SAS, Autopilots and Flight Directors - What’s in a Name?
R. Fred Polak | Editor
What an industry we work in! It’s full of acronyms, abbreviations and terms that, to an outsider, seem like a foreign language — And it is! To some of us, the terms seem to merge together until we find it difficult to separate and identify just what is what. SAS, AP, FD, CPL, HSI, ADI ... and the list goes on and on.
In the world of avionics, rest assured that whatever system is understood the least will be blamed the most. The “autopilot won’t hold altitude.” “Can’t fly heading select with SAS engaged.” To those of you that have been around these systems for a while, you know what we are talking about. Typically, SAS, autopilot and flight director systems and modes of operations are lumped together and referred to as an Automatic Flight Control System (AFCS). There can be various combinations of single and dual AFCS installations available for a particular helicopter model. The AFCS is designed to reduce pilot workload, improve mission reliability and enhance safety of flight.
To better understand what SAS, autopilots and flight director systems are supposed to do, let’s define their functions and take away some of the mystery surrounding them. Since the flight director system can be flown separately from the autopilot, let’s start with it. What follows is general in nature and not tied to a specific helicopter model or avionics manufacturer.
Flight Director (FD) — The flight director provides the pilot and/or autopilot with computed lateral and vertical steering commands to fly the helicopter along a desired lateral and vertical flight path. Think of it as the pilot’s navigation tool box. Just as there are different parts or segments to each flight (takeoff, climb, cruise, descent, approach and landing), the flight director has different lateral and vertical modes the pilot can use in each of these segments. The flight director steering commands are presented on the lateral and vertical command bars on the ADI/EADI.
The flight director is divided into four basic parts:
• Sensors (raw data)
• Controller (mode selector)
• Loads (command bars and autopilot)
The sensors provide the raw data to be processed by the computer. The flight director mode selector (controller) tells the computer which raw data to use, depending on pilot mode preference. The computer processes the raw data and gain scales the information to be displayed on the ADI/EADI command bars and/or to the autopilot.
For the flight director to compute a steering command, the following has to be considered:
• What is the pilot’s desired helicopter lateral and vertical navigation position?
• What is the helicopters actual lateral and vertical navigation position?
• lf there is a difference between desired and actual position, correct for the difference and control the speed at which the correction takes place.
Autopilot (AP) — As complex as some of today’s autopilot systems have become, they all can be narrowed down to providing at least one main function. That function is stability. In essence, the autopilot takes care of the routine repetitive tasks and allows the pilot the ability to concentrate on other flight concerns.
Today’s modern helicopter autopilots can be three-axis or four-axis systems. A three-axis system provides pitch, roll and yaw axis stabilization around the pilot’s desired attitude and heading reference. In a four-axis system, there is also a collective axis, where the autopilot provides collective power control. These systems are considered limited authority systems in that for short-term external disturbances, the cyclic control does not change position. For long-term disturbances (i.e. change in CG or fuel burn), the cyclic control is allowed to move to a new position which extends the authority of the AFCS.
To accomplish this, the autopilot system must detect changes in helicopter attitude and respond to those changes more quickly and smoothly than its human counterpart.
For an autopilot to maintain this stability, it must:
• Know what the pilot’s desired helicopter attitude is
• Know what the actual helicopter attitude is
• Compare the two
• If there is a difference or error:
• Correct for the difference or error and
• Control the speed of the correction.
What is needed for both autopilots and flight directors to do their jobs is almost identical. The difference lies in what data the two systems are using. In the flight director, it is navigation position data, and in the autopilot it is helicopter pitch and roll attitude.
The autopilot typically has several modes of operation. The most common are referred to as Stability Augmentation System (SAS), Attitude Retention (ATT) and Coupled (CPL). The most basic of these modes is SAS.
SAS (Stability Augmentation System) — When the SAS mode is engaged, it supplies short-term attitude and attitude rate stabilization for use in hands-on flying. It is referred to as a stability augmentation system because it stabilizes the helicopter against outside disturbances, and augments or helps pilot input. The SAS mode is designed so that pilot control motions (pitch and roll) are enhanced while helicopter motions caused by outside disturbances are counteracted. This mode of operation improves basic helicopter handling qualities.
When helicopter motion (a wind gust) is detected, a stabilizing control signal proportional to the amplitude and rate of the motion is generated in the AFCS computer and routed to the appropriate actuators. SAS is generally used during low and slow maneuvering where the pilot may be making constant attitude changes in preparation for landing. SAS by design is flown hands-on the control stick, and no flight director modes can be flown while SAS is engaged.
ATT (Attitude Retention Mode) — The ATT mode is designed to be a hands-off mode. In the ATT mode, the autopilot pitch and roll channels are engaged to maintain the attitude existing at the moment of autopilot engagement. Should an external upset (wind gust) be detected, a stabilizing signal is generated by the AFCS computer to stop the helicopter from moving away from the reference attitude and bring it back to the reference attitude position.
If a change in attitude is desired, the pilot simply pushes the beep trim switch on the cyclic control to obtain a small attitude change, or the Force Trim Release (FTR) switch on the cyclic control, and then moves the cyclic to fly the helicopter to the desired new attitude. When the pilot releases the selected switch, the system again resumes the attitude retention function holding the new attitude position. When collective (power) changes are made, it is generally necessary to push the FTR switch and establish new cyclic positions. As the new trim condition is established, releasing the FTR switch causes the system to maintain the new control position.
Coupled (CPL) Mode — In the CPL mode, the autopilot(s) follow flight director lateral and vertical steering commands to achieve the pilots desired flight path. In coupled flight, the AFCS computer is truly flying the helicopter. The pilot becomes an avionics system manager and sets up the lateral and vertical navigation scenario while monitoring the AFCS for proper operation. The AFCS also allows the pilot to de-couple the flight director from the autopilot(s). This allows the pilot to manually fly the flight director commands as presented on the ADI/EADI. The AFCS can still be in ATT mode for basic stabilization; it just does not see the flight director commands.
In the SAS, ATT and CPL modes discussed, the yaw axis works to hold the helicopter heading and perform yaw damping. Flight director commands never go to the yaw axis. So, you ask, what’s in a name? Let’s see what we have determined:
• Flight directors are for navigation
• Autopilots are for stability
• Basic autopilot modes are SAS, ATT and CPL
• The autopilot cannot couple to the flight director in SAS
• Flight director commands can be flown manually while the autopilot is engaged in the ATT mode
SAS is a hands-on mode and ATT is a hands-off mode
Being able to separate the functionality of autopilots and flight directors helps to more quickly and accurately determine where the problem lies when the pilot writes up a snag in the logbook. If the pilot cannot manually fly the flight director command, then the autopilot cannot fly it either. If the pilot can fly the flight director command manually and it does not work in the CPL mode, then the autopilot is at fault.
Lastly, since what we have discussed is general in nature, for specifics about your helicopter’s actual modes of operation and terminology applied, always refer to the helicopter’s Aircraft Maintenance Manual (AMM) and the avionics OEMs System Maintenance Manual (SMM) and Pilot’s Operating Handbook (POH). Hope this helps make sense of what’s in a name.