Navaid Substitution – Flying Ground-based Approaches in FMS/LNAV

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Navaid Substitution – Flying Ground-based Approaches in FMS/LNAV

Flight Technical Services Q&A

By Jim Johnson
April 28, 2021

The airspace system is full of RNAV approaches and these types of operations are now considered ordinary. They’re accurate, reliable, and available for just about any runway. Since there are still a number of ground-based (VOR, NDB, ILS/LOC) approaches, most of which are “coded” and available in your navigation database, a common question remains: can I fly a ground-based approach using only FMS lateral and vertical guidance if the approach is in my nav database? Another way of asking the question is: can I substitute my FMS guidance for the raw data from the navaid?

And the Answer is. . .

You can fly a ground-based approach procedure using FMS guidance provided – 

  • Your AFM doesn’t prohibit it (no surprise here). There’s a pretty good chance your AFM will prohibit using the FMS/LNAV to fly a localizer-based approach altogether since lateral accuracy requirements on an ILS or LOC procedure are much more stringent than an RNAV approach as you get closer to the runway
  • You monitor the underlying navaid in the final approach segment to ensure the aircraft is tracking the intended path adequately. Hence, the navaid and onboard receiver must be operational
  • You have no position integrity (degrade) annunciations. 

Reference: FAA regulation 91.205(d)(2) and Advisory Circular (AC) 90-108. Most non-US regulatory authorities issue similar requirements, however each must be confirmed.



Why is this so confusing?

This became a bit cloudy over the years as many readers will recall the GPS overlay program in the 1990s. Many NDB and VOR approach charts had the words “or GPS” in the title. For these approaches, if the aircraft had a GPS that was approved for instrument approaches, it didn’t matter if the underlying NDB or VOR was operating, and pilots didn’t have to monitor the raw data (green needles) to ensure the aircraft was tracking the correct lateral path. To date most regulatory agencies have pulled the “or GPS” out of the title and created stand-alone RNAV (GPS) or RNAV (GNSS) approaches in their place.

Why the Requirement

When an RNAV Instrument procedure is developed, the regulatory authority is responsible for confirming that the data that defines the lateral and vertical paths is “coded” correctly in the FMS navigation database. Additionally, it is flight checked to ensure it flies as designed and coded. When ground-based procedures like VOR or NDB approaches are developed or revised, the regulatory agency does not verify FMS coding and flight checks are only completed using the ground-based navaid raw data. Even though these procedures are retrievable from your navigation database, it is only because your data provider has “coded” them. Without a flight check of the coding, the regulatory agencies will not allow the approach to be flown down to the published minimums using only FMS guidance. Even though the data providers do an excellent job coding these procedures, raw data must still be monitored.

The FAA has offered Part 121 and 135 operators special authorizations to fly ground-based approaches without having to monitor the underlying navaid, however the requirements for getting this approval requires onerous simulation or actual flight checks.

Summary

FMS coding for ground-based (VOR, NDB, ILS/LOC) approaches has not been flight checked.

Monitoring the underlying navaid is required only during the final approach segment; FMS guidance can be used on its own for the initial and intermediate segments of the approach.

Since raw data must be monitored, the navaid and onboard receivers must be operative.

 

Jim Johnson is Sr Manager Flight Technical Services and can be reached at James.Johnson2@Honeywell.com.