The weather conditions at airports are often the cause of delays, diversions and cancellations. Strong winds and poor visibility are often the main culprits, and it is only thanks to technologies first developed in the 1970s that modern commercial aircraft can safely land in such conditions. How do pilots land at airports in poor weather, and how do they decide whether to attempt a landing in the first place? Enter instrument approaches.
The key to the equation is visibility, specifically the pilot’s ability to see the runway on approach. Visibility can be measured in Kilometers (and sometimes statute miles) in the air, referred to as meteorological visibility, or meters on the runway (referred to as runway visual range, or ‘RVR). The visibility is the key factor taken into consideration when deciding whether to continue an approach down to the ground. That decision to continue is made at the decision height or altitude. This is the altitude at which the call is made to continue or abandon the approach and initiate a go around. This is measured either as Decision Height – which is the height above ground level (AGL) or Decision Altitude (DA) which is the altitude measured using the pressure setting currently set on the aircraft’s altimeter. If the pilot can not see the runway or lead in lights at that height, they will execute a missed approach, following the prescribed procedure on their approach chart.
The International Civil Aviation Organization (ICAO) has established a standardized approach category system to define the minimum weather conditions and equipment required for safe and efficient landings. These categories, known as CAT 1, CAT 2, CAT 3A, CAT 3B, and CAT 3C approaches, are crucial in ensuring safe landings, especially during adverse weather conditions. Let’s have a look at the absolute minimum meteorological conditions required to fly these approaches.
| Category | Minimum Decision Height (DH) | Minimum Runway Visual Range (RVR) | Equipent required |
|---|---|---|---|
| CAT 1 | 200 feet AGL (above ground level) | 550 meters | Instrument Landing System (ILS) and autopilot |
| CAT 2 | 100 feet AGL | 350 meters | Enhanced ILS and fail-passive landing system |
| CAT 3A | 0 feet | 200 meters | Highly advanced ILS and fail-operational landing system |
| CAT 3B | 0 feet | 50 meters | Highly advanced ILS, and fail-operational landing system |
| CAT 3C | 0 feet | 0 meters | Extremely advanced ILS, autopilot, and fail-operational landing system |
Misty morning #autoland (filmed by off duty crewmember) at @HeathrowAirport in 125m the other morning. Note the how autopilot nails the centreline lights on rollout #avgeek #aviation pic.twitter.com/LhOIHGrDTH
— Craig (@crgmat) February 17, 2023
CAT 1 and CAT 2 approaches are most common at airports, while CAT 3 approaches are typically reserved for major international airports and are less frequently used due to their advanced equipment and training requirements. Some approaches may be inherently challenging due to common poor weather events or local geography. The Automatic Landing System installed at the airport can be either ‘fail-passive’ or ‘fail-operational’. In the event of a failure of its redundant systems, a fail-passive system means there is no significant out of trim condition or deviation of flight path but the landing can not be completed automatically. A fail-operational system can still support the approach, flare and automatic landing if its redundant systems fail, hence it is required for CAT 3 approaches.
Check out a low visibility approach flown down to minimums in our Atlantic Airways trip report.
Where are the world’s CAT 3C approaches?
Whilst some major airports (such as LHR and JFK) and some aircraft are certified to land in what is effectively zero visibility, there are presently no systems available that allow an aircraft to taxi to the gate in such conditions. As such, CAT 3C approaches are typically not flown in practice. The lowest operational conditions are CAT 3B.
It’s also worth noting that CAT 3C systems can be three to four times more expensive to operate than CAT 3B.
These categories are vital for ensuring safe landings in different weather conditions. They provide a structured framework for equipping aircraft with the necessary technology and ensuring pilot proficiency to perform precision approaches and landings. The more advanced the CAT category, the greater the level of automation and redundancy required, allowing for landings in progressively poorer visibility conditions.
Cover photo: BAF70, JetPhotos
Update – December 2023
The International Civil Aviation Organisation (ICAO) has updated its classification system for instrument approaches. This has been driven by the introduction of new technologies and techniques, and the proliferation of Performance Based Navigation (PBN) methods. Whilst this has been implemented in an effort to simplify some confusing and overlapping terminology, the categorisation has, in fact, become a little more complex.
Instrument approaches are now initially categorised based on their decision height. A ‘Type A’ approach has a decision height of over 250 feet, and a ‘Type B’ approach below 250 feet.
Type A approaches encompass all the traditional non-precision approaches based on 2D VOR/DME, NDB and LOC sources. They also include ‘Approach with Vertical Guidance’ (APV) approaches. The difference is that this new structure makes it clear that APV approaches are considered to be 3D (but not precision approaches).
This restructure inherently emphasises the prioritisation of PBN-based approaches over conventional navaids and ILS, as the industry moves towards this as the primary means of approaching airports on instruments.
