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As a result, in such conditions, the flight control systems commands the engines to increase thrust without pilot intervention and with an accuracy that no pilot could achieve.
Human Factors Considerations
The F/a-18D Hornet that slammed into a residential neighborhood in San Diego last December came from the first family of fighter jets with full fly-by-wire technology, where a flight control computer gathers data from on-board sensors to control flaps and other control surfaces that were mechanically driven on planes decades ago. But for all their high-tech appeal, do fly-by-wire systems distance pilots from the feel and behavior of their airplanes to the point that crashes become more likely (Milstein)?
In aviation, human factors is dedicated to better understanding how humans can most safely and efficiently be integrated with the technology. That understanding is then translated into design, training, policies, or procedures to help humans perform better (Human Factors).
The term "human factors" has grown increasingly popular as the commercial aviation industry has realized that human error, rather than mechanical failure, underlies most aviation accidents and incidents.
Because technology continues to evolve faster than the ability to predict how humans will interact with it, the industry can no longer depend as much on experience and intuition to guide decisions related to human performance. Instead, a sound scientific basis is necessary for assessing human performance implications in design, training, and procedures just as developing a new wing requires sound aerodynamic engineering (Human factors).
Because improving human performance can help the industry reduce the commercial aviation accident rate, much of the focus is on designing human-airplane interfaces and developing procedures for both flight crews and maintenance technicians (Human factors).
Even if a faulty flight computer is not directly to blame for this crash, fly-by-wire systems put distance between pilots and the airplanes they fly, so that first signs of problems might be obscured by the computer's automatic corrections. Decades ago, when pilots controlled airplanes mechanically with levers, cranks and pushrods, they felt resistance from wind and could intuitively sense if something wasn't right. Like power steering in cars, fly-by-wire makes flying easier and often smoother because computers are doing more of the work. But it also separates pilots from that touch-and-feel connection with the mechanics of the airplane (Milstein).
John Cox, an aviation consultant and former commercial pilot, said that fly-by-wire technology can sometimes mask damage to an airplane by keeping it flyable even when human pilots couldn't. That could be good, if it allows a plane to get away from populated areas before crashing, but bad if pilots do not know there's a problem. "Fortunately the systems are very good about annunciating problems -- if something goes wrong, they tell you," says Cox (Milstein).
For real-time technology, human-factors development is the task of collecting usability data from man-in-the-loop testing for components that will have a human interface (Why Use...).
An example of usability testing is the development of fly-by-wire flight controls.
Systems developers and testers have always assumed that human compensation is measurable, or, at least, that a cognizant and trained tester is able to identify and detect compensation. More than one study conducted at the Wright-Patterson large amplitude multi-mode aerospace research simulator (LAMARS) facility indicates that this is not necessarily true. Test pilots were able to compensate sufficiently to fly and meet defined performance standards on intentionally crippled aircraft flight control designs. These flight control systems (FCS) were designed to trigger pilot-induced oscillations, but, in most cases, test pilots could compensate sufficiently to prevent pilot-induced oscillations and to control the simulated aircraft (Alford).
Anecdotally, this points to a colossal deficiency in the test of highly augmented aircraft systems, such as fly-by-wire flight control systems, that has been borne out by multiple aircraft accidents in actual aircraft designs: natural pilot compensation is sufficient to allow faulty designs to reach production and operational service while hiding critical handling qualities cliffs that can lead to loss of an aircraft. This observation, if applied across the gamut of human factors experimentation, has vast ramifications for test and evaluation and development of all human interface systems (Alford).
From a human factors viewpoint, it is imperative that these systems take on roles, and provide functions, that are the most supportive to the pilot, given the stress, time pressure and workload they may experience following a FCS fault. For example, highly sophisticated fault recovery systems may be able to fly the aircraft following dramatic FCS failures without even notifying the pilot; however, such systems are not only expensive, but may not be able to compensate for all failures, may fail themselves, or may allow a pilot, believing he or she is flying a sound aircraft, to put the aircraft into a dangerous condition (Pritchett).
