1 engine missed 1

10.01.2005

Submission:

The 2.5% PANs-OPS missed approach gradient was never linked directly to the Annex 6 green page material. When the ILS criteria was updated in the 1970s (round about OCP/4) it was based on a BCAR for Category II ILS operations which involved a missed approach from the minimum Categotry II DH of 100 ft. The FAA criteria was quite different.

A driving force behind the Cat II development at the time was to get the Trident to 100 ft DH at Heathrow - and under the UK Special DOA scheme (13th draft), the glidepath antenna was a problem. Increasing the missed aproach gradient above 2.5% raised the side surface above the antenna and solved the problem. When OCP combined the data extrapolation originaly proposed by the FAA and the model used in the UK DOA scheme, it also included this concept of a variable missed approach gradient. Furthermore, the Panel saw no reason why the same concept could not be applied to non-precision procedures and added it to the criteria for all procedures.

So much for the history - but what of the anomaly between the 2.5% PANS-OPS and the aircraft performance calculations. Here one has to ask where is the acident evidence to suggest that we have a problem - and there is none. There are very very few accidents (if any) where deficiencies in the PANS-OPS criteria have been identified as a causal factor. Indeed the original derivation of the 10E-7 target level of safety was itself over pessimistic, since it was based on all accidents (one order better than the then current total rate of 10E-6). It should have been based on landing accidents involving collision with critical obstacles. This suggests that the PANS-OPS safety target is really about one or two orders more remote (i.e. between 10E-8 amd 10E-9).

So, do not worry too much about the discrepancies between PANS-OPS and the AFM - concentrate on avoiding the CFIT accidents that really do kill people.

Robert Woodhouse
Procedure designer, ICAO OCP member (retired), Canada