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Why Does Autopilot Not Have Control Of Rudder?  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (1 year 11 months 2 weeks 6 days 11 hours ago) and read 6281 times:
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HI,

Was just recently watching one of the Air Crash/mayday episodes about the 747 in flight upset were the pilot more or less bent the plane during the recovery.

Just curious about why it was decided for this aircraft type anyway for the autopilot not to have authority over movement of the rudder, other than yaw damper of course.

Cheers

24 replies: All unread, jump to last
 
User currently offlineamccann From United States of America, joined Mar 2008, 171 posts, RR: 0
Reply 1, posted (1 year 11 months 2 weeks 6 days 11 hours ago) and read 6269 times:

From my limited classroom knowledge of flight control systems I presume the lack of autopilot authority over the rudder is due to a lack of necessity. During phases of flight, not in an emergency or failure of any variety, when the autopilot is engaged the primary function of the rudder is for coordinated flight. However this completely changes when say the number 1 engine on a 4 engine airplane fails, then the rudder is used to counter the moment created from asymmetric thrust.


What one person receives without working for, another person must work for without receiving. - Ronald Reagan
User currently offlinetb727 From United States of America, joined Jun 2005, 1545 posts, RR: 8
Reply 2, posted (1 year 11 months 2 weeks 6 days 10 hours ago) and read 6214 times:

Yeah it's just not needed, yaw damper takes care of anything with the rudder. As far as using the autopilot when you have an engine out, on the 727 you just use rudder trim. Just look at the yoke and put a little rudder trim towards the side that the yoke is down until it's level. I'm not sure how wing mounted airliners work but that's what we practice.


Too lazy to work, too scared to steal!
User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 3, posted (1 year 11 months 2 weeks 6 days 8 hours ago) and read 6151 times:

Quoting smartt1982 (Thread starter):
Just curious about why it was decided for this aircraft type anyway for the autopilot not to have authority over movement of the rudder, other than yaw damper of course.

Allow me to clarify a couple of things:

1- The yaw damper system, while independent from the autopilot, is part of the aeroplane's auto flight control.
2- Many 747 had a triple channel A/P, one for yaw control which provided steering (using rudder) during an automatic roll-out.

As to why it was designed that way, I guess it was deemed unnecessary since the Y/D provide all necessary corrections for turning & periodic oscillations.

Quoting tb727 (Reply 2):
Just look at the yoke and put a little rudder trim towards the side that the yoke is down until it's level. I'm not sure how wing mounted airliners work but that's what we practice.

Pretty much the same.
Yoke leveled-Ball slightly out-Rudder as required.

Regards,
B747FE.



"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
User currently offlineRJSampson From United States of America, joined Jul 2010, 35 posts, RR: 0
Reply 4, posted (1 year 11 months 2 weeks 6 days 7 hours ago) and read 6109 times:

During an autoland, how does the aircraft swing the nose out of a crab before touchdown in a stiff crosswind if the AP doesn't have rudder authority?

User currently offline26point2 From United States of America, joined Mar 2010, 786 posts, RR: 0
Reply 5, posted (1 year 11 months 2 weeks 6 days 6 hours ago) and read 6070 times:

Autoland used in very low vis appproaches = fog = calm wind. No need for rudder input.

Rumour has it the new Global 7000 and 8000 will come with a 3 axis autopilot.


User currently offlineMD11Engineer From Germany, joined Oct 2003, 13792 posts, RR: 63
Reply 6, posted (1 year 11 months 2 weeks 6 days 6 hours ago) and read 6053 times:

Quoting 26point2 (Reply 5):
Autoland used in very low vis appproaches = fog = calm wind. No need for rudder input.

Not in HHN. The airport is located on a mountain top. If we have fog, it means that we are actually already in the clouds. So the often have CAT 3 B conditions with strong side wind (up to storm force).

Jan


User currently offlineMax Q From United States of America, joined May 2001, 4057 posts, RR: 19
Reply 7, posted (1 year 11 months 2 weeks 6 days 5 hours ago) and read 6032 times:

Quoting RJSampson (Reply 4):


During an autoland, how does the aircraft swing the nose out of a crab before touchdown in a stiff crosswind if the AP doesn't have rudder authority?

