Is there any evidence that the A320 family is significantly (from a statistically point of view) safer, has lower operation costs, better dispatch reliability, or less maintenance-intensive than the 737 family? Why isn't every airline buying A320s instead of 737s?
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Originally posted by Evan View Post...I just feel a yawn coming on.
Originally posted by Evan View PostBut now that you got me going on the issue, let's not lose perspective. I never said anything about the number of components; I said mechanical complexity. Mechanical means something moves against something else, creating friction and wear and requiring maintenance and periodic replacement. Solid state components do not have this concern. Software is easily debugged and upgraded. And mechanical cables, pullies, springs and brackets weigh more than electrical wiring.
Originally posted by Evan View PostAnd remember, we are only talking about the flight control system here. The 737MAX still carries a full raft of flight management computers, FADEC/EEC, coms, IFE and even FBW spoilers.
Originally posted by Evan View PostThe only reason it is not completely FBW is because Boeing didn't want to pay for it. The only reason it is not using more composites is because Boeing didn't want to pay for it.
Originally posted by Evan View PostAnd on that two-sided coin of safety, for every bizarre pilot-error accident there are probably countless (including unreported) incidents of the protections saving the day and the flight continuing normally. If you call that a two sided coin flip, it is a very weighted one and will always land with the FBW side-up.Be alert! America needs more lerts.
Eric Law
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Originally posted by Gabriel View Post...has lower operation costs, better dispatch reliability, or less maintenance-intensive than the 737 family?
My Audi is 26 years old, has ~240,000 miles on it, and has a cable-operated throttle. The original cable is still in place and working fine, and if it fails a replacement part costs about $20 and can be installed in about half an hour. And diagnosing a problem is so simple my dog could do it... if the cable snaps the throttle won't open, if it sticks the throttle won't close.
The newer equivalent cars have "drive-by-wire" motorized throttles. The throttle bodies almost never make it to 100K miles and cost $350 (that's a bare minimum discounted price... at a dealer you'll pay $600). A new one can be installed in about half an hour. And diagnosing one requires a specialized computer and even with that is extremely difficult - there's no direct indication on the computer that the throttle plate is not responding correctly to position commands. And that's only one of the two components that frequently fail in the system... the throttle pedal assembly with potentiometers on it also costs $hundreds - I don't know what their failure rate is. The tradeoff is slightly better driveability and maybe an additional 0.5 MPG? Not worth it IMHO.Be alert! America needs more lerts.
Eric Law
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Originally posted by Gabriel View PostIs there any evidence that the A320 family is significantly (from a statistically point of view) safer, has lower operation costs, better dispatch reliability, or less maintenance-intensive than the 737 family? Why isn't every airline buying A320s instead of 737s?
I believe that the A320 is a safer airplane IF the crews are properly trained for standard, supplemental and unusual situations. The A320 requires a deeper systems-level understanding. We don't want pilots monkeying around with FCC CB's in midflight (that one still amazes me...). If the operators are not training pilots well and lack the culture to do so, no airplane is safe.
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Originally posted by elaw View Post...most of which don't have the direct ability to cause the plane to crash if they fail.
You can't because it's never happened. Engineers had to deal with a very high level of paranoia to get FBW certification. The FCC's are dual channel units that monitor themselves, and there are multiple-levels of redundancy beyond that. And beyond that there is direct law and beyond that is mechanical flight control, which has never and will never be needed.
Now, name one crash caused by the failure of flight control cables. I can think of at least two off the top of my head.
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Originally posted by Evan View PostI believe that the A320 is a safer airplane IF the crews are properly trained for standard, supplemental and unusual situations. The A320 requires a deeper systems-level understanding.
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Originally posted by Evan View PostName one crash directly caused by a flight control computer failure. Just one.
Now, name one crash caused by the failure of flight control cables. I can think of at least two off the top of my head.
My issue with FBW isn't the low-level components of the system which I agree are designed and tested to very high standards... presumably ones expected to provide a similar level of reliability to mechanical systems.
My issue is what I'll call the "AF447 syndrome": that the way they're sometimes implemented, FBW systems have the ability to, often abruptly during a high-stress situation, significantly change their behavior in ways that change or add challenges to the pilots in controlling the aircraft. Especially if the pilots aren't trained well or don't fully understand what they were taught.
