AOA and the A320 Questions Questions...
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AOA and the A320 Questions Questions...
Hi All:
I'm a pilot in training and new to the A320, and I am having difficulty getting an answer I can live with regarding "alpha prot" vs. "alpha floor". I'm hoping there's an A320 technophile/aerodynamics-guru out there who can help.
Here's the issue: The book says that "alpha prot" (i.e. 45 bank limit, speed brakes retract, sidestick commands AOA, pitch up trim inhibited; did I miss anything?) occurs at a "pre-determined" AOA; I assume calculated by the FACs. Now the book also says that "alpha floor" (i.e. TOGA A/THR, speedbrakes retract, etc.) occurs at a "pre-determined" AOA between "alpha prot" and "alpha max". They even give us a pretty Cl chart showing the points on the Cl vs. AOA chart; and clearly "alpha floor" is a higher AOA than "alpha prot". And these are all AOA right? Nothing to do with airspeed or energy.
However, I am told that in reality, "alpha floor" has nothing to do with "alpha prot". As it was explained to me, "alpha prot" will only be encountered when the aircraft slows to the speed as indicated on the PFD speed tape. (top of the barber pole), and then if I continue to slow, I will encounter "alpha floor". But when I ask what will happen if I yank full back stick at say, 240 knots, they say I could encounter "alpha floor" but not "alpha prot". HOW IS THIS POSSIBLE IF THESE PROTECTIONS ARE PURELY AOA?? Weren't we all told you could stall an aircraft at any speed? So it would seem I could induce a high AOA at ANY speed, and thereby encounter not only "alpha floor" but "alpha prot" first...Will the G-load protection keep me from inducing an AOA high enough to trigger "alpha prot" until the airpeed bleeds off?
Thanks in advance for your help! I'm very glad to be training on the "Bus"; it's a wonderful aircraft.
d
I'm a pilot in training and new to the A320, and I am having difficulty getting an answer I can live with regarding "alpha prot" vs. "alpha floor". I'm hoping there's an A320 technophile/aerodynamics-guru out there who can help.
Here's the issue: The book says that "alpha prot" (i.e. 45 bank limit, speed brakes retract, sidestick commands AOA, pitch up trim inhibited; did I miss anything?) occurs at a "pre-determined" AOA; I assume calculated by the FACs. Now the book also says that "alpha floor" (i.e. TOGA A/THR, speedbrakes retract, etc.) occurs at a "pre-determined" AOA between "alpha prot" and "alpha max". They even give us a pretty Cl chart showing the points on the Cl vs. AOA chart; and clearly "alpha floor" is a higher AOA than "alpha prot". And these are all AOA right? Nothing to do with airspeed or energy.
However, I am told that in reality, "alpha floor" has nothing to do with "alpha prot". As it was explained to me, "alpha prot" will only be encountered when the aircraft slows to the speed as indicated on the PFD speed tape. (top of the barber pole), and then if I continue to slow, I will encounter "alpha floor". But when I ask what will happen if I yank full back stick at say, 240 knots, they say I could encounter "alpha floor" but not "alpha prot". HOW IS THIS POSSIBLE IF THESE PROTECTIONS ARE PURELY AOA?? Weren't we all told you could stall an aircraft at any speed? So it would seem I could induce a high AOA at ANY speed, and thereby encounter not only "alpha floor" but "alpha prot" first...Will the G-load protection keep me from inducing an AOA high enough to trigger "alpha prot" until the airpeed bleeds off?
Thanks in advance for your help! I'm very glad to be training on the "Bus"; it's a wonderful aircraft.
d
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From what I was told, the aircraft also looks at rate of deceleration in determining alpha floor. In your highspeed example you certainly could achieve a very high pitch angle which in turn, if you keep the nose up would result in a high AOA as ac decels, aircraft will go directly to Alpha floor, bypassing alpha prot. If you do this more gradual, you should get alpha prot first.
I guess you could aliken it to highspeed protection. If you slowly lower the nose you can get Vmo+16, but if you shove it forward you can attain Vmo+30, however the ac will decel to Vmo+16.
