What is the minimum number of IRS units required for the FMC to provide LNAV guidance?

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Multiple Choice

What is the minimum number of IRS units required for the FMC to provide LNAV guidance?

Explanation:
LNAV guidance in the FMC relies on a navigation reference provided by the IRS. The IRS delivers a stabilized position and velocity reference that the FMC uses to compute the aircraft’s lateral path along the flight plan. With at least one IRS functioning, the FMC has a valid reference to generate LNAV guidance, and GPS can supplement that data if available. However, having zero IRS units leaves no dependable reference for the FMC to base lateral navigation on, so LNAV cannot be provided. GPS alone does not count as an IRS, so it does not meet the requirement. Two IRS units would offer redundancy but are not required for LNAV to function; a single IRS is sufficient.

LNAV guidance in the FMC relies on a navigation reference provided by the IRS. The IRS delivers a stabilized position and velocity reference that the FMC uses to compute the aircraft’s lateral path along the flight plan. With at least one IRS functioning, the FMC has a valid reference to generate LNAV guidance, and GPS can supplement that data if available. However, having zero IRS units leaves no dependable reference for the FMC to base lateral navigation on, so LNAV cannot be provided. GPS alone does not count as an IRS, so it does not meet the requirement. Two IRS units would offer redundancy but are not required for LNAV to function; a single IRS is sufficient.

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