Aviation Topic of the Week
By Michael Oxner,
January 18, 2004
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 In fact, they may not be published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).
As you can see here, there are 5 DMRs in Canada. They are:
- 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
- Eastern Quebec and Labrador, requiring 1,500 feet
- The Gaspe Peninsula and northwestern New Brunswick, requiring
1,500 feet
- The island of Newfoundland, requiring 1,500 feet
- 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.
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.
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.
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.