Originally Posted by
TheOddOne
discount,
I'm puzzled by your assertion that diesel generators are used for backup. We certainly don't use them like that.
Our SOP is to use a ring main as its own backup. Thus, a break in the ring DURING any period of LVPs is covered by the supply coming around the other way, so to speak.
If the ring is interrupted before the onset of LVP, then it can't be used as its own backup. What we do then is to use the Diesel generators as the PRIMARY source and the mains as BACKUP. That way, we get the 1 sec changeover required for CATIII. This doesn't cost any more, just a bit of lateral thinking to give the result.
I'll also repectfully disagree with you about there being a difference between LVPs due low cloud and low horizontal surface visibility. Yes, we often get situations where we are 200' cloud ceiling but several km under it. We still need LVPs in this circumstance, so that when crews cloudbreak at 200', there is a reasonable chance that the a/c is aligned with the runway. I reckon you've got less than 15 secs between a 200' cloudbreak and the flare in a big a/c. That's supposed to be possible to hand-fly for a CATI approach. Thus we protect for this condition.
Cheers,
TheOddOne
I am aware that the main way to achieve the one second change over time is to use diesels as primary, with mains as secondary, at many airports. Also there is the double ring. However, see the paragraph below as to whether this was required or not.
I wish to continue the debate about the split in low visibility and low cloud procedures further. If you had no CAT III system, and only a CAT I ILS, would you still put in low visibility procedures for low cloud and high visibility operations? I would suggest not. The change in the ILS signal deflection when using CAT I holds for departing aircraft and flying to a 200'DH is still manageable. The main signal deflections are from large tails vacating at the end of the runway or from big wings in overflight. Yes, there are other reflections etc. A 737 old generation I seem to remember gave a return to CAT III signal when vacating most of the way down a runway and its tail was 60 metres from centreline, for example. At the CAT I hold, it does not eat all the dynamic ILS tolerance up.
The main debate should be on the total disconnect between JAR OPS 1 requirements and CAP 168 requirements. If it was high visibility and daylight, why would the crew need all of the other lights apart from PAPI, they can shoot the approach with the lights out anyway! Many operators do not even require a PAPI, but I will not go against Annex 14 in this area. I feel that no PAPI = no approaches. I still feel that the lower risk option was to continue operations with the ILS.