smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Why Does Autopilot Not Have Control Of Rudder?

Mon Apr 30, 2012 11:00 pm

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
 
amccann
Posts: 124
Joined: Sat Mar 08, 2008 9:14 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Mon Apr 30, 2012 11:20 pm

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 avatar
tb727
Posts: 1798
Joined: Thu Jun 30, 2005 1:40 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 12:36 am

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!
 
B747FE
Posts: 217
Joined: Mon Jun 21, 2004 11:09 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 2:20 am

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 avatar
rjsampson
Posts: 106
Joined: Sat Jul 31, 2010 12:00 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 3:23 am

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?
 
26point2
Posts: 836
Joined: Wed Mar 03, 2010 6:01 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 4:20 am

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.
 
MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 4:38 am

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
Je Suis Charlie et je suis Ahmet aussi
 
Max Q
Posts: 5695
Joined: Wed May 09, 2001 12:40 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 5:21 am

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.
 
vc10
Posts: 1352
Joined: Thu Nov 15, 2001 4:13 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 8:52 am

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 avatar
Francoflier
Posts: 3795
Joined: Wed Oct 31, 2001 12:27 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 9:36 am

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.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
Clydenairways
Posts: 1116
Joined: Thu Jan 11, 2007 8:27 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 9:55 am

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.
 
bueb0g
Posts: 656
Joined: Fri Jul 02, 2010 5:57 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 1:41 pm

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?
 
fr8mech
Posts: 6721
Joined: Mon Sep 26, 2005 9:00 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 2:26 pm

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.
 
vc10
Posts: 1352
Joined: Thu Nov 15, 2001 4:13 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 3:41 pm

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
 
BoeingGuy
Posts: 4031
Joined: Fri Dec 10, 2010 6:01 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 4:05 pm

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.
 
Pihero
Posts: 4247
Joined: Mon Jan 31, 2005 5:11 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 4:14 pm

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
 
vc10
Posts: 1352
Joined: Thu Nov 15, 2001 4:13 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 5:40 pm

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
 
Pihero
Posts: 4247
Joined: Mon Jan 31, 2005 5:11 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 6:39 pm

Be diplomatic : I don't wish to be involved in a divorce lawsuit.

Cheers !
Contrail designer
 
Bellerophon
Posts: 522
Joined: Thu May 09, 2002 10:12 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 6:55 pm

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)
 
Tristarsteve
Posts: 3374
Joined: Tue Nov 22, 2005 11:04 pm

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 7:43 pm

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.
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 8:05 pm

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 :-)
 
Pihero
Posts: 4247
Joined: Mon Jan 31, 2005 5:11 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 8:14 pm

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
 
Pihero
Posts: 4247
Joined: Mon Jan 31, 2005 5:11 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Tue May 01, 2012 10:39 pm

.... and the Caravelle, too.
I'm really getting old !
Contrail designer
 
737tdi
Posts: 1116
Joined: Sun Sep 02, 2007 10:05 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Wed May 02, 2012 12:41 am

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.
 
ROSWELL41
Posts: 755
Joined: Mon Aug 20, 2001 3:50 am

RE: Why Does Autopilot Not Have Control Of Rudder?

Wed May 02, 2012 11:39 pm

The A320 autopilot has control of the rudder as welll.

Who is online

Users browsing this forum: No registered users and 11 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos