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Spectators Balcony (Spotters Corner) If you're not a professional pilot but want to discuss issues about the job, this is the best place to loiter. You won't be moved on by 'security' and there'll be plenty of experts to answer any questions.


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Old 7th Mar 2011, 23:23   #1 (permalink)
 
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Why to autopilots have no rudder authority?

I was told recently that autopilots, even on the heavy metal Boeing and Airbus airliners, have no control over the rudder. First off, is this really true? I notice that in simulators, even the more realistic x-plane, I can kick a rudder when the AP is engaged, while I have to fight to move the ailerons and elevator, so it would seem that even the sims have this modeled in.

If this is true, how are coordinated turns done with the AP engaged? Also, wouldn't aircraft with AutoLand need some amount of automated rudder control?

Or...do said aircraft's flight computers use asymmetric spoileron and aileron deployment to induce yaw instead of using the rudder?
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Old 8th Mar 2011, 00:02   #2 (permalink)

 
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You need to look to the specific aircraft type for an answer. Yes, many transport category aircraft use a two-axis autopilot. The B742, for example, uses a two-axis autopilot. The autopilot system controls the airplane it pitch and roll. An independent yaw damper system provides support on the vertical axis in yaw, by reducing sideslipping yaw that can become dutch roll.

During approach, when the airplane is configured to land, the yaw damper system also provides an elementary turn coordination feature. This occurs automatically with flap extension.

Most large airplanes, indeed most transport category aircraft, require very little rudder input for most of the flight; a characteristic of "jet" pilots is a tendency to be lazy with their feet. Feet are often flat on the floor with not much rudder input at all. The B742 is very stable and takes almost no rudder input, most of the time. The exception is an engine-out, but even then the rudder is so large that it is powerful, and it's possible to overcontrol if one gets too aggressive with the rudder. Throwing full rudder in also causes roll, and once the contrl wheel goes past 1.8 units of roll spoilers extend as well as ailerons deflecting, and both yaw and roll are affected again. It can be entertaining to watch someone overcontrol in one axis in a simulator, because it can quickly turn to a juggling act as each of the other axes goes haywire.

The short answer is that the rudder isn't needed much during normal flight operations. It's not moved much, either, except for engine-out situations or assymetrical thrust.
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Old 8th Mar 2011, 00:36   #3 (permalink)
 
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Quote:
If this is true, how are coordinated turns done with the AP engaged?
With the L1011 type, SAS is used...Stability Augmentation System.
SAS provides full time, turn coordination and yaw damping...and during autoland maneuvers, runway alignment and rollout.
A superb arrangement...works good, lasts a long time.
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Old 8th Mar 2011, 01:22   #4 (permalink)
 
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You're talking individual types here and of the four large aircraft types I've flown, only one of those didn't have an autopilot rudder channel (747 - both classic and -400).

The 747 does have a yaw damper which also adds a rudder input to prevent adverse yaw in some circumstances. The 747 has two sets of ailerons, inboard and outboard. Only the inboard ones are used at higher speeds as a result of wing twisting and possible aileron reversal. On the classic, the outboards become active when the flaps are selected, on the 744 at 238 knots.
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Old 8th Mar 2011, 04:22   #5 (permalink)

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BA 747-436 has a rudder channel for Autoland.

The autopilot rudder commands are added only during a multi autopilot approach and landing. To taxi off the runway you have to remember to disconnect the auto-pilot.
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Old 8th Mar 2011, 11:15   #6 (permalink)
 
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On many types the autopilot does control the rudder but only during approach and landing and in the immediate after take-off phase in the event of an engine failure. Other than that, as others have said, the yaw damper channel co-ordinates turns.
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Old 8th Mar 2011, 20:51   #7 (permalink)
 
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Very interesting information here.

Allow me to ask why there are sometimes multiple autopilot on/off switches (Command switch)? Are these for different channels, for instance say the captain only wants the A/P to handle pitch and therefore only the elevator channel is turned on?
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Old 8th Mar 2011, 21:37   #8 (permalink)
 
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It's not usually that complex (fortunately). You'll often find there's a CMD switch for each of the autopilots ( e.g. three switches for the Left, Centre , and Right autopilot), hence allowing the pilots to select which one of the autopilots is doing the driving.

Some newer types (e.g. 777) have a CMD switch for each pilot, if either is pressed a random autopilot out of the three is engaged.
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Old 9th Mar 2011, 00:31   #9 (permalink)
 
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It's not usually that complex (fortunately). You'll often find there's a CMD switch for each of the autopilots ( e.g. three switches for the Left, Centre , and Right autopilot), hence allowing the pilots to select which one of the autopilots is doing the driving.

