As several instances of unreliable airspeed/primary airspeed failures have been released by the NTSB (read this article by Reuters: U.S. probes altitude, speed data on two Airbus A330s), I was reminded that unreliable airspeed is one of the "memory items" I was taught while getting my A320 type rating.
Memory items are checklists for emergency situations that you are required to memorize, as immediate action is required by the crew (i.e. you might be dead by the time you pull out a checklist). Now there are a lot of situations where pilots know how to act without needing a checklist (engine failures, stalls, etc.) but there are some where the response isn't instinctual, so a memory items checklist is needed. I've heard of some airlines that had more than 20, luckily on the A320 we had 3.
Here's the memory item for "unreliable airspeed indication."
MEMORY ITEMS - If safe conduct of flight is affected:
Note: respect all stall warnings if in ALTERNATE LAW
1. Adjust pitch/thrust:
- Below THR RED ALT -- 15 degrees/TOGA
- Above THR RED ALT and below 10,000' -- 10 degrees/CLB
- Above THR RED ALT and above 10,000' -- 5 degrees/CLB
2. AUTOPILOT................OFF
3. FLIGHT DIRECTOR..........OFF
4. AUTOTHRUST...............OFF
5. Flaps....................Maintain current CONFIG
6. Speedbrakes..............Check retracted
7. Gear.....................UP
When at or above MSA or circuit altitude, level off for troubleshooting.
Basically, these pitch attitudes and power settings are there to keep you from stalling and give you a decent climb rate. It seems Airbus believes most unreliable airspeed problems are expected to be on takeoff, as the dividing line on pitch attitudes and power settings are based on whether you've passed the thrust reduction altitude, which is usually around 1,000' above ground level, where the pilots reduce engine power from TOGA (takeoff/go around - maximum power) to CLB (climb power, which the engines can sustain for long periods, unlike TOGA power).
After this there is a checklist which deals with resetting the ADRs and has tables of the correct pitch attitude and power setting for climb, cruise, descent and landing at a range of different altitudes in case the ADRs do not come back online. From what I've gathered from the news articles, in both instances it seems the crews were able to reset the ADRs successfully.
As with most Airbus emergencies related to data failures, it's more likely than not that the computers can be restarted quickly and the flight can continue normally. This is, as I mentioned before on another post, not an easy airplane to fly when things go wrong. Combine an emergency such as unreliable airspeed with a situation that Airbus may not have considered, such as the suspected severe thunderstorm activity, and it may have been a recipe for disaster that enveloped Air France flight 447.
Thursday, June 25, 2009
Wednesday, June 24, 2009
Those boring announcements are there for a reason...
“All right, you can go ahead and let them up now.”
In pilot-speak, this is the captain telling me, the first officer, that he or she is satisfied that the flight is going to be smooth and the passengers should be free to move about the cabin. When I’m not flying the leg I get to make the announcement.
“*BING* Good morning, folks, from the flight deck. We’re currently at our cruising altitude of 36,000 feet. It looks like our ride should be nice and smooth today so I’m going to go ahead and turn off the ‘fasten seat belt’ sign. You’re free to move about the cabin but please keep your seatbelts on whenever you’re seated, just in case we run into any unexpected turbulence. Also if you could try and keep the aisles clear our cabin crew would really appreciate it. We’re showing an on time arrival, when we get a little closer I’ll be back to let you know what the weather looks like. Thank you for flying with us and have a great flight.”
Of course I don’t think 90% of the passengers actually listen to this announcement. I can’t blame them; before I was an airline pilot I didn’t either. But there’s a reason we make the announcements we do.
The most ignored direction we give our passengers is “please keep your seatbelts on whenever you’re seated.” I understand that they don’t want to wear the seatbelts any more than necessary. After all, if the flight has been smooth so far, and if we hit any turbulence, they’ll have time to get their seatbelts on, right?
I realized a few weeks ago that I didn’t really have a good way of explaining turbulence and what causes it to appear out of, literally, the clear blue sky. So here’s what I hope is a good explanation, concentrating on high-altitude turbulence that jets encounter.
