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Which of the following statements accurately describes a Stability Augmentation System (SAS)?
  • A
    Utilizes rate gyros or a vertical reference system.
  • B
    Offers long-term attitude hold capabilities.
  • C
    Offers short-term attitude hold capabilities.
  • D
    Exclusively relies on a vertical reference system.

Refer to figures.
Generally, a helicopter’s Stability Augmentation System (SAS) is a device used in helicopters to improve their stability in flight. It works by using sensors to monitor the helicopter's movements and then making small adjustments to the control inputs to help maintain stability.

Analytically, the helicopter is stabilized about its 3 axes which pass through the center of gravity (C of G):

  • Normal axis (yaw)
  • Longitudinal axis (roll)
  • Lateral axis (pitch)

Stability is the property of a body to resist displacement or, if displaced, its response and has:

  • Static (immediate) and
  • Dynamic (subsequent) values.

Static stability is the initial response to displacement, and it can be:

  • Positive (the helicopter returns back to its original position)
  • Neutral (the helicopter remains at its displaced attitude)
  • Negative (the helicopter continues moving away)

Dynamic stability is what the helicopter does over time when displaced, and it can be:

  • Positive;
  • Neutral and
  • Negative (fig.).

Here comes SAS, which:

  • Improves the handling characteristics of the helicopter throughout the flight envelope by damping out the oscillations and providing positive dynamic stability.
  • Holds a selected attitude for a short period of time (slowly, the helicopter will diverge away).
  • Can be selected or deselected on an autopilot panel, which varies with aircraft type but with the same principles or maybe from a 'SAS’ Override' button upon the flight controls.
  • Is a part of the AFCS but does not fly the helicopter. It is considered an 'inner loop' system.

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