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How safe is (airbus) fly by wire? Airbus A330/340 and A320 family emergency AD

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How safe is (airbus) fly by wire? Airbus A330/340 and A320 family emergency AD

Old 30th Dec 2012, 22:25
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Originally Posted by 737Jock
And yes there are incidents accountable to airbus FBW, SNPL mentioned 4 where it played a factor!
Which four? Apologies, I cannot read French.
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Old 31st Dec 2012, 01:14
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Cool

Hi,

FBW is no more dangerous than another mode .. it's FBC (Fly By Computers) that can be dangerous
Side note .. I wonder why FBW for those latest generation aircraft
A Tiger Moth is a real FBW aircraft .. the latest generation aircraft are FBEW (Fly By Electrical Wires)

Last edited by jcjeant; 31st Dec 2012 at 01:14.
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Old 31st Dec 2012, 11:24
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Main problem with Airbus is that they tell you it is straightforward, they tell you that the aircraft will look after you, they tell you all about the protections and how they work, they tell you about what happens when the protections are degraded.

What they don't tell you is that because of all these design "improvements" you, the pilot, have to watch it far more closely than a "conventional" aircraft.

Typical case in point is if you are flying an approach with the autopilot disconnected (standard Cat 1) and you have reason to Go Around. A conventional aircraft will pitch up for you as the power comes up, an Airbus will simply fly you into the ground at a higher speed, unless you do some thing about it!
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Old 31st Dec 2012, 13:17
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Quote from Bengerman:
"...if you are flying an approach with the autopilot disconnected (standard Cat 1) and you have reason to Go Around. A conventional aircraft will pitch up for you as the power comes up, an Airbus will simply fly you into the ground at a higher speed, unless you do some thing about it!"

That's a very sweeping statement.
First: if you're talking about jets, you are assuming engines mounted under the wings.
Second: in that case, you are right to say that the conventional aeroplane will lift its nose beautifully. From then on, particularly during flap retraction, you or the APauto-trim will be working furiously to stop the beast sitting on its tail. Have you ever done it?

Last edited by Chris Scott; 31st Dec 2012 at 13:21. Reason: Returning paragraph spacing to what I pasted into the system (as usual!)
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Old 31st Dec 2012, 13:26
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an Airbus will simply fly you into the ground at a higher speed, unless you do some thing about it
- was this not one of the 'surmises' on the Tripoli AB crash?
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Old 31st Dec 2012, 14:56
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Useful discussion of A320 Go-Around mode, SRS and MAN TOGA here:

http://www.pprune.org/tech-log/37144...ound-mode.html

Rule Number 1 : Read the flippin' FCOM.
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Old 31st Dec 2012, 15:06
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Bengerman refers to a manual handling issue, not a managed GA.

Read the flippin' thread?

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Old 31st Dec 2012, 15:16
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That may or may not be so - Bangerman's post is not clear as to whether he's referring to the TOGA buttons fitted to some types (which do use their own automation system) or the aerodynamic tendency of podded-engine designs as Chris Scott describes. Until we get clarification either way, I'm just trying to provide some useful background info.
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Old 31st Dec 2012, 15:22
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From Bengerman, I get an auto OFF surmise, and it is clear at least to me he does not refer to underslung thrust, but Airbus g command Pitch, which moots the effect, in any case.

Follow?

Last edited by Lyman; 31st Dec 2012 at 15:23.
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Old 31st Dec 2012, 15:28
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Yes I follow - but I'm not going to presume until the intent is confirmed one way or the other. "Stable flightpath" pitch command or no, if you have positive pitch and select TOGA then the aircraft will climb.

Also, I'd appreciate it if you didn't try to pick holes in what I'm saying to elicit a response. I don't want to get into a slanging match with you - frankly, your dismissiveness of any line of inquiry that doesn't fit your preconceived notions has run its course with my patience - and I'd rather keep schtum and see what others have to say. Hope you're OK with that.

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Old 31st Dec 2012, 15:35
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Perhaps I am misreading your purpose, and you are merely trying to supply background. Because if one looks closely it could appear that your purpose is to smear scent on the trail that is not the animal we track....

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Old 31st Dec 2012, 16:21
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OK.