Some newer types (e.g. 777) have a CMD switch for each pilot, if either is pressed a random autopilot out of the three is engaged.
Are you talking redundancy here or an autopilot for each of the three axes?
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Old 9th Mar 2011, 06:12   #10 (permalink)

 
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Allow me to ask why there are sometimes multiple autopilot on/off switches (Command switch)? Are these for different channels, for instance say the captain only wants the A/P to handle pitch and therefore only the elevator channel is turned on?
Think of each switch as an "ON-OFF switch. The difference for the autopilot is that one can use some of it's capability, or all of it.

If three switches are installed, that's for three different autopilots. Generally only one autopilot is in use at a time. For certain operations, such as autoland, more than one autopilot is used. Certain things must take place during an approach to land, or one or more autopilots will be excluded and dropped out of use.

Each autopilot swith controls all the axes of the airplane that are autoflight funcitons. This means that if Autopilot A is engaed, it engages each o the pitch, roll, or yaw functions assigned.

Autopilots may have a "manual" and a "command" Command engages more components and more effectively, than Manual. Manual is generally a fail-down mode which is capable of less than the full autopilot can do. In manual mode, the autopilot may only capable of holding altitude and heading. It's a limited feature Generally one moves the associated lever to manual to hold present pitch attitude with a wing-leveler. That's it.

Each autopilot is fully capable of doing the things the others can do, and to do it independently of the others.
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Old 9th Mar 2011, 12:43   #11 (permalink)
 
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Each autopilot is fully capable of doing the things the others can do, and to do it independently of the others.
Thanks guppy. So that beinh said..your quote above, they ARE there for redundancy? Why are they all fired up for AutoLand if they are all "equal" to each other? Could you give me some real life examples of one, two, and three autopilots being used in manual and/or command mode? Thanks! Sorry if I have not fully grasped this just yet.

edit: rather, why would the pilot flying chose the center autopilot versus the "left", or why the left versus the right or center, etc...?
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Old 9th Mar 2011, 16:02   #12 (permalink)

 
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You could say redundancy, on the basis that if one fails another can be used. When we speak of redundancy in an aircraft, however, it usually implies multiple systems that are in use simultaneously, which allow a fail-down capability.

In normal use, where one autopilot system is used, the other autopilot systems are off. You could think of them as "spares." Each of the autopilot systems is generally attached to a different guidance system, and in many cases, different hydraulic systems. In the airplane I fly, we have four different hydraulic systems three autopilots, and three different IRS gyro systems that work for each individual autopilot system.

The autopilots, in turn, receive inputs from a number of different sources, including air data computers, INS or IRS gyro systems, FMS/FMC management computers, etc. Sub systems such as autothrottles receive input from engine instrumentation or engine sensors, as well as autopilot panels (pilot inputs), performance management computers, etc. For the most part, these systems that feed the autopilots, and the autopilots that control aircraft systems, are independent. Some share components; the autothrottle, for example.

During certain phases of flight, landing on an ILS approach, autopilots may be used in concert with each other. In the airplane I fly, an additional option is available when selecting the navigation mode for the approach, which permits more than one autopilot to be used. In this mode, the electrical solenoids that normally lock out other autopilots and prevent multiple autopilots from being used at the same time, are removed. A computer closely monitors the inputs of each autopilot, and if one gives an indication of doing something that's not in harmony with the approach data or which appears to be in error, the offending autopilot is "cammed out," or dropped out of the loop.

During an instrument approach, certain criteria need to be fulfilled at certain altitudes and certain places during the approach; if these needs aren't met, the autopilot system won't allow the approach to proceed with multiple autopilots engaged. If this is the case, the approach may need to be abandoned, or may need to be continued to higher landing minimums. When flying to low minimums, referred to as "category III" minimums, multiple autopilots may be used to satisfy requirements of the approach. The approach allows the airplane to be flown much lower than a standard instrument approach, including autoland approaches in which the airplane flies all the way to a touchdown.

The use of multiple autopilots could be viewed as extra oversight. Just as using two pilots to fly the ILS and monitor the progress of the approach offers a higher level of awareness and safety, so does the use of more than one autopilot, especially during low minimum approaches. You could also look at their use as redundancy, like wearing two sets of latex gloves during an emergency medical call. One breaks, you've got a second already lined up and flying the approach, "in the loop," ready to go.

In the airplane I fly, I can only engage one autopilot at a time in normal operations. When first engaged, I announce "Autopilot A to Command" (or Autopilot B, etc, depending on which one is selected). I fly an older airplane, which requires me to select the vertical speed mode and navigation modes, and select the altitude mode I need, as well as the autothrottle mode (if used), all independently. By doing this, I have selected the autopilot I intend to use, and then selected all the functions that I intend to input into the autopilot, to tell it what I want it to do.