On the 757 / 67 The Autopilot does take control of the rudder on an autoland, if there is a crosswind it will lower the upwind wing slightly and apply opposite rudder to maintain runway centreline.


After touchdown it will track the centreline by following the localiser using the rudder and nosewheel steering (autopilot has to be disengaged to leave the runway centreline afer rollout )



The best contribution to safety is a competent Pilot.
User currently offlinevc10 From United Kingdom, joined Nov 2001, 1397 posts, RR: 16
Reply 8, posted (1 year 11 months 2 weeks 6 days 1 hour ago) and read 5937 times:

Again the memory is getting vague now , but I seem to remember that above a certain speed the feel unit makes it very hard if not impossible for the pilot to move the rudder. This is because at high speeds like cruise, movement of the rudder could affect aircraft drastically and I believe could also damage the structure, therefore if pilot restrained from moving rudder better do the same for the autopilot, well at least at high speeds

littlevc10


User currently offlinefrancoflier From France, joined Oct 2001, 3613 posts, RR: 11
Reply 9, posted (1 year 11 months 2 weeks 6 days 1 hour ago) and read 5910 times:

Quoting Max Q (Reply 7):
On the 757 / 67 The Autopilot does take control of the rudder on an autoland,

Ditto on the 744, not sure how it goes on the 777, but I'm guessing it's similar.



Looks like I picked the wrong week to quit posting...
User currently offlineclydenairways From Ireland, joined Jan 2007, 1194 posts, RR: 0
Reply 10, posted (1 year 11 months 2 weeks 6 days ago) and read 5898 times:

Quoting MD11Engineer (Reply 6):

Not in HHN. The airport is located on a mountain top. If we have fog, it means that we are actually already in the clouds. So the often have CAT 3 B conditions with strong side wind (up to storm force).

Same with NOC in the west of Ireland.


User currently offlinebueb0g From United Kingdom, joined Jul 2010, 623 posts, RR: 0
Reply 11, posted (1 year 11 months 2 weeks 5 days 21 hours ago) and read 5781 times:

Quoting vc10 (Reply 8):
but I seem to remember that above a certain speed the feel unit makes it very hard if not impossible for the pilot to move the rudder

Never been made aware of this, which aircraft were you thinking? In fact, I'm sure that's not correct, as pilots are able to deal with engine out scenarious in cruise using the rudders.



Roger roger, what's our vector, victor?
User currently offlinefr8mech From United States of America, joined Sep 2005, 5091 posts, RR: 12
Reply 12, posted (1 year 11 months 2 weeks 5 days 20 hours ago) and read 5758 times:

Quoting vc10 (Reply 8):
Again the memory is getting vague now , but I seem to remember that above a certain speed the feel unit makes it very hard if not impossible for the pilot to move the rudder. This is because at high speeds like cruise, movement of the rudder could affect aircraft drastically and I believe could also damage the structure, therefore if pilot restrained from moving rudder better do the same for the autopilot, well at least at high speeds

This is called a rudder ratio or rudder limiter system. In simple system, when an aircraft reaches a preset speed, the rudder is limited in movement. As I recall, on the DC8, the flaps provide an input to the rudder through standard Douglas monkey motion in order to restrict the rudder.

I'm thinking, that on the B757/B767 the rudder input by the pilots/autopilot is limited to 4 deg, but the yaw damp gets 6 degs. Numbers are probably off, but I recall that the yaw damp had more authority than the rudder pedal.



When seconds count...the police are minutes away. Never leave your cave without your club.
User currently offlinevc10 From United Kingdom, joined Nov 2001, 1397 posts, RR: 16
Reply 13, posted (1 year 11 months 2 weeks 5 days 19 hours ago) and read 5713 times:

Quoting bueb0g (Reply 11):
Never been made aware of this, which aircraft were you thinking? In fact, I'm sure that's not correct, as pilots are able to deal with engine out scenarious in cruise using the rudders.

Well like me they are both getting old now, but on both the VC-10 and the Concorde feel units made it very hard to input rudder demands at high speeds

littlevc10


User currently offlineBoeingGuy From United States of America, joined Dec 2010, 2878 posts, RR: 7
Reply 14, posted (1 year 11 months 2 weeks 5 days 18 hours ago) and read 5685 times:

In general, on Boeing airplanes the autopilot only gives aileron control for turns, as the OP apparently knows.