B737-400 auto throttle in, auto pilot out, CAT 1 approach G/A from, say, 200'. In the time it takes to say "Go Around, Flap X" apply nose down trim to counter over-rotation from pitch couple in a light aircraft....If you do not then you have to push quite hard! All B737 pilots know this!!

Airbus in same config has no pitch couple so after power shoved into TOGA aircraft will simply accelerate UNLESS pitch up is manually applied...result one smoking hole!

The title of the thread is "How safe is (airbus) fbw?

Last edited by Bengerman; 31st Dec 2012 at 16:23.
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Old 31st Dec 2012, 16:46
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Originally Posted by Bengerman
The title of the thread is "How safe is (airbus) fbw?
As far as I'm concerned the answer is and has always been "not perfect, but nevertheless as safe as anything else out there".

Thanks for the clarification - as I read you the procedure would naturally be different in a FBW Airbus than it would in a 734, but a difference in procedure between types is not a safety hazard as long as the correct procedure for that type is followed. I suspect you'd find similar behavioural differences and quirks between Boeing and MD/Lockheed types if you'd flown in that era (which you may have done) - the late 411A made a pastime of enumerating the benefits (as he saw them) of the L-1011's characteristics versus Boeing types.

My non-ATPL status aside, can it also not be said that the Airbus system actually provides for a simpler procedure? Because trim is automatic one does not have to worry about it - simply point the nose where you want it to go with the sidestick, select TOGA and everything else is handled by the system.

"Conventional" is not a synonym for "what Boeing do", and neither Boeing, Airbus nor any other manufacturer can successfully lay claim to having engineered the safest flight control system (although, salesmanship being what it is, they all try to make that claim).

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Old 31st Dec 2012, 18:23
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Go-Arounds

Bengerman,
I carefully addressed your manual go-around scenario, but you missed my points completely. (Are you a B737 pilot?) I'll start again.

In Direct Law, the Airbus would experience a marked pitch up, because of its under-slung engines, like a B737. A Fokker 100 or B717 would not, because of their engine configurations.

In Normal Law, the A320 pilot simply eases the stick back to achieve the desired pitch change. That is, he/she initiates and controls the pitch attitude, which is intuitive. Later, as the pilot lowers the pitch and the flaps are progressively retracted, the autotrim will respond to keep the elevator close to neutral, and stick movements will be minimal.

You accept that the B737 usually needs to be trimmed forward during the go-around, to stop over-rotation. Hardly intuitive, is it? You presumably also know that it's going to need a great deal more down-trimming during flap retraction. Pilots do what they have to do.

Perhaps I should add that, having flown the VC10 and BAC 1-11, which fall into the B717 category; the B707, A310 and DC10, which fall into the B737 category; and the A320, I've done manual go-arounds in all of them.

Chris
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Old 31st Dec 2012, 18:26
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Dozy,
Clandestino is spot-on: Strasbourg and Bangalore. (Right now, I can't remember which came first).

Strasbourg
resulted in a mod to the FCU, and the later addition of the selected rate to the relevant FMA on the PFD (VS or FPA). Because of limitations of space, the relevant FCU selector knob and associated selected-rate window are dual-function: VS or FPA. Changing from VS (actually HDG/VS) mode to FPA (actually TRK/FPA) mode is achieved by a push-button, confirmed by an adjacent indicator. Originally, IIRC, a selected VS of 3300ft/min DOWN would be displayed in the selected-rate window as -33. A selected FPA of 3.3 deg DOWN would be displayed as -3.3, IIRC. The similarity of the two readings is thought to have misled the crew but, as Clandestino also points out, that assumes the crew also failed to ensure that the relevant FMA was indicating FPA (if the theory is correct, it would have been indicating VS). The FCU modification changed the selected VS displayed from hundreds of feet/min to ft/min, giving -3300 in the above case. Once the FMA addition was incorporated, we encouraged crews to use that as the primary read-out, once you had found the knob and started turning it. That way, you observe the changing rate on the PFD, and minimise interruption of your flight-instrument scan.

