GS-Mini, Auto-thrust, and Short Runways – Airbus A320/330/340
Thread Starter
GS-Mini, Auto-thrust, and Short Runways – Airbus A320/330/340
Quote from PJ2 ["EK A330 Heavy Landing at BHX" Mar01/17:29]:
Until my company "made it known" that if one disconnected the autothrust, one was "on one's own" if anything happened, I flew every approach on the 320 series and 340/330 series aircraft with manual thrust. It simply kept one "closer" to the airplane in terms of situational awareness and it is what I taught during line indoctrination training.
[Unquote]
With little information available on the circumstances of the above incident, I've so far resisted the temptation to contribute to the discussion on that thread. But the comments re GS-Mini – some better informed than others – have taken me back six years to recall my intimate relationship with the A320 autothrottle (a "curate's egg", if ever I had one) and "managed" speed, including GS-Mini.
When I did my A320 conversion in 1988, just before the type was certificated, it was put to me and my co-pilot that, with the exception of the FBW flight controls, there were two ways to handle the cockpit flight systems:
1) ALL MANUAL;
2) ALL AUTOMATIC.
Since then (I think Northwest was the first airline to mandate it), there has been a steady movement towards recommending or mandating the use of autothrottle (A/Thr) during "manual" flight, even on the approach below 1000ft aal. Personally, during my 14 years on type, I rarely used it for manual approaches; once fully visual and configured for landing. The exceptions were approaches where visibility was tricky, or when visual contact was not made until late on. I did many windy arrivals into the fairly short runway at Jersey; almost invariably with manual thrust.
Manual thrust worked even better than on my previous types, because (a) the levers are a joy to use (once manual thrust has been established) and (b) GS-Mini avoids the necessity for large changes of thrust during wind-shear. The tendency of the A/Thr, on the other hand, to be slow to reduce IAS – as the "managed speed" falls in accordance with the headwind component – was yet another reason for my policy.
Managed speed should be treated as a TARGET speed. Unfortunately, many pilots seem to regard it as a MINIMUM speed, chasing it like mad if using manual throttle. And the A/Thr often seems to have the same idea, particularly when the target is trending downwards. If so, this is quite wrong. Even VAPP is not a minimum speed in a gust. The only minimum speed is VLS. The latter has a degree of fat built into it, certainly on European aeroplanes. [Yes, I also remember the "Seven-oh".]
This tendency of the autothrottle to bring the aeroplane to the threshold at a speed above VAPP on a head-windy day is unsatisfactory, and potentially hazardous on a short runway. A318 operations will soon be starting, if they haven't already, at London-City. Without trying to anticipate any special SOPs that may be in hand, I very much hope that the operators, including the pilot-managers of my former employers, will be aware of what I am talking about. Remember, zero headwind on the ground can be preceded by headwinds low on the approach, particularly at night.
So the way the A/Thr interprets "managed speed" and GS-Mini can lead to an excessive threshold speed with little headwind to help you stop on a short runway.
There have been some recent modifications to the GS-Mini logic, apparently, but I am going to reserve specific comment on GS-Mini until more information comes to hand on the modifications. In the meantime, it is worth emphasising that the overall package of the A320 thrust and speed control was probably the best of the seven jet types I flew – despite the non-driven throttle-levers, rather than because of them.
Chris
Until my company "made it known" that if one disconnected the autothrust, one was "on one's own" if anything happened, I flew every approach on the 320 series and 340/330 series aircraft with manual thrust. It simply kept one "closer" to the airplane in terms of situational awareness and it is what I taught during line indoctrination training.
[Unquote]
With little information available on the circumstances of the above incident, I've so far resisted the temptation to contribute to the discussion on that thread. But the comments re GS-Mini – some better informed than others – have taken me back six years to recall my intimate relationship with the A320 autothrottle (a "curate's egg", if ever I had one) and "managed" speed, including GS-Mini.
When I did my A320 conversion in 1988, just before the type was certificated, it was put to me and my co-pilot that, with the exception of the FBW flight controls, there were two ways to handle the cockpit flight systems:
1) ALL MANUAL;
2) ALL AUTOMATIC.
