HeliSASTM Stability Augmentation System and Autopilot for Helicopters



HeliSAS was developed by Hoh Aeronautics, Inc. with the objective of providing a low-cost, lightweight stability augmentation system for helicopters.  Hoh Aeronautics, Inc. accomplished many years of helicopter research during the development of the U.S. Army handling qualities specification for rotorcraft, ADS-33E-PRF.  Many of the concepts developed therein were used to create a low-cost, lightweight helicopter SAS/AP system for the civil market.  The original work to develop HeliSAS was accomplished under Phase 1 and Phase 2 NASA Small Business Innovative Research (SBIR) contracts.

The proof-of-concept test aircraft was a Robinson R44 helicopter that was purchased by HAI specifically to develop HeliSAS.  A flying prototype was completed in 2005 and the intellectual property licensed to Cobham in March 2006.  Cobham provided the funding to achieve FAA certification, which was achieved for the R44 in October of 2009.


The HeliSAS system weights only 9 lbs plus the weight of the attitude gyro.  The system will accept any solid state attitude heading reference system (AHRS) that provides standard ARINC 429 outputs.  Alternatively, an especially designed panel mounted gyro built by Castleberry instruments can be used as the attitude source for HeliSAS.

The heart of the system is an attitude-command-attitude-hold stability augmentation system (SAS).  This means that the rotorcraft will always return to a level attitude if disturbed, and will resist pilot inputs away from a level attitude with approximately 3 lbs of force.  HeliSAS is designed as a “fly through” system so that the pilot can easily maneuver with the system engaged throughout the flight envelope from liftoff to landing, including autorotation.  There is significant confusion in the rotorcraft industry regarding the definition of SAS and the difference between an attitude SAS and a rate SAS.  This are discussed in an article that can be addressed by the following link – Rate vs. Attitude SAS.

Recoveries from bank angles up to 90 degrees have been demonstrated in flight test.  If faced with the possibility of spatial disorientation, the pilot can simply let go of the controls and be assured that the helicopter will maintain level flight.  In addition, the attitude command attitude hold feature greatly simplifies the hover task, especially in windy or gusty conditions.  Non-helicopter pilots have been able to hover with no training with HeliSAS engaged.

The Autopilot functions are specifically designed to be intuitive and many pilots report that the system is “easy to operate and does what you expect it to do”.  The autopilot functions include:

·         Heading select and hold

·         Altitude hold

·         VOR/ILS tracking including all-angle localizer intercepts

·         LNAV/VNAV approaches


HeliSAS has been designated by the FAA as a “new and novel design”, and therefore required significant scrutiny for certification.  That is because the SAS is based on a parallel-servo mechanization.  This means that the servos are simply attached directly to the controls, and that all SAS corrections are reflected in cyclic motion.  This was previously thought to be unacceptable because the pilot would object to the cyclic moving in contradiction to his or her control inputs.  Research conducted by HAI for the U.S. Army (using the Canadian National Research Council variable stability Bell 205) showed that if the control algorithms are phased properly, it is possible to use a parallel-servo design that is not objectionable to the pilot because the cyclic moves as expected.  In fact most pilots do not even notice the motion of the cyclic.

This differs from most other helicopter stability augmentation systems which are based on a series design wherein the cyclic does not move in response to SAS actuator inputs.  This involves significant complexity in that the series servo must be limited (usually to less than 10% of travel), and an additional parallel servo is required to keep the series servo centered.  This requires a total of 4 servo-actuators for a two axis system as well as magnetic brakes and cyclic centering springs in each axis  In addition, the FAA requires a control-position instrument for each axis to advise the pilot where the swashplate is relative to full travel, since its motions are not reflected in the cyclic.

Advantages of the parallel system are:

·         Installation is greatly simplified because there is only one servo to connect to the flight controls in each axis, and it is not necessary to cut into the flight control system.  No calibration or adjustments are required during installation or for the life of the system

·         There are significantly fewer components to fail than with a series system.

·         When the SAS is disengaged, the pilot is back to the basic helicopter.  There is no feel spring with potential mis-trim at disconnect to worry about.

·         The pilot always knows where the main rotor actuator is because it is reflected directly in the cyclic.  Control margins are just as obvious as without a SAS.

·         Any system failure can be immediately observed in erratic cyclic motion and easily overridden by the pilot.

·         There is no possibility of servo saturation.  Testing for the U.S. Army showed that servo saturation was highly undesirable and can result in rapid and unexpected rotorcraft motions (such as “digging in” during a quickstop).

·         Research conducted for the U.S. Army showed that an attitude command attitude hold SAS is far more effective as a safety enhancement than a Rate SAS.  In fact the Rate SAS was shown to be of no value in conditions of poor visibility.

The parallel SAS implementation was refined during the extensive 5 year HeliSAS development and is protected by U.S. Patent 7108232.

Because HeliSAS was classified as new and novel, it was subject to a “Special Condition” by the FAA.  This meant that the certification was accomplished under the very stringent 14 CFR Part 29 regulations for heavy commercial helicopters.  This included an extensive systems safety analysis, rigorous environmental testing (DO 160F) including the new and very stringent HIRF requirements.  The Special Condition also required that the system meet the same software testing as very sophisticated fly-by-wire aircraft (DO178B Level A).  These analysis and test results were reviewed and approved by both the FAA Los Angeles Airplane Certification Office and the FAA Rotorcraft Directorate in Ft. Worth. 

Return to Home Page