Aviation Topic of the Week
By Michael Oxner, January 18, 2004


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This week's topic:
Minimum IFR Altitudes

A long time ago, I had a request for descriptions of what is covered in altitudes in the charts, such as 25 NM safe altitudes and so forth. First, I'll offer my apologies for not getting back to this earlier. Second, we'll have a look at the subject matter.

Definition of Minimum IFR Altitude
Determination of Minimum IFR Altitudes
Types Published and Tolerances
    Enroute Charts
    Approach Plates

Definition of Minimum IFR Altitude

The definition of Minimum IFR Altitude is quite vague in a number of published references. After being frustrated myself on many occasions in the past, I've come to accept that it is left intentionally vague. The important part of looking for safe IFR altitudes is to find the lowest one useable for the airspace you're in. That would be the minimum. Because there are different phases of flight, there are different altitudes applicable in each case. That's not to say you couldn't use one to justify flight lower than another, just that it may be easier to relate a certain altitude to a phase of flight. They all have one thing in common, the fact that they are assessed as being safe.

The minimum IFR altitude for a given airspace may be any one of several that can be found covering airspace that is published on IFR enroute charts, approach plates, or other such chart. In fact, they may not be published at all. There are cases where assessments have not been made and therefore minimum IFR altitudes are not found on any chart. What's a pilot to do in such a case? Refer back to the regulations for IFR flight. Our pilot bible, the AIP Canada, refers us to CAR 602.124 for this aspect of IFR flight. It states that, "Except when taking off or landing, aircraft in IFR flight shall be operated at least 1,000 ft above the the highest obstacle within a horizontal radius of 5 NM of the aircraft. Exceptions to this are flights within designated mountainous regions, but outside areas for which minimum altitudes for IFR operations have been established." Ok, so what does this mean?

Determination of Minimum IFR Altitudes

The simple fact is that it's virtually impossible for any pilot or ATC to be 100% aware of what terrain and obstructions exist, exactly where they are and exactly where the aircraft is in relation to them at all times. Though there are exceptions (like radar vectoring areas where obstructions and aircraft are both painted on the screen), this is where the publications come in. For commonly used areas, airspace planners have assessed the regions for terrain and obstructions by use of charts and databases to determine what the highest obstacle for any given area is. The only possible way to do this massive task is to determine what the airspace is to be used for so you have parameters to work within to determine the altitudes. For example, airways have defined dimensions, discussed in greater detail below, which have to be assessed. Once they have determined the obstructions and terrain within those dimensions, they can say definitively that the lowest safe altitude within these dimensions is XXXX number of feet above sea level and publish that value. For that to mean anything to a pilot, he has to have an idea of what dimensions he has to work with. With any published altitude, there is a description of what that altitude means in the AIP or in a given publication such as the approach plates in the Canada Air Pilot (CAP).

For those areas that don't have minimum IFR altitudes published, a pilot will have to determine what a safe altitude is. He should consult charts with terrain and obstructions printed, such as Visual Navigation Charts (VNC) and World Aeronautical Charts (WAC). These can be used to determine altitudes that should be safe, but again, the pilot should consider the likelihood of errors to be encountered in navigation. And, as mentioned above, add 1,000 feet to meet requirements. If operating in a Designated Mountainous Region (DMR), add an appropriate buffer instead of 1,000 feet if there hasn't been an IFR altitude established, such as a Minimum Enroute Altitude. What are these values to add? Have a look at the chart below, taken from the Designated Airspace Handbook (DAH).

DMRs As you can see here, there are 5 DMRs in Canada. They are:
  1. The Rocky Mountains of western Canada, requiring a buffer of 2,000 feet above the highest obstacle within a 5 NM radius of the aircraft
  2. Eastern Quebec and Labrador, requiring 1,500 feet
  3. The Gaspe Peninsula and northwestern New Brunswick, requiring 1,500 feet
  4. The island of Newfoundland, requiring 1,500 feet
  5. Baffin Island and the eastern islands of the Canadian Arctic Archipelago, requiring 2,000 feet
So if there is no airway, approach segment or other altitude published that is useable, the pilot must determine the highest obstacle in the area in which he plans to operate and add 1,000, 1,500 or 2,000 feet to that value, depending on where he is. This is something that really can only be done in pre-flight due to the chart-intensive work. It can take time to review the needed charts, time that often isn't available in the cockpit. This is why preparation is again the key, like so many other areas of safe cockpit procedures.


Types Published and Tolerances

We'll start here with Airways. For those in the know, the standard dimension considered for protected airspace is the airway width. For a VHF airway, based on VORs, the width is 4 NM either side of the centerline out to a distance of 50.8 NM from the facilities. From there, lines that splay out from the centerline at an angle of 4.5° either side are joined to cover the area between the facilities. It kind of looks like a diamond sitting on top of a bar. For the VHF airway, airspace planners assess obstacle clearance within this polygon, plus they add a 2 NM buffer on either side of this shape. So over a VOR facility itself, and within 50.8 NM of it on the airway, the Minimum Obstruction Clearance Altitude, or MOCA, will consider 4+2 NM either side of the track, for a total width of 12 NM. DMRs are considered for airway MOCAs, so the added altitude above the highest obstacle is 1,000, 1,500 or 2,000 feet as applicable. Please note that the MOCA and the Minimum Enroute Altitude (MEA) on airways are separate entities. The MOCA ensures obstacle clearance, while the MEA ensures obstacle clearance and signal coverage from the NAVAIDs on which the airway segment is based. ATC will not normally issue clearance below the MEA for an aircraft flying the airway, but may approve it if requested for reasons such as icing, turbulence, etc. This may become important in areas with long airway segments overlying low land for which the MOCA may be low but the MEA is high due to the distance between facilities. Another area where this can be important is where terrain obstructs NAVAID signals forcing a high MEA (there is at least one airway in western Canada with an MEA over FL180) even though the MOCA may be substantially lower. This is becoming more and more important with the advent of more sophisticated NAV systems that don't rely on ground-based NAVAIDs. GPS, for example, can take you anywhere. How do you know how low you can go if there is no published altitude for your area? Often it's best to use airway tracks, even if navigating "GPS direct" as it's often called, simply because there are altitudes published on these tracks. In such a case, ATC may clear you to maintain a lower altitude (one that's at or above the MOCA) since he knows you're not relying on the signal coverage (and hence requiring the MEA) to get you there. Here's a diagram, hopefully enough to clarify what I described above.