Since then (I think Northwest was the first airline to mandate it), there has been a steady movement towards recommending or mandating the use of autothrottle (A/Thr) during "manual" flight, even on the approach below 1000ft aal. Personally, during my 14 years on type, I rarely used it for manual approaches; once fully visual and configured for landing. The exceptions were approaches where visibility was tricky, or when visual contact was not made until late on. I did many windy arrivals into the fairly short runway at Jersey; almost invariably with manual thrust.
Manual thrust worked even better than on my previous types, because (a) the levers are a joy to use (once manual thrust has been established) and (b) GS-Mini avoids the necessity for large changes of thrust during wind-shear. The tendency of the A/Thr, on the other hand, to be slow to reduce IAS – as the "managed speed" falls in accordance with the headwind component – was yet another reason for my policy.
Managed speed should be treated as a TARGET speed. Unfortunately, many pilots seem to regard it as a MINIMUM speed, chasing it like mad if using manual throttle. And the A/Thr often seems to have the same idea, particularly when the target is trending downwards. If so, this is quite wrong. Even VAPP is not a minimum speed in a gust. The only minimum speed is VLS. The latter has a degree of fat built into it, certainly on European aeroplanes. [Yes, I also remember the "Seven-oh".]
This tendency of the autothrottle to bring the aeroplane to the threshold at a speed above VAPP on a head-windy day is unsatisfactory, and potentially hazardous on a short runway. A318 operations will soon be starting, if they haven't already, at London-City. Without trying to anticipate any special SOPs that may be in hand, I very much hope that the operators, including the pilot-managers of my former employers, will be aware of what I am talking about. Remember, zero headwind on the ground can be preceded by headwinds low on the approach, particularly at night.
So the way the A/Thr interprets "managed speed" and GS-Mini can lead to an excessive threshold speed with little headwind to help you stop on a short runway.
There have been some recent modifications to the GS-Mini logic, apparently, but I am going to reserve specific comment on GS-Mini until more information comes to hand on the modifications. In the meantime, it is worth emphasising that the overall package of the A320 thrust and speed control was probably the best of the seven jet types I flew – despite the non-driven throttle-levers, rather than because of them.
Chris
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glad with ATHR... but detest 'RETARD'
Chris, My airline has a policy to keep ATHR on (unless it is u/s).. and so far I found no problem with it. In my opinion it does a pretty good job, considering the G/S mini which limits the change in thrust... Yes, may be I have not scruitinized it well enough like you did...
in my knowledge, managed speed is the target IAS (=GS mini + current headwind component). it is not the minimum speed for sure. when I fly with manual ATHR (in simulator only) MAGENTA IS MY TARGET.. occasionally I may trend below it .. but thats o.k. as long I remain above the amber band.
My previous airliner did not have this feature and my experience with 'ATHR off' does not makes 'ATHR ON' any different...
One thing I hate most about airbus is the RETARD call... at times, I found it most irritating.. During my early days on airbus, I screwed up few landings, because I retarded the thrust levers listening to the RETARD callout only, rather based on the requirement...
cheers
in my knowledge, managed speed is the target IAS (=GS mini + current headwind component). it is not the minimum speed for sure. when I fly with manual ATHR (in simulator only) MAGENTA IS MY TARGET.. occasionally I may trend below it .. but thats o.k. as long I remain above the amber band.My previous airliner did not have this feature and my experience with 'ATHR off' does not makes 'ATHR ON' any different...
One thing I hate most about airbus is the RETARD call... at times, I found it most irritating.. During my early days on airbus, I screwed up few landings, because I retarded the thrust levers listening to the RETARD callout only, rather based on the requirement...
cheers
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Autothrust
Steering manually for me also means using thrust manually.
In gusty conditions I prefer a selected speed and use judgement as to how accurately I have to maintain it, rather than a magenta target speed that is way above Vls, and of which I never now when it is going to come down to sensible values.
Another thing I don't like about managed speed on approach, is the fact that the magenta bug is not always your target anyway - the green symbols (dot, S and F) will be your target if the magenta bug is below them, but then again not if you fly a Flap 3 approach, with Flap 3 selected: the F is then not removed as in a full flap approach, but still the maganta bug below the F will be the target.
