747 flight controls

[747 forever]

Hi guys,
I just want to know few things about this great jet. First when the jet banks does the pilot apply rudder like you would do to a cessna to keep the turn indicator`s ball in the center.
Second although it is not for flight control but when landing when would you disengage the autopilot? I know that on takeoff after you pass 200 feet you can engage it.
Thanks

[Pontius]

does the pilot apply rudder like you would do to a cessna to keep the turn indicator`s ball in the center.

No need to, mate. The aircraft will actually chuck in a bit of rudder to keep the 'ball' centred, so the rudder is really just there for resting your feet (4 engines) or keeping it straight (3 or 2 engines). The pendants will now spout off about how you can use rudder (yes, you can, but nobody does) and how the turn compensation only works up to certain bank angles (it does but I've never done aeros in an airliner, so not really relevant). In short, on most jet airliners (in the air), you don't use rudder in normal operations once you're airborne. You will use them during takeoff (especially with a crosswind) and you will use them during landing (with a crosswind) but climb, cruise and descent; they're there for when a donk fails

when would you disengage the autopilot?

All depends on the pilot flying. Clearly, if you're doing an autoland then the autopilots stay engaged throughout. If you're doing a manual landing then MOST of the time guys will disengage the A/P around 1000'. Sometimes they'll disengage it earlier and hand-fly the whole approach. Sometimes they'll leave it until later and they can get a better feel for the approach conditions. Generally though (and different companies have different rules, so note my generalisation), you'll fly it manually when you've completed the landing checklist and the aircraft is in a stable, landing configuration. I'll reiterate the fact that companies will have their own rules but, generally, this is the same for all airliners, not just the Jumbo.

PS: Sorry if the reply seems a bit 'simple' but I don't know the age/experience level of the op.

[Mercenary Pilot]

I know that on takeoff after you pass 200 feet you can engage it.

250' to be exact, at least on the -400.

[NSEU]

We might want to distinguish between "can" and "should" at this point. As far as I know, the A/P allows engagement immediately after liftoff. The mode will be track hold (laterally) and a combination of V/S and speed (vertically) until LNAV and VNAV kick in at 50' and 400' respectively.

Some don't have much faith in the A/P at low altitudes and leave engagement until later.

I assume we are talking about the 747-400 and not earlier series 747's or the 747-800?

[forget]

Hasn't Boeing decided to call their new 747 the 747-8 rather than 800. I'll give it a year or so. The 800 it will be. Marketing whippersnappers!

[SNS3Guppy]

I just want to know few things about this great jet. First when the jet banks does the pilot apply rudder like you would do to a cessna to keep the turn indicator`s ball in the center.

Yes, and no.

Yes, the rudder works conventionally, just the same as most airplanes, but no, it's not really needed that much.

I fly the Classic (747-100, -200), and while the rudder does have a turn coordination feature, it's not active all the time; just when flaps are down. When it's not active, the airplane is really a two-axis autopilot (pitch and roll). It's designed such that for most of the flight, the airplane flies straight and doesn't need much rudder input at all. One of the features of the rudder system is ratio changing, which changes the amount the rudder is actually capable of deflecting, depending on airspeed. At slow speeds, it deflects as much as 25 degrees, but at speeds above 170 knots, it only deflects 4 degrees, even though the pilot may apply full rudder.

It's a really big rudder, and too much rudder input is stressful on the airframe, unnecessary, and causes significant roll as well as yaw.

The autopilot has no input to the rudder. The 747 actually has two rudders; an upper, and a lower. The upper is the "captain's rudder," and the lower is the "FO's rudder." In practice, there's no difference, really. Each is controlled by a separate yaw damper, which is automatic and requires no pilot input. The yaw damper only has four degrees of authority. It's the yaw damper that actually supplies the turn information to the cockpit indicators (rate of turn): upper to the captain, lower to the copilot. The pilot has no control over the yaw damper, except a switch to turn it on or off. The yaw damper provides turn coordination when the airplane is slow (when flaps are extended), but the pilot has no indication of what's going on; the rudder pedals don't move, and there's no cockpit feedback.

