Tweet
Originally posted by Myndee
View Post
If the question is 'computer flight control and FBW in general: bad or good', than I have to break it to you that every new aircraft introduced now and into the foreseeable future will have these features. Customers (operators) are looking for lower operating costs, and FBW computer flight control provides lower fuel cost and reduced mechanical maintenance costs. And it provides greater range and added safety features to boot.
Now, if your question is 'Airbus vs Boeing FBW philosophy: which is better', that is an ongoing debate. FBW is here to stay (until we get to FBWiFi or FBL or FB4G or something). Both Airbus and Boeing FBW aircraft function similarly during take-off, landing and automated flight segments (autopilot(s) engaged). The principal difference between Airbus and Boeing FBW philosophy lies in the means of pilot/system interaction and the level of authority that pilot is given in manual flight where automated flight is the norm. Boeing seems to trust the pilot more, and has chosen to make the system emulate conventional aircraft characteristics without restricting the pilot from exceeding the safe operating envelope. It gives more physical feedback to the pilot but places a higher workload on the pilot as well. Airbus seems to trust the computers more and has given authority to the computer to restrict the pilot from exceeding the safe operating envelope. It gives most physical feedback to the computers instead of the pilots but, in doing so, has also reduced some of the traditional piloting workload.
The two most obvious places to notice this are in the protections and the trim characteristics. Boeing stall and envelope protections are really only deterrents, which the pilot can override by force. They are there only to inform the pilot, and they assume the pilot will always know the correct thing to do. Airbus protections are actual limitations. The pilot can pull or roll or push all he wants but the plane will not exceed a safe envelope. This assumes the pilot may not always know the correct thing to do.
As for trim characteristics, the Boeing system retains (artificially when in normal law) the self-correcting aspect of positive static speed stability. When the pilot lets the speed depart from the trim speed for the attitude he is flying, he will notice this as control force pressure, as in a conventional aircraft, and will have to retrim to remain at that attitude without having to apply constant force on the column. This self-correcting tendency is known as static speed stability. But at any given time during manual flight the Boeing may or may not be in trim and the pilot needs to constantly correct trim for changes in speed or attitude.
In comparison, the Airbus system essentially automates this retrim procedure, and therefore provides feedback to the computers but no feedback to the controls. The aircraft is never out-of-trim, regardless of attitude or airspeed and so the pilot workload is reduced (aside from minor corrections on the sidestick if the aircraft deviates from its intended flight path.). Since the Airbus computers will not allow the aircraft to fly too slow or pitch up too high, and has overspeed protections as well, there is no need for traditional static speed stability 'feel'.
Those are the main differences in normal manual flight, oversimplified perhaps, but essentially it gives you the picture. Both give superior flight characteristics and allow for less drag and greater range and efficiency than conventional aircraft. Both are immensely well protected against failure of the FBW control system itself.
Since these commercial jets are intended to fly in automation from just after lift-off all the way down to flare, these systemic differences are usually a non-issue, and therefore both are very safe, BUT, and here is the relevant thing: both Airbus and Boeing automated flight require redundancy, both will disconnect if redundancy is lost, and neither system can be considered fully redundant because air data systems are not redundant, since both rely on external sensors that can be expected to fail simultaneously to a common environmental fault, like ice ingestion. Therefore, aircraft control needs to be fault tolerant, adaptable to manual flight without systems that rely on air data (such as stall protection).
This is where the debate centers.
In manual flight, with air data lost and stall protections unavailable, it is essential that the pilot has a means of stabilizing the plane and remaining within a safe speed envelope until the problem clears itself up and autopilot can be restored. Boeing does this via traditional control column feedback. If the aircraft is flying too slow, the nose will lower due to the aerodynamics of positive static stability and the pilot will feel the column pull away from him. To maintain the flight path he will either have to increase airspeed or retrim for the slower speed. It works in reverse for overspeed. Therefore, the pilot can approximate the correct airspeed by 'feel' (but he can also retrim and fly outside the safe envelope, which could lead to disaster). The Boeing uses a 'load-factor demand law' that will normally hold the flight path (altitude in level flight), however:
• An established flight path remains unchanged unless the pilot changes it through a control column input, or if the airspeed changes and the speed stability function takes effect.
Even if he doesn't adjust the manual pitch trim, because stall protections are lost the system will also restore static speed stability at the limits of the speed envelope:
At the flight envelope limit, the aircraft is not protected:
. In high speed, natural aircraft static stability is restored with an overspeed warning
. In low speed, the auto pitch trim stops at Vc prot (below VLS), and natural longitudinal static stability is restored, with a stall warning at 1.03 VS1g.
. In high speed, natural aircraft static stability is restored with an overspeed warning
. In low speed, the auto pitch trim stops at Vc prot (below VLS), and natural longitudinal static stability is restored, with a stall warning at 1.03 VS1g.
In certain failure cases, such as the loss of VS1g computation or the loss of two ADRs, the longitudinal static stability cannot be restored at low speed. In the case of a loss of three ADRs, it cannot be restored at high speed.
Now, in my view, the newer Boeing 777 system is more conducive to manual flight, when air data is unreliable and the aircraft can be safely flown using traditional pilot instincts ASSUMING THE PILOT DOES KNOW WHAT TO DO.
This, in itself, DOES NOT make the Airbus systems unsafe however. Airbus has provided a means to stabilize and control the aircraft during everything from a single system failure (no effect at all) to a full electrical failure (mechanical cable control) ASSUMING THE PILOT KNOWS WHAT TO DO. The requirements for unreliable airspeed procedure are different from traditional piloting, but they are no more difficult to perform. They just require new instincts, which are developed through proper training.
Ideally, I would like to see Airbus integrate additional systems in response to what will be learned here, perhaps even a means to allow the autopilot to continue in an unreliable airspeed mode, at least temporarily. Unreliable airspeed is a transient condition that usually clears up within a few minutes. Also, the detent-range A/THR is unsafe in my opinion and needs to be servo-driven. But, based on the fact that the great majority of fatal crashes are caused by poor pilot judgment (or reaction), I still prefer the Airbus protection philosophy, although they clearly need to make some better provisions for when those protections are lost and the pilot lacks direct 'feel' for static speed stability (the BUSS back-up speed scale system is an indication that they recognize this need).
All that said, none of the evidence released thus far indicates that this crash was related to these aspects of system philosophy, and therefore the result would have been the same in a 777 or a 707.
Comment