Once I select an autopilot, the other autopilot(s) are locked out. I can select a mode for the instrument approach which removes that lockout, and which allows more than one autopilot to be engaged; that mode is only available for an instrument approach, and only available for certain approaches in certain conditions.

There's no particular reason why one might choose this autopilot or that. As convention, generally the pilot in the left seat engages autopilot A, and the pilot in the right seat uses autopilot B, but there's no reason why either one couldn't use the opposite autopilot. In our airplanes, the autopilot and flight director (a device on the attitude display indicator, or attitude indicator, which gives guidance on flying the airplane) are tied to a particular side of the cockpit. That is to say, autopilot B will match what's seen on the first officer's display. There shouldn't be any difference between the captain and first officer displays, but they are independent systems, and if the first officer engages autopilot A but views his own instrumentation and flight director, it won't necessarily be showing him what's going on because he's using a different computer and autopilot than he's using for his own flight director.

The difference is generally very minimal. In some airplanes using much older technology such as straight INS units, it's possible to have different information displayed on one side of the cockpit than the other. This is due to drift in the INS units, and other factors. One pilot may be seeing a course needle deflected to one side, suggesting he needs to turn that direction, and a flight director telling him to turn in that direction, even though the autopilot soldiers on, straight ahead. The autopilot is working off data on the other side of the cockpit, and the other pilot shows on his instrumentation what's reflected in the actions of the autopilot. For these reasons and traditional convention, generally each pilot will use his or her "own" autopilot, though it's not at all necessary.

Sometimes a problem may develop with an autopilot or autopilot channel. A document in the airplane known as a "minimum equipment list" allows us to fly with inoperative items or equipment, so long as certain requirements are met. An autopilot may be "MEL'd" out of service, in which case the operative autopilot(s) will be used. The choice of which one to use may be dictated what's functioning at the time.

It's also important to remember while discussing autopilot and their use that the autopilot doesn't fly the airplane. It's a control system, just as using the control column is a control system. In neither case is the pilot actually moving the control surfaces of the airplane, but making an input to the airplane. When manually moving the control wheel, for example, I move a cable, which moves a hydraulic valve, which sends hydraulic fluid to an actuator at a control surface, which moves the surface. When making an autopilot input, I tell the autopilot where I want the airplane to go, or what I want it to do. I may roll a small wheel on the center pedestle or on the forward instrument panel, for example, rather than moving the control column; the autopilot is just another method of controlling the airplane.

Autopilots often don't have rudder inputs because the rudders are often used very little, particularly in phases of flight in which the autopilot is in play.
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Old 10th Mar 2011, 14:23   #13 (permalink)
 
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Wow thanks for taking the time Guppy.

I understand it now. It's sort of like the way the flight dynamics computers on the space shuttle work. There are maybe 3 different computers that control the gimbal of the SRB nozzles as well as the SSME's as well as the orbiter's aerodynamic control surfaces during the launch phase. I believe that those computers are all linked to a central "moderator" computer that ensures that they are all in agreement. If they are, a command is sent out to the control entities. This prevents a small glitch in one computer from causing a really bad day. Now if one computer completely fails, the other takes over seamlessly, because it's already "in the loop" as you say. I might not be remembering the exact details of the orbiter's computer(s) perfectly well but you get the general idea. By the way, here in 2011 the shuttle's computers are equivalent to an IMB 5150 which has an 8088 processor and was the cat's meow...in 1981. Drifting a bit here...

So allow me to ask, what aircraft(s) do you fly, and what is their mission? Would love to know, thanks.

I think I may have been wrong about you...you are quite a resource to this forum. Incredibly knowledgeable to say the least.
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Old 10th Mar 2011, 15:57   #14 (permalink)

 
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I'm afraid I don't know any more about the shuttle than what shows up on the Discovery Channel. As I understand it, the Shuttle is some old technology. Far and away above anything I do, though.

Even with older airplanes, upgrades in avionics continue to give new life. Look at the B52; it was thought to be ready for retirement two decades ago, and now it looks like it will fly at least another 25 years.

Personally, I'm an ag aviator (crop duster) that gets sidetracked into doing a little bit of this or that on the side. I like fabric airplanes and things that go low and slow with the nosewheel somewhere other than the nose and a little character to go with the wings. Or helicopters...but then who doesn't?
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Old 10th Mar 2011, 18:01   #15 (permalink)
 
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Cool about the 'dusting.

So what aircraft do you fly that have multi-channel A/P's?
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Old 10th Mar 2011, 18:42   #16 (permalink)

 
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At the moment, the Boeing 747.
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Old 10th Mar 2011, 20:15   #17 (permalink)
 
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How does one go from crop dusters to 747's on a whim? Interesting LOL.
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Old 10th Mar 2011, 20:15   #18 (permalink)

 
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I wear several hats.
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