Although I'll admit the situations aren't 100% equal, I think a parallel can be drawn between a FBW system suddenly changing modes and reversed mechanical flight controls or certain other mechanical failures. In each case the airplane should be controllable, if the pilot quickly and correctly assesses what's going on and responds correctly in a timely manner. But in the real world there have been numerous situations where accidents happened or almost happened because of pilots reacting improperly to unexpected operation of the automation systems (and yes that sometimes goes beyond FBW). Just as there have been numerous accidents that happened or almost happened because mechanical flight controls malfunctioned in a way that the aircraft *could* have been controlled but wasn't.
Also just for the record, I'm completely in agreement with you about the "cool factor" of FBW and some of its other benefits like reduced weight. And I agree that in certain situations it seems like it should be able to reduce the likelihood of an accident. I just think that if it's not implemented properly or if certain other factors come into play (like pilots' improper reaction to what the system is doing) it has the potential to cause or help cause an accident. Just like a mechanical system could if a hose burst or a cable broke or something.
But statistically, and I think in the airlines' view, FBW doesn't make a difference in accident rates. And the potential economic benefits of FBW could easily be offset by increased costs in maintaining / repairing such a system, as well as any other costs associated with the particular airplane. And other than trying to not kill people, that's what they care about most: the cost of operating the entire airplane, not just one system. The airlines buying non-FBW aircraft apparently think they're just as safe as the FBW ones, and that they'll cost less to operate.Be alert! America needs more lerts.
Eric Law
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Originally posted by elaw View PostMy issue is what I'll call the "AF447 syndrome": that the way they're sometimes implemented, FBW systems have the ability to, often abruptly during a high-stress situation, significantly change their behavior in ways that change or add challenges to the pilots in controlling the aircraft. Especially if the pilots aren't trained well or don't fully understand what they were taught.
AF447 is the result of an abrupt autopilot disconnection, non-existent CRM and subsequent human factors that led to a total loss of situational awareness. None of this was caused by FBW. The sudden loss of autoflight would also happen on a 737 and so might the resulting human factors. I agree that, in manual flight, a yoke with direct feedback is an advantage but the FBW 777 and 787 both provide this. What I wanted to see rolling out of Everett (or Renton) was a short-haul, narrow-body version of the 787. FBW, 50% composite and designed to take it through the next half century of service.
The airlines buying non-FBW aircraft apparently think they're just as safe as the FBW ones, and that they'll cost less to operate.
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Silly discussion Evan and elaw.
The 373 has hydraulic controls that move the control surfaces, not cable and pulley, and the A320 has hydraulic controls, not electric. The difference is that, between the pilot stick/yoke and the hydraulic valve that actuates the hydraulic piston that actuates the flight control in BOTH airplanes, in one case there is a computer (well, several) that interpret the pilot inputs and provides valve commands (via electric wires) to cause the piston and surface control to move as needed for the desired response, and in the other there are cables and pulleys that go directly from the stick to the valves.
In the Boeing approach to FBW, there is not much difference from the pilot's POV under normal circumstances because the control system emulates the response of a traditional plane (although the FBW will oppose more resistance to commands that will put the plane at the edge or outside the normal flight envelope) and for the Airbus approach there is a bigger difference because the control laws depart from the response of a traditional airplane.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Originally posted by elaw View PostSee Gabriel's comment... there's no statistical evidence that FBW makes aircraft safer. I agree that it seems like it *should*, but there's no evidence that it actually does.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Ah okay I misinterpreted your remark.
For what it's worth, everything I've ever read about airliner safety has said there's not enough data to indicate any one model of plane is any safer or less safe than any other and I guess in my head I extrapolate that to mean there's also not enough data to indicate FBW aircraft are more or less safe than non-FBW. But I suppose that might not be true as the number of aircraft that fall into each of those categories is larger than the number of any one aircraft model (duh) so the data could statistically be more meaningful.Be alert! America needs more lerts.
Eric Law
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Originally posted by Gabriel View PostSilly discussion Evan and elaw.
The 373 has hydraulic controls that move the control surfaces, not cable and pulley, and the A320 has hydraulic controls, not electric. The difference is that, between the pilot stick/yoke and the hydraulic valve that actuates the hydraulic piston that actuates the flight control in BOTH airplanes, in one case there is a computer (well, several) that interpret the pilot inputs and provides valve commands (via electric wires) to cause the piston and surface control to move as needed for the desired response, and in the other there are cables and pulleys that go directly from the stick to the valves.
