on almost all ifr flights atc will assign an altitude to aircraft as part of a clearance or an enroute instruction however there's 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 and root low chart here's the area over central wyoming let's have a look at this airway victor 298 between the boysen reservoir and muddy mountain vrs the numbers above the black box indicating the airway number depicts some minimum altitudes for this route the bottom number with an asterisk at the start is the minimum obstruction clearance altitude or mocha the top number is the minimum and root altitude or mea if we're flying at or above the mocha 10 300 feet we're guaranteed obstruction clearance along the whole route specifically we have a thousand feet of clearance above the highest obstacle within four miles of the airway centerline or 2 000 feet of clearance and designated mountainous terrain in addition to obstacle clearance we're also assured some navigational coverage from the two vor stations making up this segment of the airway as long as we're within 22 miles of either vor station we'll 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 boyson reservoir and are able to pick up that signal for this reason it's common for ifr flights to only take place at or above the minimum and root altitude 11 000 feet here at the mea not only do we have the obstruction clearance guaranteed just like the mocha but we get signal coverage along the entire root of flight 91 miles on this segment this means that at any point along the segment of victor 298 will be able to navigate either by flying outbound on the radio from muddy mountain or inbound along the radial to boyson reservoir we may even be able to receive both vor signals along some portions of the route typically we should be able to switch which vor we're using to navigate at the halfway point of the segment here that would be half of the 91 miles in the box or 45 and a half miles from either vor sometimes due to terrain or reception issues we may not be able to switch at the halfway point and here we've placed a big mountain in front of boysen reservoir because it's blocking the signal we won't be able to use it at our halfway point the mea isn't high enough to allow us to do so in these cases you'll see a zigzag symbol like this called a changeover point we'll locate it closer to boise reservoir indicating that we should only switch which vor we're navigating off of when we get to this point which is just 25 miles from boyce and reservoir or 66 miles from muddy mountain like we said you'll usually be at or above the mea however sometimes it makes sense to fly at a lower mocha when icing conditions are present at higher altitudes for example you may be able to ask atc to descend down to the mocha and look for warmer or clearer conditions you could do this if you're within 22 miles of either navid you can also descend to a mocha if 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 and the middle one in blue is called the gnss mea this is the minimum and root altitude you're allowed to fly if you have an approved gps on board and the lower altitude of the asterisk is still the mocha let's look at why having a gps on board lets 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 4500 feet we have no trouble receiving the signal if we descend to 3000 feet though the signal is blocked by terrain now if we have 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 guys up there are always watching notice the gps mea is published along with the mocha and then on this airway they happen to be the same altitude in the absence of a published gps mea the aircraft can still be assigned the lower mocha 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 lindy fix and has a flag with an r on it this is the minimum reception altitude or mra and it's listed underneath the fixed name at 5500 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 the north off of this airway is being used to identify the lindy fix the mra is the lowest altitude where we'd be able to not only navigate along the victoria way but also determine the lindy 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 effects here lindy 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 3500 feet at the lindy 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 3500 feet initially and then when reaching lindy would start a climb to the next higher mea of 5500 feet we'll continue westbound to the usorfix there's no break in the airway line like at lindy so no change in altitude is needed here notice on the left that there's two altitudes listed these are directional meas we're westbound so the arrow indicates that rmea after the wu not fix is 4 000 feet so at wound we can descend down to that lower mea directional meas are usually used because changing to higher altitudes requires 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 will need to already be at 5500 feet and will stay at that altitude until reaching lindy where we can descend to the lower mea of 3500 these details add a little bit more complexity to the minimum and root altitudes and when to climb or descend if we look at main here we can see one more complexity built into our altitude planning here's victor 39 notice that at both the neats and label fixes there's a flag with an x on it this is the minimum crossing altitude at neats it's listed that if we're northeast bound along the airway the mca is 4500 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 3500 feet we'll need to plan our climb so that we cross needs at 4500 then we'll continue in the climb to the next mea which is 6000 feet there's no break in the airway at lymer so we can continue past there but label has another mca this one is 7000 feet so once again we'll need to start our climb so we cross label at 7000 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 now and these tan colored numbers are found throughout the unroot charts and depict altitudes and hundreds of feet each number applies to an entire quadrant of the chart bound by the lines of longitude and latitude in this box this quadrant the altitude that applies called an off-route obstruction clearance altitude or oroka is 15 600 feet if these look familiar to you from private training it's because they're 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 chartered airway you could see that on a charted airway which traverses this quadrant like victor 328 here the minimum and root altitude is only 10 000 feet a good deal lower than the aroca we can look at the sectional chart of the same area to get an idea why the airway cuts through a relatively flat portion of this quadrant in the west and south but in the northeast of the quadrant there's high mountainous terrain the entire quadrant has just one a roca 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 typically only want to know about aroca 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 oroco sign by atc provided they can radar identify you here we're looking at new mexico and you can see the various arocas listed atc has a similar chart of the area which looks a bit like this it's broken into more sectors in the quadrants on the enroute chart and is able to more closely isolate off certain parts of the airspace these numbers here are called minimum vectoring altitudes or mva and then 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 i remember once approaching my home airport in imc while a thunderstorm cell was sitting on top of the lone instrument approach i knew that if i could get sight of the airport i could cancel ifr and fly direct to the field for landing avoiding the weather entirely i asked atc to descend to the minimum vectoring altitude which allowed me to break out below the clouds spot the airfield and proceed direct under vfr anyways mvas can be good to know for reasons like that or again if you're looking to duck below icing since atc can't vector you below the mva you'll sometimes hear the question can you maintain your own terrain and obstruction clearance below such and such altitude that altitude is the mva you're responsible for not hitting anything until you're high enough for atc to vector you around by the way you can check out these mva charts on the faa website but they come with the big caveat that the 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 as we mentioned atc will assign altitudes but you'll need to be aware of these minimums 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 clearance altitudes a glance at the enroute chart will tell you everything you need to know if this was helpful please click subscribe so that you can stay up to date on every new training video coming out each tuesday and friday and get access to posts and articles on the community page that'll take your training even further it just takes one click and it's so worth it