Understanding Runway Visual Range (RVR) with Vaisala Expert Jarmo Pilli

In the world of aviation, weather plays a crucial role in ensuring safe and efficient flight operations. One of the key parameters used to assess weather conditions at airports is Runway Visual Range (RVR). To shed light on this important topic, we had a detailed Q&A session with Jarmo Pilli, an expert in RVR from Vaisala. In this blog, we will explore the insights shared by Jarmo, providing a comprehensive understanding of RVR and its significance in aviation.

What is Runway Visual Range (RVR)?

Jarmo Pilli: Runway Visual Range, commonly referred to as RVR, is a critical parameter used to indicate the operational conditions when a pilot's visibility of the runway is significantly reduced. Essentially, it measures how far a pilot can see the runway markings or lights from the cockpit. This information is crucial for pilots to prepare their flight operations correctly, especially during adverse weather conditions.

When is RVR Required?

Jarmo Pilli: RVR becomes essential when aerodrome visibility drops below 800 meters. Under such conditions, airports can only operate using precision approach methods that rely on RVR readings. Note that, for early indication of current operative conditions, RVR reporting starts when visibility drops below 1500m. For instance, Instrumented Landing Systems (ILS) are used, and specific RVR values are required for different categories of operations:

• CAT I: Runway TDZ, RVR 550m or more
• CAT II: Runway TDZ and MID, RVR 350m or more
• CAT III: Runway TDZ, MID, and END, RVR 175m (IIIA) or 50m (IIIB), 0m (IIIC, not in use).

How is RVR Assessed?

Jarmo Pilli: RVR assessments can be conducted manually or automatically. In manual assessments, an observer calculates the visible number of runway edge lights and converts this into RVR in meters. However, this method is impractical as it requires stopping flight operations. Therefore, automated RVR assessments are also commonly used for category I runways, whilst automated RVR is mandatory for category II and III runways.

How Does Automated RVR Assessment Work?

Jarmo Pilli: Automated RVR assessments use three main data sources: Meteorological Optical Range (MOR), Background Luminance (of the sky), and runway light intensity. These values are measured and processed using ICAO-defined computing methods to provide RVR values for specific runway locations such as TDZ, MID, or END points. The computation is complex and involves iterative processes.

Why is MOR Critical in RVR Assessment?

Jarmo Pilli: MOR is the most critical value in RVR assessment. Unlike background luminance and runway lights, the percentage error in MOR measurement directly translates to a similar percentage error in RVR value. For example, during bright daylight, MOR equals RVR, but at night, the RVR value in meters can be three times or more the MOR value. Therefore, accurate MOR measurement is vital for reliable RVR readings.

How is MOR Measured?

Jarmo Pilli: ICAO Annex 3 standards allow only two technical measurement solutions for MOR: transmissometers and forward scatter sensors. While transmissometers have been the gold standard due to their accuracy and fail-safe measurement principle, forward scatter sensors can be used in certain operative environments. However, forward scatter sensors have limitations and may not always meet the operationally desirable accuracy.

Why Choose Forward Scatter Sensors?

Jarmo Pilli: Forward scatter sensors are often chosen because they require only one mast, making them cheaper to purchase and install compared to transmissometers, which have two masts. They can be adequate for airports with common low visibility conditions like fog, provided they are high-quality and well-maintained.

Where are RVR Sensors Installed at Airports?

Jarmo Pilli: ICAO provides recommendations for the installation of RVR sensors. They should be placed at a lateral distance of not more than 120 meters from the runway centerline and about 300 meters along the runway from the threshold.

In Category II and III operations, a mid-point sensor is also required. If the runway is very long (>3600m), two mid-point sensors are recommended.

RVR sensors are installed approximately 2.5 meters above the runway to estimate the pilot's view from the cockpit.

Conclusion

Understanding RVR and its assessment is crucial for maintaining safe flight operations, especially during adverse weather conditions. The insights shared by Jarmo Pilli highlight the importance of accurate RVR measurements and the role of advanced technologies in ensuring reliable data. As aviation continues to evolve, the significance of RVR in enhancing safety and efficiency remains paramount.