One thing gets lost in this debate about the system accuracy – that regardless of the needles, the one thing you do when you see the first lights through the murk, is….. nothing. If the approach is stable, the aircraft should be on the right trajectory, so maintaining attitude an power until you see the actual runway is a good idea.
The “clean” break out of low cloud is COMPLETELY different from an approach with low visibility. As I wrote earlier, I had so far only ONE of them, where I couldn’t really see the threshold, only the lights, and it was a big eye-opener. I was glad that I remembered the advice of my instructor – when that happens, do NOTHING, keep the attitude and wait for the threshold to appear.
The more detailed explanation he gave was: With low visibility, the “instinctive” tendency is to push (the only thing remotely horizontal is very low in the visual field), hence looking up, seeing the required reference, and then keeping scanning the AI to make sure you keep the attitude is important. So the minimum scan would be Outside – AI – ASI – AI – ALT – AI – Outside. You should have enough visual reference to no longer need the DI in the scan.
So his advice was similar to Timothy’s, except that he didn’t suggest to continue using the ILS needles.
Whether or not you then include he cross-pointer in that scan is less important. So even if you are inclined to stick with Dave – listen to Timothy if it comes to remaining on instruments for at least part, until you see the runway and touchdown zone well enough, which could be as low as 100ft.
Cobalt wrote:
The more detailed explanation he gave was: With low visibility, the “instinctive” tendency is to push (the only thing remotely horizontal is very low in the visual field), hence looking up, seeing the required reference, and then keeping scanning the AI to make sure you keep the attitude is important. So the minimum scan would be Outside – AI – ASI – AI – ALT – AI – Outside. You should have enough visual reference to no longer need the DI in the scan.
Having done a couple, I completely agree with this. A 550m vis approach is very poor and you do need to maintain stability in the descent until you have enough visual reference of the runway to land. If you are stable this is not that hard. If not…
To clarify, I haven’t argued against using instruments as a cross check. I have (and continue to) argue against using the ILS GP for guidance below system minima as a matter of course. Conversely, the centreline signal (localiser) is virtually bomb proof all the way to the far end of the runway.
Dave – so what would you do in an absolute emergency? Everything socked in, low on fuel, no where to go.
You need to get it on the runway, approach is stable and maybe some visual clues at DH, maybe not.
I appreciate your position is not to go below in “normal” circumstances.
Firstly, there’s a lot of what-ifs there and there is more than enough history of people getting it wrong under similar circumstances (I’m thinking Metroliner at Cork).
Personally, I would maintain attitude and power. I would not chase the GP on a CAT I system.
But, the Cork one had fuel to go elsewhere.
The zero-zero scenario is where you are out of options.
Obviously this should never happen, especially with mobile wx data which works ok enough most of the time.
Peter wrote:
Obviously this should never happen
Indeed.
Cork had options, which only makes it more of a waste. However they flew below minima and, when they finally made the correct decision, ran out of luck and skill.
Dave_Phillips wrote:
I would not chase the GP on a CAT I system.
At what point does your use of the GP become a “chase”?
Oh you know what I mean. I would not follow the GP below system minima.
Peter wrote:
Obviously this should never happen, especially with mobile wx data which works ok enough most of the time.
Unlikely, but it is not just to do with the weather. There are technical reasons why you might need to get it on the ground – now.
I think you are right Dave, no one is going below DH with options (or they shouldnt be, of course there will be those that think they know better) but it seems to me this is also summed up by Cobalt and Timothy in that at DH if you are stable on G/S and centreline and continue without changing anything the outcome should be a lot better than landing in a field in the same conditions.
