Vertical Navigation | Instrument Procedures

One of the advantages of some GPS and multi-sensor FMS RNAV avionics is the advisory VNAV capability. Traditionally, the only way to get vertical path information during an approach was to use a ground-based precision NAVAID. Modern RNAV avionics can display an electronic vertical path that provides a constant-rate descent to minimums.

Since these systems are advisory and not primary guidance, the pilot must continuously ensure the aircraft remains at or above any published altitude constraint, including stepdown fix altitudes, using the primary barometric altimeter. The pilots, aircraft, and operator must be approved to use advisory VNAV inside the FAF on an instrument approach.

VNAV information appears on selected conventional nonprecision, GPS, and RNAV approaches (see “Types of Approaches” post). It normally consists of two fixes (the FAF and the landing runway threshold), a FAF crossing altitude, a vertical descent angle (VDA), and may provide a visual descent point (VDP) [Figure 1A].

Aircraft instrument procedure vertical navigation

Figure 1A. VNAV information

The VDA provides the pilot with advisory information not previously available on nonprecision approaches. It provides a means for the pilot to establish a stabilized descent from the FAF or step-down fix to the MDA. Stabilized descent is a key factor in the reduction of controlled flight into terrain (CFIT) incidents. However, pilots should be aware that the published angle is for information only − it is strictly advisory in nature. There is no implicit additional obstacle protection below the MDA. Pilots must still respect any published stepdown fixes and the published MDA unless the visual cues stated 14 CFR §91.175 are present, and they can visually acquire andavoid both lit and unlit obstacles once below the MDA. The presence of a VDA does not guarantee obstacle protection in the visual segment and does not change any of the requirements for flying a nonprecision approach.Pilots may use the published angle and estimated/actual groundspeed to find a target rate of descent from the rate of descent table published in the back of the U.S. Terminal Procedures Publication. This rate of descent can be flown with the Vertical Velocity Indicator (VVI) in order to use the VDA as an aid to flying a stabilized descent. No special equipment is required.

In rare cases, the LNAV minima may have a lower HAT than minima with a glide path, due to the location of the obstacles and the nonprecision MAP. This should serve as a clear indication to the pilot that obstacles exist below the MDA, which must be seen in order to ensure adequate clearance. In those cases, the glide path may be treated as a VDA and used to descend to the LNAV MDA, as long as all of the rules for a nonprecision approach are applied at the MDA.When there are obstacles in the visual area that could cause an aircraft to destabilize the approach between the MDA and touchdown, the IAP will not show a vertical descent angle in the profile view. The charts currently include the following statement: “Descent Angle NA” or “Descent Angle NA-Obstacles” [Figure 1B ].

Figure 1B. Descent Angle N/A

Like flying any other IAP, the pilot must see and avoid any obstacles in the visual segment during transition to landing. A constant-rate descent has many safety advantages over non-precision approaches that require multiple level-offs at stepdown fixes or manually calculating rates of descent. A stabilized approach can be maintained from the FAF to the landing when a constant-rate descent is used. Additionally, the use of an electronic vertical path produced by onboard avionics can serve to reduce CFIT, and minimize the effects of visual illusions on approach and landing. Some countries even mandate the use of continuous descent final approaches (CDFAs) on non-precision approaches.

Wide Area Augmentation System

The Wide Area Augmentation System (WAAS) offers an opportunity for airports to gain ILS like approach capability without the purchase or installation of any ground-based navigation equipment at the airport. Today, WAAS is already being used at more than 900 runways across the United States to achieve minimums as low as 200 feet height above HAT/one-half mile visibility.

Benefits Of WAAS In The Airport Environment

WAAS is a navigation service using a combination of GPS satellites and the WAAS geostationary satellites to improve the navigational service provided by GPS. WAAS achieved initial operating capability (IOC) in 2003. The system is owned and operated by the FAA and provided free of direct user charges to users across the United States and most of Canada and Mexico.

