarely to select flaps 3 or full with very high Vapp on the 'classic' A320 series.

Oh, and I will be using manual thrust with managed Vapp in stormy conditions. I have tried manual and A/THR in stormy conditions and I can do it better then Airbus autothrust. But then again, I use manual thrust in 98% percent of my landings. Don't try it in a storm when you never do it otherwise! Why am I better then A/THR? Because I'm not stupid enough to blindly chase the Vapp and go to idle at 300' ft when the speed increased to Vapp +10 kts with a long enough runway. Airbus automation is very good, but it has limits. Use it wisely but don't be a slave to it. If the automatics don't behave as they should, then take over and fly it manually. Sadly, not all crews are trained anymore to be able to do that.

But hey, don't listen to me. I only have 20 years experience and around 14000 hrs on the A320.
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Old 13th Jun 2019, 19:06
  #52 (permalink)  
 
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Originally Posted by sabenaboy View Post
The GS mini function is a great Airbus characteristic. As always, the pilot should not be slave to it and a little bit of common sense should always be allowed.
For instance when there's one of those autumn Atlantic storms passing over Western Europe. Strong winds around 45-50 kts at airport level and even 65-70 around 3000' agl are not uncommon.
Big and sudden wind shifts are not be expected, but below 3000' ft and increasing to airport level it will be turbulent and gusty due to the wind blowing over ground obstacles.

In such a case, if you fly managed speed with autothrust on, the GS mini speed will be very close to Vne and you risk exceeding the max flaps speed when selecting the next flaps setting and gusts do occur. (I don't fly NEO's) I have seen GS mini speeds of 185 kts in conf 2 and 170 in conf 3.
I'm not talking about tornado or microbust conditions here, but gusty, RELATIVELY stable storm wind. So what I do when I see extremely high Vapp's at a relatively high altitude is indeed go selected speed (Vfe next - 10 or 15 kts) for a short while and selecting next flaps setting when reducing through Vfe next, and then lower down go to managed speed again. You will not fall out of the sky in the conditions I described, but it will will keep you away from a flaps overspeed. And no, this is not aerobatics as Vilas suggested in reply #42, but common sense. I read here that on the NEO Airbus reduced the Vapp increment due to wind at higher altitudes. If that's true, that's a good thing and it proves the logic of going selected speed temporarely to select flaps 3 or full with very high Vapp on the 'classic' A320 series.

Oh, and I will be using manual thrust with managed Vapp in stormy conditions. I have tried manual and A/THR in stormy conditions and I can do it better then Airbus autothrust. But then again, I use manual thrust in 98% percent of my landings. Don't try it in a storm when you never do it otherwise! Why am I better then A/THR? Because I'm not stupid enough to blindly chase the Vapp and go to idle at 300' ft when the speed increased to Vapp +10 kts with a long enough runway. Airbus automation is very good, but it has limits. Use it wisely but don't be a slave to it. If the automatics don't behave as they should, then take over and fly it manually. Sadly, not all crews are trained anymore to be able to do that.

But hey, don't listen to me. I only have 20 years experience and around 14000 hrs on the A320.
SPOT ON.

Very wise comment. I flew many years for a major EU carrier on A320 and A340 and it didn't bother the training departement to fly selected SPD approaches and using manual thrust in windy conditions.

BTW I don't know about any incident/accident not using GS mini....

Last edited by gearlever; 13th Jun 2019 at 19:40.
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Old 14th Jun 2019, 09:11
  #53 (permalink)  
 
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Rather you than me. I am not a test pilot.
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Old 14th Jun 2019, 09:57
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Originally Posted by gearlever View Post
SPOT ON.

Very wise comment. I flew many years for a major EU carrier on A320 and A340 and it didn't bother the training departement to fly selected SPD approaches and using manual thrust in windy conditions.

BTW I don't know about any incident/accident not using GS mini....
There is a difference between briefly going to selected speed for flaps extension to avoid a possible VFE exceedance in case of high gusty headwinds and flying the whole approach in selected speed, which is against Airbus philosophy unless directed by a checklist. Regarding the use of manual thrust, Airbus tells you that if You do not find its performance satisfactory you are obviously entitled by all means to go manual thrust. In my company the OFDM monitors the GS mini value, and flying selected speed and thus going below the GS mini threshold would result in a low energy state and level 2 exceedance.
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Old 14th Jun 2019, 12:10
  #55 (permalink)  
 
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Originally Posted by Uplinker View Post
Rather you than me. I am not a test pilot.
Who are you replying to, Uplinker? And what are you replying about?