While flying at high altitude you can encounter turbulence both within clouds and in clear air. The former is caused by convective currents.
Inside a cumulus cloud, air is cooling and sinking as well as warming and rising. You’ve seen this if you’ve watched a thunderstorm “build” (as warm, unstable air is lifted) and dissipate (as the warm air cools, it condenses, becomes saturated with water, and sinks again – this is when it’s raining). All these convective currents buffet the airplane as it flies through them. Luckily most jets fly above where most cumulus clouds thunderstorms form, with the exception of very strong thunderstorms. These can have vertical extends well above 45,000 feet, and these we fly around.
This kind of turbulence can be very dangerous, but since flying into thunderstorms is so risky, we use weather radar to go around all but the smallest cumulus clouds. Contrary to popular belief, we don’t TRY to make the ride rough. But sometimes, due to traffic or just the widespread nature of a storm system, we can’t go around it. However, since your flight crew knows that a cumulus cloud will hold at least some turbulence, we would never have the seat belt sign off if we anticipated flying through any.
The other common form of turbulence we encounter is called mountain wave turbulence. This is caused by air currents moving over the tops of mountains. On the leeward side of the mountain the air becomes very turbulent. I haven’t flown across any tall mountain ranges so I can’t offer much insight on mountain wave turbulence besides that.
The most critical form of turbulence with concern to passenger safety is Clear Air Turbulence, or CAT. This is turbulence that occurs without any sort of visual cue or warning, and is the reason we ask passengers to keep their seatbelts fastened whenever they’re seated.
CAT is caused as an aircraft moves between different bodies of air that are moving in different directions at different speeds. This occurs most often around the jet stream or frontal systems. I’ve flown with captains that were good enough at reading weather reports that they could tell by the “winds aloft” report where there would be some turbulence. Winds aloft is a weather report that lists the speed and direction of winds at different altitudes at certain locations – over commonly used navigation facilities and airports. The reporting starts at 3000 feet (with a few exceptions) and continue up to FL390 (39.000 feet).
By interpreting the change in wind speed and direction along the route, you can determine where you’re most likely to encounter CAT. If we use this technique to establish the likelihood of hitting CAT on the flight, we can anticipate the need to keep the fasten seat belt sign lit as well as ask the flight attendants to remain seated until we are sure the danger is past.
A quick search of the NTSB’s Aviation Accident Database revealed over 30 aircraft incidents involving CAT over the last 10 years. In the majority of the accidents the only injured people were the flight attendants, who are especially vulnerable to turbulence since they are not seating during the majority of the flight. Most of these incidents resulted in G loads to the airplane of less than +2Gs and -1Gs, which is about what you feel on a roller coaster. Imagine being on a roller coaster without your seatbelt on!
In several accident reports I’ve read, the flight crew had no reason to expect CAT. At most, weather was observed several miles ahead. In the vast majority of cases, no warning was given to the cabin. In a Northwest Airlines incident in 1972, five flight attendants and nine passengers were injured, two of them seriously, when the Boeing 747 entered “an area of unforecast and unexpected severe clear air turbulence when numerous occupants did not have their seatbelts fastened.” (NTSB report number: NTSB-AAR-72-27)
So please, when the flight crew asks you to keep your seatbelt on whenever you’re in your seat, do it! I guarantee your captain and first officer are doing the same.
(For more details on weather, pick up “Aviation Weather,” another great FAA publication (AC 00-06A).)
In pilot-speak, this is the captain telling me, the first officer, that he or she is satisfied that the flight is going to be smooth and the passengers should be free to move about the cabin. When I’m not flying the leg I get to make the announcement.
“*BING* Good morning, folks, from the flight deck. We’re currently at our cruising altitude of 36,000 feet. It looks like our ride should be nice and smooth today so I’m going to go ahead and turn off the ‘fasten seat belt’ sign. You’re free to move about the cabin but please keep your seatbelts on whenever you’re seated, just in case we run into any unexpected turbulence. Also if you could try and keep the aisles clear our cabin crew would really appreciate it. We’re showing an on time arrival, when we get a little closer I’ll be back to let you know what the weather looks like. Thank you for flying with us and have a great flight.”