WAAS improves the navigational system accuracy for en route, terminal, and approach operations over all the continental United States and significant portions of Alaska, Canada, and Mexico. This new navigational technology supports vertically-guided instrument approaches to all qualifying runways in the United States. Vertically-guided approaches reduce pilot workload and provide safety benefits compared to non-precision approaches. The WAAS enabled vertically guided approach procedures are called LPV, which stands for “localizer performance with vertical guidance,”and provide ILS equivalent approach minimums as low as 200 feet at qualifying airports. Actual minimums are based on an airport’s current infrastructure, as well as an evaluation of any existing obstructions. The FAA plans to publish 300 WAAS approach procedures per year to provide service to all qualifying instrument runways within the NAS.

Advantages Of WAAS Enabled LPV Approaches

The advantages of WAAS enabled LPV approaches include:

LPV procedures have no requirement for ground-based transmitters at the airport.
No consideration needs to be given to the placement of navigation facility, maintenance of clear zones around the facility, or access to the facility for maintenance.
LPV approaches eliminate the need for critical arealimitations associated with an ILS.
From a pilot’s viewpoint, an LPV approach looksand flies like an ILS, but the WAAS approach is more stable than that of an ILS.
WAAS equipped users can fly RNAV and basicrequired navigation performance (RNP) procedures, as well as LPV procedures, and the avionics costsare relatively inexpensive considering the totalnavigation solution provided.

RNAV (GPS) approach charts normally have four lines of approach minimums: LPV, LNAV/VNAV, LNAV, and Circling. Figure 2 shows how these minimums might be presented on an approach chart, with the exception of Ground Based Augmentation System (GBAS) Landing System (GLS). This enables as many GPS equipped aircraft to use the procedure as possible and provides operational flexibility if WAAS becomes unavailable. Some aircraft may only be equipped with GPS receivers so they can fly to the LNAV MDA. Some aircraft equipped with GPS and FMS (with approach-certified barometric vertical navigation, or Baro-VNAV) can fly to the LNAV/VNAV MDA. Flying a WAAS LPV approach requires an aircraft with WAAS-LPV avionics. If for some reason the WAAS service becomes unavailable, all GPS or WAAS equipped aircraft can revert to the LNAV MDA and land safely using GPS only, which is available nearly 100 percent of the time. Some locations will have an LP line of minima on an RNAV (GPS) approach chart; but the use of LP is being phased out. At locations with obstacle penetrations in the missed approach segment, there might be two lines of minima for the same type of navigation- one line with higher approach minima without a specified climb gradient and another line with lower approach minima with a specified climb gradient in the event of missed approach.

Figure 2. RNAV GPS approach minima

LPV identifies WAAS approach with vertical guidance (APV) approach minimums with electronic lateral and vertical guidance capability. LPV is used for approaches constructed with WAAS criteria where the value for the vertical alarm limit is more than 12 meters and less than 50 meters. WAAS avionics equipment approved for LPV approaches is required for this type of approach. The lateral guidance is equivalent to localizer accuracy, and the protected area is considerably smaller than the protected area for the present LNAV and LNAV/VNAV lateral protection. Aircraft can fly this minima line with a statement in the AFM that the installed equipment supports LPV approaches. In Figure 2, notice the WAAS information shown in the top left corner of the pilot briefing information on the chart depicted. Below the term WAAS is the WAAS channel number (CH 56202), and the WAAS approach identifier (W35A), indicating Runway 35L in this case, and then a letter to designate the first in a series of procedures to that runway [Figure 2].LNAV/VNAV identifies APV minimums developed to accommodate an RNAV IAP with vertical guidance, usually provided by approach certified Baro-VNAV, but with vertical and lateral integrity limits larger than a precision approach or LPV. Many RNAV systems that have RNP 0.3 or less approach capability are specifically approved in the AFM. Airplanes that are commonly approved in these types of operations include Boeing 737NG, 767, and 777, as well as the Airbus A300 series. Landing minimums are shown as DAs because the approaches are flown using an electronic glide path. Other RNAV systems require special approval. In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because DA on the LNAV/VNAV vertical descent path is farther away from the runway threshold than the LNAV MDA missed approach point.Also shown in Figure 2, is the LNAV minimums line. This minimum is for lateral navigation only, and the approach minimum altitude is published as a MDA. LNAV provides the same level of service as the present GPS stand alone approaches. LNAV supports the following systems: WAAS, when the navigation solution will not support vertical navigation; and GPS navigation systems which are presently authorized to conduct GPS approaches.

Circling minimums that may be used with any type of approach approved RNAV equipment when publication of straight-in approach minimums is not possible.

