In our paper, Safety is emphasised for a simple reason; for conventional fixed and rotary-wing aircraft, accidents linked to adverse aircraft-pilot-couplings (APCs) have continued to occur throughout the history of flight. Often attributed to ‘pilot error’, such accidents are linked with flawed understanding of the underlying aeronautical science by the collective community of pilots, operators and even designers. As tiltrotors, and other unconventional rotorcraft, approach entry into commercial transport service, it is considered important that all potential sources of adverse APCs be thoroughly investigated and fully understood, with risk mitigation strategies defined.

  To stress this point, in the concluding section of the paper, the authors write,

 “A principal recommendation from this study is that the theory should be applied in the handling qualities analyses for new rotorcraft configurations, with coverage of practical manoeuvres throughout the flight envelope. Extensive investigations are required because, as described in the paper, these low frequency or non-oscillatory APCs are often hidden in the closed-loop dynamics, with subtle and insidious consequences”.

In this blog, the authors have chosen to highlight this recommendation because of its importance to safety.

  The idea that behaviour might be ‘hidden’ within closed-loop dynamics, while elusive, is not new. It was first revealed in the Wright brothers’ aircraft during the birth of practical aviation. Wilbur’s and Orville’s solutions to the APC ‘problems’ they encountered have profound lessons for us today; these are discussed in previous Aeronautical Journal papers by the first author. The pilot and the aircraft are elements of an integrated system, in which feed-back and feed-forward control strategies are used to maintain or change flight condition. Efforts to constrain one flight state to be constant can expose the pilot-vehicle system to instability. In contrast, on its own, the aircraft might be stable and the pilot might be adopting a control strategy considered to be effective, and safe. Yet, as the strength of control is increased, a different flight state diverges, perhaps hidden from the pilot’s attention.

  Early analysis of stability of flight under constraint was cast in the form of so-called ‘effective’ stability derivatives, e.g. Neumark’s speed stability X_ueff and Pinsker’s weathercock stability N_veff, as discussed in the paper. Such linear analysis sheds insight into the fundamental aeronautical science at work in these closed-loop instabilities, providing designers and flight instructors with the knowledge and tools to mitigate the risks to safety. Later, the first author, with his project supervisor Ronald Milne, presented a theory to predict the kinds of control strength, the gains in the feedback loops, required to drive the coupled system unstable. APCs were described in terms of the morphing of the flight modes, reflected in loci of eigenvalues and eigenvectors from linear analysis. The paper on this subject, titled The Strongly Controlled Aircraft, was published in the RAeS’s Aeronautical Quarterly in 1971, barely a lifetime since Wilbur and Orville’s first APC experiences.

  The application of this ‘stability under constraint’ theory to tiltrotor aircraft has revealed a range of dynamic behaviours, some expected, some unusual. Concerning flightpath control, the classical surge-mode instability for flight at speeds below minimum power is shown to apply to the tiltrotor in helicopter mode, but can be alleviated in conversion and airplane modes through appropriate design of the thrust-power management system. Furthermore, the impact of flightpath control with cyclic or elevator on the short–period mode is seen to be a trade-off between the stabilising pitch attitude and de-stabilising incidence contributions to the flightpath angle. In airplane mode, roll control with ailerons can expose a potential yaw instability in the presence of adverse yaw.

  These findings come from linear analysis and while considerable physical insight can be gained, the authors recognise the shortcomings, and limited ranges of application, of their approximations. APC behaviour can be nonlinear and tiltrotors are likely to have automatic flight control functions that might compete for authority in manoeuvring flight, eg stability augmentation, structural load alleviation and flight envelope protection. In his seminal report on APCs, Understanding and Preventing Unfavourable Pilot-Vehicle Interactions, Duane McRuer makes the point that, “These features combine to make pilot-vehicle-system behavior at the margins of the control-effector/control-function envelope a multidimensional surface of bewildering complexity.” 

  Bewildering complexity should not deter engineers from investigation of course. Rather, analysis methods that address complexity are required and applied to the wide range of novel rotorcraft configurations to ensure that their future operational record is APC-accident free. For this to be successful, such methods must derive from simulation models with sufficient fidelity that they can predict such behaviour. As government authorities and industry grapple with the complexities of using modelling and simulation for the certification of an aircraft’s flight characteristics, the importance of reliable predictions is particularly germane.

  Prediction is one side of the coin but having criteria for acceptability, and hence safety margins, is the other. Existing handling qualities standards for fixed and rotary-wing aircraft do not fully address the kind of low-frequency, non-oscillatory APCs described in the paper.  For example, requirements for flight on the back-side of the power curve are described in terms of trim characteristics, rather than closed-loop stability.  Requirements for yaw stability of fixed-wing aircraft are described in terms of effective static stability, rather than closed-loop dynamic stability. Handling qualities standards are evolving rapidly as the advanced air mobility industry grows and the authors highlight the opportunity to bring APCs onto the agenda.

  The paper also addresses the human element in the APC conundrum and, to address this, the authors make a recommendation related to the way pilots are trained to deal with APCs should they be unfortunate enough to encounter them. This links with the simulation fidelity issue discussed previously in this blog.

 “As tiltrotors and other novel rotorcraft enter commercial operations it is recommended that the problem-based-learning (PBL) approach described in this paper be included in basic and continuation pilot-training programmes. Since much of the training will use flight simulators, it is recommended that an assessment of the fidelity requirements for such training be undertaken in concert with the development of the PBL material.”

  While the research reported in the paper builds on the very real and often tragic history of aircraft-pilot-couplings, the authors’ primary motivation is to guide designers and operators along a pathway to maximise future safety.

The potential impact of adverse aircraft-pilot couplings on the safety of tilt-rotor operations is an open access article available to read in Volume 126 Issue 1304 of The Aeronautical Journal.

This journal is the world’s oldest aerospace publication currently in production and has been published by the Royal Aeronautical Society since 1897.

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