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for flight planning, preflight procedures at the
aircraft, procedures prior to RVSM airspace entry,
inflight (en route) procedures, contingency procedures
and post flight.
d. The following paragraphs either clarify or
supplement Appendix 4 practices and procedures.
4-6-6. Guidance on Severe Turbulence
and Mountain Wave Activity (MWA)
a. Introduction/Explanation
1. The information and practices in this
paragraph are provided to emphasize to pilots and
controllers the importance of taking appropriate
action in RVSM airspace when aircraft experience
severe turbulence and/or MWA that is of sufficient
magnitude to significantly affect altitude-keeping.
2. Severe Turbulence. Severe turbulence
causes large, abrupt changes in altitude and/or
attitude usually accompanied by large variations in
indicated airspeed. Aircraft may be momentarily out
of control. Encounters with severe turbulence must
be remedied immediately in any phase of flight.
Severe turbulence may be associated with MWA.
3. Mountain Wave Activity (MWA)
(a) Significant MWA occurs both below and
above the floor of RVSM airspace, FL 290. MWA
often occurs in western states in the vicinity of
mountain ranges. It may occur when strong winds
blow perpendicular to mountain ranges resulting in
up and down or wave motions in the atmosphere.
Wave action can produce altitude excursions and
airspeed fluctuations accompanied by only light
turbulence. With sufficient amplitude, however,
wave action can induce altitude and airspeed
fluctuations accompanied by severe turbulence.
MWA is difficult to forecast and can be highly
localized and short lived.
(b) Wave activity is not necessarily limited to
the vicinity of mountain ranges. Pilots experiencing
wave activity anywhere that significantly affects
altitude-keeping can follow the guidance provided
below.
(c) Inflight MWA Indicators (Including Turbulence).
Indicators that the aircraft is being
subjected to MWA are:
(1) Altitude excursions and/or airspeed
fluctuations with or without associated turbulence.
(2) Pitch and trim changes required to
maintain altitude with accompanying airspeed
fluctuations.
(3) Light to severe turbulence depending
on the magnitude of the MWA.
4. Priority for Controller Application of
Merging Target Procedures
(a) Explanation of Merging Target Procedures.
As described in subparagraph c3 below, ATC
will use “merging target procedures” to mitigate the
effects of both severe turbulence and MWA. The
procedures in subparagraph c3 have been adapted
from existing procedures published in FAA
OrderJO7110.65, Air Traffic Control, paragraph
5-1-8, Merging Target Procedures.
Paragraph5-1-8 calls for en route controllers to
advise pilots of potential traffic that they perceive
may fly directly above or below his/her aircraft at
minimum vertical separation. In response, pilots are
given the option of requesting a radar vector to ensure
their radar target will not merge or overlap with the
traffic's radar target.
(b) The provision of “merging target procedures”
to mitigate the effects of severe turbulence
and/or MWA is not optional for the controller, but
rather is a priority responsibility. Pilot requests for
vectors for traffic avoidance when encountering
MWA or pilot reports of “Unable RVSM due
turbulence or MWA” are considered first priority
2/14/08 AIM
Operational Policy/Procedures for Reduced Vertical Separation Minimum (RVSM) in the 4-6-5
Domestic U.S., Alaska, Offshore Airspace and the San Juan FIR
aircraft separation and sequencing responsibilities.
(FAA Order JO 7110.65, paragraph 2-1-2, Duty
Priority, states that the controller's first priority is to
separate aircraft and issue safety alerts).
(c) Explanation of the term “traffic permitting.”
The contingency actions for MWA and severe
turbulence detailed in paragraph 4-6-9, Contingency
Actions: Weather Encounters and Aircraft System
Failures, state that the controller will “vector aircraft
to avoid merging targets with traffic at adjacent flight
levels, traffic permitting.” The term “traffic permitting”
is not intended to imply that merging target
procedures are not a priority duty. The term is
intended to recognize that, as stated in FAA
OrderJO7110.65, paragraph 2-1-2, Duty Priority,
there are circumstances when the controller is
required to perform more than one action and must
“exercise their best judgment based on the facts and
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