Your remark about how well the thrust levers handle - agree.
Overall thrust response from the engines (IAE) - coming off the idle stop they are awfully slow to accelerate, but once at a reasonable RPM, their response is OK.
In gusty conditions I prefer a selected speed and use judgement as to how accurately I have to maintain it, rather than a magenta target speed that is way above Vls, and of which I never now when it is going to come down to sensible values.
Another thing I don't like about managed speed on approach, is the fact that the magenta bug is not always your target anyway - the green symbols (dot, S and F) will be your target if the magenta bug is below them, but then again not if you fly a Flap 3 approach, with Flap 3 selected: the F is then not removed as in a full flap approach, but still the maganta bug below the F will be the target.
Your remark about how well the thrust levers handle - agree.
Overall thrust response from the engines (IAE) - coming off the idle stop they are awfully slow to accelerate, but once at a reasonable RPM, their response is OK.
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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!
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!
Last edited by Norman Stanley Fletcher; 8th Mar 2008 at 23:52. Reason: edited for typo!
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Norman,
Interesting post...although I stand to be corrected, I believe that some A320's in the basic configuration calculate VAPP as :VLS + 5 + 1/3 TWR HWC, as opposed to VLS+Max (5, 1/3 TWR HWC).
Also, I believe at 700 ft RA the current speed target is memorised by the autothrust to ensure stability. Something to keep in mind.
I was trying to figure out where you got 9kts from your THWC in the example you gave, then realised it should of been labelled 1/3 THWC (just woken up, need coffee).
Cheers.
Interesting post...although I stand to be corrected, I believe that some A320's in the basic configuration calculate VAPP as :VLS + 5 + 1/3 TWR HWC, as opposed to VLS+Max (5, 1/3 TWR HWC).
Also, I believe at 700 ft RA the current speed target is memorised by the autothrust to ensure stability. Something to keep in mind.
I was trying to figure out where you got 9kts from your THWC in the example you gave, then realised it should of been labelled 1/3 THWC (just woken up, need coffee).
Cheers.
Last edited by JSF1; 8th Mar 2008 at 09:33.
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Hi JSF1
You are right about the 1/3THWC - a typo which is now corrected.
Regarding the 700' RA, what you are saying is not strictly correct. There is a facility known as 'data lock' which freezes the values in the PERF App page. You can still write stuff in but it will not affect the system after 700' RA. The gs mini function is not, however, affected by this and the gust will be added as I described earlier right the way down to landing. It is just that the datum wind (tower wind) cannot be altered after this point - the variable 'gust' is still calculated and added to VAPP and that is reflected in the variable approach speed below this height.
I cannot comment about older Airbuses as the ones I fly are all less than 5 years old. I have in the past flown older ones and they all used the same formula. FCOM 4.3.25 says 'The FMGC computes this approach speed, using the formula: VAPP = VLS + 1/3 of the headwind components (limited to VLS + 5 as a minimum and VLS + 15 as a maximum).' I am therefore sticking to my guns regarding the mechanisation of the function. I hope that helps - thanks for your input.
Cheers
NSF
You are right about the 1/3THWC - a typo which is now corrected.
Regarding the 700' RA, what you are saying is not strictly correct. There is a facility known as 'data lock' which freezes the values in the PERF App page. You can still write stuff in but it will not affect the system after 700' RA. The gs mini function is not, however, affected by this and the gust will be added as I described earlier right the way down to landing. It is just that the datum wind (tower wind) cannot be altered after this point - the variable 'gust' is still calculated and added to VAPP and that is reflected in the variable approach speed below this height.
I cannot comment about older Airbuses as the ones I fly are all less than 5 years old. I have in the past flown older ones and they all used the same formula. FCOM 4.3.25 says 'The FMGC computes this approach speed, using the formula: VAPP = VLS + 1/3 of the headwind components (limited to VLS + 5 as a minimum and VLS + 15 as a maximum).' I am therefore sticking to my guns regarding the mechanisation of the function. I hope that helps - thanks for your input.
