There's a common item that confuses many of us while going through IFR Ground School. On almost all IFR flights, ATC will assign an altitude to aircraft as part of a clearance or an enroute instruction. However, there are about a dozen types of minimum altitudes we need to know about for purposes of planning, emergencies, and general situational awareness.
The air traffic control system also uses these minimums when assigning altitudes or approving flight plans. Almost all of these altitudes are depicted on our IFR low enroute chart.
Here, over central Wyoming, the airway Victor 298 has some numbers above the black airway box depicting some minimum altitudes for this route. The bottom number, with an asterisk at the start, is the Minimum Obstruction Clearance Altitude, or MOCA. The top number is the Minimum Enroute Altitude, or MEA.
If we are flying at or above the MOCA, 10,300 feet, we are guaranteed obstruction clearance along the whole route. Specifically, we have 1000 feet of clearance above the highest obstacle within 4 miles of the airway centerline, or 2000 feet of clearance in designated mountainous terrain.
In addition to obstacle clearance, we are also assured some navigational coverage from the two VOR stations making up this segment of the airway. As long as we are within 22 nautical miles of wither VOR station, we will be able to use them to navigate along the route. If we go beyond 22 miles, because of our lower altitude, we may not be able to pick up either VOR station, due to terrain limiting our line of sight of the signals at a greater distance. Without navigational coverage, our VOR receiver will switch to OFF mode, and we won’t be able to navigate along the airway, until we get within 22 miles of the next VOR station at Boysen Resevoir, and are able to pick up that signal.
For this reason, it’s common for IFR flight to only take place at or above the minimum enroute altitude, 11000 feet here. At the MEA, not only do we have the obstruction clearance guaranteed just like at the MOCA, but we get signal coverage along the entire route of flight, 91 miles on this segment. This means that at any point along this segment of Victor 298, we’ll be able to navigate either by flying outbound on the radial from Muddy Mountain, or inbound on the radial to Boysen Reservoir. We may even be able to receive both VOR signals along some portions of the route.
Like we said, you’ll usually be at or above the MEA. However, sometimes it makes sense to fly at the lower MOCA. When icing conditions are present at higher altitudes, you may be able to ask ATC to descend down to the MOCA and look for warmer or clearer conditions. You can do this if you’re within 22 miles of either navaid.
You can also descend to a MOCA is you’re equipped with an approved GPS in addition to your VOR receivers, which brings up another important minimum altitude on the charts. Here’s a chart over Iowa. Having a look at Victor 138, we see that there are three altitudes listed, one of them in blue. The top one is the MEA still, 4,500 feet, the middle one in blue is called the GNSS MEA or GPS MEA more commonly, this is the minimum enroute altitude you’re allowed to fly if you have an approved GPS on board. And the lower altitude with the asterisk is still the MOCA.
Let’s look at why having a GPS on board let’s you fly a lower altitude. At the standard MEA, we rely on VOR signals to navigate the airway. These signals start at ground level, so we need to have line of sight with the station in order to receive them. At the MEA of 4,500 feet, we have no trouble receiving this signal. If we descent to 3,000 feet, though, the signal is blocked by terrain.
Now, if we have a GPS on board, we can take advantage of the satellite network, which is based in space and is able to broadcast down to our aircraft. No matter how low you’re flying, the electric eyes up there are always watching. Notice the GPS MEA is published along with the MOCA, and that on this airway they happen to be the same altitude. In the absence of a published GPS MEA, aircraft can still be assigned the lower MOCA as long as they have a working GPS.
Let’s move a little further south and have a look at another minimum altitude. This one is at the LINDE fix, and has a flag with an R on it. This is a Minimum Reception Altitude or MRA, and it’s listed underneath the fix name at 5,500 feet. Fixes along airways are often identified using cross radials, in other words, they’re the intersection of radials from two or more VORs.
Usually some of these VORs lie off the victor airway, you can tell at the top of the map that a VOR that lies to the north, off of this airway, is being used to identify the LINDE fix. The MRA is the lowest altitude where we would be able to not only navigate along the victor airway, but also determine the LINDE fix using all the off route navaids.
We can get around the MRA requirement if we also have a DME on board and can use that to identify the fix. Here, LINDE is 73 DME from the Omaha VOR to the Southwest. And of course we could be using GPS to identify these fixes as well.
Now, let’s have a closer look at this airway, victor 172, to learn a little bit more about how these minimum altitudes work. An aircraft flying east to west would start at the initial MEA listed at 3,500 feet. At the LINDE intersection, the airway line is broken up by two sideways T symbols, meaning the MEA changes at that point. So our aircraft would fly at 3,500 feet initially, and then when reaching LINDE, would start a climb to the next higher MEA, of 5,500 feet.