Airway

For an LF/MF airway which is based on NDBs, or a VOR and an NDB, the process is similar. The dimensions are 4.34 NM either side plus a splay of 5° for the protected airspace for separation purposes. For obstruction clearance assessments, the width is 4.34 NM either side, plus a buffer of 4.34 NM either side, for a total width of 17.36 NM at the facility. Again, DMRs are considered as with VHF airways above, and the 5° splay makes the area of assessment much wider for long airway segments that run further than 49.66 NM (the point at which the 5° splay and the 4.34 NM lines meet) from either facility for the leg.

The next altitudes published on enroute charts that we'll cover is the Area Minimum Altitudes, or AMAs. These are often bounded by the solid latitude and longitude lines and are published as a large digit (or two) indicating thousands of feet, and a slightly smaller digit indicating hundreds of feet. So, for example, 49 would be 4,900 feet. The AMAs include no buffer outside the lines that bound them on the chart but do include DMR coverage. If they border a DMR so that only part is inside it, the whole AMA has the appropriate altitude for the DMR added to it. For AMAs partially or wholly overlying American airspace, 2,000 feet is added to the altitude determined as safe during obstacle assessment.

100SA On to approach plates, there are several that can be used, some more widely than others. We'll start with the wide range, 100 NM Safe Altitude, formerly known as the Emergency Safe Altitude. The altitude was established for use in the event of an emergency or loss of orientation. A quick glance at the approach plate could give you a safe altitude as long as you're within 100 NM of the aerodrome. If you're looking at the approach plate outside of this distance for safe altitudes, one might have to ask why, I suppose. In any case, the 100 NM Safe Altitude is always based on a radius of the Aerodrome Reference Point (ARP). There is no buffer associated with it beyond the 100 NM radius, but it does take into account the DMRs. Just like the AMA mentioned above, it also includes 2,000 feet of obstacle clearance if the area partially overlies US airspace.

25MSA Focusing a little shaper now, the 25 NM Safe Altitudes, also known as quadrantal altitudes, are a little more useful for a number of reasons. The wide-sweeping nature of the 100 NM Safe Altitude automatically means a high altitude unless you're well clear of mountainous regions, US Airspace and over very flat land. The 25 NM Safe Altitudes, first off, only consider a 1,000 foot obstacle clearance, even in DMRs. These meet the definition for "areas for which minimum altitudes for IFR operations have been established" in the second part of the regulation mentioned above. There is a buffer outside the 25 NM radius these ones are named for as well. 4 NM outside the 25 NM are assessed for obstacles and terrain, making the total radius 29 NM. Also, where practical, the quadrantal altitudes come by their name because they are often split more finely by quadrant. Divided on the north-south and east-west lines, the quadrants are assessed individually, providing more specific altitudes for use. Again, 4 NM is considered beside the "pie shapes" that emerge. See the diagram for a better explanation. Where two or more quadrants have the same altitude, they are merged and labeled with only one value, rather than kept separate. RNAV (GPS) approaches normally have one altitude for the whole circle. The example at left is the 25 NM Safe Altitude from an outdated copy of the VOR 27 Approach at CYFC. The northeast quadrant is 2,000 feet and this considers 4 NM beyond the range of 25 NM from the VOR, as well as 4 NM west of the north line and 4 NM south of the east line. The bearings published as reference are in degrees magnetic unless otherwise specifies (°T indicates true bearings for the Northern Domestic Airspace, for example).

The other altitudes assessed on approach segments consider varying widths of airspace in a somewhat more complex system. There are, as with airways, a primary and a secondary surface. The primary surface is the main segment which must have a consistent amount of clearance throughout. The secondary surface is a sloping surface that rises from the boundary of the primary surface to the edge of its own boundary. The airspace is considered to have the same width as the enroute structure until the Intermediate Fix (IF) and then narrow from there to the width appropriate for the type of approach (ILS is narrower than NDB or VOR approach, for example) at the Missed Approach Point (MAP). An ILS, for example, has a primary surface that is 500 feet either side of the localizer at the MAP. It then widens again depending on the nature of the missed approach segment. Obviously 1,000 feet of obstacle clearance can't be considered in all of these cases, such as on final descent inside the Final Approach Fix (FAF), but it is considered elsewhere, like on the transitions between the enroute navaids and approach navaids. For examples, a VOR on an airway with a published transition to an NDB which is the FAF for an approach, or a DME arc transition to a straight-in.

As you can see, situation awareness is paramount. You have to know where you are before you can consider yourself safe by any one of these altitudes. Especially when the altitudes you're using aren't based on a NAVAID which provides a direct measure of distance like a VOR/DME, and you have to make sure by other means of where you are.




How much about Minimum IFR Altitudes is still vague? Drop me a line if I raised more questions. My e-mail address is moxner@nbnet.nb.ca. Thanks for taking the time to read, and I hope I helped clear this up a little.