A Complete Manual for Conducting International Flight Operations

Navigation System Accuracy Check

Navigation System Accuracy Check

Each crewmember shall conduct a navigation system accuracy check prior to conducting Class II navigation. The requirements for the navigation accuracy checks are outlined in AC-91-70B. The intent of the check is to verify that aircraft position is in agreement with the GPS, IRS and ground based navigation equipment stated positions. While in Class 1 navigation, the availability of sesnors for SBAS equipped aircraft is Hybrid IRS, GPS-D (Differential), GPS, IRS, DME/DME and VOR/DME. When established in Class 2 navigation the DME/DME and VOR/DME will be dropped from the position solution. It is therefore critical that crewmembers verify the correct alignment and accuracy of the IRS and GPS system prior to flight. If a significant discrepancy develops between GPS stated position and IRS position in flight, it is not recommended to update the position as this has attributed to gross navigational errors while in Class II navigation. Crewmembers shall also be aware of their individual aircraft limitations with respect to IRS latitude limitations and IRS alignment times while at extreme Northern Latitudes and Southern Latitudes.

The image to the right is a navigation accuracy check conducted prior to a Pacific oceanic crossing. In this case the Navaid is MQO- Morro Bay, the RAW R/DME is the position of the aircraft referenced from reading the present DME and radial from the ground based Navaid. FMS R/DME may be obtained from the position sensors page and is usually a line select key at the bottom of the page. FMS1, 2, 3 can be verified that the FMS position is consistent with the GPS indicated position, or verifying that the FMS position is consistent with the appropriate taxi stand latitude and longitude. IRS1, 2, 3 may be viewed on the NAV, Position Sensors page on Honeywell FMS installations and will indicate the accuracy of the IRS relative to the GPS position. It is not unusual to see some variation in IRS position down to the tenths of degree prior to flight.


At the conclusion of a flight, the IRS position may indicate significant differences if they are not updated from a GPS system. If a significant discrepancy exists between the IRS and GPS position, the crewmember should consider updating the position or realigning the IRS systems. Keep in mind the minimum IRS temperature limitations for alignment, +10 degrees C in the Gulfstream, as well as the fact that the aircraft must be kept completely stationary during the alignment process. If the flight crew is not able to accomplish the navigation accuracy check on the ground, it shall be accomplished in flight, but prior to the coast-out waypoint. It is also recommended that a navigation accuracy check is conducted prior to the coast-in waypoint to verify position accuracy when transitioning back to Class I navigation.


Revision date: July 29, 2015

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