We’ll continue westbound to the USORE fix. There’s no break in the airway line like at LINDE, so no change in altitude is needed here. Notice on the left that there are two altitudes listed, these are directional MEAs. We’re westbound, so the arrow indicates that our MEA after the WUNOT fix is 4,000 feet. So at WUNOT we can descend down to that lower MEA.
Directional MEAs are usually used because changing to higher altitudes require us to maintain a minimum climb gradient, which is dependent on our speed and altitude. So for some directions of flight due to terrain or airspace it might not be possible to use that lower MEA.
Here, flying back eastbound, we’ll need to already be at 5500, and we’ll stay at that altitude until reaching LINDE, where we can descend to the lower MEA of 3,500. These details add a little more complexity to the minimum enroute altitudes and when to climb or descend.
If we look at Maine here, we can see one more complexity built into our altitude planning. Here’s victor 39. Notice that at both the NEETS and LABEL fixes, there is a flag with an X in it. This is a Minimum Crossing Altitude, MCA. At NEETS, it’s listed that if we’re northeast bound along the airway, the MCA is 4,500 feet.
Normally, we can start our climb when we reach the fix where the MEA changes, and as long as we maintain our minimum climb gradient, we’re fine. But here, we have a further restriction that forces us to start the climb earlier, so we can cross the fix at the minimum crossing altitude.
Starting at the MEA of 3,500 feet, we’ll need to plan our climb so that we cross NEETS at 4,500. Then, we’ll continue in the climb to the next MEA, which is 6,000 feet. There’s no break in the airway at LIMER, so we can continue past there, but LABEL has another MCA, this one at 7,000 feet. So once again, we’ll need to start our climb so we cross LABEL at 7,000 feet, and then stay at that new MEA along the rest of the segment. So these MCAs have to be taken into account in our route planning too.
Here’s another minimum altitude we don’t get as much chance to work with. We’re back in Wyoming. These tan colored numbers are found throughout the enroute charts and depict altitudes in hundreds of feet. Each number applies to an entire quadrant of the chart bound by lines of longitude and latitude. In this box, this quadrant, the altitude that applies, called an Off Route Obstruction Clearance Altituide, or OROCA, is 15,600 feet.
If these look familiar to you from private it’s because they are very similar to the maximum elevation figures you see on sectional charts. They provide obstruction clearance for all areas within the quadrant, not just along a charted airway.
You can see that on a charted airway which traverses this quadrant, like victor 328 here, the minimum enroute altitude is only 10,000 feet, a good deal lower than the OROCA. The airway cuts through a relatively flat portion of this quadrant in the west and south. But in the northeast of the quadrant, there is high mountainous terrain.
The entire quadrant has just one OROCA altitude, and it has to be able to provide clearance for the entire area, so it needs to be higher than the MEA to accommodate. You’d typically only want to know about OROCA for emergency situations or for situational awareness.
If you’re flying an off route course, you might be able to get a lower altitude than the OROCA assigned by ATC, provided they can radar identify you. Here we’re looking at New Mexico, and you can see the various OROCAs listed. ATC has a similar chart of the area, which looks a bit like this.
It’s broken into more sectors than the quadrants on the enroute chart, and is able to more closely isolate off certain parts of airspace. These numbers are called Minimum Vectoring Altitudes. They are the lowest that air traffic control is allowed to assign you heading vectors and maintain terrain and obstruction clearance. ATC may allow you to descend to the minimum vectoring altitude.
You can check out these MVA charts on the FAA website, but they come with the big caveat that they’re not to be used for navigation purposes, unlike the published altitudes on the charts we’re working with here. It’s just a nice thing to know.
Here’s one last altitude to look at. On the western side of the Chesapeake Bay, victor 170 has what’s called a Maximum Authorized Altitude. VOR signals travel line of sight, and sometimes, you might be high enough up to receive two signals on the same frequency, which could confuse your receiver. You could also be bumping up against the top of some airspace above this altitude, so for these reasons sometimes an airway will have a maximum authorized altitude.
These are the altitudes to consider when flying the enroute portion of an IFR flight. As we mentioned, ATC will assign altitudes, but you’ll need to be aware of these altitudes. In case of lost communications, you’ll need to fly the higher of the assigned, expected, or minimum altitude along a route of flight, and you’ll need to be aware of terrain and obstruction clearances altitudes. A glance at the enroute chart will tell you everything you need to know.