Minimum Safe Altitude (MSA)
A minimum safe altitude (MSA) is calculated pre-flight so that you have a known altitude that is clear of obstacles and terrain. However, when flying VFR you don’t always have to fly above the MSA.
In good weather and with the ground in sight, you can safely fly below your 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 pilot who gets caught out by bad weather (and they will be wishing they hadn’t left the ground in the first place!)
There are two methods for calculating a Minimum Safe Altitude (MSA):
MSA: The 5nm Method
One method for calculating the 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.
If the highest point is an obstacle:
In this example, the highest obstacle is 1220ft. We round this up to 1300ft, then add 1000 feet. So the MSA is 2300ft.
1220 FT ➔ 1300 FT ➔ 2300FT
Although this obstacle is just outside the 5nm area, it is worth including anyway given its elevation and distance from your track of about 5.5nm!
If the highest point is a spot height:
In this example, the highest spot height within 5nm of our track is at 1750ft. We round this up to 1800ft and add 300 feet for unmarked obstacles to get 2100ft. Lastly, add 1000 feet to get the MSA of 3100ft.
1750 FT ➔ 1800 FT ➔ 2100 FT ➔ 3100 FT
MSA: The MEF Method
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 2100 FT ➔ Add 1000ft ➔ MSA of 3100 FT
The MEFs are 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.
The PLOG
An MSA should be calculated for each leg of your route and entered into your PLOG. For this leg from Ross Airfield to Milson Airfield, we have calculated an MSA of 3100ft using the 5nm method (incidentally, the MSA using the MEF method is the same for this leg, however this will not always be the case):
Want to know more? See Skybrary’s article on Minimum Safe Altitude here: https://skybrary.aero/articles/minimum-safe-altitude-msa