Cat 1 minima

[back to Boeing]

Cat 1 minima are generally 200’/550m. Out of curiosity. How long have those limits been imposed and why. Did some person one day just decide “yup that’s what pilots can fly to so that’ll be the minima”. Or is there a more technical reason?

[Tomaski]

Cat 1 minima are generally 200’/550m. Out of curiosity. How long have those limits been imposed and why. Did some person one day just decide “yup that’s what pilots can fly to so that’ll be the minima”. Or is there a more technical reason?

ILS technology goes back to the 1930's. I'm not sure when the 200' basic minimum was adopted, but it's been at least 40 years, and I presume it is a function of the position tolerances of the equipment (ground and aircraft). Assuming there are no issues with terrain, obstacles or antenna interference, lower minimums are possible with the additional equipment and crew training, but all of those cost money to provide. Airports and operators have to decide if the need for the lower minimums justify the investment.

[safetypee]

Extracts from “The RAE Contribution to All-Weather Landing“

ICAO ~ 1949 adopted ‘Calvert’ approach lighting pattern.
… airworthiness authorities and airline operators promulgated limiting ‘decision heights’ and RVR below which safe manual landings could not have a high probability of success. Typical figures at this time were 200 feet decision height and 800 metres RVR and, as we shall see later, were classified as Category 1 operations.

All-Weather Operations Panel (AWOP) … considered broad criteria. In 1965 they were adopted by ICAO and remain much the same today (2011).
Category, Minimum Decision Height (feet), Minimum RVR (metres)
1 200 ft, 800m
2 100 ft, 400 m

N.B. In later years, research has demonstrated and has been accepted for certification, a range of minima depending on type of approach aid, lighting pattern, aircraft equipment and performance. There are wide ranging variations in DH and visibility minima.

https://www.aerosociety.com/media/4856/the-rae-contribution-to-all-weather-landing.pdf

[RubberDogPoop]

Min visibility 800m perhaps?

[BalusKaptan]

Ceiling RVR Lighting required
Cat 3B 0’ 100/100/100 Not required.

Cat 3B 17’ 125/125/100 1 centreline light. (France only with DH)

3B: If one RVR temporarily u/s, may continue ops 3B with other 2 controlling.

Cat 3A 50’ 200/150/Advisory 3 centreline lights.

3A: If Mid point temp. u/s, RO maybe used as Mid & is controlling.

CAT 2 100' 350/150/Advisory App environment and threshold.

CAT 1 200' 550

Vis is in metres

[safetypee]

RDP, ‘… perhaps visibility.’
Perhaps, but remember that the paper was written from a research viewpoint; also that the means of measuring RVR (equipment) and determining SVR for DH calculation were the focus in the early years - RVR was the scientific metric.
The initial driver for ‘Cat 1’ was the electronic principle and accuracy of ILS - post WW 2, which was flown manually. Thus the visual aids supported the need for significant manoeuvre in the vertical and lateral flight paths; the ‘Calvert’ lighting pattern aiding both.
Initial autopilots’ accuracy were little better than manual flight, but workload arguments enabled reduced visibility operation. Thence the focus was on improved accuracy and decision making, what features - visual scene were required to determine the relative flight path and enable manoeuvre.

Latter research expanded the DH - RVR envelope, ILS accuracy, and particularly auto-flight capability including auto-land, which reduced the need for manoeuvre, and has subtlety changed the nature of the decision in each category.
These aspects must be considered from a safety viewpoint, which reflects the complicated regulatory approach for DH - RVR required today (JAR and thence EASA). There are some ‘corruptions’ of these principles, unsupported by research, which depend on ‘unquantifiable’ human performance - what the pilot actually sees, features used, and accuracy of manual flight.

[aterpster]

ILS technology goes back to the 1930's. I'm not sure when the 200' basic minimum was adopted, but it's been at least 40 years, and I presume it is a function of the position tolerances of the equipment (ground and aircraft). Assuming there are no issues with terrain, obstacles or antenna interference, lower minimums are possible with the additional equipment and crew training, but all of those cost money to provide. Airports and operators have to decide if the need for the lower minimums justify the investment.
Before I got my instrument rating, which was 1958. You may recall it was 200 feet only with a middle marker. If the middle marker was inop or not received, it was then 250. Visibility without ALS was 1 mile, which later became 3/4 mile. With ALS it became 1/2 mile or RVR 2,600, which later became 1/2 mile or RVR 2,400, and RVR 1,800, with TDZ and CL lighting.