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400 ft limit by Rich Hanson

During the past two or three years
there has been much discussion
concerning why 400 feet above ground
level (AGL) is identified as an altitude
constraint for model aircraft.
The criteria is first mentioned in
FAA’s Advisory Circular (AC 91-57)
entitled “Model Aircraft Operating
Standards,” published in June 1981. The
AC states, “Do not fly
model aircraft higher
than 400 feet above the
surface,” and goes on to
say, “When flying
aircraft within 3 miles of
an airport, notify the
airport operator, or when
an air traffic facility is
located at the airport,
notify the control tower,
or flight service station.”
Advisory Circulars are advisory in
nature and are not regulatory per se;
however, operators are expected to give
the guidance appropriate consideration in
their flight operations.
AMA’s experience has shown that
model aircraft seldom create any hazard
to manned aircraft. Our requirement to
follow AMA’s See and Avoid Guidance,
yield the right of way to all man-carrying
aircraft, and to maintain visual contact
while utilizing a spotter, when
appropriate, mitigates any risk posed by
model aircraft.
The guidance in AMA’s Safety Code
to remain at or below 400 feet above
ground level altitude when within three
(3) miles of an airport diminishes any
additional risk presented by the increased
density of low altitude manned aircraft
traffic arriving and departing from
airports, ensuring the safe environment
we maintain elsewhere.
AMA has taken its situational
awareness of the hobby and provided
guidance in the National Model Aircraft
Safety Code that states, “Model aircraft
pilots will … Not fly higher than
approximately 400 feet above ground
level within three (3) miles of an airport,
without notifying the airport operator.”
This language is slightly confusing,
but what it is saying is when within three
(3) miles of an airport you should contact
the airport and remain at or below 400
feet AGL, and only operate above 400
feet when you have coordinated the
operations with the airport authority or
air traffic control facility if one exists at
the airport.
But, why 400 feet?
To understand the significance of 400
feet, you need to understand FAA’s basic
approach to traffic separation. Aircraft
operating in the National Airspace
System (NAS) are separated by
procedures and directives that use all
three axes of flight to maintain
separation: heading, speed, and altitude.
Aircraft flying under visual flight
rules (VFR) and headed in a westerly
direction are expected to maintain an
even cardinal altitude plus 500 feet, i.e.
8,500 feet
above mean sea
level (MSL).
traveling in an
direction are
expected to stay
at odd altitudes
plus 500 feet,
i.e. 7,500 feet
MSL. This results in a 1,000-foot
separation between aircraft heading on
potentially converging courses. Another
related rule states that aircraft operating
below 10,000 feet MSL
are limited to a maximum airspeed of
250 knots.
The 400 feet AGL guideline
was identified using a similar traffic
separation approach. With the exception
of takeoffs and landings and a few
helicopters and sport aviation aircraft,
manned aircraft generally operate at 500
feet AGL and above. Keeping model
aircraft at 400 feet and below maintains a
theoretical 100-foot separation.
Of course, judging the altitude of a
model aircraft is extremely difficult.
This is why your best approach is to
always maintain visual contact with
your aircraft, yield the right of way to
all man-carrying aircraft, and see and
avoid all aircraft, using a spotter when
appropriate. MA
—Rich Hanson



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