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Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
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Glossary

The following technical terms are not defined in the report and may be unfamiliar to some readers.

A

Aircraft bandwidth (ωbw).

A measure of the range of frequencies over which a pilot can exert good closed-loop control without excessive compensation (e.g., excessive pilot lead or anticipation). At the nominal aircraft attitude to controller input (θ/δp) bandwidth frequency, ωbwθ, the θ/δp phase angle is -135° (see Figure 6-1).

Amplitude ratio.

The ratio of the amplitudes of the steady-state output and input when the input is a sine wave. This ratio is often expressed in decibels (dB) where [amplitude ratio]dB = 20 log10 × [amplitude ratio].

B

Bandwidth.

A measure of the highest frequency sinusoidal input that a linear system can track with reasonable fidelity. Often defined as the frequency where the amplitude ratio of the system is 3 dB below the zero frequency value.

Bobweight effect.

The overall effect of unbalanced masses (bobweights) intrinsic to or deliberately introduced into various locations throughout a mechanical control system. Bobweight effect can serve the positive purpose of providing cues to the pilot regarding accelerations of the aircraft.

Bode diagram.

A Bode diagram presents system transfer function or frequency response data plotted in rectangular coordinate form. The amplitude ratio (output-to-input) expressed in dB and phase angle are plotted against frequency on a log scale.

Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×

Buffet.

Buffet refers to the unsteady aircraft motion caused by flow conditions over parts of the aircraft, typically during transonic flight or low-speed conditions preceding a stall.

C

C*U integrator.

''C*U" is the popular name for a particular pitch axis control law that Boeing uses in the flight control system of the 777. As seen in Figure 2-10, the C*U integrator provides a feedback signal that incorporates pitch rate, normal acceleration, speed error, and column position. Thus, in steady state, column commands result in an incremental speed change. The other terms in the C*U signal are combined with the column feed forward and pitch rate signals to provide the desired effective aircraft dynamics.

Carefree flight.

A type of flying in which the pilot is free to maneuver the aircraft in a "carefree" manner with little or no concern for particular task constraints. Carefree flying can be an exploratory experiment to discover latent, unanticipated APC susceptibilities.

Cliff.

The "cliff" metaphor is used to convey a sense of unexpected, dramatic, and excessively large changes in aircraft motion associated with relatively slight changes in pilot activity. When cliff-like changes result from an incremental increase in the amplitude of the pilot's output, the pilot-vehicle system is not behaving like a linear system. Instead, this indicates the presence of significant nonlinearities either in the dynamics of the effective aircraft or in the pilot's behavior. Many, if not all, Category II and III PIOs exhibit cliff-like behavior.

Closed-loop feedback system.

A combination of control system elements in which command variables are compared with desired output variables. If the outputs differ from the desired values, corrective signals are sent to control actuating elements to bring the controlled variables to their proper values.

Cooper-Harper Pilot Rating (CH PR).

A numerical flying (or handling) qualities rating (1–10) a pilot assigns to an aircraft and piloting task that indicates the workload the task required and the performance that could be obtained. A rating of 1 indicates optimum handling qualities.

D

Divergence.

An unstable system response characterized by an output of increasing amplitude when the input, itself, is bounded. Divergences are associated with aperiodic (non-oscillatory) APC events.

F

Flight envelope.

The bounds within which a certain flight system can operate, especially a graphic representation of these bounds showing the interrelationships of operational parameters.

Flight management computer.

A computerized system found in the cockpits of modern commercial aircraft that can automate many of the tasks normally performed by the pilot. These tasks include route planning, navigation, fuel management, and aircraft control.

Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×

Fly-by-light.

A control system that uses optoelectronic systems to transmit control information by light through fiber-optic cables.

Fly-by-wire.

A control system that uses conventional electronic systems to transmit control information via electrical cables.

Flying qualities.

Aircraft characteristics that govern the ease or precision with which the pilot can accomplish specific tasks.

