Use of Barometric Vertical Navigation (VNAV) for Instrument Approach Operations

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Use of Barometric Vertical Navigation (VNAV) for Instrument Approach Operations Using Decision Altitude

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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
Page 2 Par 3
(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
10/19/00 AC 90-97
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
AC 90-97 10/19/00
Page 4 Par 7
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
10/19/00 AC 90-97
Par 7 Page 5
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
Page 6 Par 9
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