Ground-Based Augmentation System (GBAS)

The United States version of the Ground-Based Augmentation System (GBAS) has traditionally been referred to as the Local Area Augmentation System (LAAS). The worldwide community has adopted GBAS as the official term for this type of navigation system. To coincide with international terminology, the FAA is also adopting the term GBAS to be consistent with the international community. GBAS is a ground-based augmentation to GPS that focuses its service on the airport area (approximately a 20–30 mile radius) for precision approach, DPs, and terminal area operations. It broadcasts its correction message via a very high frequency (VHF) radio data link from a ground-based transmitter. GBAS yields the extremely high accuracy, availability, and integrity necessary for Category I, II, and III precision approachesand provides the ability for flexible, curved approach paths. GBAS demonstrated accuracy is less than one meter in both the horizontal and vertical axis. [Figure 3]

Figure 3. GBAS architecture

The GBAS augments the GPS to improve aircraft safety during airport approaches and landings. It is expected that the end state configuration will pinpoint the aircraft’s position to within one meter or less with a significant improvement in service flexibility and user operating costs.GBAS is comprised of ground equipment and avionics. The ground equipment includes four reference receivers, a GBAS ground facility, and a VHF data broadcast transmitter. This ground equipment is complemented by GBAS avionics installed on the aircraft. Signals from GPS satellites are received by the GBAS GPS reference receivers (four receivers for each GBAS) at the GBAS equipped airport. The reference receivers calculate their position using GPS. The GPS reference receivers and GBAS ground facility work together to measure errors in GPS provided position.The GBAS ground facility produces a GBAS correction message based on the difference between actual and GPS calculated position. Included in this message is suitable integrity parameters and approach path information. This GBAS correction message is then sent to a VHF data broadcast (VDB) transmitter. The VDB broadcasts the GBAS signal throughout the GBAS coverage area to avionics in GBAS equipped aircraft. GBAS provides its service to a local area (approximately a 20–30 mile radius). The signal coverage is designed support the aircraft’s transition from en route airspace into and throughout the terminal area airspace.

The GBAS equipment in the aircraft uses the corrections provided on position, velocity, and time to guide the aircraft safely to the runway. This signal provides ILS look alike guidance as low as 200 feet above touchdown. GBAS will eventually support landings all the way to the runway surface. Figure 4 is an example of a GBAS (LAAS) approach into Newark, New Jersey.

Figure 4. GLS approach at Newark, New Jersey

Required Navigation Performance (RNP)

The operational advantages of RNP include accuracy, onboard performance monitoring and alerting which provide increased navigation precision and lower minimums than conventional RNAV. RNP DAs can be as low as 250 feet with visibilities as low as 3/4 SM. Besides lower minimums, the benefits of RNP include improved obstacle clearance limits, as well as reduced pilot workload. When RNP capable aircraft fly an accurate, repeatable path, ATC can be confident that these aircraft are at a specific position, thus maximizing safety and increasing capacity.

To attain the benefits of RNP approach procedures, a key component is curved flight tracks. Constant radius turns around a fix are called “radius-to-fix legs (RF legs).”These turns, which are encoded into the navigation database, allow the aircraft to avoid critical areas of terrain or conflicting airspace while preserving positional accuracy by maintaining precise, positive course guidance along the curved track. The introduction of RF legs into the design of terminal RNAV procedures results in improved use of airspace and allows procedures to be developed to and from runways that are otherwise limited to traditional linear flight paths or, in some cases, not served by an IFR procedure at all. Navigation systems with RF capability are a prerequisite to flying a procedure that includes an RF leg. Refer to the notes box of the pilot briefing portion of the approach chart in Figure 5.

Figure 5. RNAV RNP approach procedure with curved flight tracks

In the United States, operators who seek to take advantage of RNP approach procedures must meet the special RNP requirements outlined in FAA AC 90-101, Approval Guidance for RNP Procedures with Authorization Required (AR). Currently, most new transport category airplanes receive an airworthiness approval for RNP operations. However, differences can exist in the level of precision that each system is qualified to meet. Each individual operator is responsible for obtaining the necessary approval and authorization to use these instrument flight procedures with navigation databases.

