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Advisory Vertical Guidance (Advisory VNAV) A Useful and Flexible Safety Enhancement. S.V. Massimini, DSc June 2011. Safety Benefits of Vertically-Guided Approaches.

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Advisory vertical guidance advisory vnav a useful and flexible safety enhancement

Advisory Vertical Guidance (Advisory VNAV)A Useful and Flexible Safety Enhancement

S.V. Massimini, DSc

June 2011

Safety benefits of vertically guided approaches
Safety Benefits of Vertically-Guided Approaches

  • Analysis of Controlled Flight Into Terrain (CFIT) accidents* has shown that the accident rate for Nonprecision Approaches (NPA) versus Precision Approaches (PA) is nine times higher for Air Carrier Aircraft and two times higher for Air Taxi aircraft.

    • Therefore, vertically-guided approaches provide a substantial safety benefit (especially at runways without ILS/GBAS or where ILS/GBAS equipment is inoperative)

  • A principal safety objective of the U.S. WAAS program was to provide vertically-guided approaches to all instrument runways

*Bregman et. al., 1997, “Safety Benefits of Precision vs. Non Precision Approaches,”

The MITRE Corporation, McLean VA . Consistent with Flight Safety Foundation 1996

study, and other analyses of safety versus vertical guidance.

Vertically guided approaches capability
Vertically Guided ApproachesCapability

  • Vertical guidance can be provided by

    • ILS or MLS

    • GBAS

    • SBAS

      • RNP, RNP AR or LPV

    • Barometric Vertical Navigation (Baro VNAV)

      • GPS with approved Baro VNAV (with RNAV, RNP, or RNP AR)

Nonprecision runways
Nonprecision Runways

  • Some runways do not have vertical guidance

    • No ILS available

      • Economic reasons

      • Siting reasons

      • Glide path equipment inoperative

    • No GBAS available

      • Economic reasons

      • Ground equipment inoperative

    • SBAS vertical guidance not available

      • Vertical guidance not certified/approved

      • Ionospheric anomaly area

    • Lack of aircraft equipage for APV/Baro VNAV

Advisory vertical navigation vnav an alternative
Advisory Vertical Navigation (VNAV)An Alternative

  • A nonprecision approach where the aircraft avionics provide a vertical path to the runway

  • The aircrew use GPS or other guidance to fly the nonprecious approach and monitor altitude so as to comply with any required altitudes

    • Including level off at the Minimum Descent Altitude (MDA) if the airport is not in sight

  • The aircraft can fly a stabilized descent to the runway

Advisory vnav
Advisory VNAV

Aircraft computes and flies 2.91º glide slope per the vertical path angle (VPA) on the procedure.

VPA will comply with stepdown, but pilot must ensure the altitude is met.

Pilot must level off at MDA if field is not in sight.

What is required for advisory vnav
What is Required for Advisory VNAV?

  • GNSS or GNSS-overlay nonprecision approach with a published Vertical Path Angle (VPA)

    • The angle for a published VPA must comply with all stepdown fixes between the final approach fix and the runway threshold

    • The VPA is available in the avionics approach database

      Note: Advisory VNAV is not generally provided on runways where GNSS-based vertical guidance is provided by other approaches

  • GNSS equipped aircraft with

    • Approved baro VNAV avionics or

    • Advisory baro VNAV avionics or

    • SBAS

Sbas and advisory vnav
SBAS and Advisory VNAV

  • Advisory VNAV is included in the SBAS Minimum Operation Performance Standards (MOPS) as an optional capability

    • Garmin and other avionics manufacturers provide advisory VNAV using SBAS on all runways where a VPA is provided

      • Nearly all nonprecision GPS approaches in the U.S.A. except for circling approaches

        Note: Some manufacturers provide advisory VNAV using baro VNAV

  • No explicit integrity monitoring is required for the vertical signal since the pilot is responsible for complying with the mandatory altitudes for the nonprecision approach

Aircrew training
Aircrew Training

  • Advisory VNAV provides the pilot a glideslope to fly a stabilized approach

  • However, the pilot must complywith required stepdown altitudes

  • Aircrew training for advisory VNAV should be accomplished prior to flying the procedure


  • Vertically-guided approaches provide a safety benefit by reducing CFIT

  • Where a only a nonprecision approach is available, advisory VNAV may provide vertical guidance

    • Aircrew must comply with altitude restrictions at stepdown fixes

    • Aircrew must level off at MDA if field is not in sight at minimums

    • Aircrew training is essential

  • Advisory vertical guidance may be provided by SBAS or by approved/advisory baro VNAV systems