The following is off-topic, and will be old-hat to pilots of aircraft with full EFIS, but may be valuable news to others:
Operating Airbuses (and other modern types) demands the constant use and monitoring of the FMAs (flight-mode annunciators). Incorporated into the PFD, they enable you to confirm you have the desired autoflight mode, while minimising the time your scan is removed from the PFD. (The same applies to the indices of selected speed, selected HDG/TRK, and selected altitude.)

Some FMA changes are worthy of mandatory callouts, but these are all worthless unless made by the pilot NOT operating the FCU. (When the AP is engaged, FCU selections are normally made by the PF.) Unfortunately, no two fleets agree which calls should be made (calling them all would be bedlam). Where available, FMAs are the primary indicators of autoflight configuration, with or without the AP.

Moving on to Bangalore, the same applies. For the descent, the practice of dialling zero feet into the altitude selector and selecting OP DES ("open descent") with A/THR was always a hostage to fortune, but where it became insidiously dangerous was when the PF elected to break off instruments, and fly a visual approach without turning off both flight directors (FD1 and FD2). The reason is that OP DES forces the A/THR into idle mode, which is clearly indicated as IDLE on the thrust FMA. The FD gives pitch-attitude commands to maintain the speed selected by the crew (and displayed on the ASI). If and when the PF ceases to follow those commands, the speed will rise or fall, the thrust staying at idle (unless manual thrust is used). We quickly learned to turn off both FDs, and monitor the thrust FMA change to SPD (speed mode), in which the autothrust controls the speed. Sadly, that lesson had apparently not been learned by the crew at Bangalore. (Note my paragraph above.) After the accident, Airbus introduced an automatic mode change of the A/THR from IDLE mode to SPD mode in the event that the current speed falls below (IIRC) VLS.
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Old 31st Dec 2012, 18:45
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Thanks for the clarification Chris, so if I've understood you correctly the ergonomics problems were primarily autoflight- rather than FBW-related.

Last edited by DozyWannabe; 31st Dec 2012 at 18:47.
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Old 31st Dec 2012, 19:05
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IT148

Following the IT148 crash, the display windows were widened to include extra digits IIRC.
This is where Boeing has an edge as all relevant display windows are totally physically separate.
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Old 31st Dec 2012, 19:11
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A320 Flying Modes

There are a total of 17 different flying modes on the A320 with all the possible combinations of A/P on/off, A/THR on/off, OP/DES, OP, CL, OP/CL and what have you...from an article entitled "Modal Confusion".
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Old 31st Dec 2012, 19:23
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Originally Posted by Kimon
Following the IT148 crash, the display windows were widened to include extra digits IIRC.
Almost - the relevant display was given different multi-segment components to display a small "00" when in V/S mode.

This is where Boeing has an edge as all relevant display windows are totally physically separate.
Boeing and Airbus bought the autoflight interface components from the same supplier - Honeywell. The change (or similar changes in multi-function windows) was applied across all affected types, including some from both manufacturers - e.g. the 757 and 767 had a shared annunciator window for V/S and FPA mode.

Regarding flight laws (which are a distinct concept from AP/FD/ATHR), they are grouped into three - Normal, Alternate and Direct. Alternate has distinct sub-categories depending on the failure that caused it to activate - but in real terms all the pilot has to keep in mind above Direct is that only Normal Law has hard protections and autotrim is active in Alternate.

Turning the autoflight off renders the alphabet soup you refer to moot (just as it does in all modern airliners).

Last edited by DozyWannabe; 31st Dec 2012 at 20:02.
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Old 31st Dec 2012, 21:03
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Originally Posted by HN39
Agreed. But even without knowing the threshold value, it is unlikely that alpha-floor would be activated due to freezing of the alpha sensors below M.53 in a typical climb speed schedulerly in the climb, in the graph before the acceleration to 300 kt at FL100.
Thank you, in the meantime, apparently according to your graph, for Mach 0.5 alpha-max is around 9 deg and alpha-stall warning around 8. therefore it is not entirely far fetched to think that a light 330 with AoA probes blocking around 10 deg early after takeoff would encounter the favorable conditions for alpha-floor activation before reaching the Mach number of .53 ...

Note : Is it possible the dashed blue line should be above the solid blue one ... ?
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