hikoushi

Hello!

Since you folks here always seem to have the answers, I have a question for you. A few, really.

I've been flying the 717 for about 4 years and am just about to start training on the A330. Finally bit the bullet and decided to go take a crack at the big French whale. Looking forward to it immensely, and in the meantime enjoying the last few months of honest flying in an honest airplane. Which brings up the question.

What the hell does it feel like to physically fly the stupid thing? I mean, let's say you have the autothrust OFF and the autopilot OFF. You are level at say, 10,000 feet at 250 knots. You pull the throttles to IDLE without touching the sidestick. What does it do? In every aircraft I've flown up to this point, which includes the 717 as well as a smattering of various regional airline and GA planes prior, the plane will drop it's nose and ATTEMPT to capture it's trimmed airspeed, and enter a descending phugoid of some kind while trying to sort itself out. Instead of letting it bob around drunkenly, we apply a few gentle "guiding" nudges of pressure and maybe a small trim change will establish a flight-idle descent at that same speed.

I understand that in Normal Law the sidestick basically commands a load factor, or "G" request (I haven't hit ground school yet so stop me if I'm wildly wrong at any point). Does that mean that the Airbus, if I idled the throttles without touching the stick (A/P and A/THR OFF) that instead of dropping the nose, the plane would try to maintain 1 g by holding altitude and bleeding off speed until it hit Alpha Floor (or whatever the hell that is called), or would it pitch down like a "normal" airplane and just do so very gently and precisely while staying close to 250? What does it do? I am again talking all manual hand flying here. Is it maintaining flight path vector, or airspeed if you move the thrust levers around without moving the stick?

Also, again, stupid question here. I am hand flying the airplane in level flight with the autothrust off. I select (I believe you call it) open descent / FLCH at 250 knots. I pull the throttles to idle and pitch down for the flight director. Airplane stabilizes in the descent. I let go of the stick. I push the throttles forward about halfway without touching anything else. What happens? Does the airplane make any pitch adjustments to maintain the selected airspeed, or does it maintain it's attitude and flight path, and let the speed increase? Again, this is with everything OFF.

Just curious about how the airplane actually FLIES. Will get all the deep technical stuff in the schoolhouse when the time comes, just really curious how the airplane behaves itself when you actually try to physically fly it like an airplane. Don't care about behaviour with degraded control laws, etc; will get to that once I get my head unplugged re: normal law handling characteristics.

Specifically referring to the A330 here, but assuming it is the same for all Airbus models, so please chime in. Thank you in advance!

TyroPicard

If you are able not to think of the A330 as a "stupid thing" you might pass the course....

Jonty

The aircraft is pitch stable. Not speed stable. In all cases it will try to maintain the selected pitch until you reach one of the reversion limits (normal law).

windshear-a-head

Not sure if the 717 has it but it is a very similar feel to CWS on Boeing. The 330 was my first Airbus too, I'm not keen to be honest, at first its a very un-natural way to fly, eventually though like most things it becomes second nature and you realise how little you have to actually touch it (your best mates **** etc).

In gusty/windy conditions the 330 especially can become a handful.

The real joy comes with ECAM mixed with QRH.

Good luck.

Silvio Pettirossi

Well, I can try to answer how the a320 would react and AFAIK, it should be the same on the a330;

First case: With A/THR and autopilot off in level flight, thrust at idle, airspeed will slowly bleed off while the plane maintains level flight/1g as you said. Reaching alpha floor AOA it will go into TOGA, speed will increase with the plane still in level flight until a few knots above Vmo/Mmo where it will start climbing in the overspeed protection, always remaining in TOGALOCK.

Second case: In open descent, again with every thing off an thrust levers at idle, you pitch down following the FD bar to maintain 250kt. If you let go off the stick it will maintain 1g in the descent you established. With no further corrections on the stick it will not maintain 250kt but only 1g. Now you increase thrust. This will increase your speed but still it will try to remain at the 1g descent untill again, you hit overspeed protection and it will pitch up.

This is writen out of memory without manuals at hand so everyone correct me if I am wrong.
The Bus, in normal law, will probably not stall, spin, overspeed or overstress itself if left unatended but it will happily fly in to the ground.

Stuck_in_an_ATR

One correction. If you set the thrust to idle, the Bus will maintain flightpath until it reaches Valpfaprot. THEN it will pitch down to maintain Valphaprot. Alpha floor won't kick in until you pull through Valphaprot towards Valphamax (or so I was taught, no manuals at hand presently).