Cheers
NSF
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Norman,
Your post makes GS mini function very clear. One more problem that I have seen and is more practical problem is when you are following a non-Airbus aircraft on approach. Although the same wind is experienced by both aircraft ( say CRJ/ERJ/Boeing and us sitting in the Bus) but preceding aircraft is not flying GS mini. The end result is the separation (between the two aircrafts) reduces and it can lead to a situation where you may have to go around or you'd have to fly selected speed that defeats the purpose of GS mini.
So in a situation like this what would you do?
Your post makes GS mini function very clear. One more problem that I have seen and is more practical problem is when you are following a non-Airbus aircraft on approach. Although the same wind is experienced by both aircraft ( say CRJ/ERJ/Boeing and us sitting in the Bus) but preceding aircraft is not flying GS mini. The end result is the separation (between the two aircrafts) reduces and it can lead to a situation where you may have to go around or you'd have to fly selected speed that defeats the purpose of GS mini.
So in a situation like this what would you do?
Norman,
Thank you for what is the best explanation of GS mini I have seen. I have cut and pasted it into my own notes.
My company encourages manual flying and it's written into our ops manual that we should practice it - including manual thrust.
I haven't found the non moving levers a problem at all. I'm one of the few people who likes the system and thinks it's good! When I converted to the aircraft, I was sceptical at first but them programed myself to think of them as a 'thrust rating' switch, much as the THR button worked on the Boeing's MCP but more intuitive as the 'switch' worked like real 'throttles'. I told myself that they needn't actually be there at all. I was pleasantly suprised to find that they do work in 'manual' - and rather well. The only problem I have encountered is that the 321 seems to be programmed with the 320 software and it's response to speed changes is a bit slow as I get the impression it hasn't accounted for the icreased mass. I tend to like to use man thrust in turbulent conditions as I find that I can be pro-active whereas the sytem is reactive - and slow!
PS. I haven't found anyone who can explain that Boeing THR button!
Thank you for what is the best explanation of GS mini I have seen. I have cut and pasted it into my own notes.
My company encourages manual flying and it's written into our ops manual that we should practice it - including manual thrust.
I haven't found the non moving levers a problem at all. I'm one of the few people who likes the system and thinks it's good! When I converted to the aircraft, I was sceptical at first but them programed myself to think of them as a 'thrust rating' switch, much as the THR button worked on the Boeing's MCP but more intuitive as the 'switch' worked like real 'throttles'. I told myself that they needn't actually be there at all. I was pleasantly suprised to find that they do work in 'manual' - and rather well. The only problem I have encountered is that the 321 seems to be programmed with the 320 software and it's response to speed changes is a bit slow as I get the impression it hasn't accounted for the icreased mass. I tend to like to use man thrust in turbulent conditions as I find that I can be pro-active whereas the sytem is reactive - and slow!
PS. I haven't found anyone who can explain that Boeing THR button!
Last edited by Dan Winterland; 10th Mar 2008 at 03:02.
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Nice explanation of the GS-mini, but, non beeing a Bus pilot, I would like to know why was it necessary to install that on the bus, and why other manufacturers didn't use it?
What would be the difference between A and B ,on the same approach,besides the higher target speed (due to gust )?
I know that Boeing has a feature in the AT which allows the speed to increase ,and not correct it very fast ,and has faster reaction in case of speed decrease. Is this a different way to handle gusts?
From your explanation I tend to believe that a classic plane may not be safe to fly during gusty wind, which is not the case,I think.
What would be the difference between A and B ,on the same approach,besides the higher target speed (due to gust )?
I know that Boeing has a feature in the AT which allows the speed to increase ,and not correct it very fast ,and has faster reaction in case of speed decrease. Is this a different way to handle gusts?
From your explanation I tend to believe that a classic plane may not be safe to fly during gusty wind, which is not the case,I think.
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Thanks Dan - glad to be of assistance.