Frequency response.

The response of a component, instrument, or control system to input signals at varying frequencies. For example, the frequency response characteristics of a servoactuator are defined as the steady-state relationship of the output amplitude to the input amplitude and the output-to-input phase difference when the input is subjected to constant amplitude sinusoidal signals of various frequencies.

G

Gain.

In general, the ratio of output to input of a control system element. For elements with low-pass filter-like characteristics, gain is the amplitude ratio at zero frequency. Pilot gain is the sensitivity with which the pilot reacts to a given percept. If the situation is urgent, the pilot is likely to react with large corrective inputs even for small system errors. When this happens, the pilot is said to be exhibiting high gain. More relaxed responses imply a lower pilot gain.

H

Handling qualities.

See flying qualities.

L

Limit.

See "rate limit" and "position limit."

Limit cycle.

The name given to a system oscillation in which the frequency and amplitude are determined by the nonlinear properties of the system.

N

Neuromuscular system.

The system that governs human movement, generally consisting of nerves (neuro-), which provide commands and feedback from and to the central nervous system, and muscles ( -muscular), which generate the forces necessary for movement.

P

Peak magnification ratio.

The maximum amplitude ratio for a system determined across all input frequencies.

Phase lag.

The phase difference between the input and output of a system in which the input is a sine wave. The "lag" applies when the output, or response, lags in time behind the input or command.

Phase margin.

A measure of system stability defined as the phase lag to be added to achieve 180° of phase lag at the open-loop frequency response corresponding to the 0 dB amplitude ratio.

Pipper.

A small symbol, typically a circle or square, that appears in the gun sight (head-up display) of a fighter aircraft indicating pointing errors between

Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×

the target and the chase aircraft. Pipper errors are typically described in terms of angular displacement such as "mils" (milliradians).

Position limit.

The maximum allowable deflection for an aircraft control surface or other effector, based on either mechanical limits associated with the actuation system or on lower limits imposed by the flight control system.

Proprioceptive.

Perceptions of forces and movements of the pilot's body (including limb).

R

Rate limit.

The maximum allowable rate of deflection for an aircraft control surface or other effector, based on either the maximum rate at which the actuation system can reposition the control surface or on lower limits imposed by the flight control system.

S

Stability augmentation system (SAS).

A subsystem of the flight control system that uses sensors, actuators, etc., to augment the basic dynamic properties of the aircraft. When considered as an entity, the SAS is essentially a closed-loop regulator control system. SAS signals are introduced in series with pilot inputs so the SAS signals do not cause stick motion or forces, but still serve to modify the effective aircraft dynamics. On older aircraft, the SAS generally has limited authority.

U

Upset.

An upset refers to a sudden, large change in aircraft attitude that was not deliberately commanded by the pilot. Upsets are frequently caused by atmospheric turbulence.

W

Washout filters.

Washout filters remove the low frequency components of commanded cab motion in moving-base simulators.

Workload.

The total of the combined physical and mental demands upon a person.

Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×
Page199
Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×
Page200
Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×
Page201
Suggested Citation:"Glossary." National Research Council. 1997. Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions. Washington, DC: The National Academies Press. doi: 10.17226/5469.
×
Page202
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Adverse aircraft-pilot coupling (APC) events include a broad set of undesirable and sometimes hazardous phenomena that originate in anomalous interactions between pilots and aircraft. As civil and military aircraft technologies advance, interactions between pilots and aircraft are becoming more complex. Recent accidents and other incidents have been attributed to adverse APC in military aircraft. In addition, APC has been implicated in some civilian incidents.

This book evaluates the current state of knowledge about adverse APC and processes that may be used to eliminate it from military and commercial aircraft. It was written for technical, government, and administrative decisionmakers and their technical and administrative support staffs; key technical managers in the aircraft manufacturing and operational industries; stability and control engineers; aircraft flight control system designers; research specialists in flight control, flying qualities, human factors; and technically knowledgeable lay readers.

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