One of the prac exam questions, asks what the vis min will be, if the approach lighting is inop. See attached.

Two questions.

1. In MOS 91 15.10. What's the difference between -
   "the approach lighting system normally available beyond 420 m from the
   runway threshold is inoperative". Where the min is set to 800m.
   And
   "the approach lighting system normally available for the
   runway is inoperative" where the min is set to 1500m.


2. If the min is accepted as 1500. Why use a higher min of 1600 listed on the chart, like the question answer suggests?

Cheers
Matthew

Part 91 para 15.10 (f) says you should use 1500m if the approach lighting is not available. However, on this occasion, the approach plate shows a separate requirement based on the missed approach gradient you are capable of.

If your missed approach gradient is limited to 2.5% then the visibility required is 1.6km if you have the actual QNH (see the second line under CATEGORY on the approach chart). That's a separate issue that must be considered, so the approach lighting is really irrelevant in this case.

Thanks Bob.

I still would have thought that 1500 would override both the 4.4% and 2.5% missed approach gradient visibility, if the runway lighting is not available.

1500 v 1600 is probably not that much to worry about, but I was thinking about how to get these types of questions correct in the exam and can't see why 1600 overrides the MOS.

In this question. I chose differently again and chose the 800m visibility option (Part 91Para 15.10 (d)), because it says if the approach lightning normally available beyond 420m from the runway, is not available, so I assumed, take the most conservative.

Para 15.10 (f), with the 1500m limitation, has essentially the same wording, but without the "beyond 420m" caveat. I don't understand what the difference is between (d) and (f). Are they two different types of approach lighting?

I would have thought that if you have to choose between a required visibility of 1500m and 1600m, the most conservative choice would be 1600m.

Simple approach lighting systems (SALS) extend only 420m from the threshold and are used mainly for runways that have no instrument approach associated.

Precision approach CAT 1 lighting systems extend up to 900m from the threshold and are used for ILS or RNP approaches.

You might find this link handy simpleflying.com/runway-markings-lights-guide/
Bob

Haha, yep, I should have thought about that before writing conservative.

Thanks for the light clarification. ????

Matthew