I am somewhat confused by the MDA and circling minima. Specifically question 4 of set 4 on page 316 of the book.

It says you are flying a straight in approach for runway 27 NDB into Wynyard and become visual at 1150. The visibility is 3km.

The SI minima is 1100 with visibility of 5km.

Now you are higher than the cloud level of 1100, but you can't see 5km. Why at this point don't you have to go to your alternate?

The answer seems to be to do with when you can descend within the circling area, namely if you are visual and you stay 300ft above obstacles.

All okay, but the question doesn't say how far away you are, but the answer says you can continue the approach until you are in the circling area then fly a normal circuit.

But then what is the point of the visibility minimum, and what is the point of the circling minimum of 1100 and 2.4km?

Common sense tells me if I can see the runway I am going to head towards it and then fly a visual circuit. But then what are the rules for? I fear I am missing something important here.

The best way to understand this is to consider the 'circling visibility' published in the DAP to be 'night circling'. That's what it actually is. As soon as I become Prime Minister, I'm going to change it to NIGHT CIRCLING.

If the approach has a straight-in minimum published as well as a (higher) circling minimum, by day you can maintain that lower minimum to the circling area providing you can maintain the required circling visibility to and within the circling area.

If for some reason you cannot land on the nominated runway, you can circle as required, by day, and maintain the required obstacle clearance for your category visually. You certainly cannot do that a night!

Thanks Bob. Makes sense in respect to altitude for the straight in approach.

Where does the SI visibility minimum come into it though? Is this more for planning purposes? If you as in this example come out of cloud you know your altitude and you are higher than the SI minimum. But this question says you can see 3km, but the SI visibility minimum is 5km.

What you say above is about being able to see obstacles and stay clear of them which makes sense, but then what is the point of the SI visibility minimum? There does not seem to be any time you would use it unless it was just for planning?

When you break visual at 1150' with 3km vis, chances are that you will not see the runway; the SI approach requires a vis of 5km.

At this point you will have two options
1) fly the missed approach and then … whatever
2) Continue onwards for a circling approach - by day only.

For option 2 (by day only)
Maintain the SI MDA until within the circling area - the chart indicates that you can maintain the SI MDA to the missed approach point with zero vis.

Once within the circling area, you may change to a circling approach if you have a vis of 2.4 km (you have 3km) and can maintain obstacle clearance visually. You can only maintain obstacle clearance visually by day. The circling vis of 2.4km would be required by day, simply to keep the runway in sight from any point within the circling area.

Notwithstanding the above, changing from a SI approach to a "circling approach" will have its problems if you are unfamiliar with the aerodrome and have not planned ahead.