TECH TIPS - HeliSAS with Genesys ADAHRS

The Genesys ADAHRS is becoming the default external attitude source for the Genesys HeliSAS when not connected to a Garmin G500. This month’s Tech Tips will focus on the installation and alignment of the ADAHRS with a few “tricks” to keep the HeliSAS functioning correctly.

The ADAHRS provides an external pitch and roll reference that is used to compare with the internal pitch and roll sensors of the HeliSAS Flight Control Computer (FCC) using an ARINC429 digital serial bus. While the ADAHRS can also provide altitude, airspeed, vertical speed, outside air temperature, and heading, this information is ignored in the FCC. However, this does not mean that the Pitot, Static, and heading are not connected in the aircraft.

A typical installation of the ADAHRS consists of the ADAHRS module with connection to the aircrafts’ Pitot and Static system and Magnetic Sensing Unit (MSU). The Outside Air Temperature Probe is not used.

Location of the ADAHRS module and MSU and mounting brackets are normally identified in the installation documents. This article will focus more on general installation issues than specific for an airframe type.

The ADAHRS must be mounted with at least one of the sides parallel to the centerline of the airframe and another side parallel to the waterline within ±0.5°. This is a must as it provides a reference for the ADAHRS module. Adjustment of any mounting errors in pitch or roll can be adjusted in the ADAHRS Maintenance Utility when performing the Leveling Function. This is comparable to installing shims with older AHRS or vertical gyros.

The aircrafts’ Pitot and Static lines must be connected to the ADAHRS module to provide airspeed and altitude aiding for attitude stability within the ADAHRS module. The fittings for the Pitot and Static ports on the ADAHRS can be removed and replaced with fittings that match the Pitot and Static lines. The ADAHRS Installation Manual identifies the thread type and size for new fittings on the ADAHRS module.

The MSU must be mounted in the location that is magnetically quiet within the aircraft. While the installation document will identify the location of the MSU, it may not include external items to look for. Most MSU installations are located in the tail boom area of the aircraft. Typically within these areas are electrical wiring harnesses for position lights and strobes. Experience has shown that incandescence position lights and strobes will cause magnetic interference if the wiring is allowed to run next to the MSU. In some cases, it may be necessary to move the existing wiring harness to increase the distance between the harness and the MSU.

Once the ADAHRS has been installed, the installer must perform an alignment of the ADAHRS for Leveling, Compass Swing, and Compass Alignment. Prior to starting any of these functions, the installer should select the Set ADAHRS Options function and set the Airspeed Filter, Altitude Filter, and VSI Filter to a value of “0 of 15”.

The Leveling Function is used to set the orientation of the ADAHRS module as installed in the aircraft. The Pitot/Static port orientation identifies the location of the ADAHRS module ports as installed in the aircraft.

Adjustments of pitch and roll offsets are entered during this time.

It is crucial that the aircraft has been leveled prior to starting this function. Any errors in the leveling may cause intermittent mis-compares which would result in disconnects of the HeliSAS in flight.

The Compass Swing Function is used to set the magnetic compass table inside of the ADAHRS module. While heading is not provided to the FCC, the ADAHRS uses heading to further stabilize the attitude output. If the heading is off or starts to drift because it has not be aligned properly, the attitude may start to drift causing disconnects of the HeliSAS in flight. When performing the Compass Swing Function, all aircraft systems (engine, air conditioning, position lights, etc.) that are used in flight must be energized and operating. The aircraft should be picked up into a hover and a pedal turn should be performed for the clockwise and counter-clockwise turns as indicated in the ADAHRS Maintenance Utility. The turns should not exceed approximately 10 degrees per second when performed.

The Compass Alignment Function is used to compensate for slight offsets at the four primary cardinal points. This is similar to performing the N/S and E/W compensation on a Whiskey compass or directional gyro. The Compass Alignment Function should be performed immediately after the Compass Swing Function to ensure the offsets match the magnetic table.

A maintenance technician should be located outside of the aircraft to direct the Pilot to the correct heading using a Compass Rose or Master Compass. These offset adjustments are critical to heading operation and ultimately attitude stability. The technician should verify that the final offsets for all 4 cardinal points should not exceed ±5° and have a split of no more than 5°. If any one offset is greater than 5° or the maximum split between all 4 cardinal points is greater than 5°, the Compass Alignment Function should be repeated.

Troubleshooting of the ADAHRS is performed using the Display ADAHRS Values and Status Flags Function in the ADAHRS Maintenance Utility.

This function allows a technician to view the status of each module within the ADAHRS and the MSU for validity and operation. The page is divided into seven major sections, however only the top three provide the majority of information needed to troubleshoot the ADAHRS.

The top section displays the sensor module within the ADAHRS. The sensor module contains three accelerometers and three rate sensors to calculate pitch and roll attitude.

The AHRS Hdwr Stat and AHRS Data Stat display any error codes that may generated if a failure occurs within the sensor module.

The second section displays the MSU data. The three-axis magnetometer data is sent to the ADAHRS module to calculate aircraft heading.

The MSU Hdwr Stat and MSU Data Stat display any error codes that may be generated if a failure occurs within the MSU.

The third section displays the ADC module data within the ADAHRS module.

The ADC Hdwr Stat and ADC Data Stat display any error codes that may be generated within the ADC module. Alt Corr displays the altitude correction value and IAS Corr displays the airspeed correction value. Since the OAT Probe is not installed, the OAT failure bit (ADC Hdwr Stat = 0x04 and ADC Data Stat = 0x84) will always be displayed.

Assistance with troubleshooting of the ADAHRS is always available by contacting Genesys Product Support at (800) 872-7832 or support@genesys-aerosystems.com.

The Genesys ADAHRS is a very reliable and robust product that provides attitude to the HeliSAS. Proper installation and alignment is key to successful operation. I hope these tips will help you further understand and aid you in the installation of the ADAHRS.