FBW Handling qualities in gusty conditions

[Jwscud]

The discussion on the AMS bad weather threads have got me thinking. How do the Boeing and a Airbus FBW types handle in gusty conditions? Does the FBW pick up a dropped wing for you?

Having flown only old fashioned types that require frequent and positive control inputs to keep on path I would be interested to hear how the control techniques differ - a lot of Airbus guys talk about "stirring the pot" as something to avoid. Are you basically trying to correct solely for long term flight path trends rather than shirt term attitude excursions that the FBW will correct for you?

[The African Dude]

The discussion on the AMS bad weather threads have got me thinking. How do the Boeing and a Airbus FBW types handle in gusty conditions? Does the FBW pick up a dropped wing for you?

Having flown only old fashioned types that require frequent and positive control inputs to keep on path I would be interested to hear how the control techniques differ - a lot of Airbus guys talk about "stirring the pot" as something to avoid. Are you basically trying to correct solely for long term flight path trends rather than shirt term attitude excursions that the FBW will correct for you?

The Airbus FBW will usually return the aircraft to the previous attitude following a disturbance, however the aircraft has been knocked off and needs to be returned to the correct trajectory. The inputs from the pilot are then a roll rate and g-load demand (A320), then stick-neutral once back on the correct trajectory.

What this means is that you are correcting for long-term flight path trends but of course there will be short term disturbances that need to be checked and dealt with too.

It's just as much work IMHO because ultimately, as with old-fashioned types, the pilot is still adjusting inputs to achieve the rate of response desired in the moment.

[wiggy]

The discussion on the AMS bad weather threads have got me thinking. How do the Boeing and a Airbus FBW types handle in gusty conditions? Does the FBW pick up a dropped wing for you?

Not a trainer or a TP so going out on a limb here but as far as the Boeing (777) is concerned, no, it won't pick up a dropped wing...leaving aside extreme situations where envelope protection might be a player to all intents and purposes to the end user in a cross wind landing it behaves the same way the "old fashioned" types did...which I like....

[FCeng84]

The 787 is the first Boeing commercial transport with maneuver demand control augmentation in all axes. Pitch, roll, and yaw pilot control inputs command pitch rate, roll rate, and side slip angle respectively. In the presence of atmospheric disturbances with the pilot controls centered the control system strives to keep pitch rate, roll rate, and sideslip angle zero. This means that the airplane response to an atmospheric disturbance will be less than for an un-augmentated configuration.

The pilot will need to make small inputs to return the airplane to the desired attitude and longer term inputs to manage attitude such that the desired trajectory is achieved. A key to reduced pilot workload is to let the system manage the initial, high frequency response to a disturbance and only make pilot inputs as needed to control attitude and trajectory long term.

[vapilot2004]

Each operator generally provides their own limitation guidance.

On the Airbus, the FBW is not designed to correct track errors during crosswind approaches (what do you want - the aircraft to do everything for you?) so one must put in the correction manually - the near lock to lock inputs required often surprise those new on type.

Design considerations (Boeing and Airbus) include 1. low speed rudder and roll authority (sideslip & crab) and 2. wingtip and engine pod clearance (sideslip). Both manufacturers recommend the crab/de-crab method as it eliminates the latter dictating limits.

Manufacturer Published Max Demonstrated:
A320 - Dry Runway/Good Braking Reported: 33 knots Landing (gusting to 38)
B738 - Dry/Wet Runway/Good Braking: 40 knots Landing (continuous)

[Piltdown Man]

I unable to comment on the types mentioned above but I can tell you about the Embraer 190. The rudder and elevator control is excellent. It goes exactly where you point it and as quickly as you expect. But the ailerons, put simply, are horrible. The aircraft has a superb rate of roll, but you can't use it. The main reason is that there is no force feedback apart from springs and their loading feels binary, complete with a "clunk" in the centre. As a result, it is difficult to fly the aircraft with any degree of accuracy in gusty weather. Rather ironically, these are not fully FBW but sort of cable controlled. The FBW elements are the roll spoilers.

PM

[Jwscud]

Each operator generally provides their own limitation guidance.

On the Airbus, the FBW is not designed to correct track errors during crosswind approaches (what do you want - the aircraft to do everything for you?) so one must put in the correction manually - the near lock to lock inputs required often surprise those new on type

That was what I was trying to get at - you make control inputs to manage the flight path - keep the aircraft on glide and centreline while the FBW does the donkey work of keeping the roll/pitch attitude you have commanded to maintain your desired path.

[vapilot2004]

That was what I was trying to get at - you make control inputs to manage the flight path - keep the aircraft on glide and centreline

The case for any aircraft, yes? And I agree, JW, we seem to share an understanding here.

while the FBW does the donkey work of keeping the roll/pitch attitude you have commanded...

Nice as it is, the FBW is not always quick enough to restore a previously commanded attitude, particularly on approach with gusting or variable winds. Countering the gusts is a mostly a manual affair.

