Advisory

Circular

SUBJECT: Use of Barometric Vertical

Navigation (VNAV) for Instrument Approach

Operations Using Decision Altitude

Date: 10/19/00

Initiated by: AFS-420

AC No: 90-97

Change:

1. PURPOSE. This document provides guidance for the operational use of barometric

vertical navigation (VNAV) equipment to perform 14 CFR Part 97 area navigation (RNAV)

instrument approach procedures published with a VNAV Decision Altitude (DA). This

document identifies the types of equipment that the Federal Aviation Administration (FAA)

has determined to be acceptable for barometric VNAV approach operations.

2. RELATED TITLE 14, CODE OF FEDERAL REGULATIONS (14 CFR) PARTS.

a. Part 91, General Operating and Flight Rules, sections 91.175 and 91.205.

b. Part 97, Standard Instrument Approach Procedures.

c. Part 121, Certification and Operations: Domestic, Flag, and Supplemental Air

Carriers and Commercial Operators of Large Aircraft, section 121.567.

d. Part 125, Certification and Operations: Airplanes Having a Seating Capacity of 20

or More Passengers or a Maximum Payload Capacity of 6,000 Pounds or More, sections

125.287 and 125.325.

e. Part 129, Operations: Foreign Air Carriers and Foreign Operators of U.S. -

Registered Aircraft Engaged in Common Carriage.

f. Part 135, Air Taxi Operators and Commercial Operators, section 135.165.

3. RELATED PUBLICATIONS.

a. Advisory Circulars (AC).

(1) AC 20-129, Airworthiness Approval of Vertical Navigation (VNAV) Systems for

Use in the U.S. NAS and Alaska.

(2) AC 20-130, Airworthiness Approval of Navigation or Flight Management

Systems Integrating Multiple Navigation Sensors, latest edition.

(3) AC 20-138, Airworthiness Approval of Global Positioning System (GPS)

Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System, latest

edition.
AC 90-97 10/19/00

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(4) AC 120-29, Criteria for Approving Category I and Category II Landing Minima

for FAR 121 Operators.

b. FAA Orders.

(1) FAA Order 8400.10, Air Transportation Operations Inspector’s Handbook.

(2) FAA Order 8700.1, General Aviation Operations Inspector’s Handbook.

4. DEFINITIONS.

a. Area Navigation (RNAV). This is a method which permits aircraft navigation along

any desired flightpath within the coverage of the associated navigation aids or within the

limits of the capability of self-contained aids, or a combination of these methods.

b. Barometric Vertical Navigation (VNAV). A function of certain RNAV equipment which presents computed vertical guidance to the pilot referenced to a specified

vertical path. The computed vertical guidance is based on barometric altitude and is

typically computed as a geometric path between two waypoints or an angle based on a

single waypoint. 

NOTE: The GPS augmentation system known as the Wide Area Augmentation

System (WAAS) will also possess a VNAV capability and may be capable of

performing to the lateral navigation (LNAV)/VNAV minima, however, this document only addresses barometric VNAV.

c. Decision Altitude (DA). In an approach with vertical guidance, DA is a specified

altitude in mean sea level at which a missed approach must be initiated if the required

visual references to continue the approach have not been established.

d. Lateral Navigation (LNAV). The function of RNAV equipment that computes,

displays, and provides lateral guidance to a profile or path.

e. Required Navigational Performance (RNP). A statement of the navigational

performance necessary for operation within a defined airspace.

f. RNP Level or Type (RNP-X). A value, in nautical miles (NM), from the intended

horizontal position within which an aircraft would be at least 95 percent of the total flying

time.

5. BACKGROUND. Controlled flight into terrain (CFIT) is a major cause of aircraft

accidents during the approach phase of flight. Instrument approaches, without the vertical

guidance, are particularly prone to CFIT. Various forums, including the United States

Secretary of Transportation Safety Summit and the FAA Administrator’s Safer Skies

Initiative, have recommended that whenever and wherever possible vertical guidance

should be provided on all instrument approaches. A timely and cost-effective solution is

to make use of the instrument flight rule (IFR) approved barometric VNAV capability of

certain aircraft to perform guided, stabilized descent paths on instrument approach

procedures which otherwise have no vertical guidance. In order to support these
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Par 5 Page 3

initiatives, the FAA will publish Part 97, i.e., public RNAV Instrument Approach

Procedures (IAP), which provides for VNAV operations to a decision altitude.

