Flight Operations Course
Course 6 – Minimum flight altitudes
Minimum flight altitudes
Minimum flight altitude = The lowest altitude at which aircraft may safely operate.
A non-directional beacon (NDB) just transmits a simple signal that equipment
on your aircraft can detect and locate in relation to your position. An instrument inside
your aircraft points to the NDB, and you can turn to fly towards the NDB, away from it,
etc. You don't actually know where you are in relation to the NDB as far as points of the
compass are concerned (north of it, south of it, etc.), you just know where it is in relation
to wherever you are.
A VOR (Very High Omni Range transmitter) signal is more complex, and as a
result it also allows equipment on your aircraft to determine where you are in relation to
the VOR station, in terms of compass headings. The VOR signal tells your aircraft which
"radial" it is on, meaning one of 360 imaginary lines radiating outward from the VOR, in
relation to magnetic north. For example, if you're on the 090 radial, you're due east of
the VOR station, and if you're on the 225 radial, you're southwest of the station.
The radials of the VOR give you more flexibility for navigation. Not only can you
fly towards or away from a VOR, but you can do so in a specific direction. If you track a
NDB, you only know that you're moving towards it ... from somewhere. If you track a
VOR, however, you can set your instruments to fly towards it along a specific radial, so
you're not only approaching the VOR but you're also approaching it from a known
direction. VORs also often have DME equipment installed, so you know not only where
you are in terms of direction from the VOR, but also in terms of distance, which allows
you to find your exact position with just one VOR/DME station.
An operator shall establish minimum flight altitudes and the methods to
determine those altitudes for all route segments to be flown which provide the required
terrain clearance.
The method for establishing minimum flight altitudes must be approved by the
Authority.
Where minimum flight altitudes established by States overflown are higher than
those established by the operator, the higher values shall apply.
An operator shall specify the method by which it is intended to determine
minimum flight altitudes for operations conducted over routes for which minimum flight
altitudes have not been established by the State flown over or the responsible State,
and shall include this method in the operations manual.
An operator shall take into account the following factors when establishing
minimum flight altitudes:
(1) The accuracy with which the position of the aircraft can be determined;
(2) The probable inaccuracies in the indications of the altimeters used;
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Flight Operations Course
Course 6 – Minimum flight altitudes
(3) The characteristics of the terrain (e.g. sudden changes in the elevation) along
the routes or in the areas where operations are to be conducted.
(4) The probability of encountering unfavorable meteorological conditions (e.g.
severe turbulence and descending air currents); and
(5) Possible inaccuracies in aeronautical charts.
In fulfilling the requirements prescribed above due consideration shall be given
to:
(1) Corrections for temperature and pressure variations from standard values;
(2) The ATC requirements; and
(3) Any contingencies along the planned route.
Methods for the establishment of Minimum Flight Altitudes
The following are examples of some of the methods available for calculating
minimum flight altitudes.
1. KSS Formula
1.1. Minimum obstacle clearance altitude (MOCA). MOCA is the sum of:
 The maximum terrain or obstacle elevation whichever is highest; plus
 1 000 ft for elevation up to and including 6 000 ft; or
 2 000 ft for elevation exceeding 6 000 ft rounded up to the next 100 ft.
The lowest MOCA to be indicated is 2 000 ft.
From a VOR station, the corridor width is defined as a borderline starting 5 nm either
side of the VOR, diverging 4° from centerline until a width of 20 nm is reached at 70 nm
out, thence paralleling the centerline until 140 nm out, thence again diverging 4° until a
maximum width of 40 nm is reached at 280 nm out. Thereafter the width remains
constant.
From an NDB, similarly, the corridor width is defined as a borderline starting 5
nm either side of the NDB diverging 7° until a width of 20 nm is reached 40 nm out,
thence rallying the centerline until 80 nm out, thence again diverging 7° until a maximum
width of 60 nm is reached 245 nm out. Thereafter the width remains constant.
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Flight Operations Course
Course 6 – Minimum flight altitudes
MOCA does not cover any overlapping of the corridor.
1.2. Minimum off–route altitude (MORA).
MORA is calculated for an area bounded by every second LAT/LONG square on
the Route Facility Chart (RFC)/Terminal Approach Chart (TAC) and is based on a
terrain clearance as follows:
 Terrain with elevation up to 6 000 ft (2 000 m) – 1 000 ft above the highest terrain
and obstructions;
 Terrain with elevation above 6 000 ft (2 000 m) – 2 000 ft above the highest
terrain and obstructions.
2. Jeppesen Formula
MORA is a minimum flight altitude computed by Jeppesen from current ONC or
WAC charts. Two types of MORAs are charted which are:
i. Route MORAs e.g. 9 800a; and
ii. Grid MORAs e.g. 98.
Route MORA values are computed on the basis of an area extending 10 nm to
either side of route centerline and including a 10 nm radius beyond the radio
fix/reporting point or mileage break defining the route segment.
MORA values clear all terrain and man–made obstacles by 1 000 ft in areas
where the highest terrain elevation or obstacles are up to 5 000 ft. A clearance of 2 000
ft is provided above all terrain or obstacles which are 5 001 ft and above.
A Grid MORA is an altitude computed by Jeppesen and the values are shown
within each Grid formed by charted lines of latitude and longitude. Figures are shown in
thousands and hundreds of feet (omitting the last two digits so as to avoid chart
congestion). Values followed by ± are believed not to exceed the altitudes shown.
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Flight Operations Course
Course 6 – Minimum flight altitudes
3. ATLAS Formula
Minimum safe En–route Altitude (MEA). Calculation of the MEA is based on
the elevation of the highest point along the route segment concerned (extending from
navigational aid to navigational aid) within a distance on either side of track as specified
below:
i. Segment length up to 100 nm – 10 nm (See Note 1 below).
ii. Segment length more than 100 nm – 10% of the segment length up to a
maximum of 60 nm (See Note 2 below).
Note 1: This distance may be reduced to 5 nm within TMAs where, due to the number
and type of available navigational aids, a high degree of navigational accuracy is
warranted.
Note 2: In exceptional cases, where this calculation results in an operationally
impracticable value, an additional special MEA may be calculated based on a distance
of not less than 10 nm either side of track. Such special MEA will be shown together
with an indication of the actual width of protected airspace.
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Flight Operations Course
Course 6 – Minimum flight altitudes
The MEA is calculated by adding an increment to the elevation specified above
as appropriate:
Elevation of highest point
Not above 5 000 ft
Above 5 000 ft but not above 10 000 ft
Above 10 000 ft 10% of elevation plus
Increment
1 500 ft
2 000 ft
1 000 ft
NOTE: For the last route segment ending over the initial approach fix, a reduction to
1000 ft is permissible within TMAs where, due to the number and type of available
navigation aids, a high degree of navigational accuracy is warranted.
The resulting value is adjusted to the nearest 100 ft.
Minimum safe Grid Altitude (MGA). Calculation of the MGA is based on the
elevation of the highest point within the respective grid area.
The MGA is calculated by adding an increment to the elevation specified above
as appropriate:
Elevation of highest point
Not above 5 000 ft
Above 5 000 ft but not above 10 000 ft
Above 10 000 ft 10% of elevation plus
The resulting value is adjusted to the nearest 100 ft.
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Increment
1 500 ft
2 000 ft
1 000 ft