Just a guess, anyway, welcome to the "busdriver" club. Great airplane.
I guess you could aliken it to highspeed protection. If you slowly lower the nose you can get Vmo+16, but if you shove it forward you can attain Vmo+30, however the ac will decel to Vmo+16.
Just a guess, anyway, welcome to the "busdriver" club. Great airplane.
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Try a session in the sim - low level through the Swiss Alps at max chat. You'll be rolling and pulling with a high airspeed but A.FLOOR will keep activating due to the dynamic predictions of the FACs, even though your speed goes nowhere near the orange and black barber pole.
Also f*cking good fun
Think of A.FLOOR as an instructor/Captain sitting next to you who notices you perform a manuever which will lead to a stall - he leans over and applies thrust for you.
The sidestick usually commands g-load across the entire speed range, except in the orange/black range below a-prot. In this speed range, which is dependent in config, height, attitude (via the FACs) the sitck actuall commands an AOA.
So if you hold the stick at a certain deflection in this speed range, you will fly a constant (although high) AOA. At some particular AOA, however, depending on your current manuever, thrust setting etc. the aeroplane says "that's enough of that, mate" and gives you A.FLOOR.
So they are linked and they aren't. In a sense. Sort of. Welcome to the 21st century!
PS had loads of flight deck visits on a charter the other day - all Glaswegians, who are not afraid to speak their minds. Their minds all said, at some point or otheronversation, "so you don't really so much, do you?" Grrrr
Also f*cking good fun
Think of A.FLOOR as an instructor/Captain sitting next to you who notices you perform a manuever which will lead to a stall - he leans over and applies thrust for you.
The sidestick usually commands g-load across the entire speed range, except in the orange/black range below a-prot. In this speed range, which is dependent in config, height, attitude (via the FACs) the sitck actuall commands an AOA.
So if you hold the stick at a certain deflection in this speed range, you will fly a constant (although high) AOA. At some particular AOA, however, depending on your current manuever, thrust setting etc. the aeroplane says "that's enough of that, mate" and gives you A.FLOOR.
So they are linked and they aren't. In a sense. Sort of. Welcome to the 21st century!
PS had loads of flight deck visits on a charter the other day - all Glaswegians, who are not afraid to speak their minds. Their minds all said, at some point or otheronversation, "so you don't really so much, do you?" Grrrr
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The answer:
Think of Alpha Floor as some kind of energy protection.
It comes on when a pre-determined AOA between Alpha-Prot and Alpha-Max is reached, you know that, but auto thrust is also triggered with the stick close to maximum aft deflection and with a nose high attitude.
How do I know that?? No, I'm not French!!!
But I got this nice little booklet from a friendly Airbus rep called : 'A319/A320/A321 Electrical Flight Control System (a post-graduate course).....you know what?? Let me just use up a few kilobytes and quote the section:
'Furthermore, Alpha-floor protection has been installed on all Airbus aircraft; it is a form of energy protection. When reaching a pre-determined AOA (depending on Slat/Flap configuration) between Alpha-prot and Alpha-max, autothrust is triggered (after a short time delay for confirmation and calculation) and in order to provide the most appropriate assistance to the pilot, the engines are accelerated to TOGA thrust. However , the pilot can manually reduce the thrust, if the thrust is estimated too high according to the aircraft attitude. This protection is also activated with the stick close to the maximum aft deflection and with either a nose-up attitude, or the AOA protection active......'
Think of Alpha Floor as some kind of energy protection.
It comes on when a pre-determined AOA between Alpha-Prot and Alpha-Max is reached, you know that, but auto thrust is also triggered with the stick close to maximum aft deflection and with a nose high attitude.
How do I know that?? No, I'm not French!!!