Desi - you make a good point. My own experience is that the situation you describe does not happen in practice. I have had to think about this one and the conclusion I have come to is that the main problem occurs when inside 4 miles to run to the airport. At that point you can fly your own speed anyway as all previous ATC constraints are always removed. Let us say your approach speed is now 150kt instead of 125kts for the last 4nm. The chances are that in these gusty conditions the other aircraft will have put on a speed increment as well so approach speeds may be nearer. In the time it takes for the first aircraft to land (115 secs) the first aircraft will travel 4.8nm. Therefore there is less than a mile to be concerned about. The min recommended spacing is 3nm for medium to medium so you need to start at 4 miles apart to be sure. Ultimately there is nothing wrong with going around if you get too close. My basic thoughts are that this is an airmanship issue - the same problem can arise due to ATC spacing and so forth. You have to use best judgement on the day, but as I say, experience suggests this is rarely a problem. Hope that helps.
Alexban - I am not a Boeing pilot so cannot speak for the logic of their design. I can, however, say that the logic of the Airbus design is to provide the lowest safe approach speed. All it is doing is removing the thinking that the conventional pilot has. The way the conventional pilot overcomes the big gust problem is to add a 'sensible' increment onto the approach speed, which is taken to be up to 15ts generally. That increment is added automatically by the Airbus and in theory means it is one less thing to think about. As I have tried to explain, it is important to understand how the Airbus calculates its approach speed or you start to think it is some wierd and bizarre facility which takes the pilot out the loop. It is completely logical and is only doing what most other pilots do anyway. Regarding your statement that other aircraft are not unsafe so it cannot be strictly correct what I am saying, I would disagree slightly. If on a conventional aircraft the pilot does not add a safe increment to his approach speed, he is indeed in danger of stalling in extreme conditions. Any commercial pilot worth his salt would always make some sort of adjustment so Boeings are not dropping out the sky around the world! Nonetheless, like all aircraft they are capable of gross mishandling with potentially catastrophic results. The key thing is that the adding of increments is done automatically on the Airbus but on other aircraft it is left to the judgement of the individual pilot to decide on the appropriate approach speed. Both types are still aircraft and still require air over the wings to keep airborne - it is just the method of achieving that which varies. On balance, I prefer the Airbus way, but it has taken some time to get used to its strange ways!
Desi - you make a good point. My own experience is that the situation you describe does not happen in practice. I have had to think about this one and the conclusion I have come to is that the main problem occurs when inside 4 miles to run to the airport. At that point you can fly your own speed anyway as all previous ATC constraints are always removed. Let us say your approach speed is now 150kt instead of 125kts for the last 4nm. The chances are that in these gusty conditions the other aircraft will have put on a speed increment as well so approach speeds may be nearer. In the time it takes for the first aircraft to land (115 secs) the first aircraft will travel 4.8nm. Therefore there is less than a mile to be concerned about. The min recommended spacing is 3nm for medium to medium so you need to start at 4 miles apart to be sure. Ultimately there is nothing wrong with going around if you get too close. My basic thoughts are that this is an airmanship issue - the same problem can arise due to ATC spacing and so forth. You have to use best judgement on the day, but as I say, experience suggests this is rarely a problem. Hope that helps.
Alexban - I am not a Boeing pilot so cannot speak for the logic of their design. I can, however, say that the logic of the Airbus design is to provide the lowest safe approach speed. All it is doing is removing the thinking that the conventional pilot has. The way the conventional pilot overcomes the big gust problem is to add a 'sensible' increment onto the approach speed, which is taken to be up to 15ts generally. That increment is added automatically by the Airbus and in theory means it is one less thing to think about. As I have tried to explain, it is important to understand how the Airbus calculates its approach speed or you start to think it is some wierd and bizarre facility which takes the pilot out the loop. It is completely logical and is only doing what most other pilots do anyway. Regarding your statement that other aircraft are not unsafe so it cannot be strictly correct what I am saying, I would disagree slightly. If on a conventional aircraft the pilot does not add a safe increment to his approach speed, he is indeed in danger of stalling in extreme conditions. Any commercial pilot worth his salt would always make some sort of adjustment so Boeings are not dropping out the sky around the world! Nonetheless, like all aircraft they are capable of gross mishandling with potentially catastrophic results. The key thing is that the adding of increments is done automatically on the Airbus but on other aircraft it is left to the judgement of the individual pilot to decide on the appropriate approach speed. Both types are still aircraft and still require air over the wings to keep airborne - it is just the method of achieving that which varies. On balance, I prefer the Airbus way, but it has taken some time to get used to its strange ways!
