AIM 

6/17/21 

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

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keeping can follow the guidance provided 

below. 

(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 
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 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

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8, 

Merging Target Procedures. Paragraph 5

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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 
target. 

(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

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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

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6

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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

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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 
capability. 

b.  Pre

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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 
(GTG), SIGMETs and PIREPs. 

c.  Pilot Actions When Encountering Weather 

(e.g., Severe Turbulence or MWA) 

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Operational Policy/Procedures for Reduced Vertical Separation Minimum (RVSM) in the 

Domestic U.S., Alaska, Offshore Airspace and the San Juan FIR