What Is MSL in Aviation?
Pilots · 5 min read
MSL stands for Mean Sea Level, a measurement of altitude that is used by pilots for both aircraft and drones.
Performance-based navigation is a modern method of air navigation services in the commercial aviation industry. In the early days, aircraft navigation specifications were defined as sensors and identification signals such as beacons and waypoints.
In the early days of aviation, aircraft were not equipped with a range of smart instruments to measure flight position and other data. Instead, aircraft pilots used to rely on visual cues to guide them through the sky, mostly various visual landmarks such as bonfires or natural features.
However, with the rapid development of commercial aviation through the first half of the 20th century, aircraft started reaching new heights and flying in high altitudes while relying on visual landmarks became a challenge. Hence newer aircraft were produced with gadgets like high-precision gyros and radio transmitters to be able to contact ground based navigation aids as radio beacons, which would help pilots plot their routes more precisely.
The greatest breakthrough in navigation, however, happened in the 1960s with the introduction of GPS. Ever since then, navigation systems and navigation infrastructure have become so advanced that modern day aspiring cadets need to go through intricate training courses of flight management and navigation systems to be able to obtain their licenses.
Historically, aircraft navigation has been heavily governed by a sensor specific method, specifically VOR beacons as well as instruments onboard to guide the pilot in respect to a specific waypoint. This manner of navigation, as you can imagine, was quite limiting since the pilot was constricted to following a singular line through the air.
In the 1970s as airspace design became more complex with ever-growing air traffic and obstacle clearance requirements, RNAV as a concept was introduced. RNAV or area navigation allows the pilot to know their present position at any given time with or without the use of ground-based navaids. This eliminated the need to follow radio beacons and let the aircraft move to and from two random points in a given area. Using area navigation in turn led to more efficient, greener routes. But more on the advantages later.
In simple language, PBN is a concept that exists under the RNAV and reshapes the navigational ability of an aircraft from sensor-equipment-based to performance based. ICAO defines PBN as “Area navigation based on performance requirements for aircraft operating along an ATS route, on instrument approach procedures or in a designated airspace.”
PBN procedures state that pilots can have enhanced navigation capability if they can use RNAV to travel their desired path and also monitor their onboard equipment with respect to how accurately it is able to measure your position error. The focus of PBN is to continuously monitor your position, compare it to outside information and make sure within a certain value range.
Two elements make up what we know as PBN today – Area Navigation (RNAV) and Required Navigation Performance (RNP). Not to mention, PBN procedures also include an aircraft’s navigational performance standards. However, since some concepts and definitions of RNAV and RNP are quite inconsistent around the globe, their implementation in operations has caused confusion among air navigation service providers, regulators and other stakeholders.
So, PBN is a relatively new approach between air service providers to resolve this miscommunication, to define and clarify the application and procedures of RNAV and RNP from approach procedures to all areas of a flight. Moreover, PBN implementation aims to make sure the efficiency RNAV brings to aviation is properly employed and to ensure that pilots fly with appropriate equipment during appropriate circumstances and procedures.
Performance requirements of PBN are communicated via navigation specifications, such as Advanced RNP (A-RNP), RNP 0.3, RNP 1, among others and the FAA has listed navigation specifications intended for specific types of procedures.
Performance based navigation (PBN) exists under the definition of area navigation or RNAV, but in this context, RNAV simply means ‘area navigation’. Area navigation is a concept which does not require an airplane to be heading directly to or from a radio navigation station/beacon on any desired flight path, meanwhile PBN instructs the operators to use RNAV for a certain route or approach if they can comply with other data.
A navigation requirement where on board performance monitoring and alerting systems are installed is called required navigation performance (RNP). In contrast, the navigation system that does not require such specifications is called area navigation specification (RNAV).
The performance-based navigation concept is based on three basic components known as The Navigation Specification, the navigation aid infrastructure and, navigation application. The navigation specification identifies performance requirements in terms of precision, integrity, and continuity for intended operations in a specific airspace.
Navigation specifications also describe the process of achieving performance requirements by identifying the requirements of proposed navigation performance. Additional requirements for PBN are pilot training, knowledge, and an operating certificate.
The Navaid infrastructure describes the ground equipment requirements or, if used, space-based navigation requirements such as GPS. Appropriate Navaid infrastructure is essential for complying with PNB requirements. Navigation application is the application of navigation specification and Navaid infrastructure in combination with ATS routes, instrument approach procedure and instrument flight rules in the context of airspace utilization.
PBN includes En-route applications, terminal airspace applications and, approach procedure applications. Sophisticated approach systems such as instrument landing system ILS, microwave landing system MLS, and Global Navigation Satellite Systems landing system GLS are not part of the PBN concept because they are not based on area navigation techniques.
The ground and space-based navigation aids are named Navaid Infrastructure. These navigational aids are mentioned in Navigation Specification. Ground-based navigation aids are navigation systems based on the ground and provide navigational services visual flight rules VFR operations. It includes Very High-Frequency VHF Omnidirectional Range (VOR) and Nondirectional Radio Beacon (NDB).
Furthermore, there are three types of VOR navigational stations, including VOR, VOR plus distance measuring equipment VOR-DME and, VOR plus military tactical air navigation system vortac (used for military applications only). Satellite-based navigation systems include GPS navigation using GNSS satellites.
To comply with the PBN concept, an aircraft must meet the specific requirements where RNAV and RNP precision must be 95 percent of the total flight time. Required navigation performance is a system that requires equipment installed on aircraft to carry onboard performance monitoring and alerting systems.
The PBN concept also includes navigation specifications that define flight crew and aircraft requirements that are essential to carry out navigation applications inside a specific airspace concept.
In both RNP and RNAV navigation specifications, numerical designation defines lateral navigation accuracy in terms of nautical miles. It must be accomplished for at least 95 percent of the flight time operating within specific airspace.
This information is mentioned in ICAO Doc 9613, named Performance-based Navigation Manual. FAA mentions the same information in AC 90-105 named Approval Guidance for RNP Operations and Barometric Vertical Navigation in the United States National Airspace System and remote and Oceanic Airspace.
PBN marked a transition from sensor-based to performance-based navigation; it has several benefits over the sensor-specific mechanism of improving airspace and obstacle clearance standards, including the elimination of the requirement to retain sensor-specific routes and operations, as well as the expenses involved with them.
It also eliminates the requirement for every new adaptation of navigation systems to build sensor-specific procedures, which would be prohibitively expensive.
It explains the usage of RNAV and RNP systems by offering a restricted set of navigational criteria designed for worldwide usage.
PBN makes the operating clearance task quicker for carriers and can provide operational benefits such as enhanced safety, flight efficiency, airspace availability, expenses reductions, and decreased effect on the environment such as reducing noise and greenhouse gas pollution.
As PBN is being implemented in countries, airports and airspaces around the world, it is becoming increasingly important for young and seasoned pilots alike to be introduced to this concept.
Aeroclass has released a course of PBN fundamentals, during which the students also get a broader explanation of the PBN concept and navigation infrastructure. Throughout the course the students will also identify the benefits of Performance Based Navigation and describe differences between RNAV and RNP. Take a peek at the course and the instructor in the short trailer:
To sum up the article, it is quite clear that PBN procedures provide operational benefits to many stakeholders involved in aviation. This advanced form of air navigation not only enhances safety, and increases airport operation efficiency, but it also permits aircraft operation to be smoother. Moreover, PBN application reduces carbon footprint – a pain point of modern aviation – since aircraft fly shorter routes and burn less fuel.