On the 777 and 787 the autopilot does sort of have control of the rudder in flight. There's a fly-by-wire features called the Rudder Aileron Cross-Tie. It will put in a little bit of rudder to assist in turns sometime. Also, they have a fly-by-wire feature called Thrust Assymetry Compensation that will give a rudder input if an engine fails. On the 777 it actually looks at a loss of thrust on one engine; on the 787 it looks at inertial yaw rate.

As others have noted, the Autopilot will control the rudder below 1500 feet during an autoland on Boeing airplanes (at least non-737 models). Boeing airplanes have a runway alignment submode during autoland that will remove crab angle at 500 ft and/or 200 ft. It's a bit more sophisticated on the 777 and 787.

The 777 and 787 have so-called "Backdrive Actuators" to backdrive the controls so the crew can clearly see what the autopilot is doing, unlike another manufacturer who thinks it's not important to have the pilot in the loop as long as the computers are running the show (woooo-hoooo, I'll be I hear about this one!). Anyway, the backdrive actuators do not backdrive the rudder pedals when the autopilot is engaged, except below 1500 feet during an autoland (when Land 3 or Land 2 autoland status is annunciated). Then they engage and backdrive the rudder pedals so the crew can feel autopilot inputs to the rudder also.

I hope this helps.


User currently onlinePihero From France, joined Jan 2005, 4128 posts, RR: 76
Reply 15, posted (1 year 11 months 2 weeks 5 days 18 hours ago) and read 5678 times:
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The Tristar - once again - was the forerunner of the introduction of the yaw axis in the A/P system, especially when the xwind landing technique was forward slip iso crabbing angle.
Then cat III requirements demanded the yaw axis - with nosewheel parallel control - and basically everybody has it, whether as original feature or as an option ( case of the 737 )

Quoting vc10 (Reply 13):
both the VC-10 and the Concorde feel units made it very hard to input rudder demands at high speeds

You are quite right. One of my favourite airplanes... epitome of British elegance. I re-discovered this Flight article on flying the "Ten". Every guy who took her was smitten, and years and years later, although on the 1011, they were still having very fond memories of her.

Flying the ten



Contrail designer
User currently offlinevc10 From United Kingdom, joined Nov 2001, 1397 posts, RR: 16
Reply 16, posted (1 year 11 months 2 weeks 5 days 17 hours ago) and read 5597 times:

Pihero,

Thanks for posting that link "props ,piston and old jets etc" I have only had time to glance over it but it looks like it could get me I trouble with the wife again "You are not reading about old aeroplanes again!!!!!! " ,but I will find the time.

I loved the article about the "ignition engine analyzer" which I had some experience with on the Lockheed Constellation, but although I got to understand what it was telling you, to understand and recogonize all the faults it could inform you about was way beyond me. I thought the important ones to remember were the ones which said you had to shut the engine down [2010 June]

A great web site

littlevc 10


User currently onlinePihero From France, joined Jan 2005, 4128 posts, RR: 76
Reply 17, posted (1 year 11 months 2 weeks 5 days 16 hours ago) and read 5559 times:
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Be diplomatic : I don't wish to be involved in a divorce lawsuit.

Cheers !



Contrail designer
User currently offlineBellerophon From United Kingdom, joined May 2002, 583 posts, RR: 59
Reply 18, posted (1 year 11 months 2 weeks 5 days 15 hours ago) and read 5538 times:

Pihero

Quote:
...The Tristar - once again - was the forerunner of the introduction of the yaw axis in the A/P system...


Oooooh....  

That will be news to the avionics and auto-flight engineers on the Trident design team!   

The Trident, with Auto Rudder Control and Kick Off Drift, was years ahead of the L-1011 in incorporating yaw control into the auto-flight system.

The Trident 1C made its maiden flight on 09 Jan 62, and performed the worlds first fully automatic approach and landing in commercial service on 19 June 1965, some five years before the L-1011 even made its maiden flight.