Of course I don’t think 90% of the passengers actually listen to this announcement. I can’t blame them; before I was an airline pilot I didn’t either. But there’s a reason we make the announcements we do.
The most ignored direction we give our passengers is “please keep your seatbelts on whenever you’re seated.” I understand that they don’t want to wear the seatbelts any more than necessary. After all, if the flight has been smooth so far, and if we hit any turbulence, they’ll have time to get their seatbelts on, right?
I realized a few weeks ago that I didn’t really have a good way of explaining turbulence and what causes it to appear out of, literally, the clear blue sky. So here’s what I hope is a good explanation, concentrating on high-altitude turbulence that jets encounter.
While flying at high altitude you can encounter turbulence both within clouds and in clear air. The former is caused by convective currents.
Inside a cumulus cloud, air is cooling and sinking as well as warming and rising. You’ve seen this if you’ve watched a thunderstorm “build” (as warm, unstable air is lifted) and dissipate (as the warm air cools, it condenses, becomes saturated with water, and sinks again – this is when it’s raining). All these convective currents buffet the airplane as it flies through them. Luckily most jets fly above where most cumulus clouds thunderstorms form, with the exception of very strong thunderstorms. These can have vertical extends well above 45,000 feet, and these we fly around.
This kind of turbulence can be very dangerous, but since flying into thunderstorms is so risky, we use weather radar to go around all but the smallest cumulus clouds. Contrary to popular belief, we don’t TRY to make the ride rough. But sometimes, due to traffic or just the widespread nature of a storm system, we can’t go around it. However, since your flight crew knows that a cumulus cloud will hold at least some turbulence, we would never have the seat belt sign off if we anticipated flying through any.
The other common form of turbulence we encounter is called mountain wave turbulence. This is caused by air currents moving over the tops of mountains. On the leeward side of the mountain the air becomes very turbulent. I haven’t flown across any tall mountain ranges so I can’t offer much insight on mountain wave turbulence besides that.
The most critical form of turbulence with concern to passenger safety is Clear Air Turbulence, or CAT. This is turbulence that occurs without any sort of visual cue or warning, and is the reason we ask passengers to keep their seatbelts fastened whenever they’re seated.
CAT is caused as an aircraft moves between different bodies of air that are moving in different directions at different speeds. This occurs most often around the jet stream or frontal systems. I’ve flown with captains that were good enough at reading weather reports that they could tell by the “winds aloft” report where there would be some turbulence. Winds aloft is a weather report that lists the speed and direction of winds at different altitudes at certain locations – over commonly used navigation facilities and airports. The reporting starts at 3000 feet (with a few exceptions) and continue up to FL390 (39.000 feet).
By interpreting the change in wind speed and direction along the route, you can determine where you’re most likely to encounter CAT. If we use this technique to establish the likelihood of hitting CAT on the flight, we can anticipate the need to keep the fasten seat belt sign lit as well as ask the flight attendants to remain seated until we are sure the danger is past.
A quick search of the NTSB’s Aviation Accident Database revealed over 30 aircraft incidents involving CAT over the last 10 years. In the majority of the accidents the only injured people were the flight attendants, who are especially vulnerable to turbulence since they are not seating during the majority of the flight. Most of these incidents resulted in G loads to the airplane of less than +2Gs and -1Gs, which is about what you feel on a roller coaster. Imagine being on a roller coaster without your seatbelt on!
In several accident reports I’ve read, the flight crew had no reason to expect CAT. At most, weather was observed several miles ahead. In the vast majority of cases, no warning was given to the cabin. In a Northwest Airlines incident in 1972, five flight attendants and nine passengers were injured, two of them seriously, when the Boeing 747 entered “an area of unforecast and unexpected severe clear air turbulence when numerous occupants did not have their seatbelts fastened.” (NTSB report number: NTSB-AAR-72-27)
So please, when the flight crew asks you to keep your seatbelt on whenever you’re in your seat, do it! I guarantee your captain and first officer are doing the same.