Last edited by Norman Stanley Fletcher; 10th Mar 2008 at 11:21.
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hey!
I have another question...also 'approach target speed' related and wondering if someone could help me out.
If you're flying in strong headwind conditions it is very likely you will have a rather high target speed (the magenta bug), higher than F speed in config 2.... Flying with autothrust on...the target speed is than the magenta bug instead of F speed...correct?
But flying manual thrust...is it necessary to keep the magenta bug...or can you decelerate towards F speed...
Who can help me out?
Thanks a lot!
I have another question...also 'approach target speed' related and wondering if someone could help me out.
If you're flying in strong headwind conditions it is very likely you will have a rather high target speed (the magenta bug), higher than F speed in config 2.... Flying with autothrust on...the target speed is than the magenta bug instead of F speed...correct?
But flying manual thrust...is it necessary to keep the magenta bug...or can you decelerate towards F speed...
Who can help me out?
Thanks a lot!
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My simplified view:
Crossing the threshold you want to be at a certain AIRSPEED. This equates to a certain GROUND speed.
GS Mini makes sure the aircraft maintains at least this speed across the ground during the approach.
Example.
Airspeed = 130 kts. Headwind 5 kts. Ground speed = 125 kts.
Say at 500 ft. Headwind = 40 kts. Airspeed would 165kts but Groundspeed still = 125 kts.
Therefore when the headwind suddenly drops off, the aircraft already has the correct groundspeed which combined with the tower headwind is the correct amount of energy.
FIS.
Crossing the threshold you want to be at a certain AIRSPEED. This equates to a certain GROUND speed.
GS Mini makes sure the aircraft maintains at least this speed across the ground during the approach.
Example.
Airspeed = 130 kts. Headwind 5 kts. Ground speed = 125 kts.
Say at 500 ft. Headwind = 40 kts. Airspeed would 165kts but Groundspeed still = 125 kts.
Therefore when the headwind suddenly drops off, the aircraft already has the correct groundspeed which combined with the tower headwind is the correct amount of energy.
FIS.
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Alex
My interpretation, as an ex Airbus pilot and current Boeing pilot:
GS mini is an excellent feature in providing protection to wind variations on the approach. Most relevantly, imho, when you have microburst activity.
Consider the scenario where you have a tower wind component of 10kts Headwind:
Instantaneously on the approach you encounter 50kts HWC - GS mini will add up to 40 kts IAS (50-10) giving a constant GS approach (=constant power). It will also help protect you when (in a microburst) the 50 kt HW becomes a 50 kt TW.
The system is not perfect, but it sure helps in microbursts.
My interpretation, as an ex Airbus pilot and current Boeing pilot:
GS mini is an excellent feature in providing protection to wind variations on the approach. Most relevantly, imho, when you have microburst activity.
Consider the scenario where you have a tower wind component of 10kts Headwind:
Instantaneously on the approach you encounter 50kts HWC - GS mini will add up to 40 kts IAS (50-10) giving a constant GS approach (=constant power). It will also help protect you when (in a microburst) the 50 kt HW becomes a 50 kt TW.
The system is not perfect, but it sure helps in microbursts.
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First things first, congrats NSF to your promotion...
As ever your explanation regarding G/S mini makes it for us simple souls easier to understand.
However, DESIpilot has a valid point, although not strictly applicable to the 320 series, for the A340-600 it is a common problem.
In my company there was a much higher than average number of go-arounds due to loss of separation between the preceding A/C and the 600 on the approach due to the G/S mini function especially in stronger wind condition this has lead the ATC having to increase the separation on approach.
I suppose a way round this prolem would be to go in selected speed until 4 miles then hitting managed speed however this could cause an un-stable approach if not monitored.
I think that G/S mini is a great function but still has its flaws.
As ever your explanation regarding G/S mini makes it for us simple souls easier to understand.
However, DESIpilot has a valid point, although not strictly applicable to the 320 series, for the A340-600 it is a common problem.
In my company there was a much higher than average number of go-arounds due to loss of separation between the preceding A/C and the 600 on the approach due to the G/S mini function especially in stronger wind condition this has lead the ATC having to increase the separation on approach.