Interestingly, the Trident dealt with crosswinds by keeping the wings level on approach and kicking off the drift just prior (but not always!) to touchdown, whereas the L-1011 and DC10 both used a wing-down technique.

A persistent rumour on the Trident fleet in BEA was that, having heard about the advanced auto-flight system that had been developed for the Trident, Lockheed had come over to Hatfield to see for themselves, were impressed, and subsequently recruited many of the auto-flight and avionics engineers from Hatfield to work on the L-1011 systems.

Whether this was true or not I don't know, but there were many pilots in BEA - who flew both aircraft types during their careers - who believed the L-1011 avionics were indeed a (much improved) "Mark2" version of the Trident system.

Best Regards

Bellerophon

(Proud possessor of 7,000+ hours on the Trident)


User currently offlineTristarsteve From Sweden, joined Nov 2005, 3929 posts, RR: 34
Reply 19, posted (1 year 11 months 2 weeks 5 days 15 hours ago) and read 5509 times:

Quoting Bellerophon (Reply 18):
The Trident, with Auto Rudder Control

Thanks for that.
The Trident was my first aircraft licence, I was A and C at BEA.
Reading your entry reminds me of the huge difference in systems.
The Trident with its triple channel Cat3B autpilot, and the B737-200 with single channel and originally no autoland.
Amazing what a few foggy days at LAP can achieve! ( No landings at all for 15 days in the smog in Nov 1962)

And then, when there was a defect on the Trident Autoland, the avionics guys were out in the hangar, with all the hydraulics pressurised and the aircraft bouncing around for hours. Today even I can run an A320 autoland check in 10 minutes with no test equipment.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6262 posts, RR: 4
Reply 20, posted (1 year 11 months 2 weeks 5 days 14 hours ago) and read 5498 times:

In general (and I know that there's exceptions to every rule, for example, all the Cat III systems above   ), I would say that AA 587 proved beyond a shadow of a doubt that the rudder is the most dangerous flight control surface to use in a jet transport. We all saw what happens when applied improperly. Most transport jets are flown with the feet flat on the floor except in an engine out or a crosswind, or taxiing past tiller speeds. Now you want to give control of that particular control surface to someone who might on occasion act up? (his name is George    ). I'd be willing to wager that the systems mentioned in previous posts with rudder authority only use it on approach...


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently onlinePihero From France, joined Jan 2005, 4128 posts, RR: 76
Reply 21, posted (1 year 11 months 2 weeks 5 days 14 hours ago) and read 5487 times:
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Quoting Bellerophon (Reply 18):

The Trident, with Auto Rudder Control and Kick Off Drift, was years ahead of the L-1011 in incorporating yaw control into the auto-flight system.

I must really getting barmy senile !
How could I have forgotten the Trident ?
Of course, you're 1925 % right.

Regards



Contrail designer
User currently onlinePihero From France, joined Jan 2005, 4128 posts, RR: 76
Reply 22, posted (1 year 11 months 2 weeks 5 days 12 hours ago) and read 5399 times:
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.... and the Caravelle, too.
I'm really getting old !



Contrail designer
User currently offline737tdi From United States of America, joined Sep 2007, 633 posts, RR: 1
Reply 23, posted (1 year 11 months 2 weeks 5 days 10 hours ago) and read 5356 times:
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Quoting vc10 (Reply 16):
Pihero,

Thanks for posting that link "props ,piston and old jets etc" I have only had time to glance over it but it looks like it could get me I trouble with the wife again "You are not reading about old aeroplanes again!!!!!! " ,but I will find the time.

I loved the article about the "ignition engine analyzer" which I had some experience with on the Lockheed Constellation, but although I got to understand what it was telling you, to understand and recogonize all the faults it could inform you about was way beyond me. I thought the important ones to remember were the ones which said you had to shut the engine down [2010 June]

A great web site

littlevc 10




Ditto, what a great little site. I love reading old aviation stuff and this is going to be read from front to back.


User currently offlineROSWELL41 From United States of America, joined Aug 2001, 764 posts, RR: 1
Reply 24, posted (1 year 11 months 2 weeks 4 days 11 hours ago) and read 4988 times:

The A320 autopilot has control of the rudder as welll.

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