MSA
 

Having observed the discussion over on the "Unpublished let downs" thread, it is taken as read that to remain at or above MSA in IMC is "safe" yet to even consider a descent below it is "unsafe" unless on a published let down procedure (of whatever sort), precision or non-precision.

MSA is based on a vertical separation of 1,000 feet (apart from over mountainous terrain of course) and has been for very many years. The question is, why is it so? Why was that particular figure chosen?

Supplementary question, why is it 2000' in mountainous terrain (at least in the US)? On the face of it banging into the side of a mountain is no worse than banging into any other terrain. I guess it must be because it's considered harder to maintain an altitude in the mountains, due to updrafts/downdrafts (just ask Steve Fossett), so an additional 1000' for "accuracy of level keeping".

It's generally thought to be a good idea to have a red line, decided in advance, which you don't pass "just a little bit, just this once".

We'll all have some of our own personal red lines, re crosswind, spare fuel[#], whatever. Others are laid down for us, such as the 90 day rule and MSA. I'm quite happy with this, it saves me having to work out my own, which in the case of MSA would be on the basis of not enough experience.

[#] Yes, I once called the fuel truck because the aircraft had 1 gallon less in it than I'd calculated, including generous reserves.

Gertrude, you state that you are happy to have others decide on the "red line" for you, which is good. Would you feel the same way if the regulator had set a limit based on a higher altitude, say 1500 feet, rather than 1,000 feet, or a lower one, say 500 feet?

Could it be as simple that if you add 1,000 ft to the highest point there is less margin for computation error than using, say, 500 ft?

I did wonder about that possibility.

Does PITHBBLOT mean anything to you? In case you haven't seen it before it's a handy mnemonic for remembering the errors of the barometric altimeter (it'll certainly familiar to pilots taught within the British mil). The one that we're interested in here is the 'O', orographic error. Windspeed tends to increase over higher terrain (a not dissimilar example is increased speed over an aerofoil section), and as we know as speed of the air increases the pressure decreases. This has the effect of creating a lower atmospheric pressure over mountainous terrain. This in turn would result in an aircraft effectively flying at reduced terrain separation over those mountainous areas if flown purely by reference to a barometric altimeter ("high to low, down you go"). Conventional wisdom suggests that this error starts to become significant when flying over terrain of 3000ft or more, hence many organisations describe increased safety altitudes in those areas.

If I thought a regulatory limit was unreasonably restrictive I would have to decide what to do about it. Which might mean shrugging and putting up with it, or it might mean going and reading the RIAs and background research to try to understand how it had been arrived at.

If I thought it unreasonably dangerous, which I probably would with a 500' MSA, then I would impose my own higher limits.

Cor, you learn something every day. Obvious when you think about it. Thanks.

Extract from the Canadian AIM - AIR 1.5 Pressure Altimeter:

TP 14371 - Transport Canada Aeronautical Information Manual (TC AIM) - Transport Canada

In IMC or VFR on top, you won't be able to predict downdraft positions. Light aircraft cannot maintain altitude in them, and can descend into cloud below the hills.

Badbackanddeaf, that post was very enlightening and one to bear in mind when going over mountains (not applicable to me in my li'l ol' microlight!)

N5296S, I was going to answer along the lines that you need to avoid 'curl over' as the air descends on the downwind side of the mountain. Very dangerous in strong wind conditions.

Tell me about it. I once hit a full-blown rotor flying over the 8000' ridge north of LA. I lost 2000' in about 10 seconds.

I wasn't asking "why shouldn't you fly close to the terrain in mountains", which is somewhat obvious, at least unless you are in close to dead calm conditions. I was asking why the MSA is 1000' higher in mountains. I guess I somewhat answered my own question there though.