(For more details on weather, pick up “Aviation Weather,” another great FAA publication (AC 00-06A).)
Tuesday, June 9, 2009
Oh Murphy, I hate you
Last Thursday I finally got to take my CFII checkride. For the first time before a checkride I felt calm, I felt like I knew the material backwards and forwards and could handle the flying portion of the checkride. I attribute this to the fact that I had a week's notice and spent plenty of time that week studying. No last-minute rush to finish up the lesson plans or realizing you've missed studying a critical section of knowledge.
I even managed to eat breakfast that morning, another pre-check first for me. Normally I'm so nervous that I can't stomach anything. But this time I had some semblance of calm knowing that this would be my last FAA checkride for a very long time. I probably won't have another checkride that isn't for work, unless I choose to get my multi-engine instructor rating in the future, or go for seaplane ratings.
FAA checkrides involve an oral exam, consisting of knowledge/judgement-based questions, and a flight check. The FAA has what's called "pratical test standards," which give guidance to examiners on what to test candidates on. It's basically a checklist of certain items they need to hit on during your test. Some of this is accomplished in the oral and some of it on the flight, or a combination thereof.
I won't hit much on my checkride itself other than that the oral and flight both went very well, with the exception of what follows. But I did pass :)
Murphy's Law (as defined by UrbanDictionary.com):
1. If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it.
2. The law that says anything that can go wrong, will go wrong.
My flight school has two Cessna 172SPs, SA and PJ. I was scheduled to take SA on my checkride, but the manager of the flight school asked if I could take PJ. SA only had about 5 more hours before it needed a 100-hour inspection (required of all airplane for hire) and she wanted to try and extend that over the weekend. So I preflighted PJ, filed a flight plan and we got ready to go. PJ was very reluctant to start but we attributed this to the engine and the outside air being hot (fuel injected airplanes are very susceptible to vapor lock). We taxied to the runway to do our engine runup...and the left magneto was bad (there are 2 on each airplane, and they help generate the power for the spark plugs, more or less). After attempting to clear the mag several times (this is accomplished by leaning the mixture and running the engine at high RPMs to "burn off" whatever is choking the spark plugs) we brought the airplane back to the ramp.
So here we are, filing a revised flight plan and taking SA, the original airplane we were suppose to take. Whoops.
And in case that wasn't enough, now we got to argue with Tampa Approach.
Normally Tampa Approach is fantastic. But that day was a GREAT flying day, both for VFR and IFR. The ceiling was just under 3000 feet so you could get some decent actual instrument time in. We had filed IFR just in case and picked up our flight plan with Tampa. We requested and received a hold so that I could demonstrate holding, and then requested a full VOR approach into Lakeland Regional. And were denied due to traffic. So we canceled our IFR flight plan (now seeing that the clouds were high enough so that we could operate under VFR), contacted Lakeland and asked for the full VOR approach. Lakeland said they could accommodate us and instructed us to proceed direct to the LAL VOR. When we were about 4 1/2 miles north of the VOR Lakeland said "Oh SA, Tampa said they can take you now."
This struck me as odd, but ok, let's talk to Tampa. I check in with them and hear this: "SA turn 360 immediately you are entering Lakeland's airspace!"
Me: "Um...SA turning 360 but sir we were just talking to Lakeland, they cleared us for the full VOR approach and then told us we could talk to you."
ATC: "Oh. Standby. OK SA heading of 290 vectors for the full VOR approach."
.....Me: "290 on the heading, vectors for the full VOR approach."
After another 10 minutes of vectoring we're back to where we were, 4 miles from the VOR and cleared for the approach *facepalm* Sometimes I really don't understand ATC.
At one point the examiner said, "well you know what they say about Murphy's Law" and I just responded "I hate that guy."
But all in all I passed with flying colors, got complimented on my crosswind landing technique (apparently this is the FAA's new "topic of concern") and came back just in time to be there when one of Jim's students got back from his first solo (woohoo!!). Just with a little more excitement than I would've preferred!
