FMS Sensor Logic

- James Albright (a former G450 driver)

Updated: 2018-01-01

Since the dawn of what we considered high performance long range navigation, when GPS became standard equipment, we preached the navigation priority: GPS, DME/DME, VOR/DME, and then IRUs. Well, all that has changed with Hybrid IRUs. But how those fall back from GPS is less than clear. You are more likely to see GPS, Hybrid IRUs, DME/DME, and then VOR/DME. After a lot of discontent from the masses, in late 2012, Honeywell set the record straight. Most of their explanation is quoted verbatim here.

Before Hybrid IRU's

Photo: MCDU Progress Page 1, GPS

[Honeywell DirectTo, 2012 Q4] Prior to FMS 7.1 ( EPIC) and 6.1 (FMZ 2000), the FMS uses a linear hierarchy to determine which sensor should be used for navigation. If the EPU value of any sensor is deemed unacceptable by the FMS, then it will 'fail down' to the next available sensor. The fail down logic in older software versions uses the following hierarchy:

  1. GPS

  2. DME/DME

  3. VOR/DME

  4. IRS (if installed)

  5. Dead Reckoning

The Gulfstream manuals don't say this. In fact, the only reference in the GV series is in the classic GV AFM Limitations when talking about RNP-10. Nevertheless, this is the way Honeywell says it worked on these older systems.

Hybrid IRU Position Sensor

[G450 Aircraft Operating Manual, §1-34-30, ¶7] The selected sensor for the FMS position is chosen by comparing the EPU values of all available sensors that have not been deselected by the pilot, and choosing the sensor with the lowest value. Sensor accuracy is the produced Figure of Merit (FOM) for GPS and Hybrid IRU, and the computed EPU for IRS drift and radio position. Once a sensor is selected, that sensor remains selected unless the EPU of another sensor is at least 5% lower than the EPU of the selected sensor. For an instrument approach selected from the NAV database, the selected sensor functions in one of two ways: When the database record indicates that the chosen approach has a required sensor (e.g., GPS required), then that sensor is locked as the selected sensor at three miles from the FAF and remains the selected sensor throughout the approach. When the database record does not indicate that the chosen approach has a required sensor, the sensor with the best accuracy when the aircraft is three miles from the FAF is locked as the selected sensor for the remainder of the approach.

[G450 Aircraft Operating Manual, §2B-32-70]

  • The FMS uses a performance based sensor selection scheme that uses the EPU of each sensor to select the best performing sensor. The GPS sensors and the Hybrid IRS function produce a figure-of-merit (FOM), which is used as the EPU value for those sensors. The EPU is modeled by the FMS for the IRS, DME/DME, and VOR/DME sensors. The sensor currently selected for use is displayed on the PROGRESS page 1.

  • The available FMS sensors for selection are as follows:

    • Hybrid IRS

    • GPS-D

    • GPS

    • IRS

    • DME/DME

    • VOR/DME

    • Degrade

    • DR

  • Note: GPS-D is GPS with SBAS. The availability of GPS-D and GPS are mutually exclusive, and GPS-D is dependent on the presence and reception of the SBAS signal. When the SBAS signal is present, and the GPS sensor is the selected sensor for the FMS, then the annunciator on PROGRESS page 1 is GPS-D. If no SBAS signal is present, then the annunciator on PROGRESS page 1 is GPS.

The FMS will use the sensor with the lowest EPU and continue to use that until another sensor beats it by 5%. Normally that would only be a contest between GPS and the Hybrid IRU. But there have been documented cases where that wasn't true. Honeywell finally addresses this in 2014 . . .

With Hybrid IRUs

Photo: MCDU Progress Page 1, GPS-D

[Honeywell DirectTo, 2012 Q4]

  • As Performance-Based-Navigation (PBN) becomes more prevalent throughout worldwide airspace, Honeywell has modified the logic of the EPIC and FMZ 2000 FMS systems to ensure that the best sensor is being used for navigation. Sensor accuracy and integrity is generally measured by the Estimated Position of Uncertainty (EPU). An EPU is assigned for each sensor that is available to the FMS, and is calculated using various means.

  • With software versions 7.1 (EPIC) and 6.1 (FMZ 2000), the FMS chooses the best available sensor based on the lowest EPU. “Best” sensor means that, if the current sensor in use is at least less than 5% worse than any other sensor, the FMS will continue to use it as long as it meets all validity checks. In other words, for the FMS to use another sensor, the new sensor has to be performing at least 5% better than the sensor currently in use. The exception is when DME/DME or VOR/DME is being considered. In this case, a radio source must have an EPU least 40% better than a GPS or IRS EPU to be considered for navigation.

This is breaking news: DME/DME and VOR/DME will not be called upon until they are significantly better than GPS or the IRU's. Given the IRU's have become so good, you are more likely to see HYBRID or IRS as a sensor choice.

  • The incorporation of this new logic also means that crews are more likely to encounter IRS Navigation mode because the newer Hybrid Inertial Reference Units are constantly being updated by the GPS, and generally have a lower EPU than DME/DME. Additionally, the inclusion of SBAS will yield an even lower EPU for the IRS. Subsequently, if the GPS EPU rises to a value that is 5% worse than the IRS EPU, then IRS will be used by the FMS. For example, if the aircraft is using GPS with SBAS (GPS-D mode) with an EPU of 0.01, then the IRS should also have a similar EPU because its position is constantly being updated with the D-GPS position. If, however, the SBAS signal is lost, the GPS sensor would immediately increase to a higher value (e.g., .11). It is then possible that the system would revert to IRS for a short period because the IRS EPU would be better than the normal GPS EPU. As a result of IRS coasting, the IRS EPU would then grow to match the GPS EPU; then the FMS would resume using normal GPS. If normal GPS is then lost, the FMS will continue to coast in IRS Navigation mode until it is 40% better than the IRS EPU.

  • Whenever the EPU value exceeds the RNP value for a given phase of flight, various CAS messages or FMS messages will be annunciated. Additional CAS messages may also be annunciated if the EPU reaches an unacceptable level to conduct an RNAV approach, regardless of phase of flight. For example, when operating in terminal airspace, the EPU would have to exceed 1.0 in order to receive an “UNABLE RNP” Message. Additionally, if the EPU reaches a value above 0.5, other CAS Messages are annunciated on certain EPIC platforms, alerting the crew that an RNAV approach may not be possible. It is important to note that automatic reversion to IRS due to GPS issues will not generate a CAS message until the IRS EPU reaches a level above the current or next leg RNP value. Flight crews should review their OEM guidance to determine applicable CAS messages and associated flight crew procedures.