A minimum safe altitude (MSA) is calculated so that we have a known altitude that is clear of obstacles and terrain. However, we don’t always have to fly above the MSA.
In good weather and with the ground in sight, we can safely fly below our MSA. If you lose sight of the surface or the weather deteriorates, you will have an MSA written down that you can climb above to keep yourself separated from obstacles and terrain.
If you have no instrument flying qualifications, you should be cancelling your flight or diverting before the weather forces you above the MSA. The MSA is a last resort for a VFR private pilot who gets caught out by bad weather (and they will be wishing they hadn’t left the ground in the first place!)
One method for calculating MSA is to find the highest terrain or obstacle anywhere within 5nm of your planned track. This requires some time spent studying the chart for the highest obstacle elevations and spot heights.
In this example, the highest obstacle within 5nm of our track is at 2320ft. We round this up to 2400ft, then add 1000 feet. So the MSA is 3400ft.
2320 FT ➔ 2400 FT ➔ 3400FT
In this example, the highest spot height within 5nm of our track is at 1299ft. We round this up to 1300ft and add 300 feet for unmarked obstacles to get 1600ft. Lastly, add 1000 feet to get the MSA of 2600ft.
1299 FT ➔ 1300 FT ➔ 1600 FT ➔ 2600 FT
Every half degree of latitude and longitude is marked on the chart with a black line and all the black lines form a grid across the chart. Maximum Elevation Figures (MEFs) state the highest possible elevation of an obstruction within each of these grid sections.
To find the MSA, look at the MEF of each grid section that your track passes through. Take the highest of these MEFs and add 1,000 feet (MEFs already add the 300ft for potential unmarked obstacles).
Highest MEF of 2400 FT ➔ Add 1000ft ➔ 3400 FT
This is a quick and easy way of determining the MSA as you don’t have to scrutinise the chart for the highest spot height or obstacle. However, it has the disadvantage of covering a larger area so your MSA may be higher because of an obstacle 20nm away in the opposite corner of a grid section.