RNAV Approach Authorization

Like any other authorization given to air carriers and Part 91 operators, the authorization to use VNAV on a conventional non-precision approach, RNAV approaches, or LNAV/VNAV approaches is found in that operator’s OpSpecs, AFM, or other FAA-approved documents. There are many different levels of authorizations when it comes to the use of RNAV approach systems. The type of equipment installed in the aircraft, the redundancy of that equipment, its operational status, the level of flight crew training, and the level of the operator’s FAA authorization are all factors that can affect a pilot’s ability to use VNAV information on an approach.

Because most Part 121, 125, 135, and 91 flight departments include RNAV approach information in their pilot training programs, a flight crew considering an approach to North Platte, Nebraska, using the RNAV (GPS) RWY 30 approach shown in Figure 6, would already know which minimums they were authorized to use. The company’s OpSpecs, FOM, and the AFM for the pilot’s aircraft would dictate the specific operational conditions and procedures by which this type of approach could be flown.

Figure 6. North Platte Regional (KLBF), North Platte, Nebraska, RNAV (GPS) RWY 30

There are several items of note that are specific to this type of approach that should be considered and briefed. One is the terminal arrival area (TAA) that is displayed in the approach plan view. TAAs, discussed in the Types of Approaches post, depict the boundaries of specific arrival areas, and the MIA for those areas. The TAAs should be included in an IAP briefing in the same manner as any other IFR transition altitude. It is also important to note that the altitudes listed in the TAAs should be referenced in place of the MSAs on the approach chart for use in emergency situations.

In addition to the obvious differences contained in the planview of Figure 6, RNAV (GPS) approach procedure example, pilots should be aware of the issues related to Baro-VNAV and RNP . The notes section of the procedure in the example contains restrictions relating to these topics.RNP values for each individual leg of the procedure, defined by the procedure design criteria for containment purposes, are encoded into the aircraft’s navigation database. Applicable landing minimums are shown in a normal manner along with the associated RNP value in the landing minimums section.RNP required sensors, FMS capabilities, and relevant procedure notes are included in the Pilot Briefing Information procedure notes section. [Figure 5] RNP AR requirements are highlighted in large, bold print. RNP procedures are sequenced in the same manner as RNAV (GPS) procedures. Procedure title “RNAV” includes parenthetical “(RNP)” terminology. RF legs can be used in any segment of the procedure (transition, intermediate, final, or missed approach). RF leg turn directions (left or right) are not noted in the planview because the graphic depiction of the flight tracks is intuitive. Likewise, the arc center points, arc radius, and associated RF leg performance limits, such as bank angles and speeds are not depicted because these aircraft performance characteristics are encoded in the navigation database. RNP values for each individual leg of the procedure, defined by the procedure design criteria for containment purposes, are encoded into the aircraft’s navigation database. Applicable landing minimums are shown in a normal manner along with the associated RNP value in the landing minimums section.

When more than one set of RNP landing minimums is available and an aircrew is able to achieve lower RNP through approved means, the available (multiple) sets of RNP minimums are listed with the lowest set shown first; remaining sets shown in ascending order, based on the RNP value. On this particular procedure, lateral and vertical course guidance from the DA to the Runway Waypoint (LTP) is provided by the aircraft’s FMS and onboard navigation database; however, any continued flight below the DA to the landing threshold is to be conducted under VMC. [Figure 5]

Baro-VNAV

Baro-VNAV is an RNAV system function that uses barometric altitude information from the aircraft’s altimeter to compute and present a vertical guidance path to the pilot. The specified vertical path is computed as a geometric path, typically computed between two waypoints or an angle based computation from a single waypoint. Operational approval must also be obtained for Baro− VNAV systems to operate to the LNAV/VNAV minimums. Baro−VNAV may not be authorized on some approaches due to other factors, such as no local altimeter source being available. Baro−VNAV is not authorized on LPV procedures.For the RNAV (GPS) RWY 30 approach, the note “DME/ DME RNP-0.3 NA” prohibits aircraft that use only DME/ DME sensors for RNAV from conducting the approach. [Figure 6]Because these procedures can be flown with an approach approved RNP system and “RNP” is not sensor specific, it was necessary to add this note to make it clear that those aircraft deriving RNP 0.3 using DME/DME only are not authorized to conduct the procedure.