...to maintain your desired path.
Yes...meanwhile drift is occurring, hence the sidestick gyrations that can border on PIO. (your reference to pot stirring)

As I understand it, due to how the FBW is programmed for rate and G, control input deflection and timing will be at odds with what worked previously on a conventionally controlled aircraft. It takes practice in the actual aircraft to get 'in sync'. Sim fidelity is not adequate, in my experience.

[vilas]

Basically FBW only allows FMGS or side stick ordered changes to flight path. Anything coming out of thrust/weight couple as in GA or atmospheric disturbances it will resist. That adds to the stability. But once displaced you need to bring it back. Here comes the difference in handling, AB FBW is flown with intermittent inputs because each time the SS is out of neutral you command the computer to do something. So give an appropriate input and back to neutral and again what is required and back. Otherwise you may set up PIO.

[Aztec Kid]

FCeng84, based on your previous posts, you were involved with the design of the B777 flight control system. I must commend you and your colleagues for your brilliant success. When flying the B777, because the control feel and response are so balanced and traditional, I never had the impression that I was in a FBW aircraft.

I wonder if you could share more details regarding the distinctions between the control laws on the B777 verses the B787, especially as it would relate to cross control inputs that would be required during a sideslip to crosswind landing.

In the past, I have had reasonably good success with the sideslip technique in the B777. During the flare, this would require applying and holding aileron input, while applying and holding opposite rudder to maintain proper longitudinal alignment. Would a pilot perceive any differences with flight control laws on the B787?

[Microburst2002]

It is the same in Airbus, but it is in my opinion not very well explained in Airbus literature.

The system is stable, which means that it tends to resist displacement, but it doesn't mean that it prevents it, and by no means will it make the airplane recover the initial parameters.

In the Airbii, the best you can do in gusty conditions is to let the airplane resist gusts, and after that you bring the airplane to the desired flight path and speed. If you try to counter the gusts at the same time as the FBW does, then the coupled inputs will be excessive and you will be continuously swinging.

gust-FBW-pilot input…..gust-FBW-Pilot input…gust-FBW-pilot input and so on, instead of gust-pilot input nonstop all the way to the threshold.

FCeng84, that is interesting, the fbw yaw. Does it have a protection? I mean, normally in gusts it is not a good idea to kick the rudder to keep the ball centered (AA A300 in NY) but it is recommended to let the fin do its job naturally.

[roulishollandais]

Why are we seldom / never reading actual algorithms instead of long partial periphrases without figures ?

[vapilot2004]

Max demonstrated crosswind, from our books, is 38kts including gusts for both dry and wet runways.

Mr. ECAM, if you look under limitations in the manufacturer's certification data, there are published numbers for continuous crosswind limits and that number for the A320 is 33 knots with dry runway/good braking. This is a certification number - many airlines publish their own, often lesser limits.

I was using a commonly accepted standard to describe the limits - maximum continuous crosswind - which is also a certification standard. For whatever reason, Airbus chooses to publish their own additional limits, including gusts, naturally raising the figures.

Since the numbers are demonstrated using Normal and Direct Law, it is not a FBW limitation. My earlier post, here (http://www.pprune.org/tech-log/565237-fbw-handling-qualities-gusty-conditions.html#post9060389), includes the general design reasons for the limits if you are interested.

[misd-agin]

In my experience 777 flew normal in crosswinds. No special tricks, just be a pilot.

[FCeng84]

Aztec Kid - thanks for your response and inquiry about the lateral directional handling of the 787. As I mentioned in my earlier entry, the 777 has maneuver demand augmented control in the pitch axis provided by the C*U control law. This has been carried over to the 787 with little modification. The biggest HQ difference between 777 and 787 is the maneuver demand augmentation introduced on the 787 for lat/dir control.

The 787 includes a control function that Boeing calls "P-Beta" referring to the common nomenclature for roll rate (p) and sideslip angle (beta). This system drives the wing roll surfaces and the rudder to achieve a desired roll rate / sideslip angle combination as commanded via the wheel and pedals.

For wheel inputs alone (feet on the floor - no pedal input) the system commands to a roll rate proportional to wheel displacement. For bank angles in the +/- 30 degree range turn coordination and compensation is provided such that no column or pedal inputs are needed to fly constant flight path angle coordinated turns. At higher bank angles the turn compensation is removed such that column pull is required to keep the flight path from decreasing. In addition, the Boeing Bank Angle Protection (BAP) function comes active at when bank exceeds 35 degrees generating a wheel force that the pilot must overcome to continue to command roll in the direction of increasing bank angle. (Note that BAP was introduced on the 777 and is essentially the same on 787.)

