Jazmin Li
The use of airports with no conventional navaids as alternates is a complex topic that requires careful consideration of various factors. Navaids, or navigation aids, play a crucial role in aircraft navigation and include systems such as GPS, VOR, and RNAV. While GPS-equipped aircraft can operate without ground-based navaids, certain requirements must be met for an airport to qualify as an alternate. These requirements include weather reporting capabilities, specific weather conditions, and the availability of non-GPS approach procedures. Additionally, factors such as the number of navaids available and their impact on minimum visibility and ceiling requirements, as outlined in the 1 NAVAID and 2 NAVAID rules, also come into play. Understanding these regulations and the capabilities of different airports is essential for safe flight planning and ensuring appropriate alternates are selected.
| Characteristics | Values |
|---|---|
| Use of airport with no conventional navaid as alternate | Not allowed unless the aircraft has DME/DME equipment, in which case RNAV can be used as a backup |
| Reasons | Unmonitored navaids, lack of reporting WX, GPS-based approaches |
| Alternate airport requirements | Weather forecasts meeting certain qualifications, weather reporting capabilities, non-GPS approach procedure |
| Navaid rule | 1 NAVAID rule, 2 NAVAID rule |
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What You'll Learn
GPS-equipped aircraft don't need ground-based navaids
GPS-equipped aircraft do not need ground-based navaids or associated aircraft avionics to fly the approach. However, it is suggested to monitor the underlying approach with ground-based navaids when able. For instance, if an aircraft is equipped with a suitable RNAV system, a pilot can fly a procedure or route based on an operational VOR using the RNAV system without needing to monitor the VOR.
The use of GPS and its augmentation systems, collectively referred to as the Global Navigation Satellite System (GNSS), enables Point-Based Navigation (PBN) procedures. Aircraft with DME/DME equipment can also use RNAV as a backup during GNSS disruptions. In the absence of DME/DME equipment, the FAA retains a limited network of VORs, called the VOR MON, to provide basic conventional navigation services if GNSS becomes unavailable.
GPS-based approaches are identified by the name of the procedure and the inclusion of "or GPS" in the title, such as "VOR/DME or GPS RWY 15." Authorized procedures must be retrievable from a current onboard navigation database. This database may include information on conventional navaid approaches, which should not be confused with GPS overlay approaches.
When using GPS and WAAS at an alternate airport, flight planning must be based on specific criteria, such as RNAV (GPS) LNAV or circling minima line, or conventional approach procedures with "or GPS" in the title. At the alternate airport, pilots may plan for lateral navigation (LNAV) or circling minimum descent altitude (MDA), among other options.
In summary, while GPS-equipped aircraft do not require ground-based navaids, monitoring these navaids when possible is advisable. Additionally, the availability of backup options, such as RNAV and VOR MON, ensures safe navigation even during GNSS disruptions.
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Alternate airports must meet weather minimums
When planning a flight, it is essential to consider not just the weather conditions at your intended destination but also the weather minimums at any potential alternate airports. Alternate airports are designated as such to provide a safe landing option should the weather deteriorate or other unforeseen circumstances arise, making it impossible to land at the intended destination.
For an airport to be designated as an alternate, it must meet specific criteria regarding weather minimums. These criteria are in place to ensure that pilots can safely divert and land at the alternate airport if needed. The specific weather minimums that an alternate airport must meet can vary depending on the regulations of the aviation authority governing the flight. These may include factors such as cloud base height, visibility, and the presence of certain navigational aids.
In general, an alternate airport should have weather conditions that are equal to or better than those at the intended destination airport. This ensures that if a diversion is necessary, the aircraft can still land safely at the alternate airport. The specific weather minimums required will depend on the type of flight and the regulations under which it is operating. For example, a commercial passenger flight may have different requirements compared to a private charter flight.
It is important to note that the weather minimums for alternate airports are typically higher than those for a standard airport. This is to provide an extra margin of safety in the event of a diversion. Pilots must carefully consider these minimums when planning their flight routes and selecting alternate airports to ensure they comply with the relevant regulations and have a safe and viable diversion option.
Additionally, it is worth mentioning that not all airports with no conventional navaids can be used as alternates. The availability of appropriate navigation aids is just one factor in determining whether an airport can be designated as an alternate. Other factors, including weather minimums, runway length and condition, and the availability of necessary facilities and services, also come into play. As such, pilots should carefully review all relevant criteria before selecting an alternate airport.
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RNAV provides backup during GNSS disruption
Not all airports can be used as alternate airports. An airport may not be qualified for alternate use if it does not have weather reporting capabilities, or if the airport NAVAID is unmonitored or is Global Positioning System (GPS) based. For an airport to be used as an alternate, the forecast weather must meet certain qualifications at the estimated time of arrival. Standard alternate minimums for a precision approach are a 600-foot ceiling and 2 statute mile (SM) visibility. For a non-precision approach, the minimums are an 800-foot ceiling and 2 SM visibility.
RNAV (Area Navigation) provides a backup to continue flying PBN during a GNSS disruption. Aircraft that carry DME/DME equipment can use RNAV. For those that do not, the FAA is retaining a limited network of VORs, called the VOR MON, to provide a basic conventional navigation service if GNSS becomes unavailable. During a disruption, the MON will enable aircraft to navigate through the affected area or to a safe landing at a MON airport without reliance on GNSS. While this is not as efficient as the new PBN route structure, it provides nearly continuous VOR signal coverage at 5,000 feet AGL across the NAS, outside of the Western U.S. Mountainous Area (WUSMA).