When the airplane is moving faster, the natural streamlining tendencies of the airplane, plus that massive horizontal stabilizer, keep the airplane "coordinated," and most pilots don't touch the rudder at all.

Second although it is not for flight control but when landing when would you disengage the autopilot? I know that on takeoff after you pass 200 feet you can engage it.

We are not allowed to engage the autopilot until after flap retraction. For a normal departure (in our operation), flaps are generally retracted at 3,000' above the departure field. We definitely don't engage it at 200'.

I generally hand fly during the departure until above 18,000', and usually until about 29,000' before engaging the autopilot (let's face it, with this type of flying, one doesn't actually get to hand-fly that much, so I do it as much as I can). When to engage it depends on the operator policy, local workload, and personal preference. I prefer to hand-fly when I can, but it's also important to stay proficient with both hand flying and working through the automation.

When landing, our policy has the autopilot off by 50' below the minimum descent altitude, or during a "precision" approach, by 150' to 100' at the latest. We don't do autolandings. Most of us at my operation seem to disengage the autopilot by about 500 to 1,000'. Last night I disengaged it at 2,000 and hand-flew the approach, strictly by preference. Remember, it's a tool to help reduce workload (or specifically, to enable a pilot to devote more attention to more tasks). Hand-flying the 747 is a pleasure; it's a nice flying airplane.

[Dan Winterland]

''I fly the Classic (747-100, -200), and while the rudder does have a turn coordination feature, it's not active all the time; just when flaps are down.''

This is significant because the outer ailerons are active with Flaps extension and will consequently give more adverse yaw than the inboard ailerons. IIRC, (but it's 8 years and 2 type ratings ago) the 747-400s outboard ailerons are active from 236 knots regardless of flap position and this is when the turn co-ordiantion becomes active.

[Mercenary Pilot]

We might want to distinguish between "can" and "should" at this point. As far as I know, the A/P allows engagement immediately after liftoff.

Correct, you can engage it lower but Boeing say "The autopilot must not be engaged below a minimum engage altitude of 250 feet AGL after takeoff.", hence my comment.

[747 forever]

Thanks for all the replies! I appreciate it

[spannersatKL]

SNS3Guppy
For info some classics and the -400 has an autopilot input to the rudder.....during landing rolllout...on some classics there was fitted an LRCU box, Landing Rollout Control Unit and this put inputs in to the rudder system to maintain alignment on to the runway when on the landing rollout. I believe the 744 is similar in operation but through the FCCs instead.

Also the Yaw damper input to the rudder could be considered as an Autopilot input.....well its in the same ATA chapter 22. Though I agree it is not giving any real flight guidance.

[747 forever]

Thanks guys, little off topic but what is the cabin height of the 747-800 and 747-400? I just want to compare the heights of the cabin see if there the same or not

[SNS3Guppy]

Also the Yaw damper input to the rudder could be considered as an Autopilot input.....well its in the same ATA chapter 22. Though I agree it is not giving any real flight guidance.

The yaw damper(s) are stability augmentation systems, not autopilot inputs. It's a two axis autopilot.

From ATA 22 (22-00-00):

The autopilot system is a two-axis (pitch and roll) system which operates
the elevators and ailerons to automatically maintain altitude, airspeed
and/or guide the airplane to designated locations and make automatic
landings.

The yaw damper system is a full-time, series-connected, stability
augmentation system. Airplane periodic oscillations (dutch roll) are
detected with a rate gyro. The rudder is displaced at the proper time
to dampen out any dutch roll before it can significantly affect the
flight path of the airplane. The yaw damper system and actuators are
connected such that no rudder feedback is applied to the pedals, thereby
allowing the system to operate independently without interfering with
pilot initiated rudder commands. The yaw damper system also assists in
making coordinated turns when the flaps are down. Two systems are
installed: one for the upper rudder, and one for the lower rudder.

[NSEU]

It's a two axis autopilot.

For most of the flight, it's a two axis autopilot. During Autoland (as Spanners says), it's three axis on Classic 747s with LRCUs & rollout actuators and 747-400s.

