Successfully landing off the approach.
It was then recognised that a good measure of the suitability of weather minima is the approach success rate (ASR), i.e. the number of aircraft that achieve a landing in given conditions divided by the number of aircraft commencing an approach in these conditions, expressed as a percentage. Obviously subtracting the ASR from 100 gives the go-around rate.
The approach success rate connects three factors.
1. Total risk due to go-around. Although a go-around is accompanied by a risk level no greater than in certain other areas of flight, it is not something that crews often practise. The risk during any particular go-around is to some extent dependent on the height at which it's initiated and the crew procedures used, but the total risk of going around is best controlled by limiting the go-around rate to a sensible figure. When developing minima, it was judged that this should be less than 15%, and the approach success rate should be 85% or more. So in poor visibility, risk in go-around is largely a function of approach success rate (ASR).
Today, the availability of automation has reduced some risks in go-around, but may have inadvertently introduced others. As recently as August 2013, a French study occasioned by a concern about a number of accidents and serious incidents during go-around showed that with modern highly automated aircraft, significant problems can arise due to lack of pilot preparedness for a go-around.
With these aircraft, when a go-around is needed, regardless of the cause - lack of visual reference, unstable approach, blocked runway, etc. - it is evident that the pilot flying may lose awareness of what automated systems are doing, become confused by it, or inadvertently trigger an unwanted change in automated systems, leading to significant loss of control.
Consequently although in general the performance characteristics of modern aircraft make some aspects of go-arounds less hazardous than with much older types, other issues seem to have replaced them in making risk in go-arounds still something of significance.
2. Approach Ratio. This is the number of aircraft that actually make an approach in "above minima" weather conditions, divided by the number that could have made an approach and is a measure of the trust pilots have in their Company's minima. A low value would indicate that pilots consider the minima unrealistic and are diverting, instead of using time and fuel making what they feel will be a wasted approach.
In effect, this amounted to pilots making up their own, more conservative, minima. That is bad operational practice and would inevitably increase the chances of successful landings being missed. A well-chosen ASR will produce a high approach ratio, and is in fact exactly what we typically see today, when pilots almost always attempt an approach if the conditions are reported to be above minima.
3. Regularity. Approach ratio multiplied by ASR gives a measure of the efficiency of the operation with regard to regularity. Realistic minima will achieve a high value of both these factors, and thus a high value of their product. In other words, it prevents airlines wasting money on flights that aren't going to actually arrive at the intended destination.