The least accurate sensor authorized for RNP navigation is DME/DME. The necessary DME NAVAID ground infrastructure may or may not be available at the airport of intended landing. The procedure designer has a computer program for determining the usability of DME based on geometry and coverage. Where FAA flight inspection successfully determines that the coverage and accuracy of DME facilities support RNP, and that the DME signal meets inspection tolerances, although there are none currently published, the note “DME/DME RNP 0.3 Authorized” would be charted. Where DME facility availability is a factor, the note would read, “DME/DME RNP 0.3 Authorized; ABC and XYZ required,”meaning that ABC and XYZ DME facilities are required to assure RNP 0.3.

Hot and Cold Temperature Limitations

A minimum and maximum temperature limitation is published on procedures that authorize Baro−VNAV operation. These temperatures represent the airport temperature above or below which Baro−VNAV is not authorized to LNAV/VNAV minimums unless temperature compensation can be accomplished. As an example, the limitation will read, uncompensated Baro−VNAV NA below −11 °C (12 °F) or above 49 °C (120 °F). [Figure 5] This information will be found in the upper left hand box of the pilot briefing. When the temperature is above the high temperature or below the low temperature limit, Baro−VNAV may be used to provide a stabilized descent to the LNAV MDA; however, extra caution should be used in the visual segment to ensure a vertical correction is not required. If the VGSI is aligned with the published glide path, and the aircraft instruments indicate on glide path, an above or below glide path indication on the VGSI may indicate that temperature error is causing deviations to the glide path. These deviations should be considered if the approach is continued below the MDA.

Many systems which apply Baro−VNAV temperature compensation only correct for cold temperature. In this case, the high temperature limitation still applies. Also, temperature compensation may require activation by maintenance personnel during installation in order to be functional, even though the system has the feature. Some systems may have a temperature correction capability, but correct the Baro−altimeter all the time, rather than just on the final, which would create conflicts with other aircraft if the feature were activated. Pilots should be aware of compensation capabilities of the system prior to disregarding the temperature limitations. The information can be seen in the notes section in Figure 6.

In response to aviation industry concerns over cold weather altimetry errors, the FAA conducted a risk analysis to determine if current 14 CFR Part 97 instrument approach procedures, in the NAS place aircraft at risk during cold temperature operations. This study applied the coldest recorded temperature at the given airports in the last five years and specifically determined if there was a probability that during these non-standard day operations, anticipated altitude errors in a barometric altimetry system could exceed the Required Obstacle Clearance (ROC) used on procedure segment altitudes. If a probability of the ROC being exceeded went above one percent on a segment of the approach, a temperature restriction was applied to that segment. In addition to the low probability that these procedures will be required, the probability of the ROC being exceeded precisely at an obstacle position is extremely low, providing an even greater safety margin.

Pilots need to make an altitude correction to the published, “at”, “at or above” and “at or below” altitudes on designated segment(s) of IAPs listed at specific airports, on all published procedures and runways, when the reported airport temperature is at or below the published airport cold temperature restriction.

Pilots without temperature compensating aircraft are responsible to calculate and make a manual cold-temperature altitude correction to the designated segment(s) of the approach using the AIM 7-2-3, ICAO Cold Temperature Error Table.

No extrapolation above the 5000 ft column required. Pilots should use the 5000 feet “height above airport in feet” column for calculating corrections of greater than 5000 feet above reporting station. Pilots will add correction(s) from the table to the segment altitude(s) and fly at the new corrected altitude. PILOTS SHOULD NOT MAKE AN ALTIMETER CHANGE to accomplish an altitude correction.

Pilots with temperature compensating aircraft must ensure the system is on and operating for each segment requiring an altitude correction. Pilots must ensure they are flying at corrected altitude. If the system is not operating, the pilot is responsible to calculate and apply a manual cold weather altitude correction using the AIM 7-2-3 ICAO Cold Temperature Error Table.

Pilots must report cold temperature corrected altitudes to Air Traffic Control (ATC) whenever applying a cold temperature correction on an intermediate segment and/ or a published missed approach final altitude. This should be done on initial radio contact with the ATC issuing approach clearance. ATC requires this information in order to ensure appropriate vertical separation between known traffic. ATC will not beproviding a cold temperature correction to Minimum Vectoring Altitudes (MVA). Pilots must not apply cold temperature compensation to ATC assigned altitudes or when flying on radar vectors in lieu of a published missed approach procedure unless cleared by ATC.

