When flying IFR along airways, you have to adhere to minimum altitudes, such as the minimum enroute or minimum obstruction clearance altitude. Here, on Victor 172, if we’re flying westbound, we initially have a minimum enroute altitude of 3,500 feet. After the Linde fix, the MEA is higher at 5,500. Our only requirement here is to start out at or above 3,500, and initiate a climb at the Linde fix up to 5,500. As long as we start at the fix and maintain a normal climb gradient, we’ve followed the restrictions.
Outside of signal coverage requirements, these restrictions ensure that we get adequate obstruction clearance, and the terrain below the airway allows for us to maintain the lower MEA right up to the LINDE fix.
On some airways though, like victor 244 in Utah, the terrain doesn’t allow for a delayed climb, and we have what’s known as a minimum crossing altitude. Flying eastbound along this airway, we start with an MEA of 10,500 feet. Recall that the MEA is based on obstruction clearance and adequate signal coverage along the segment of the airway.
If we were to descend to the lower minimum altitude, the minimum obstruction clearance altitude of 8,500 feet, we would only be guaranteed signal coverage 22 miles from a VOR, but still get the same obstruction clearance. In mountainous areas, this is determined by the highest obstacle in the area below the airway, plus a buffer, plus 2000 extra feet. So outside of signal coverage considerations, that obstacle below us is at around 6,500 feet or a bit below.
Traveling eastbound, we see the ANIUM fix, with a flag with an x through it, indicating a minimum crossing altitude. Below the fix we see that victor 233 has an MCA of 12,300 feet eastbound. After that, the MOCA becomes 14,800 feet. The question is, why can’t we stay at the lower MOCA, 8,500 feet, and initiate a climb to the higher MOCA only when reaching ANIUM. The answer has to do with maintaining obstruction clearance.
Let’s first look at how that higher MOCA of 14,800 is derived. Mount Peale, with a peak of 12,721 feet, is only 2 miles from the airway centerline, so well within the 4 mile lateral boundaries for obstacle clearance. So this peak will control where that MOCA is.
Adding a buffer to that peak, and then adding the 2000 feet gets us to 14,800 for the MOCA. So what if we started our climb from the first MOCA, 8,500, to the second, 14,800, only as we crossed ANIUM?
The US terminal instrument procedures, or TERPS, guide these calculations, and it says that a normal climb will be based on certain gradients depending on what altitude we’re flying at. Starting at 8,500 MSL as we are here, it’s assumed we’ll climb at 120 feet per mile, then 100 feet per mile after passing 10,000 feet.
The fix, ANIUM, happens to be 25 miles straight line distance from that high mountain peak. If we start a climb at ANIUM, and hold a normal climb gradient, we’ll only be at 11,250 feet at Mount Peale, not enough to clear it let along make the required buffer.
This means we need to cross ANIUM higher than the MOCA we’re currently at. To figure out what altitude to cross MOCA, the FAA keeps in mind those required obstacle clearances. Starting at 8,500, we have at least 2,000 feet here in mountainous terrain. We’ve already figured out the MOCA after ANIUM by adding 2,000 feet to Mount Peale, but now we have our normal climb gradient to take into account. This climb has to take us from ANIUM, to the mountain peak, a distance of 25 miles, and get us to 14,800 feet when we get there.
So the calculation to figure out that minimum crossing altitude starts with the obstacle height. It’s 12,721, but we’ll round to 12,800 as the FAA adds a buffer based on a number of factors. The required clearance in mountainous terrain is 2000 feet, so we add that, this gives us of course the new MOCA at the obstacle of 14,800 as we figured out before. Our climb value takes into account the 25 miles between the fix and the obstacle, and applies the assumed normal climb gradient, 100 feet per nautical mile for us, giving us 2,500 feet. Backing this out from the MOCA, we get our required minimum crossing altitude of 12,300 feet, just as it’s listed in the chart.
We don’t of course need to know how to calculate the MCA when we’re flying. Just remember that normally, on an airway, we hold the minimum altitude right up until the fix, and then start the climb to the next higher minimum. When there’s an MCA, we need to climb up to and cross the fix at the MCA, then start our climb to the higher min.
On most IFR flights, we’ll be assigned an IFR altitude which might be higher than these minimums anyway. Still, knowing that terrain is what dictates these calculations can help us understand “what’s under the hood” so to speak of these enroute charts.
There are of course other factors going into this calculation, for one thing we were able to ignore the effects of altitude on signal coverage in this example, but if a MOCA like the one over this mountain peak here didn’t provide us signal coverage close to a VOR, it would need to be higher than the 2000 foot buffer we had. But this is a good illustration of how the enroute procedures are designed to always make sure the minimum altitude gives us adequate obstacle clearance.