The biggest human factors questions are the role suitable for the technology, and its specific functioning to achieve that role. Specifically, for these systems to be effective, they must meet the fundamental requirements that (1) they alert pilots to problems early enough that the pilot can reasonably resolve the fault and regain control of the aircraft and that (2) if the aircraft's handling qualities are severely degraded, the health monitoring system (HMS) provide the appropriate stability augmentation to help the pilot stabilize and control the aircraft (Pritchett).
Pilot Control or FCS?
We have discussed the technical differences between Boeing and Airbus regarding their philosophies of whether or not the pilot should be able to take over from the FCS in an emergency situation. Airbus pilots cannot take over. Boeing pilots can. This is probably the most significant human factors consideration involved with a fly-by-wire FCS.
At Airbus, the pilot is denied access to the section of the flight envelope that is outside the 2.5G limit. The aircraft is capable of operating beyond the 2.5G loading limit; however it is with an increased risk of overstressing the airframe. The Airbus concept is to prevent pilots from overstressing the airframe or stalling the aircraft by limiting possible maneuvers with a flight computer (Bannister, Downie and Hill-Ling).
The fly-by-wire system prevents the pilot from placing the plane into a climb of more than 30 degrees where there is a possibility the aircraft will lose airspeed and stall, or for banking or rolling at an angle greater than 67 degrees. The aircraft's nose-down pitch is limited to 15 degrees. These limits prevent pilot from operating the aircraft outside safe design constraints, however it is possible that the aircraft could survive more extreme maneuvers in emergencies.
The Airbus human factors decision was to exploit the advantages of hard limits which are the reduced level of emergency training required, and the reduction of human error in such emergencies. The system keeps the average pilot within the limits of his or her training and skills. It also eliminates the results of a human overreaction by the pilot of pulling back hard on the stick in the case of an impending collision with the ground, which usually results in a reduction of speed and the possibility of stalling (Bannister, et al.).
Boeing's human factors philosophy provides the pilot access to the entire safe flight envelope and beyond in the event of an emergency. The engineers have employed "soft limits" in the software of flight computers on the B777, providing pilots with audible and visual warnings when the aircraft is approaching a flight condition that could breach these limits. In addition the pilot experiences increased control forces through artificial feel systems when these limits are approached (Bannister, et al.).
When considering air transport and the safety of passengers, the pilot is seen to be ultimately responsible. It is for this reason the Douglas MD-90 has a brake bar the pilot can push the throttles through to engage maximum thrust in an emergency. In the B777 the pilot can attain direct control authority over the plane by activating a single protected switch. The Embraer EMB-145 regional jet has a single button on the control stick that deactivates the stall warning stick-pusher, the autopilot and the elevator trim system, a similar system exists on the Canadair regional jet. Airbus stands alone in this area with no single switch that allows the pilot to gain full control of the aircraft by overriding the computer (Bannister, et al.).
Statistically the accident rates for Boeing and Airbus are about the same.
It is interesting to note that, although all aircraft manufacturers weigh heavily their own human factors research, almost inevitably each manufacturer comes up with a different answer.
Fly-by-wire control systems have enabled aircraft designers to increase the maneuverability of aircraft. However, the reliability of the system today still requires backups to ensure safety.
Aircraft flight control systems." n.d. Absoluteastronomy. 03 March 2009 http://www.absoluteastronomy.com/topics/Aircraft_flight_control_systems.
Alford, L.D., Jr. "Fly-by-wire T & E. challenge [aircraft test pilot handling compensation]." Aerospace and Electronic Systems Magazine, IEEE February 2004: 3-7, Volume 19, Issue 2.
Bannister, Jonathan, et al. "Fly-by-Wire Report." 04 October 2006. Adelaide University School of Mechanical Engineering. 05 March…[continue]
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