Sachin Walls
The use of GPS for alternate airport planning is a complex topic with several considerations. Firstly, it's important to note that the availability of GPS-based approaches at alternate airports is governed by regulations and policies, such as the Aeronautical Information Manual (AIM) and the FAA's policy changes. These regulations outline specific requirements that must be met for the use of GPS in alternate airport planning. While GPS can be used for alternate airport planning, there are certain restrictions and limitations in place. One key restriction is that GPS-based IAPs cannot be used at both the destination and alternate airports; it must be one or the other. Additionally, factors such as the availability of other instrument approach procedures, weather minimums, and the capabilities of the aircraft and its equipment play a significant role in determining the feasibility of using GPS for alternate airport planning. Proper training, approval, and compliance with relevant specifications are also crucial factors to consider.
| Characteristics | Values |
|---|---|
| Can GPS be used for an alternate airport? | Yes, but not for both the destination and alternate airports. |
| Requirements | GPS users must have navigation systems with fault detection and exclusion (FDE) capability, perform a preflight RAIM prediction, and have the knowledge and approval to conduct a GPS-based IAP. |
| Alternate airport weather minimums | 800-2 |
| Alternate airport 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. |
| Use of LPV approach | Cannot be planned for but can be used if necessary. |
| Use of WAAS vertical guidance | Prohibited during alternate airport planning. |
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What You'll Learn
GPS-based IAPs at alternate airports
The use of GPS-based Instrument Approach Procedures (IAPs) at alternate airports has been a topic of discussion and clarification in recent years. Previously, pilots often could not use GPS-based IAPs as part of their alternate airport calculations. However, with advancements in technology and policy updates, the use of GPS-based IAPs at alternate airports has become more prevalent and offers additional options for pilots.
The Aeronautical Information Manual (AIM) and regulatory guidelines provide important considerations for the use of GPS-based IAPs at alternate airports. According to AIM section 1-1-19, any required alternate airport must have an approved instrument approach procedure other than GPS, ensuring availability and compatibility with the aircraft's equipment. This means that airports with both ground-based navaid IAPs and GPS-based IAPs typically show the GPS-based option as unavailable for filing as an alternate. However, the latest NOTAM provides flexibility, allowing pilots to plan for a GPS-based IAP at either the destination or the alternate airport, but not both.
For air carriers with specific authorizations, such as Ops Spec C055 issued after February 2011, there is a provision to use GPS-based IAPs at alternate airports if equipped with WAAS-approved avionics. This enables flight planning based on RNAV (GPS) LNAV minima or conventional approach procedures with GPS in the title. Additionally, WAAS users with approved baro-VNAV capability can plan for LNAV/VNAV minima, while those without baro-VNAV can still plan for LNAV at alternate airports.
It is important to note that flight planning and execution are separate considerations. While planning for an alternate airport, pilots must adhere to specific weather minimums and regulations, such as the 800-2 rule for non-precision approaches. However, if diversions occur, pilots have the flexibility to fly the most suitable approach at the alternate airport, considering its published minimums. This separation between planning and execution provides pilots with options and ensures they are well-prepared for various scenarios.
To summarize, the use of GPS-based IAPs at alternate airports has evolved, offering more flexibility to pilots. By adhering to regulatory guidelines, such as AIM and Ops Spec C055, pilots can effectively utilize GPS-based approaches at alternate airports. Proper planning, including preflight RAIM predictions and ensuring conventional approaches can be flown without GPS reliance, remains crucial. Ultimately, the integration of GPS-based IAPs at alternate airports enhances pilots' decision-making capabilities and provides valuable alternatives during flight operations.
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GPS-only destinations
However, there are provisions for certain aircraft to use GPS-based IAPs at an alternate airport. For instance, air carriers with Ops Spec C055 issued after February 2011 and WAAS-approved avionics can use GPS-based IAPs at an alternate airport, but their flight planning is limited to flying the RNAV (GPS) LNAV minima line or the minima on a GPS approach procedure.
Additionally, a GPS user without approved baro-VNAV capability planning a flight to an airport with an ILS IAP can choose an alternate airport with an RNAV (GPS) IAP with LNAV minimums, which was not allowed under previous policies.
It is important to note that when planning a flight to a GPS-only destination, the alternate airport must still meet certain weather minimums, such as the 800-2 rule, and the pilot must have the proper knowledge, training, and approval to conduct a GPS-based IAP.
Overall, while GPS-only destinations can be used as alternates, it is crucial for pilots to carefully consider the specific requirements and limitations outlined by regulatory authorities to ensure safe and compliant flight operations.
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GPS and WAAS avionics
The use of GPS for alternate airport calculations is a complex topic with some specific requirements and limitations. Firstly, let's discuss the role of GPS and WAAS avionics in aviation.
GPS, or the Global Positioning System, is a satellite-based navigation system that provides positioning, navigation, and timing services to users worldwide. In aviation, GPS has become an integral part of navigation and flight planning, offering accurate and reliable positioning information.
WAAS, or the Wide Area Augmentation System, is a crucial augmentation to GPS developed by the Federal Aviation Administration (FAA). The primary goal of WAAS is to enhance the accuracy, integrity, and availability of GPS signals for aviation use. WAAS utilizes a network of ground-based reference stations in North America and Hawaii to measure and correct small variations in GPS satellite signals. These corrections are then broadcast back to WAAS-enabled GPS receivers, improving their accuracy in determining their positions.
Now, regarding the use of GPS and WAAS for alternate airport calculations:
The Aeronautical Information Manual (AIM) has specific guidelines regarding the use of GPS approaches for alternate airports. According to AIM section 1-1-19, any required alternate airport must have an approved instrument approach procedure other than GPS that is anticipated to be operational and available at the estimated time of arrival, and the aircraft must be equipped to fly this procedure. This means that GPS alone is not sufficient for selecting an alternate airport, and other instrument approach procedures, such as ground-based navaid IAPs, must be considered.