But I got this nice little booklet from a friendly Airbus rep called : 'A319/A320/A321 Electrical Flight Control System (a post-graduate course).....you know what?? Let me just use up a few kilobytes and quote the section:
'Furthermore, Alpha-floor protection has been installed on all Airbus aircraft; it is a form of energy protection. When reaching a pre-determined AOA (depending on Slat/Flap configuration) between Alpha-prot and Alpha-max, autothrust is triggered (after a short time delay for confirmation and calculation) and in order to provide the most appropriate assistance to the pilot, the engines are accelerated to TOGA thrust. However , the pilot can manually reduce the thrust, if the thrust is estimated too high according to the aircraft attitude. This protection is also activated with the stick close to the maximum aft deflection and with either a nose-up attitude, or the AOA protection active......'
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Doolaga,
I was able to find that the AOA triggering ALPHA FLOOR is 9.5 degrees in CONFIG 0; 15 degrees in CONFIG 1,2; 14 degrees in CONFIG 3; and 13 degrees in CONFIG FULL from the FCOM 1.27.20 P4. I don’t know if this is an oversimplification of the AOA that triggers ALPHA FLOOR or not. I am sure that ALPHA PROT is not based purely on AOA, but I have not been able to find any specific values regarding the triggering of ALPHA PROT. The following text is from an old Airbus publication that explains the flight control system laws with more detail than the FCOM (the same one quoted by atomic).
The aim of the law is to stabilize as quickly as possible at Alpha-max without any significant overshoots in Alpha. A pure alpha command and control law would arrive rapidly at Alpha-max, but it would then force the aircraft into a phugoid motion with the possibility of significant overshoots of Alpha-max. Therefore, a speed feedback is introduced to provide phugoid damping and an improved trajectory. This speed feedback associated with a well chosen gain permits Alpha-max to be safely reached in a minimum time.
Alpha protection can also be entered at high speed when it gives protection at the buffet boundary. Here, the aim is to allow the pilot to use all the available maneuvering capability while staying outside the deterrent buffet and countering any tendencies to pitch up. The high-speed stall occurs at much lower Alpha than the Alpha-max adopted as the low-speed limit, and Alpha-prot and Alpha-max are shifted down as a function of Mach when in the clean configuration. Thus, full back stick will result in the aircraft stabilizing at the buffet boundary with a small margin to the deterrent buffet. Because the margin is tiny, a small pitch-down pre-command is introduced progressively at a given Alpha between Alpha-prot and Alpha-max (function of Mach) to prevent dynamic overshoots into the deterrent buffet with rapid entries into Alpha protection at high Mach.
The engineers at Airbus have spent a considerable amount of time and effort designing a control system that enables the pilot to extract the maximum performance with regard to aerodynamic and structural limitations in the minimum amount of time. The control laws seem to be very well thought out. I know that abrupt high speed and low speed maneuvers typical of the type used to prevent CFIT or a mid-air collision, have been considered. I am sure that any additional details regarding the control laws are well beyond my ability to comprehend, and probably proprietary anyway.
I have always considered Alpha Floor a mode of the Autothrust system, and Alpha Prot a mode of the flight control system. So in my mind, they are two completely separate systems that work together. As far as your question of being able to encounter Alpha Floor but not Alpha Prot, my guess (and it is only a guess) is that you would first encounter Alpha Prot and second Alpha Floor. Depending on the initial conditions, and the rate of stick movement, the limitations due to G load or pitch attitude, may be reached prior to Alpha Prot and Alpha Floor.
Enjoy flying the Airbus, it is really an amazing aircraft. Remember, that even after you complete your training, there is so much to learn about this piece of equipment. Forums such as this are a great help. Sometimes your question about a system will get a response from the engineer who was involved in the design of that system.
Regards,
The Kid
I was able to find that the AOA triggering ALPHA FLOOR is 9.5 degrees in CONFIG 0; 15 degrees in CONFIG 1,2; 14 degrees in CONFIG 3; and 13 degrees in CONFIG FULL from the FCOM 1.27.20 P4. I don’t know if this is an oversimplification of the AOA that triggers ALPHA FLOOR or not. I am sure that ALPHA PROT is not based purely on AOA, but I have not been able to find any specific values regarding the triggering of ALPHA PROT. The following text is from an old Airbus publication that explains the flight control system laws with more detail than the FCOM (the same one quoted by atomic).