I even managed to eat breakfast that morning, another pre-check first for me. Normally I'm so nervous that I can't stomach anything. But this time I had some semblance of calm knowing that this would be my last FAA checkride for a very long time. I probably won't have another checkride that isn't for work, unless I choose to get my multi-engine instructor rating in the future, or go for seaplane ratings.
FAA checkrides involve an oral exam, consisting of knowledge/judgement-based questions, and a flight check. The FAA has what's called "pratical test standards," which give guidance to examiners on what to test candidates on. It's basically a checklist of certain items they need to hit on during your test. Some of this is accomplished in the oral and some of it on the flight, or a combination thereof.
I won't hit much on my checkride itself other than that the oral and flight both went very well, with the exception of what follows. But I did pass :)
Murphy's Law (as defined by UrbanDictionary.com):
1. If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it.
2. The law that says anything that can go wrong, will go wrong.
My flight school has two Cessna 172SPs, SA and PJ. I was scheduled to take SA on my checkride, but the manager of the flight school asked if I could take PJ. SA only had about 5 more hours before it needed a 100-hour inspection (required of all airplane for hire) and she wanted to try and extend that over the weekend. So I preflighted PJ, filed a flight plan and we got ready to go. PJ was very reluctant to start but we attributed this to the engine and the outside air being hot (fuel injected airplanes are very susceptible to vapor lock). We taxied to the runway to do our engine runup...and the left magneto was bad (there are 2 on each airplane, and they help generate the power for the spark plugs, more or less). After attempting to clear the mag several times (this is accomplished by leaning the mixture and running the engine at high RPMs to "burn off" whatever is choking the spark plugs) we brought the airplane back to the ramp.
So here we are, filing a revised flight plan and taking SA, the original airplane we were suppose to take. Whoops.
And in case that wasn't enough, now we got to argue with Tampa Approach.
Normally Tampa Approach is fantastic. But that day was a GREAT flying day, both for VFR and IFR. The ceiling was just under 3000 feet so you could get some decent actual instrument time in. We had filed IFR just in case and picked up our flight plan with Tampa. We requested and received a hold so that I could demonstrate holding, and then requested a full VOR approach into Lakeland Regional. And were denied due to traffic. So we canceled our IFR flight plan (now seeing that the clouds were high enough so that we could operate under VFR), contacted Lakeland and asked for the full VOR approach. Lakeland said they could accommodate us and instructed us to proceed direct to the LAL VOR. When we were about 4 1/2 miles north of the VOR Lakeland said "Oh SA, Tampa said they can take you now."
This struck me as odd, but ok, let's talk to Tampa. I check in with them and hear this: "SA turn 360 immediately you are entering Lakeland's airspace!"
Me: "Um...SA turning 360 but sir we were just talking to Lakeland, they cleared us for the full VOR approach and then told us we could talk to you."
ATC: "Oh. Standby. OK SA heading of 290 vectors for the full VOR approach."
.....Me: "290 on the heading, vectors for the full VOR approach."
After another 10 minutes of vectoring we're back to where we were, 4 miles from the VOR and cleared for the approach *facepalm* Sometimes I really don't understand ATC.
At one point the examiner said, "well you know what they say about Murphy's Law" and I just responded "I hate that guy."
But all in all I passed with flying colors, got complimented on my crosswind landing technique (apparently this is the FAA's new "topic of concern") and came back just in time to be there when one of Jim's students got back from his first solo (woohoo!!). Just with a little more excitement than I would've preferred!
Monday, June 1, 2009
An explanation of fly-by-wire...
As the time I've written this we don't know the fate of Air France flight 447. I pray that the passengers and crew found a miracle and are safe somewhere.
In the usual pattern of 24 hour new networks, I'm spending my morning yelling at the TV while some of their "aviation experts" speculate on the fate of an airplane they know nothing about. I don't know enough details to speculate myself (turbulence? lightning? electrical failure? There are too many possible causes) but I will give my piece on how a fly-by-wire airplane is controlled, and specifically an Airbus system.