Manual definitions aside, if the Yaw Damper is controlling turn co-ordination, then it's performing a traditional pilot function, automatically. "Auto-pilot"

[Rick777]

The 747-8 is very similar to the 400 and is planned to not be a separate type. The cockpit height is about the same as the 400. From a pilot perspective the main difference is the ECLand MFDs like the 777 and 787.

[SNS3Guppy]

Manual definitions aside, if the Yaw Damper is controlling turn co-ordination, then it's performing a traditional pilot function, automatically. "Auto-pilot"

Actually, no. We use definitions because they define.

The yaw damper functions independent of the autopilot. Without the autopilot engaged, the yaw dampers are still functioning, so long as the shutoff switches aren't used to disable them.

The yaw dampers are stability augmentation devices

[NSEU]

The yaw dampers are stability augmentation devices

Agreed.. but is still found in the "Autopilot" "Autoflight" chapter of the Maintenance Manual

Your definitions may be different from ours.

[SNS3Guppy]

Agreed.. but is still found in the "Autopilot" chapter of the Maintenance Manual

No, it's not. The yaw damper is found in the autoflight ATA 22 chapter, because it's a flight control function and therefore an autoflight function, but it is not part of the autopilot.

The yaw dampers are autoflight functions, but not autopilot functions. As such, they're a flight control function, without any relationship to the autopilot. While it's fair to say that the yaw damper is part of the autoflight system, it's incorrect to say the yaw damper is part of the autopilot system, because it is not part of the autopilot. It's a two-axis autopilot: pitch and roll (not yaw).

How about defining it by what the pitch and roll computers supply,or do? The pitch and roll computers have no input to the rudders. The yaw damper is entirely independent, and operates with or without the flight director or autopilot engaged, active, or operable.

The yaw damper switches are electrical hydraulic shutoff switches. The switches either shut off hydraulic power to the yaw dampers, or they are an active part of the flight control at all times, as part of the stability augmentation function of the flight control, itself.

The autopilot computers operate the ailerons and elevators. Not the rudder. The yaw damper computers are independent from the autopilot/flight director computers and do not take output from them, nor provide input to them.

The previously cited maintenance manual thus describes the system as "On ALL EXCEPT 747SP, two identical yaw damper systems control the upper and lower rudders. Each system monitors airplane yaw rate and positions the rudder to compensate for periodic yaw oscillations (dutch roll). Correction signals are applied to the rudder packages during manual and autopilot controlled flight to displace the upper and lower rudders sufficiently to damp out any yaw oscillations of the airplane. Rudder displacement is limited to 3.6 degrees. The yaw damper system also provides a turn coordination feature which improves airplane response during turn maneuvers when the flaps are down at least 1 degree. System gains are also changed as a function of flap position. When the flaps are down, the roll attitude signal from the INS is introduced to provide rudder displacement proportional to roll rate. The roll attitude signal is not used when the flaps are up. The yaw damper system is normally engaged for all flight modes and operates full time."

Essentially, the purpose of the yaw damper is to correct for dutch roll (periodic yaw oscillations). Per the MEL, while no autopilots are required for flight (except for RVSM), at least one of the two yaw damper systems are always required. Only one may be inoperative. The autopilots may be entirely inoperative, but the yaw dampers may not.

[NSEU]

Seems we are talking about two different 747's

I'll concede that I used the expression "Autopilot" in error instead of "Autoflight"

However, all our 747-400's have FCC's (Flight Control Computers), not Pitch and Roll computers. Our FCC's are responsible for 3 axes.

Please feel free to quote the specifications for your Yaw Damper system, but ours are completely different Even looking though some notes for (long retired) Classic 747-300 notes, I see there is a relationship between the autopilot and the Yaw Damper system: The Yaw Dampers can be disengaged by the LRCUs ("autoland computers", for want of a better expression) in certain circumstances (so the systems are not completely independent)

Cheers
NSEU

P.S. Glad we were not talking about Airbuses

[SNS3Guppy]

I don't know anything about an airbus, though I can recognize one two out of three times.

I believe I made clear that I was not referring to the -400, but to the Classics -100 and -200, and it was from the mx manual for those aircraft that I quoted.