However, there are provisions for certain air carriers with Ops Spec C055 to use GPS-based IAPs at alternate airports if they have WAAS-approved avionics. In these cases, flight planning may include the use of RNAV (GPS) LNAV minima or the minima on a GPS approach procedure. Nevertheless, air carriers must still comply with their Ops Spec C055 authorization.
It's important to note that even with WAAS-capable avionics, there may be specific requirements for LPV minimums, such as dual WAAS receivers under TSO 145/146. Additionally, as per the current alternate airport planning policy, WAAS users are prohibited from planning to use WAAS vertical guidance at their alternate airport. Instead, they may plan for the LPV at their destination and the RNAV (GPS) LNAV/VNAV minima at the alternate, considering charted temperature restrictions.
In summary, while GPS and WAAS avionics play a significant role in aviation navigation and flight planning, there are specific regulations and considerations when using GPS for alternate airport calculations. Adhering to these guidelines ensures safe and compliant operations, providing pilots with reliable options during their flights.
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GPS and circling minimums
Circling approaches are considered challenging and potentially risky. They are rarely performed in normal operations, and pilots must be aware of the complexities involved. One of the challenges is maintaining visual contact with the field, which requires more favourable weather conditions and higher minimums. When breaking out of the clouds on a circling approach, pilots must be cautious and ensure they keep the airport in sight while managing their bank.
The circling approach procedure requires pilots to refer to the approach plate's plan view for obstacles and other relevant information. This information guides them on how and where to circle. Additionally, pilots should be mindful of maintaining visual conditions throughout the manoeuvre. If visual conditions are not adequate, a missed approach may be necessary.
When it comes to circling minimums, several factors come into play. Firstly, the FAA is gradually removing lines of circle-to-land minimums due to the increasing availability of RNAV (GPS) approaches and the associated complexity and cost of maintaining all circle-to-land minimums. Secondly, circling minimums are determined by specific criteria, including whether the circling approach is the only instrument approach procedure available at an airport and whether it serves a minimum operational network (MON) airport.
To ensure safety during circling approaches, pilots should refer to the protected area, which has been expanded since 2012. This protected area guarantees at least 300 feet of obstacle clearance. Additionally, pilots should be aware of the minimum visibility requirements for circling, which vary depending on the aircraft category. For example, ILS at FCM: A and B require a minimum visibility of 1 mile, C requires 1.5 miles, and D requires 2 miles.
When considering GPS and circling minimums, it is important to note that GPS can be used as a backup tool to validate distance estimation abilities. However, pilots should not solely rely on GPS during circling approaches, as it may not always be available in situations or malfunctions that necessitate such manoeuvres. Therefore, building proficiency in estimating distances and using known ground references is crucial.
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GPS and LPV approaches
The use of GPS approaches in alternate calculations is an important consideration for pilots. The Aeronautical Information Manual (AIM) previously limited the use of GPS-only approaches at alternate airports. However, with the introduction of Ops Spec C055, air carriers with WAAS-approved avionics can now use GPS-based IAPs at alternate airports. This has provided more flexibility in flight planning, allowing pilots to utilize the benefits of GPS technology at their destination or alternate airport.
When it comes to GPS and LPV approaches, it is crucial to understand the differences and applicability. LPV stands for Localizer Performance with Vertical Guidance and is a type of Area Navigation (RNAV) approach. LPV provides both lateral and vertical guidance, similar to an Instrument Landing System (ILS), but with some key differences. Unlike ILS, which is ground-based, LPV utilizes the space-based Global Navigation Satellite System (GNSS) to provide guidance to multiple aircraft simultaneously. The GNSS signal is augmented by systems like WAAS, resulting in extremely accurate lateral and vertical guidance down to a decision altitude (DA).
LPV approaches offer advantages over ILS, particularly near and below DA. The scaling on an LPV approach transitions to a linear scale as the runway is approached, making it less sensitive and easier to fly compared to ILS. LPV approaches typically provide a total course width of 700' at the runway threshold, equivalent to an ILS localizer. Additionally, LPV can bring aircraft down to 200' above touchdown with 1/2 mile visibility, making it a desirable approach option.
However, there are some considerations when it comes to LPV approaches. To utilize LPV minima, aircraft must be equipped with an LPV-capable Flight Management System (FMS) and a compatible Satellite-Based Augmentation System (SBAS) receiver. Additionally, LPV approaches may not be available at all airports due to terrain and infrastructure limitations. In such cases, alternative approaches like LNAV or LNAV/VNAV may be used. LNAV/VNAV was the first type of GPS approach with vertical guidance, initially designed for baro-aided GPS units but now compatible with most WAAS receivers.
In summary, GPS technology has enhanced flight planning and navigation options, including at alternate airports. LPV approaches, enabled by WAAS or similar augmentation systems, offer precise lateral and vertical guidance, making them a preferred choice when available. However, pilots must ensure their aircraft are equipped with the necessary technology and remain adaptable to utilize alternative approaches when LPV is not an option.
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Frequently asked questions
Yes, but only if your aircraft is equipped to fly the approach and the airport is anticipated to be operational at the estimated time of arrival.
You must have WAAS-approved avionics and conduct a preflight Receiver Autonomous Integrity Monitoring (RAIM) prediction for the approach integrity at the airport.
No, you can only use GPS for either the primary or alternate airport, but not both.
Using GPS for an alternate airport provides pilots with more flexibility and options during flight planning and can help ensure adequate fuel planning.
The standard alternate minimums for a precision approach are a 600-foot ceiling and 2 statute miles (SM) visibility.