Replying to me, I suppose...

What would I need to be a test pilot for?

1. For using manual thrust? Any pilot not able to fly without A/THR should not be in a cockpit! (Check MEL 22-30-01A)
2. For using manual thrust in stormy/gusty weather? I found this:
Originally Posted by My Airbus FCTM
In very difficult weather conditions, the A/THR response time may not be sufficient to manage the instantaneous loss of airspeed.
Not just me, but any pilot who regularly uses manual thrust will be doing at least as good a job as the A/THR in stormy weather. I feel pity for you if you fly for one of those companies that won't allow you to become and stay proficient to fly without A/THR. Any Airbus captain should be able to fly with the A/THR inop or switched off, even in stormy weather and certainly not only test pilots.

or 3. For using selected speed (when the A/THR is still on) to be able to select the next flaps setting when going through Vfe-next without risking exceeding the max flaps speed? Would you rather risk going in overspeed then using common sense to avoid it?

So, Uplinker, I'm really curious to know who and what your 'test pilot' remark was about?
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Old 14th Jun 2019, 12:21
  #56 (permalink)  
 
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flying selected speed and thus going below the GS mini threshold
We used a type of ground speed mini on B707 in mid 1970s.

If we selected Wind + Hold buttons on the INS, it would display Head (N for on nose) or Tail (S for stern) and the wind component in Kts. Large wind shifts were displayed by flashing 8s in the window. The technique employed was not to reduce power when we encountered a headwind gust, but to allow the IAS to increase, knowing we would loose the speed when the gust died.

It was nowhere near as easy as allowing FBW to adjust and simply reading the revised VAPP (with GS mini display).

Last edited by Goldenrivett; 14th Jun 2019 at 12:28. Reason: typo
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Old 14th Jun 2019, 18:11
  #57 (permalink)  
 
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But hey, don't listen to me. I only have 20 years experience and around 14000 hrs on the A320
That's very apt and humble advice. Because it's so true. A few years ago a Quantas 747 Capt with 20000hrs with copilot of 15000hrs and another occupant with 5000hrs stalled the aircraft in a hold four times because they didn't know the correct holding speed nor correct recovery procedure and another one with 20000hrs with 16000 on Airbus shut the wrong engine without recognising the correct one and yet another with 21000hrs kept dialling in NAV mode till heading went past 180 and then pulled heading to turn shorter way into a hill killing 150 people. So no one should listen to anyone except Airbus. Incidentally to fly 1/2 wind plus gust doesn't require a genius nor tons of experience. In every other non airbus aircraft even rookie Capt or copilot also flies that way because that's the only way. There's a better way provided to fly it take it or leave it. If horse riding gives you pleasure do it but those who ride Mercedes are not stupid.

Last edited by vilas; 16th Jun 2019 at 05:00.
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Old 14th Jun 2019, 19:04
  #58 (permalink)  
 
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Originally Posted by Vilas
Were you drunk or high on something when you wrote that reply?
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Old 15th Jun 2019, 06:53
  #59 (permalink)  
 
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Originally Posted by sabenaboy View Post
Were you drunk or high on something when you wrote that reply?
Not been drunk for decades now. Nothing of the sort. When you said the following
But hey, don't listen to me. I only have 20 years experience and around 14000 hrs on the A320.
I took it as sincere advise. Every incident I quoted is true and I stated them in support of your quote. But it appears that you were biting your tongue while putting it in the cheek. Sorry!
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Old 15th Jun 2019, 07:57
  #60 (permalink)  
 
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Originally Posted by Goldenrivett View Post



We used a type of ground speed mini on B707 in mid 1970s.

If we selected Wind + Hold buttons on the INS, it would display Head (N for on nose) or Tail (S for stern) and the wind component in Kts. Large wind shifts were displayed by flashing 8s in the window. The technique employed was not to reduce power when we encountered a headwind gust, but to allow the IAS to increase, knowing we would loose the speed when the gust died.

It was nowhere near as easy as allowing FBW to adjust and simply reading the revised VAPP (with GS mini display).
Great piece of aviation history, thanks for sharing !
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