Disclaimer: I hold an A320 type rating. There are differences between A320 systems and A330 systems, and I'm not as knowledgeable about the latter. So if I make an assumption about A330 systems that is incorrect, please forgive me. And feel free to comment/correct.
Fly-By-Wire
Fly-by-wire is a term used to explain how the control surfaces of an airplane are moved (control surfaces mean the ailerons, rudder and elevators, the movable pieces of the airplane that are used to control it).
The first method of moving control surfaces was by cable. When the pilot moves the yoke or stick and the rudder pedals, this directly manipulates cables that displace the control surfaces. This is still used effectively on smaller airplanes such as a Cessna 172.
The bigger an airplane gets, the larger the control surfaces must be, and the more force must be exerted by pilots to move them. So hydraulic controls became popular. The amount of force on the cables is amplified by hydraulic actuators that move the control surfaces. This is the most-utilized method used in airliners today.
Fly-by-wire airplanes operate differently. Any deflection of the yoke/stick/rudder pedals by the pilots is detected by computer sensors. The sensors determine the amount of deflection, or movement, needed in the control surfaces (using data such as airplane altitude and airspeed) and send this information to hydraulic actuators which then move the ailerons, elevators and rudder.
The major difference between cable-controlled airplanes and fly-by-wire airplanes is the use of, and dependence on, computers to control the airplane's movement.
Now onto the A320...
The A320 has 7 flight control computers:
2 ELACs: Elevator Aileron Computer (normal elevator and horizontal stabilizer control, as well as aileron control)
3 SECs: Spoilers Elevator Computer (spoiler control as well as standby elevator and stabilizer control)
2 FACs: Flight Augmentation Computer (electrical rudder control)
So you see that there are multiple levels of control for most of the control surfaces. If one set of computers was to fail you'd still have some control over the airplane.
I'm going to quickly go over the "levels of control" with the A320. Airbus calls these "laws." I'm glossing over certain points to keep this entry from becoming a novella.
In normal flight the computers are in "normal law." In normal law the pilots use the control stick to move the airplane. The airplane's computers actually prevent the airplane from stalling/overspeeding or undergoing any other maneuvers that may cause excessive stress on the airframe.
If computers start to fail on the airplane, it reverts into "alternate law." In alternate law you can still have some of the protections (i.e. stall protection) that you had in normal law.
Direct law is the next lowest level of control. I skipped over how the A320 is controlled by load factor demand and bank angle. So I'll just say that in direct law you control the A320 just like how'd you control any other airplane with a stick. Pull back and the nose will start to rise, move the stick left and the airplane will start to roll left.
Below direct law is "manual backup." If you were to lose all electrical power in the A320 your only hope of controlling the airplane is mechanical backup. This, I believe, has only happened once outside of testing. You would have to lose both engine generators, the APU (auxiliary power unit, another generator), drain your batteries and be unable to operate the RAT (ram air turbine, which is deployed from the bottom of the airplane and can provide limited power).
Pitch control is only available to the pilots through use of pitch trim, using the horizontal stabilizer of the A320 (it's called the THS, trimmable horizontal stabilizer).
Lateral control is only possible through use of the rudder pedals, which do have some direct linkage to the rudder.
When undergoing simulator training for the A320 type rating I had to try to fly the airplane using mechanic backup. It is NOT easy. It was nearly impossible to maintain level flight. Mechanical backup's purpose is to provide the pilots a way to keep the airplane stable while they troubleshoot the failed electrical system and flight control computers. I can't imagine anyone trying to land the airplane using mechanical backup. You simply don't have enough fine control.
Once again, this was all referenced from my A320 training and my A320 manuals. I don't know enough about the A330 to say whether all (or any) of this applies to the A330, but I'm going on the assumption that Airbus wouldn't have totally redesigned their fly-by-wire system for another airplane.