6. APPLICABILITY.

a. The guidance contained herein applies to all operators conducting operations

under Parts 91, 121, 125, 129, and 135 within the United States National Airspace System

(NAS) and Alaska.

b. Use of barometric VNAV, as described in this document, applies to Part 97

RNAV IAP's which are published with separate minima identified for VNAV equipment

and shown on the IAP as a DA. For ease of reference, these approaches will be referred to

as VNAV approaches in this document. Operators with barometric VNAV equipment

identified in this AC may utilize this equipment to fly LNAV/VNAV minima.

7. AIRCRAFT ELIGIBILITY AND APPROVAL PROCESS.

a. RNAV Capability. VNAV approach operations to a published DA will be

conducted as part of an RNAV instrument approach procedure and as such the VNAV

equipment must be part of an integrated RNAV system approved for IFR instrument

approach operations. Eligible aircraft are those with an Aircraft Flight Manual (AFM) or

AFM Supplement which explicitly states that the RNAV system is approved for operations

in accordance with paragraphs 7a(1), 7a(2), or 7a(3).

(1) Global Positioning System (GPS) Class A1 navigation equipment approved

under AC 20-138.

(2) Multi-Sensor Navigation System or Flight Management System (FMS)

incorporating a Class B1, B3, C1, or C3 GPS sensor approved under AC 20-130. For a

particular approach, GPS must be an active component of the aircraft’s navigation

solution.

(3) RNP Systems Approved for RNP 0.3 NM Operations or Less. Any

limitation or specific requirement needed to maintain RNP 0.3 shall be adhered to. Unless

otherwise noted on the procedure, the use of RNP systems requires GPS or DME/DME to

be an active component of the aircraft's navigation solution. If RNP .3 based on DME/DME

is not authorized, it will be noted on the procedure.

b. Barometric VNAV Capability. Eligible aircraft are those with an AFM or AFM

Supplement which explicitly states that the VNAV system is approved for approach

operations in accordance with AC 20-129. In addition, for a VNAV system to be approved

for approach operations under AC 20-129, it must have a vertical deviation indicator (VDI).

Since VDI scaling/sensitivity values vary widely, eligible aircraft must also be equipped

with and operationally using either a flight director or autopilot capable of following the

vertical path. Pilot deviation of +100/-50 feet is considered acceptable for adherence to the

depicted VNAV path

c. Data Base Requirement. The RNAV system must include a manufacturer

supplied electronic data base, which contains the waypoints and associated VNAV
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information, i.e., altitudes and vertical angles, for the procedure to be flown. Pilot/crew

modification of the LNAV/VNAV approach information is prohibited. If manual

adjustments to stored altitude information are necessary, e.g., cold temperature

adjustments, the crew should make appropriate adjustments to procedure altitudes and

revert to use of the temperature adjusted LNAV minimum descent altitude (MDA).

d. Part 91 Operator/Aircraft Approval. Part 91 operators should review their AFM

or AFM Supplement to establish that it shows navigation system eligibility as detailed in

paragraphs 7a, 7b, and 7c. Once the operator has established system eligibility, the

operator should take steps to ensure that barometric VNAV approach operations are

conducted in accordance with the guidance in paragraphs 8 and 9. After these actions have

been completed, the operator may begin to conduct barometric VNAV approach operations

to a published DA as shown on Part 97 RNAV IAP's which are published with separate

minima identified for VNAV equipment. A Letter of Authorization (LOA) is not required

when eligibility is based on the AFM and provisions of this AC. See paragraph 7f for

actions to take if the operator is not able to determine VNAV approach eligibility from the

AFM or AFM Supplement.

e. Air Carrier Aircraft/Commercial Operator Approval. Part 121, 125, 129, or

135 operators should present the following documentation to their Certificate Holding

District Office (CHDO): sections of the AFM or AFM Supplement which document

RNAV/VNAV airworthiness approval for approach operations in accordance with

paragraphs 7a, 7b, and 7c and sections of the training and operations manuals which

reflect the operating policies of paragraphs 8 and 9. Once the operator has addressed the

guidance in these paragraphs to the satisfaction of the CHDO, the operator may begin

using this barometric VNAV equipment to fly to the LNAV/VNAV DA as published on the

IAP. See paragraph 7f for guidance on actions to take if the operator is unable to

determine eligibility from the AFM or AFM Supplement.

f. Eligibility Not Based on the AFM or AFM Supplement (Special Approval).

