SCAS Hardover

• Background
• Causes
• Symptoms
• Emergency Procedure

Background

Most modern helicopters employ stability augmentation systems (SAS), or stability and control augmentation systems (SCAS), to reduce pilot workload and improve stability. These systems include actuators that work in series with pilot controls to manipulate rotor feathering and hence control the aircraft. In rare situations, an actuator can malfunction and be driven to an extreme position. This is called a hardover.

Causes

A SAS/SCAS actuator may experience a hardover due to

• a glitch in the software that commands the actuator,
• a mechanical failure in the actuator (age/wear, extreme loads, …), or
• environmental factors (extreme heat, humidity, vibrations, …).

Symptoms

The onset of a hardover typically causes an uncommanded pitch, roll and/or yaw rate. Afterward, pilot workload is increased as the stability provided by the SAS/SCAS is lost in the affected axis.

Emergency Procedure

Most manufacturers recommend first disengaging the SAS/SCAS if a hardover is detected. This prevents the chance of a “double recovery”: if the hardover releases while the pilot counters an uncommanded attitude change, then an even more extreme excursion could occur in the opposite direction (and even induce panic).

After the SAS/SCAS is disengaged, the pilot should regain control of the aircraft. This will require the affected control(s) to be in a nonstandard position—e.g. the cyclic may be offset to the left of the normal position.

If possible, the pilot may re-activate the unaffected SAS/SCAS channels. For example, if the hardover occurs in the pedal/yaw axis, then pitch SAS/SCAS may be re-enabled.

The aircraft should then be kept within a potentially limited envelope due to the missing SAS/SCAS axis. For example, most helicopters must fly at lower speeds without SAS/SCAS.

Finally, the pilot should land as soon as practical.