Quick MSA question
 

Preparing for the IMC ground exam and here's a sample question I came across:

I haven't come across any material discussing the 1800 ft figure above.

Does anyone know why the answer should be 1800 ft? I would have thought that the answer would be 1100 ft - 100ft being the lowest possible MEF, and 1000 ft of additional clearance required above that.

Wouldn't you need to add 300' for any unknown obstacles?

(That only gets you to 1300', I know.)

Ground elevation at 499' doesn't show any different from 0' on the map, and obstacles at 299' are not shown.

So there could be a hill at 499' with a 299' mast on it, nothing on the map to indicate either, and you need to be 1000' above the mast.

500 + 300 + 1000 = 1800

If I remember rightly from my IMC, it's because on a UK VFR chart terrain below 500ft is not marked and obstacles that are less than 300ft AGL are not always marked (some are).

So theoretically you could have a 495ft high hill with a 295ft high mast sat on it and neither would be marked on the chart. This gives you 790ft, add 1000ft to that, round it up and you get the 1800ft figure.

I think that's right! Happy to be corrected if not. :)

Edit: D'oh, beaten to it!!

That's very helpful, thank you.

Could this be less over the sea?

Wow - great answers.

river - As for the sea (sorry!), I'd stick with the land MSA at 1800'. Cruise ships can get up to nearly 300 feet above the waves and as for those offshore windmilly things, some of them off the coast of east Anglia are starting to get about 500 feet.

As far as I'm aware sea level tends to sit at around sea level......:)

What about Oil Rigs? they can be up to 700ft Height!

The MEF in the square containing Great Yarmouth is 400 so the MSA is 1400ft.