As to the conversion to the 'bus. Having read all the stuff about FBW control laws, etc., I expected the bus to handle totally differently to whatever I've flown before. However, once in the sim, I've found it quite pleasant to handfly and, surprisingly, quite conventional.

hikoushi

Thank you all for your responses. I think I get the idea. As an aside, if we were to fly the same FLCH / open descent at 250 knots, but this time the autoPILOT was still on, but the autoTHRUST was off (A/P engaged while manually moving the throttles), would the autopilot / FD still adjust pitch to maintain airspeed as I adjust the power? In other words, it sounds like while hand-flying with everything OFF, if you let everything go the airplane is basically always in a "speed-on-thrust" kind of CWS-ish mode. So while flying with the autopilot ON but using MANUAL thrust, does it behave basically like a Boeing or McD would and pitch up and down to maintain the selected speed as I move the throttles around? I know most people do not typically go flying around with A/THR off and A/P on, this is just for educational purposes (although we do it in the 717 quite a bit, but that is just because the autothrottles are slow, and tend to get somewhat "out of phase" and can fairly dramatically overcompensate in the typically very gusty conditions of our area of the world. On final it seems to work much better and give a much more stable and comfortable approach and landing just to leave the power set around 1.09-ish EPR and accept the minor speed variations, manually doing what needs to be done when the changes start to get large).

Again, thank you in advance. And TyroPicard, take it easy, buddy! Please understand that I mean "stupid thing" as an affectionate, sarcastic American-style term of endearment. I'm sure the Airbus is much smarter than me!

NigelOnDraft

This is the theory. In practice it will roughly maintain the attitude, so you will need to increase pitch to maintain level flight.

The theory completely falls down when you start taking flaps when left to itself it will fail to maintain attitude or level, and the AP is not much better in the short term. So some skill / satisfaction can be had in flying to a better standard than the AP

NoD

TyroPicard

No worries, hikoushi, I was trying to be Ironic. Now that I know you are from the USA I won't try again...
TP

Microburst2002

In OP DES, AP or FD will command pitch to maintain airspeed. Thrust changes will only affect rate, if in AP or if you follow the FD.

But let me explain you the concept of airbus fbw. The system is a flight path stable loop system. It will tend to maintain a straight path when the stick is released to neutral (it is springloaded to neutral and its feel is totally independent of speed or any other variable other than the stick deflection angle itself). It is not speed stable as a conventional system. In your 717 you need speed related feel in the stick because you need to trim off stick forces in order to have the airplane tend to fly in the phugoid you desire. When pushing or pulling it, you feel the aerodynamic effect that your input is likely going to have in the airplane. It is not the movement inof the yoke but the force on it which tells you if you will overstress the airframe. At low speeds you will instinctively push or pull to obtain a given pitch rate, at higher speeds you will modulate stick forces to obtain a g load. Stick forces help you to modulate your inputs.

In the airbus you don't need such forces as the system, when the stick is released, will move surfaces on its own to maintain flight path. To change flight path you will push or pull e stick. At low speeds you will move it more or less to achieve a pitch rate. At higher speeds you will move it more or less to achieve a g load. Same as in conventional airplanes. That is why it is very natural to fly. The control law does command g load blended with pitch rate at low speeds and pure g load at higher speeds. Once you achieve desired flight path you just release gently the stick and then you have trimmed it. Releasing the stick equates to trimming. No need for artificial forces. Even with a jammed stabiliser the elevators will deal with it and the airplanestill has autotrim. You still fly with a loop system demanding g load (pure in this case as younwill be in alternate law).

Boeing's fbw is similar in the control law, which demands a blend of pitch rate and g load dependent on speed. But they added a trim speed induced pitch input such that the airplane will behave as speed stable. It still has trimming switches and artificial feel.

Don't resist airbus fbw. Reject wrong notions that many have about it, even many airbus pilots. The protections and the more or less authority they give or take are certainly stuff for debate, yes, but the way it flies is absolutely natural. It is not an AP nor a CWS. You fly the bloody thing, pitch and thrust, just like the 717s., only easier because you don't have to trim

hikoushi

Outstanding information; really appreciate it. Definitely looking forward to flying it now. Happy New Year!