In the usual pattern of 24 hour new networks, I'm spending my morning yelling at the TV while some of their "aviation experts" speculate on the fate of an airplane they know nothing about. I don't know enough details to speculate myself (turbulence? lightning? electrical failure? There are too many possible causes) but I will give my piece on how a fly-by-wire airplane is controlled, and specifically an Airbus system.
Disclaimer: I hold an A320 type rating. There are differences between A320 systems and A330 systems, and I'm not as knowledgeable about the latter. So if I make an assumption about A330 systems that is incorrect, please forgive me. And feel free to comment/correct.
Fly-By-Wire
Fly-by-wire is a term used to explain how the control surfaces of an airplane are moved (control surfaces mean the ailerons, rudder and elevators, the movable pieces of the airplane that are used to control it).
The first method of moving control surfaces was by cable. When the pilot moves the yoke or stick and the rudder pedals, this directly manipulates cables that displace the control surfaces. This is still used effectively on smaller airplanes such as a Cessna 172.
The bigger an airplane gets, the larger the control surfaces must be, and the more force must be exerted by pilots to move them. So hydraulic controls became popular. The amount of force on the cables is amplified by hydraulic actuators that move the control surfaces. This is the most-utilized method used in airliners today.
Fly-by-wire airplanes operate differently. Any deflection of the yoke/stick/rudder pedals by the pilots is detected by computer sensors. The sensors determine the amount of deflection, or movement, needed in the control surfaces (using data such as airplane altitude and airspeed) and send this information to hydraulic actuators which then move the ailerons, elevators and rudder.
The major difference between cable-controlled airplanes and fly-by-wire airplanes is the use of, and dependence on, computers to control the airplane's movement.
Now onto the A320...
The A320 has 7 flight control computers:
2 ELACs: Elevator Aileron Computer (normal elevator and horizontal stabilizer control, as well as aileron control)
3 SECs: Spoilers Elevator Computer (spoiler control as well as standby elevator and stabilizer control)
2 FACs: Flight Augmentation Computer (electrical rudder control)
So you see that there are multiple levels of control for most of the control surfaces. If one set of computers was to fail you'd still have some control over the airplane.
I'm going to quickly go over the "levels of control" with the A320. Airbus calls these "laws." I'm glossing over certain points to keep this entry from becoming a novella.
In normal flight the computers are in "normal law." In normal law the pilots use the control stick to move the airplane. The airplane's computers actually prevent the airplane from stalling/overspeeding or undergoing any other maneuvers that may cause excessive stress on the airframe.
If computers start to fail on the airplane, it reverts into "alternate law." In alternate law you can still have some of the protections (i.e. stall protection) that you had in normal law.
Direct law is the next lowest level of control. I skipped over how the A320 is controlled by load factor demand and bank angle. So I'll just say that in direct law you control the A320 just like how'd you control any other airplane with a stick. Pull back and the nose will start to rise, move the stick left and the airplane will start to roll left.
Below direct law is "manual backup." If you were to lose all electrical power in the A320 your only hope of controlling the airplane is mechanical backup. This, I believe, has only happened once outside of testing. You would have to lose both engine generators, the APU (auxiliary power unit, another generator), drain your batteries and be unable to operate the RAT (ram air turbine, which is deployed from the bottom of the airplane and can provide limited power).
Pitch control is only available to the pilots through use of pitch trim, using the horizontal stabilizer of the A320 (it's called the THS, trimmable horizontal stabilizer).
Lateral control is only possible through use of the rudder pedals, which do have some direct linkage to the rudder.
When undergoing simulator training for the A320 type rating I had to try to fly the airplane using mechanic backup. It is NOT easy. It was nearly impossible to maintain level flight. Mechanical backup's purpose is to provide the pilots a way to keep the airplane stable while they troubleshoot the failed electrical system and flight control computers. I can't imagine anyone trying to land the airplane using mechanical backup. You simply don't have enough fine control.
Once again, this was all referenced from my A320 training and my A320 manuals. I don't know enough about the A330 to say whether all (or any) of this applies to the A330, but I'm going on the assumption that Airbus wouldn't have totally redesigned their fly-by-wire system for another airplane.
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