The aim of the law is to stabilize as quickly as possible at Alpha-max without any significant overshoots in Alpha. A pure alpha command and control law would arrive rapidly at Alpha-max, but it would then force the aircraft into a phugoid motion with the possibility of significant overshoots of Alpha-max. Therefore, a speed feedback is introduced to provide phugoid damping and an improved trajectory. This speed feedback associated with a well chosen gain permits Alpha-max to be safely reached in a minimum time.
Alpha protection can also be entered at high speed when it gives protection at the buffet boundary. Here, the aim is to allow the pilot to use all the available maneuvering capability while staying outside the deterrent buffet and countering any tendencies to pitch up. The high-speed stall occurs at much lower Alpha than the Alpha-max adopted as the low-speed limit, and Alpha-prot and Alpha-max are shifted down as a function of Mach when in the clean configuration. Thus, full back stick will result in the aircraft stabilizing at the buffet boundary with a small margin to the deterrent buffet. Because the margin is tiny, a small pitch-down pre-command is introduced progressively at a given Alpha between Alpha-prot and Alpha-max (function of Mach) to prevent dynamic overshoots into the deterrent buffet with rapid entries into Alpha protection at high Mach.
The engineers at Airbus have spent a considerable amount of time and effort designing a control system that enables the pilot to extract the maximum performance with regard to aerodynamic and structural limitations in the minimum amount of time. The control laws seem to be very well thought out. I know that abrupt high speed and low speed maneuvers typical of the type used to prevent CFIT or a mid-air collision, have been considered. I am sure that any additional details regarding the control laws are well beyond my ability to comprehend, and probably proprietary anyway.
I have always considered Alpha Floor a mode of the Autothrust system, and Alpha Prot a mode of the flight control system. So in my mind, they are two completely separate systems that work together. As far as your question of being able to encounter Alpha Floor but not Alpha Prot, my guess (and it is only a guess) is that you would first encounter Alpha Prot and second Alpha Floor. Depending on the initial conditions, and the rate of stick movement, the limitations due to G load or pitch attitude, may be reached prior to Alpha Prot and Alpha Floor.
Enjoy flying the Airbus, it is really an amazing aircraft. Remember, that even after you complete your training, there is so much to learn about this piece of equipment. Forums such as this are a great help. Sometimes your question about a system will get a response from the engineer who was involved in the design of that system.
Regards,
The Kid
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Kid, where did you find this stuff? It seems quite interesting to me. I understand it comes from a different source than the FCOM's. Tell me where to find it please, thanks!
PS: to all airbus FBW lovers:
Great minds think alike!
(but would also like to fly the 777 though)
PS: to all airbus FBW lovers:
Great minds think alike!
(but would also like to fly the 777 though)
Guest
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It is the same publication that Atomic named. It may also be known as AI/EV-0 472.0395/93 Issue 2. Get in touch with your Airbus Tech Rep. If you are interested in the technical details of the Airbus, take a look at these sites. They both have provided me with a wealth of information.
http://groups.yahoo.com/group/Airbus_Discuss
http://www.neosoft.com/~sky/BLUECOAT/
Regards,
The Kid
http://groups.yahoo.com/group/Airbus_Discuss
http://www.neosoft.com/~sky/BLUECOAT/
Regards,
The Kid
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Hey Fellow Bus Drivers/Aficionados:
Thank you for your thoughtful and helpful responses to my question. This board is obviously an outstanding resource.
I'm entering the stage of training now that is less academic and more pratical; and the correlations are begining to happen.
I look forward to many years of learning and appreciating the nuances and features of the aircraft.
Thanks again, I'll be back with more questions!!!
doolaga
Thank you for your thoughtful and helpful responses to my question. This board is obviously an outstanding resource.
I'm entering the stage of training now that is less academic and more pratical; and the correlations are begining to happen.
I look forward to many years of learning and appreciating the nuances and features of the aircraft.
Thanks again, I'll be back with more questions!!!
doolaga