GPS-equipped aircraft do not need underlying ground-based NAVAIDs or associated aircraft avionics to fly the approach. However, monitoring the underlying approach with ground-based NAVAIDs is suggested when able. Some GPS procedures have a Terminal Arrival Area (TAA) with an underlying RNAV approach. For flight planning purposes, TSO-C129() and TSO-C196()-equipped users (GPS users) whose navigation systems have fault detection and exclusion (FDE) capability, who perform a preflight RAIM prediction for the approach integrity at the airport where the RNAV (GPS) approach will be flown, and have proper knowledge and any required training and/or approval to conduct a GPS-based IAP, may file based on a GPS-based IAP at either the destination or the alternate airport, but not at both locations.
When using WAAS at an alternate airport, flight planning must be based on flying the RNAV (GPS) LNAV or circling minima line, or minima on a GPS approach procedure, or conventional approach procedure with "or GPS" in the title. Upon arrival at an alternate, when the WAAS navigation system indicates that LNAV/VNAV or LPV service is available, then vertical guidance may be used to complete the approach using the displayed level of service.
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GPS approaches are often marked NA
GPS-equipped aircraft do not need underlying ground-based NAVAIDs or associated aircraft avionics to fly the approach. However, monitoring the underlying approach with ground-based NAVAIDs is suggested when able. Existing overlay approaches may be requested using the GPS title; for example, the VOR or GPS RWY 24 may be requested as “GPS RWY 24.”
Localizer (LOC), localizer type directional aid (LDA), and simplified directional facility (SDF) procedures are not authorized. Overlay procedures are identified by the “name of the procedure” and “or GPS” (e.g., VOR/DME or GPS RWY 15) in the title. Authorized procedures must be retrievable from a current onboard navigation database. The navigation database may also enhance position orientation by displaying a map containing information on conventional NAVAID approaches. This approach information should not be confused with a GPS overlay approach.
Performance-Based Navigation (PBN) procedures are primarily enabled by GPS and its augmentation systems, collectively referred to as Global Navigation Satellite System (GNSS). Aircraft that carry DME/DME equipment can also use RNAV, which provides a backup to continue flying PBN during a GNSS disruption. For those aircraft that do not carry DME/DME, the FAA is retaining a limited network of VORs, called the VOR MON, to provide a basic conventional navigation service for operators to use if GNSS becomes unavailable.
When using WAAS at an alternate airport, flight planning must be based on flying the RNAV (GPS) LNAV or circling minima line, or minima on a GPS approach procedure, or conventional approach procedure with “or GPS” in the title.
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The 2 NAVAID rule allows lower additives
The 2 NAVAID rule provides more flexibility and the opportunity to get lower minimums. It is a bit more complex than the 1 NAVAID rule, which is based on either the alternate airport having only one IAP or the dispatcher choosing to use only one approach. The 1 NAVAID rule requires that the navaids used for the approach must be monitored, and performance limits such as tailwind components cannot be exceeded.
In the 2 NAVAID rule, the decision height (DH) is rounded up to the next 100-foot value, and 200 feet is added for the ceiling value. Then, the two visibilities are compared, and the higher value is chosen. This value is then increased by 1/2 sm.
For example, let's consider the following scenario:
ILS 26L Visibility Minimum: 1/2 SM
Decision Height: 200 ft
By applying the additives, we get:
ILS 26L Visibility Minimum: 1/2 SM + 1SM -> Visibility Alternate Minimum 1 1/2 SM
Decision Height: 200 ft + 400 ft -> Ceiling Minimum 600'
Using the 2 NAVAID rule, we can utilize two approaches to different runways with two different navaids. This allows us to use lower additives to the approaches but requires us to use the higher of the two derived numbers as our minimum.
For instance, we have:
ILS 26L Visibility Minimum: 1/2 SM + 1/2 SM -> 1 SM
Decision Height: 200 ft + 200 ft -> 400 ft
Alternate Minimum for ILS 26L = 1 SM and 400 ft
ILS 35 Visibility Minimum: 3/4 SM -> 1 1/4 SM
Decision Height: 200 ft + 200 ft -> 400 ft
Alternate Minimum for ILS 35 = 1 1/4 SM and 400 ft
To find the overall alternate minimums for the airport, we take the most restrictive (highest) values from the two approaches:
Visibility Alternate Minimum: 1 1/4 SM
Ceiling Minimum: 400'
Therefore, by using the 2 NAVAID rule, we were able to achieve lower additive values for the approaches while still meeting the necessary minimum requirements.
It is worth noting that there may be instances where the 1 NAVAID rule provides better minimums than the 2 NAVAID rule. Therefore, it is essential to evaluate both options and choose the most suitable one for a particular scenario.
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Frequently asked questions
No, an airport with no conventional navaid cannot be used as an alternate. An airport may not be qualified for alternate use if its navaid is unmonitored or if it only has GPS-based approaches.
For an airport to be used as an alternate, it must meet certain weather qualifications at the estimated time of arrival. The standard alternate minimums for a precision approach are a 600-foot ceiling and 2 statute miles (SM) visibility. For a non-precision approach, the minimums are an 800-foot ceiling and 2 SM visibility.
The 2 NAVAID rule allows the use of 2 approaches to different runways utilizing 2 different navaids. This results in lower additives to the approaches, but the higher of the 2 derived numbers must be used as the minimum.
