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AIM

10/12/17

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

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 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 5−1−8,

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

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 2−1−2, 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

capability.

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

(GTG), SIGMETs and PIREPs.

c. Pilot Actions When Encountering Weather

(e.g., Severe Turbulence or MWA)