"All-weather operations".
From this information, the pilots and authorities of airlines whose operations were severely disrupted by frequent fog concluded that any attempt to regularly land aeroplanes in worse than what became called "Cat 1" conditions would have to be made using the autopilot. Hence for decades the emphasis for so-called "All Weather Operations" ("AWO") has been on automatic landing (autoland) systems. In fact of course "ALL weather" is a misnomer, since there are many weather conditions that affect flight safety other than those meeting Category 2 and 3 visibility criteria.
The criteria for sufficiently reliable autopilots also led to exploration of "hybrid" systems with Head-Up Display monitoring of lower reliability autopilots. Eventually the technology for head-up displays and enhanced vision systems has made it possible for the pilot to see an artificial visual scene with a projection of the flight path superimposed on it, and some GPS-based landing systems can provide self-contained guidance.
However such systems are not yet in common use and the vast majority of poor weather but "non-AWO" approaches are still affected by exactly the same issues as had been recognised 60 and more years ago. Even today, many older aircraft make tens of thousands of approaches, in all parts of the world, with only the most basic guidance in the vertical plane.
Pre "Cat 1" minima.
Prior to 1960, the minimum altitude for instrument flight was called the "critical height": the lowest "critical height" was generally 300 ft and the lowest RVR was 350m. There was no systematic method of calculating critical heights at this time, so they were chosen by operational experience.
The slant visual range required to see a useful segment of the approach lighting from an aircraft at 300 ft is about 800m. Since the slant visual range was more often less than the 350m minimum RVR, pilots operating to these minima had little chance of seeing sufficient lights at 300 ft on an lLS glidepath to decide to continue the approach. So in these circumstances it was common practice for pilots to fly level at critical height until the approach lighting came into view, when descent was resumed at an increased gradient. This was of course steeper than that used in good visibility. Critical height was thus a ‘platform’ from which the captain could elect either to continue the approach or to go-around.
It was recognised in 1959 that in view of the problems of visual guidance in the vertical plane, such practices could lead to accidents. As a result changes were made that recognised that as the lowest height that could be used on an ILS approach was related to several factors:
- the aircraft's performance when a go-around is initiated
- the accuracy of its equipment, and
- the obstacles round the runway.
"Critical" became "DECISION" Height....
Taking these into account, the minimum altitude could be lowered, but the practice of levelling out at this critical height should cease. This created what now became "Decision Height" (or "Decision Altitude" depending on the altimeter reference).
At ICAO's 4th Air Navigation Conference (ANC) in Montreal in 1965, the US FAA presented the "United States Concept of Decision Height" as "the limit to which a pilot may descend before deciding to continue the approach to a landing by means of visual aids", and the ANC adopted the definition of Decision Height as
"a specified height at which a missed approach must be initiated if the required visual reference to continue the approach to land has not been established". Similarly it stated that "required visual reference" means
"that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position in relation to the desired flight path".
This followed correspondence between the NTSB and the FAA which included the statement "We see nothing in the definition of Decision Height which permits the decision to be made after the aircraft has descended below the prescribed height while the Captain attempts to locate the runway. Decision-making is a continuing process which culminates at a certain time in a decision to initiate a certain action. It is the Board's interpretation that "decision height" is that point at which the decision-making process must have been completed and action initiated immediately either to continue the approach to land, or execute a missed approach"
...... but with RVR still too low to be safe.
At this time, the RVR minimum was chosen so as to enable the pilot to judge his height and rate of closure with the ground, control the attitude of the aircraft, remove the drift and land the aircraft. So the minimum RVR only took note of the requirements for landing - not for the last part of the approach.
Due to the lack of knowledge at the time of the structure of fog and the relationship between Slant Visual Range and RVR, the minimum did not take into account the probability of seeing sufficient cues by decision height for the approach to be continued to a landing. Nor was it appreciated at the time that there was any risk from a high go-around rate.
Consequently, at the beginning of 1966, on runways equipped with ILS, some aircraft were being operated to 200 ft decision height and 400 m RVR, and other aircraft to 350 m RVR. It was found that only 1 aircraft in 3 made an approach in limiting conditions and of these only 1 in 2 actually landed off the approach.
These RVR values are now in the Cat 2 and Cat 3 bands, and to operate to them today requires the sophisticated equipment both in the aircraft and on the ground that we are now familiar with.