Stuck_in_an_ATR

@ Microburst

Could you please provide any reference for that? The FCOM states they the pitch law is pure "g load", without any blending.

My understanding is that the "pitch rate" law at low airspeeds is only apparent - the sidestick still commands "g", but at low speed you need significant pitch rate to achieve "g" - so the pilot mostly sees and feels the pitch rate. At higher speeds you need little pitch rate to attain desired "g-load", so the pilot feels mostly "g"...

Microburst2002

stuck in

I cannot, since that is totally "transparent" to the pilots in the FCOMs. No reference at all. However as it happens I researched a little about fbw control laws and I found that there is pitch factor blended at low speeds.

It is called a C* law. B777 has a C*U, the U being the trim speed thing. C* is a pitch rate and g load law, the proportion of both depending on speed.

Surf the internet, you will find plenty of stuff.

cheers

Gryphon

FCTM-OP-020 -OPERATIONAL PHILOSOPHY-FLIGHT CONTROLS

NORMAL LAW

CHARACTERISTICS IN PITCH

IN FLIGHT

When the PF performs sidestick inputs, a constant G-load maneuver is ordered, and the aircraft responds with a G-Load/Pitch rate. Therefore, the PF’s order is consistent with the response that is "naturally" expected from the aircraft: Pitch rate at low speed; Flight Path Rate or G, at high speed.

Microburst2002

Excellent, it had totally gone unnoticed by me till now.

as a note, when they designed these control laws, they first researched on what do pilots expect when we introduce inputs in the flight controls and the conclusion was that at low speed, we expect a pitch rate as the output, while we expect a g load as the output, at high speeds. I think they were right. when at high speed I modulate my inputs looking at the effect in vertical speed which is immediate. If the increase is too sharp, I relax the stick, if it is insufficient, I pull or push more. At low speed, I modulate looking at the effect in pitch rate, too fast, I relax, too slow, I push or pull more.

Stuck_in_an_ATR

Actually, if you read the passage from the FCTM carefully,it says nothing about blending:

When the PF performs sidestick inputs, a constant G-load maneuver is ordered, and the aircraft responds with a G-Load/Pitch rate. Therefore, the PF’s order is consistent with the response that is "naturally" expected from the aircraft: Pitch rate at low speed; Flight Path Rate or G, at high speed.

The wy I read this, is that pilot commands constant "g". Aircraft's response is g-load/pitch rate (it always is, in any aircraft!). However, at low speed one needs higher pitch rate to obtain the same g, than at hi speed.

So, for any commanded "g":

Lo speed - pitch rate is large, so the pilot sees mainly pitch rate
Hi speed - pitch rate small, so the pilot feels "g"...

Besides, I guess there's no speed input into the pitch law (alternate pitch law is the same as normal and it works even if all ADR's are lost).How would this g/pitch blending work?

gums

Salute!

Surprised to see the question and the responses.

The AF447 threads went over this and over this and ....

Way I understand it:

- A gee command for pitch that is corrected for pitch attitude, but basically a one gee command with stick neutral. So at 30 deg of climb, the FBW attempts to hold about 0.87 gee. No regard for speed until AoA gets to some limit, and then the system relaxes the gee to keep from stalling. The basic jet seems very benign, but the FBW control laws can get in the way under a few situations/conditions.

- The FBW control laws in "normal" or even a back-up mode continue with this gee implementation except some "protections are lost for max roll and pitch and other "limits".

- The hozontail tail will trim to maintain the gee command and reduce the need for back stick so the elevators can return to "neutral".

Some folks from the AF447 thread can add/correct to this description.

HazelNuts39

That is correct for the steady-state constant "g" that is eventually achieved. AIUI the 'blending' of G-Load and Pitch rate refers to the way the flight control computer controls the transition from the initial "1G" steady state to the commanded "x G" steady state.

The FCC initially orders a movement of the elevator. It monitors the response of the airplane and adjusts the elevator movement accordingly. The 'response of the airplane' is expressed in a 'feed back parameter' C* which is formed by adding the sensed pitch rate multiplied by a constant to the sensed G-Load. Thus the value of the constant determines the 'blend' between the pitch rate and G-Load terms that together form the feed back.

RetiredF4

Sometimes it comes handy to go way back....to 1986
A320: fly by wire airliner

It even answers an old discussion concerning the profile of the tailplane we had in one of the AF447 threads:

mm43

It certainly does, and I recall we agreed on the negative camber at the time.