I suppose a way round this prolem would be to go in selected speed until 4 miles then hitting managed speed however this could cause an un-stable approach if not monitored.
I think that G/S mini is a great function but still has its flaws.
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Hi Sprinbok 449
Hope the big bus and world travel is treating you well! I tend to agree with you that GS mini can catch you out - as you say say, selecting 160kts until 4nm is a good way of dealing with the problem. I see all these features as essentially just tools in the box to be used as required. Like all good tools they have moments of greatness and other moments when they are not quite the right fit for the job and improvisation is required! The secret is making the tools no longer a secret! Once you understand them and know their strengths and weaknesses you can use them appropriately to make the best job - such is the case with GS mini.
Hope the big bus and world travel is treating you well! I tend to agree with you that GS mini can catch you out - as you say say, selecting 160kts until 4nm is a good way of dealing with the problem. I see all these features as essentially just tools in the box to be used as required. Like all good tools they have moments of greatness and other moments when they are not quite the right fit for the job and improvisation is required! The secret is making the tools no longer a secret! Once you understand them and know their strengths and weaknesses you can use them appropriately to make the best job - such is the case with GS mini.
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springbok440
Quote :
"In my company there was a much higher than average number of go-arounds due to loss of separation between the preceding A/C and the 600 on the approach due to the G/S mini function especially in stronger wind condition this has lead the ATC having to increase the separation on approach."
It happened everywhere.
A simple solution is to tell ATC that you're flying a 'Bus and in these confitions, your approach speed would be ...such.
Quote :
"In my company there was a much higher than average number of go-arounds due to loss of separation between the preceding A/C and the 600 on the approach due to the G/S mini function especially in stronger wind condition this has lead the ATC having to increase the separation on approach."
It happened everywhere.
A simple solution is to tell ATC that you're flying a 'Bus and in these confitions, your approach speed would be ...such.
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Lemurian,
Thanks for your advice, we always advise ATC of our A/C type on checking in with the approach controller, indeed in LHR for an example it is a requirement and still it didnt stop all the go-arounds.
The problem is you cant always advise the controller what speed you will be doing on the approach as it depends on the winds and not just the surface wind given by ATIS or ATC but the winds all the way down on the approach and these can vary from 143Kts to 200Kts depending on the config.
As said before G/S mini can be a great tool but sometimes needs tweaking...
Thanks for your advice, we always advise ATC of our A/C type on checking in with the approach controller, indeed in LHR for an example it is a requirement and still it didnt stop all the go-arounds.
The problem is you cant always advise the controller what speed you will be doing on the approach as it depends on the winds and not just the surface wind given by ATIS or ATC but the winds all the way down on the approach and these can vary from 143Kts to 200Kts depending on the config.
As said before G/S mini can be a great tool but sometimes needs tweaking...
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ATIS wind and "cleared to land wind" versus SOP
Why does Airbus not recommend to update the tower wind after getting landing clearance?
Having inserted a too big headwind component from an old ATIS and not updating it means accepting a lower than optimum energy level over threshold...
Having inserted a too big headwind component from an old ATIS and not updating it means accepting a lower than optimum energy level over threshold...
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Norman, great explanation, I also have done some cutting and pasting to show others, funny you should mention the 180KT scenario because when I was originally trained on the aircraft, all literature on the subject indicated 177KTS MAX which is BS, have seen 180KTS on more than one occasion.
I'm one of the few people who likes the system and thinks it's good!
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GS Mini question
Hi guys,
My airline uses the stabilized approach technique for all A-320 operations and has recentely defined that crews should insert on the F-PLAN page, the VAPP on the final fix of the procedure. My question is: does this inhibit the GS Mini function of the A/THR? I've searched the FCOM and the FCTM, and PPRUNE, but without results...anyone care to shed a light on this?
Thanx!
My airline uses the stabilized approach technique for all A-320 operations and has recentely defined that crews should insert on the F-PLAN page, the VAPP on the final fix of the procedure. My question is: does this inhibit the GS Mini function of the A/THR? I've searched the FCOM and the FCTM, and PPRUNE, but without results...anyone care to shed a light on this?
Thanx!