Pilots should query ATC when vectors to an intermediate segment are lower than the requested intermediate segment altitude corrected for temperature. Pilots are encouraged to self-announce corrected altitude when flying into uncontrolled airfields.

The following are examples of appropriate pilot-to-ATC communication when applying cold-temperature altitude corrections:

On initial check-in with ATC providing approach clearance: Hayden, CO (example below).

Intermediate segment: “Require 10600 ft. for cold temperature operations until BEEAR”,

Missed Approach segment: “Require final holding altitude, 10600 ft. on missed approach for cold temperature operations”

Pilots cleared by ATC for an instrument approach procedure; “Cleared the RNAV RWY 28 approach (from any IAF)”. Hayden, CO (example below).

Intermediate Segment: “Level 10600 ft for cold temperature operations inside HIPNA to BEEAR”

Pilots are not required to advise ATC if correcting on the final segment only. Pilots must use the corrected MDA or DA/DH as the minimum for an approach. Pilots must meet the requirements in 14 CFR Part 91.175 in order to operate below the corrected MDA or DA/DH. Pilots must see and avoid obstacles when descending below the MDA. The temperature restriction at a “Cold Temperature Restricted Airport” is mutually exclusive from the charted temperature restriction published for “uncompensated baro-VNAV systems” on 14 CFR Part 97 RNAV (GPS) and RNAV (RNP) approach charts. The charted temperature restriction for uncompensated baro-VNAV systems is applicable to the final segment LNAV/VNAV minima. The charted temperature restriction must be followed regardless of the cold temperature restricted airport temperature.

Pilots are not required to calculate a cold temperature altitude correction at any airport with a runway length of 2,500 feet or greater that is not included in the airports list found at the URL above. Pilots operating into an airport with a runway length less than 2,500 feet, may make a cold temperature altitude correction in cold temperature conditions.

Airports are listed by ICAO code, Airport Name, Temperature Restriction in Celsius/Fahrenheit and affected Segment. One temperature may apply to multiple segments. Italicized airports have two affected segments, each with a different temperature restrictions. The warmest temperature will be indicated on Airport IAPs next to a snowflake symbol, in the United States Terminal Procedure Publication. The ICON will be added to the TPPs incrementally each charting cycle.

LNAV, LNAV/VNAV and Circling Minimums

There are some RNAV procedures with lower non-precision LNAV minimums [Figure 7] than vertically-guided LNAV/VNAV minimums. Circling procedures found on the same approach chart may also have lower minimums than the vertically-guided LNAV/VNAV procedure. Each RNAV procedure is evaluated independently and different approach segments have differing required obstacle clearance (ROC) values, obstacle evaluation area (OEA) dimensions and final segment types. Figure 8 explains the differences.

Figure 7. Example of LNAV and Circling Minima lower than LNAV/VNAV DA. Harrisburg International RNAV (GPS) Runway 13

Figure 8. Explanation of Minima

Airport/Runway Information

Another important piece of a thorough approach briefing is the discussion of the airport and runway environment. A detailed examination of the runway length (this must include the A/FD section of the CS for the landing distance available), the intended turnoff taxiway, and the route of taxi to the parking area, are all important briefing items. In addition, runway conditions should be discussed. The effect on the aircraft’s performance must be considered if the runway is contaminated.

FAA approach charts include a runway sketch on each approach chart to make important airport information easily accessible to pilots. In addition, at airports that have complex runway/taxiway configurations, a separate full-page airport diagram is published.

The airport diagram also includes the latitude/longitude information required for initial programming of FMS equipment. The included latitude/longitude grid shows the specific location of each parking area on the airport surface for use in initializing FMS. Figure 9 shows the airport sketch and diagram for Chicago-O’Hare International Airport (KORD).

Figure 9. Airport sketch and diagram for Chicago O’Hare International

Pilots making approaches to airports that have this type of complex runway and taxiway configuration must ensure that they are familiar with the airport diagram prior to initiating an instrument approach. A combination of poor weather, high traffic volume, and high ground controller workload makes the pilot’s job on the ground every bit as critical as the one just performed in the air.