In the Boeing approach to FBW, there is not much difference from the pilot's POV under normal circumstances because the control system emulates the response of a traditional plane (although the FBW will oppose more resistance to commands that will put the plane at the edge or outside the normal flight envelope) and for the Airbus approach there is a bigger difference because the control laws depart from the response of a traditional airplane.
Anyway, great. So all Boeing needs is a narrow-body version of this.
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Originally posted by Evan View PostYes, obviously. Cables vs wires to convey the cockpit controls to the actuators. Although the A380 and I assume the A350 also use electric-hydrostatic and electrical back-up actuators which eliminate the cost and weight of the blue hydraulic system. Going forward, I think this will be the standard FBW solution to redundancy.
I don't know the A350, but the A380 has "only" 2 main hydraulic systems and each main control surface have, as backup, the their own little, local and self-contained hydraulic system (the electro-hydrostatic system). The elecro-hydrostatic system is powered by an electric pump. And not only that, but for each control function there are different control surfaces that have their electro-hydrostatic system powered by different electric systems, to increase redundancy.
Now, this has nothing to do with FBW or not. For all these cases, the command from the stick/yoke to the cockpit can go through cables or wires.
However, electrical linear-motion technology is evolving. I think that there will be a time where the planes will get rid of hydraulic systems. But we are not there yet.
Well, Airbus uses a sidestick traducer to input pilot commmands via a flight path vector indication to a flight control computer that then flies the plane accordingly. You tell it where to go; it goes there. Not a feel-based, seat-of-the-pants type of flying, but the ultimate response is the same (only better).
For longitudinal control in an Airbus, you use the sidestick tell the plane how many Gs you want (at high speeds) or what pitch rate you want (at low speeds) and the FBW will then apply the required control inputs to comply with that. If you center the stick, the plane will stay at 1G (which means that it will keep the vertical speed, which is not exactly the same that keeping the flightpath vector) or will stay at zero pitch rate (which means hold the pitch, which is even more different from keeping the flightpath vector), even if there are changes in airspeed. That means that, in manual mode, you turn the plane form stable in speed and AoA to unstable in speed and AoA, which would be dangerous if it wasn't for the envelope protections.
In a traditional plane, as well as in Boeing's approach to FBW, the plane is naturally stable in airspeed and AoA.
Anyway, great. So all Boeing needs is a narrow-body version of this.
Final remark:
The A320 made its first revenue flight 20 years after the 737 (1988 vs 1968 ).
It has a much more extensive use of composite materials than the 373 (in flight control surfaces and in the fin) and has FBW controls (which I am not convinced that, in the practice, became any significant advantage in terms of safety, operative costs, maintenance intensity or dispatch reliability).
But it is pressurized for 8000 ft and has the "standard" humidity level of any normal airliner like the 737. So it doesn't make a difference for the pax.
It seems to me that both Boeing and Airbus followed the same path (the "lazy" path if you will) and that all things that you criticize from Boeing you should be also doing with Airbus. It would not surprise me if Boeing and Airbus held secret (and illegal) meetings to agree which path to go. If one of them had launched a clean-sheet design, the other would have had to follow through.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Originally posted by Gabriel View PostAFAIK, there are no electrical actuators in any primary control surface in any transport-category plane.
I don't know the A350, but the A380 has "only" 2 main hydraulic systems and each main control surface have, as backup, the their own little, local and self-contained hydraulic system (the electro-hydrostatic system). The elecro-hydrostatic system is powered by an electric pump. And not only that, but for each control function there are different control surfaces that have their electro-hydrostatic system powered by different electric systems, to increase redundancy.
Now, this has nothing to do with FBW or not. For all these cases, the command from the stick/yoke to the cockpit can go through cables or wires.
The back-up systems for the hydraulic actuators with a servo valve (connected to the main hydraulics) is also an electrically controlled pump.
But it is pressurized for 8000 ft and has the "standard" humidity level of any normal airliner like the 737. So it doesn't make a difference for the pax.
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Originally posted by Gabriel View PostHowever, electrical linear-motion technology is evolving. I think that there will be a time where the planes will get rid of hydraulic systems. But we are not there yet.
The really scary thing is it's not intended for use on the wing or tailplane of a fixed-wing aircraft... it's embedded in the rotor blades of a helicopter!Be alert! America needs more lerts.
Eric Law
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