Aeronautical Information Manual (AIM), page 268

Index   267 -- Page 268 -- 269


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

(c) Inflight MWA Indicators (Including Tur-

bulence). 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

(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 Proce-
dures. 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 Order JO
7110.65, Air Traffic Control, Paragraph 5-1-8,
Merging Target Procedures. Paragraph 5-1-8 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 separa-


tion. 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

(b) The provision of "merging target proce-
dures" 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
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 permit-
ting." The contingency actions for MWA and severe
turbulence detailed in Paragraph 4-6-9, Contingency
Actions: Weather Encounters and Aircraft System
Failures that Occur After Entry into RVSM Airspace,
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 Order JO 7110.65,
Paragraph 2-1-2, Duty Priority, there are circum-
stances when the controller is required to perform
more than one action and must "exercise their best
judgment based on the facts and circumstances
known to them" to prioritize their actions. Further

direction given is: "That action which is most critical

from a safety standpoint is performed first."

5. TCAS Sensitivity. For both MWA and
severe turbulence encounters in RVSM airspace, an
additional concern is the sensitivity of collision
avoidance systems when one or both aircraft
operating in close proximity receive TCAS advi-
sories in response to disruptions in altitude hold
b. Pre-flight tools. Sources of observed and
forecast information that can help the pilot ascertain
the possibility of MWA or severe turbulence are:
Forecast Winds and Temperatures Aloft (FD), Area
Forecast (FA), Graphical Turbulence Guidance

c. Pilot Actions When Encountering Weather
(e.g., Severe Turbulence or MWA)

4-6-4 Operational Policy/Procedures for Reduced Vertical Separation Minimum (RVSM) in the
Domestic U.S., Alaska, Offshore Airspace and the San Juan FIR

Page 268 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   267 -- Page 268 -- 269