Safety Altitudes

[Startri]

Help

Does anybody have a reference for the calculation of safety altitude? Both terminal area and en-route.

[GlueBall]

"Safety Altitudes" as you say are depicted on en route charts as MEAs (minimum en route altitude) and on approach charts as MSAs (minimum sector/safe altitude).

[OzExpat]

This term is used in Papua New Guinea to generically describe any altitude at which flight is safely possible in IMC, in any particular circumstance. Thus, it can relate to the lowest altitude for enroute flight, or MSA, or lowest holding altitude. We also use it to include the concept of an "area minimum altitude", which is an area enclosed between specific lat/long grids.

In each case, it relates to terrain clearance only. Thus, it is calculated by taking the elevation of the highest terrain within the particular protection area, adding the chart error value plus an allowance for tree growth. To this we add the Minimum Obstacle Clearance specified by ICAO in Pans Ops. A further allowance is made for QNH vagaries and the resultant figure is rounded up, if necessary, to the next whole 100 feet.

The elements may be somewhat different in other parts of the world.

[Dr. Nick]

You're flying in the UK right? Well Glueball is basically right that you use the MEA/MSA on the chart. If you're using your Half Mil then take the figure quoted in your square, add 300' because that is the max height of a structure in the UK without informing the authorities, then add a further 1000' for safety margin and you have your en-route MSA. If you want to be more accurate then you can do the same sums for the tallest structure within 10nm either side of your track.

Within 25nm of an aerodrome you can use the MSA quoted on your Aerad/Jeppy plate for IFR procedures directly from the plate using the numbers given directly as the safety factors are already accounted for.

Hope that helps... :cool:

[GoneWest]

...and that just goes to show that there is a lot to be said for reading something twice before coming to a firm conclusion.

I initially thought that Dr. Nick just said ONE HUNDRED feet above MEF+300....but I go with his plan of MEF+300+1000 - rounded to the next whole hundred.

Flight BELOW this altitude is acceptable - provided you know where you are, know where you are going and can clearly see the way. If you lose any one of those components - CLIMB and then screw yourself for getting that far without having the sense to turn round, and go back, earlier.