For the 787 augmented airplane response to pedal Boeing decided to stick with the philosophy of designing the augmented response to be similar to that of a well behaved unaugmented airplane. It was decided that pedal inputs should generate both sideslip and roll rate. This gives rise to the need for cross control when encountering cross winds. It was felt that pilots expect this characteristic and would find the airplane unnatural if it were removed. Retaining roll response to pedal input also helps with ground-to-air and air-to-ground transitions as the control system on ground provides direct coupling (wheel to wing roll surfaces and pedal to rudder). As such, on ground the pilot needs to make both wheel and pedal inputs to keep wings level while maintaining centerline during takeoff and landing. The intent is to provide a system that requires approximately the same amount of wheel / pedal cross control on-ground and in-air to minimize takeoff and landing transients.

The intent of the FBW augmentation designs for the 777 and 787 is to present the flight crew with airplane response characteristics that are intuitive and feel natural. The goal has been to have pilot techniques developed on earlier Boeing models translate seamlessly to these models with augmented manual control modes.

Pilot feedback regarding handling qualities on 777 and 787 would be most appreciated and helpful. Note that the 777X now under development will use the 787 control system arrangement as its baseline. This 777 derivative will have both the C*U pitch and the P-Beta roll/yaw control laws delivering maneuver demand augmented control in all three axes.

[HOMER SIMPSONS LOVECHILD]

FCeng84
Thank you sir ( madam ? ) for your interesting and informed post. A rare breath of fresh air.
I rarely post on pprune these days. Like most Professional pilots, I'm worn down by the fsm wannabies, 14 year old spotters, monday morning quarterbacks and 500 hour bank of mum and dad poseurs posting uninformed drivel about all and sundry.
Every now and then the site throws up a blast from the past - Informed professional input from someone who is actually qualified to comment on the subject matter.
Without getting into the whole A versus B chestnut- your post also highlights the difference in " design philosophy" between the two big guns. Boeing- designed for pilots - Airbus - designed for Bernard Zieglar's doorman ! :ok:

[Amadis of Gaul]

Outta curiosity, who is Bernard Zieglar?

[tarkay01]

https://en.wikipedia.org/wiki/Bernard_Ziegler

He is the architect of Airbus FBW.

[vapilot2004]

Also A of G, or anyone else that is interested, if you have a look at this thread starting at page 3 (http://www.pprune.org/tech-log/559226-a320-dual-input-side-stick-priority-3.html), member DozyWannabe leads a good discussion, with links, on a Mr. Gordon Corps, a gent who was the true pioneer of FBW technology ala Airbus, as opposed to the (apparently) well-promoted M. Ziegler.

[Rick777]

I agree with the guy who said just be a pilot. I'm typed on everything from 707 to 777 and have 4000 hrs in left seat of the A320. Just fly the airplane and don't worry about fly by wire or fly by cable.

[FCeng84]

Rick777 - thanks for the feedback. Any specific comments on the ground-to-air and air-to-ground transitions in crosswinds would be appreciated. Has your experience revealed any peculiarities that have led you to modify your technique for these takeoff and landing transitions? I'm particularly interested in how you find the cross control balance through takeoff and landing.

[Aztec Kid]

Hello FCeng84,

Thank you for such an enlightening description of the B787 flight control system. You have the ability to describe the essential elements of flight control system in a way that promotes a much better understanding by a typical line pilot, like myself.

I realize that the complexities of today’s aircraft make it impractical to include all of the system details in the FCOM. However, I do wish that Boeing could provide a supplementary document that contained details of the flight control system. You seem like you would be the perfect person to write such a document.

I am willing to share my observations of the B787 flight controls, as long as you understand these comments are from someone with the ink still drying on their type rating certificate. Since I have such limited experience on the B787, I would definitely like to hear from other who have been flying the aircraft for a while.

Overall, especially in flight, I would say that the handling characteristics are superb. The aircraft is quite responsive, with control forces are which light and well balanced.

While my crosswind experience in the actual aircraft is limited to relatively benign levels, I definitely had the sensation that the flight control system was applying rudder inputs (without rudder pedal movement) during both the takeoff and landing roll. At times I found this to be a bit disconcerting. My rudder inputs had to be made with care to avoid creating pilot induced yawing oscillations.

This tendency for PIO seems to be slightly more evident during the landing rollout in a crosswind. A number of respected colleagues have shared similar observations. I also notice when taxiing, and using only the rudder pedals, the nosewheel steering can be overly sensitive.

Other than the sensitivity of the rudder during the takeoff and landing roll, the crosswind characteristics feel quite conventional. While the similarities to the B777 are apparent, the B787 seems to have a lighter, more responsive feel. When flying the B787, I had good success with side-slips during crosswind landings, just as I did on the B777.

I apologize if my descriptions lack the technical details that you would obtain from an engineering test pilot, but I do hope they are of some use. If you should have any additional questions, please let me know.

Your posts on PPRuNe are excellent. Thank you very much for taking the time to contribute to this forum.

Sincerely,
The Kid