(1) The operator may not be able to determine VNAV approach eligibility from

the AFM or AFM Supplement. In this case, a Part 91 operator should request that the

local Flight Standards District Office (FSDO) assess the equipment for VNAV approach

eligibility while a Part 121, 125, 129, or 135 operator should request that the CHDO make

the eligibility assessment. The operator should provide the FSDO or CHDO with the

RNAV/VNAV system make, model and part number, any evidence of IFR RNAV/VNAV

approval, and pertinent information from crew operating procedures. If the FSDO or

CHDO is unable to determine equipment eligibility, it should forward the request and

supporting data through the appropriate FAA Flight Standards Regional Division to the

appropriate Aircraft Evaluation Group (AEG). The AEG will verify that the aircraft and

RNAV system meet the criteria for VNAV, and that the system can safely fly specified

VNAV vertical paths associated with instrument approach procedures applying a DA

rather than an MDA. The AEG will provide written documentation (i.e., amend Flight

Standards Bulletin Report or other official documentation) to verify the eligibility of that

equipment.

(2) For Part 91 operators, if the FAA determines that the navigation equipment

is eligible for barometric VNAV instrument approach operations to a published DA, the
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FSDO will provide documentation that the aircraft equipment is approved for these VNAV

operations.

(3) For Parts 121, 125, 129, and 135 operators, the FAA will attempt to

establish system eligibility and ensure that the operator's training and operations manuals

reflect the operating policies of paragraphs 8 and 9. Once these steps are successfully

completed, the operator may begin using this VNAV equipment to fly to the LNAV/VNAV

DA as published on the IAP.

8. VNAV OPERATING PROCEDURES (GENERAL). For barometric VNAV approach

operations, the flightcrew should be familiar with the operating procedures detailed below.

a. Actions at DA. The pilot/crew is expected to fly the aircraft along the published

vertical path and execute a missed approach procedure upon reaching DA, unless the

visual references specified in Part 91.175 for continuing the approach are present.

b. Temperature Limitation. Because of the pronounced effect of nonstandard

temperature on barometric VNAV operations, VNAV approaches will contain a

temperature limitation below which use of the VNAV DA, based on barometric

VNAV, is not authorized. If the temperature on a given day is lower than the VNAV

temperature limitation, the pilot/crew may continue use of the barometric VNAV

function but only to the published LNAV MDA. The temperature limitation will be

shown as a note on the IAP.

c. VNAV Path Mode Selection. Crews should be knowledgeable on selection

of the appropriate vertical mode(s) that command vertical navigation via the

published vertical path. Other vertical modes such as vertical speed are not applicable to barometric VNAV approach operations.

d. Remote Altimeter Setting Restriction. Use of barometric VNAV DA is

not authorized with a remote altimeter setting. A current altimeter setting for the

landing airport is required. Where remote altimeter minima are shown, the VNAV

function may be used but only to the published MDA.

9. PILOT KNOWLEDGE. Pilots/crews should be knowledgeable in the following areas:

a. Charting. The aeronautical instrument approach chart that will promulgate

LNAV/VNAV procedures including, but not limited to, temperature and altimeter source

limitations for barometric VNAV operations.

b. Nonstandard Temperature Effect on Barometric VNAV Operations.

Barometric altimeters are calibrated to indicate true altitude under International

Standard Atmosphere (ISA) conditions. If on a given day the temperature is warmer

than ISA, the true altitude will be higher than indicated altitude. Conversely, on a

day colder than ISA, the true altitude will be lower than indicated altitude. These

errors increase in magnitude as the altitude above the altimeter setting source

increases.
AC 90-97 10/19/00

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c. VNAV Failure Modes and Mode Reversions. The pilot/crew should be

knowledgeable of failures and mode reversions, which adversely impact the aircraft’s

ability to conduct VNAV approach operations. In addition, pilots/crews should be

aware of contingency actions, i.e., reverting to LNAV MDA, following VNAV failures.

L. Nicholas Lacey

Director, Flight Standards

 Service