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ARISTOTEL: Addressing complex pilot-aircraft coupling issues in the sky

A highly trained pilot is at the controls of a helicopter. She is attempting to lift off and deliver a payload that is suspended, like a pendulum, below the craft. Suddenly a gust of wind tilts the rotor slightly; the pilot attempts to right the helicopter, but over-compensates. A life-threatening situation has emerged in a matter of seconds.

Tags: Air

Pilots and their machines need to work together like a married couple. But like any relationship, there can sometimes be a communication breakdown. The time delay between a pilot's reaction to a sudden event, their input and the aircraft's reaction, for example, can cause serious problems.

This phenomenon is known in the industry as aircraft-pilot-couplings and rotorcraft-pilot-couplings (A/RPCs). A ground-breaking EU-funded project, entitled ARISTOTEL ("Aircraft and rotorcraft pilot couplings: tools and techniques for alleviation and detection'), has focused on improving our understanding of how pilots immediately react to an unexpected trigger – such as a sharp gust of wind – and attempted to build this into future pilot training and aircraft development.

Understanding pilot – aircraft interaction

"A/RPCs used to be known by the rather simpler term "Pilot Induced Oscillations', but this changed in 1995," explains project coordinator Dr Marilena Pavel, Assistant Professor at the faculty of aerospace engineering at Delft University (NL). "This is because it was realised that these oscillations were not just due to pilot instabilities, but better described as to do with a pilot's interaction with the aircraft."

A/RPCs have been an issue since the advent of human flight. "Back in 1902, the Wright Brothers were also experiencing stability problems, and in trying to control their craft, were over-compensating," says Dr Pavel. "This was partly due to the time delay involved, from the input of the pilot to the machine responding. So the pilot sometimes over-adjusts, rather than waiting for the aircraft to respond. This is essentially the same problem we may have today."

In fact, the significant level of automation in modern aircraft design could potentially lead to more instances of dangerous A/RPCs. While automation helps to relieve pilot workload and enables flights to operate in poor weather and limited visibility, it also increases the chances of pilots "fighting' against an automated control system. Again this is due to the time delay between pilot input and the machine responding; there is a risk that the pilot might not wait for the plane's automated system to adjust to an unexpected trigger before acting.

"This time delay between a pilot's reaction to a sudden event, their input and the aircraft's reaction will always be there," says Dr Pavel. "It might be a matter of milliseconds, but this can be crucial, for example in a helicopter carrying a swinging "pendulum' load below. This swing can be very difficult to control, and we've seen A/RPCs come into play here. The real danger is reaching a point of instability from which you cannot recover."

Tools for the future

Accepting that A/RPCs will always be a factor in aircraft design and flight, the project focused on better prediction methods. "We don't have good A/RPC prediction tools, and we know even less about rotorcraft, such as helicopters," says Dr Pavel. "So we wanted to develop these tools, based on a better understanding of pilot behaviour."

ARISTOTEL sought to assess the transition time between pilots' behaviour before a sudden trigger and after, and how exactly they adapt to external inputs. Pilots may be unaware that they are contributing to the unstable situation (unconscious input). They may try to grip the controls harder in demanding situations which induces involuntary motion – especially dangerous for helicopters carrying a swing load.

"There are lots of factors that go into a pilot providing unconscious input, and we wanted to predict them," says Dr Pavel.

Such findings will have direct implications for the aerospace industry. By incorporating them in the design process of future aircrafts, flight safety has the potential to be improved and pilots better trained to recognise A/RPCs and take proper action.

"This will help to minimise the risk of pilots losing control of their aircraft," says Dr Pavel. "In this sense, the project will enhance European aircraft safety, and will also strengthen the competitiveness of the European aeronautics industry through the development of more time- and cost-effective design tools."

Indeed, having just held its final workshop in September 2013, the project can look back on three years of cross-border cooperation and discovery. Some of the end results include a revolutionary way of looking at pilot decisions and control during an A/RPC.

"We also developed new A/RPC design guidelines and criteria for the industry, to unmask A/RPCs during design. Protocols and guidelines on how to test for A/RPC in the simulator have also been developed and published," notes Dr Pavel.

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