BOM (Broadcasting Outer Module) Company Standards
Effective Date
12/6/17
Page
9
of
12
Document No.
42
Revision
1
Code Number
D-0005
Just like a compass, the magnetometer inside the AHRS is sensitive to ferrous metals, or
magnetic fields produced by Landing Lights Wiring etc. Placing the AHRS close to a magnetic
compass is not a good idea, as this would affect the readout on both the magnetic compass and
the AHRS inside the BOM. As a rule of thumb, one foot is recommended. However, this is not
always possible, thus, the AHRS has internal algorithms that “learn” your aircraft
configuration as you fly. The heading might be off during initialization, but as soon as the
airplane starts turning, the AHRS starts compensating for any errors.
Be aware that this is magnetic heading and there will always be a magnetic deviation
(declination) to true north that varies according to your latitude and longitude. If you are
comparing the heading output on the AHRS to a GPS source, take into consideration that GPS
may be reporting “True Track”. You can add/subtract the corresponding magnetic deviation at
your current location to obtain True Heading.
9.1.3
Roll, Pitch and Yaw
The AHRS has three MEMs gyros and a 3-axis accelerometer that measures your airplane’s
attitude. When the AHRS is turned on, it requires a two-minute interval to calibrate itself. You
might see the horizon shifting ± 5 °during this self-calibration process and a flashing behavior
of the horizon. To achieve better performance, it is recommended that the aircraft stays in a
steady position (or taxing) during this two-minute period. During flight, the instrument will
calculate the aircraft’s attitude based on accelerations and rotation rates and you can expect the
horizon to have an accuracy of ± 3 ° degrees. Fast airplanes such as Jets and airlines with high
acceleration rates at take-off and landings, may experience a pitch up error right after take-off
until the airplane stabilizes. In this situation, it is recommended to perform a + 15 ° bank turn
to the left for 10 seconds, then a + 15 ° bank turn to the right at the beginning of the flight. The
instrument will operate in a full 360 degrees of turn and may be used in light aerobatic type
maneuvers. The gyros are rated for 500 °/sec max turn rates. When the maximum turn rate is
exceeded, the AHRS is temporarily disabled. This is indicated by a flashing behavior (pitch
goes from 0 to 90 degrees, and roll from 0 to 180 degrees). The instrument automatically resets
itself within 4 seconds if kept steady during that time, otherwise the instrument will recover
within 15-40 seconds depending on the amount of error induced during recovery. This will not
cause any harm to the instrument. Note: Moving the instrument with your hand will most likely
trigger the excess rotation alarm unless simulating smooth aircraft behavior.
9.1.4
Indicated Airspeed and Altitude
The BOM has pressure transducers installed, one for static pressure and another one for the
dynamic pressure. Having access to pitot-static information the AHRS can transmit Indicated
Airspeed and Pressure altitude at 29.92 inHg. To adjust the altitude due to barometric pressure
