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7-3-1. General
a. Every aircraft generates a wake while in flight.
Initially, when pilots encountered this wake in flight,
the disturbance was attributed to "prop wash." It is
known, however, that this disturbance is caused by a
pair of counter-rotating vortices trailing from the
wing tips. The vortices from larger aircraft pose
problems to encountering aircraft. For instance, the
wake of these aircraft can impose rolling moments
exceeding the roll-control authority of the encountering aircraft. Further, turbulence generated within the
vortices can damage aircraft components and
equipment if encountered at close range. The pilot
must learn to envision the location of the vortex wake
generated by larger (transport category) aircraft and
adjust the flight path accordingly.
b. During ground operations and during takeoff,
jet engine blast (thrust stream turbulence) can cause
damage and upsets if encountered at close range.
Exhaust velocity versus distance studies at various
thrust levels have shown a need for light aircraft to
maintain an adequate separation behind large turbojet
aircraft. Pilots of larger aircraft should be particularly
careful to consider the effects of their "jet blast" on
other aircraft, vehicles, and maintenance equipment
during ground operations.
7-3-2. Vortex Generation
Lift is generated by the creation of a pressure
differential over the wing surface. The lowest
pressure occurs over the upper wing surface and the
highest pressure under the wing. This pressure
differential triggers the roll up of the airflow aft of the
wing resulting in swirling air masses trailing
downstream of the wing tips. After the roll up is
completed, the wake consists of two counter-rotating
cylindrical vortices. (See
FIG 7-3-1.) Most of the
energy is within a few feet of the center of each
vortex, but pilots should avoid a region within about
100 feet of the vortex core.
FIG 7-3-1
Wake Vortex Generation
7-3-3. Vortex Strength
a. The strength of the vortex is governed by the
weight, speed, and shape of the wing of the generating
aircraft. The vortex characteristics of any given
aircraft can also be changed by extension of flaps or
other wing configuring devices as well as by change
in speed. However, as the basic factor is weight, the
vortex strength increases proportionately. Peak
vortex tangential speeds exceeding 300 feet per
second have been recorded. The greatest vortex
strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW.
b. Induced Roll
1. In rare instances a wake encounter could
cause inflight structural damage of catastrophic
proportions. However, the usual hazard is associated
with induced rolling moments which can exceed the
roll-control authority of the encountering aircraft. In
flight experiments, aircraft have been intentionally
flown directly up trailing vortex cores of larger
aircraft. It was shown that the capability of an aircraft
to counteract the roll imposed by the wake vortex
primarily depends on the wingspan and counter-
control responsiveness of the encountering aircraft.
2. Counter control is usually effective and
induced roll minimal in cases where the wingspan
and ailerons of the encountering aircraft extend
beyond the rotational flow field of the vortex. It is
more difficult for aircraft with short wingspan
(relative to the generating aircraft) to counter the
imposed roll induced by vortex flow. Pilots of short
span aircraft, even of the high performance type, must
be especially alert to vortex encounters.
(See FIG 7-3-2.)
FIG 7-3-2
Wake Encounter Counter Control
3. The wake of larger aircraft requires the
respect of all pilots.
7-3-4. Vortex Behavior
a. Trailing vortices have certain behavioral
characteristics which can help a pilot visualize the
wake location and thereby take avoidance precautions.
1. Vortices are generated from the moment
aircraft leave the ground, since trailing vortices are a
by-product of wing lift. Prior to takeoff or touchdown
pilots should note the rotation or touchdown point of
the preceding aircraft. (See
FIG 7-3-4.)
2. The vortex circulation is outward, upward
and around the wing tips when viewed from either
ahead or behind the aircraft. Tests with large aircraft
have shown that the vortices remain spaced a bit less
than a wingspan apart, drifting with the wind, at
altitudes greater than a wingspan from the ground. In
view of this, if persistent vortex turbulence is
encountered, a slight change of altitude and lateral
position (preferably upwind) will provide a flight
path clear of the turbulence.
3. Flight tests have shown that the vortices from
larger (transport category) aircraft sink at a rate of
several hundred feet per minute, slowing their
descent and diminishing in strength with time and
distance behind the generating aircraft. Atmospheric
turbulence hastens breakup. Pilots should fly at or
above the preceding aircraft's flight path, altering
course as necessary to avoid the area behind and
below the generating aircraft. (See
FIG 7-3-3.)
However, vertical separation of 1,000 feet may be
considered safe.
4. When the vortices of larger aircraft sink close
to the ground (within 100 to 200 feet), they tend to
move laterally over the ground at a speed of 2 or
3 knots. (See
FIG 7-3-5.)
FIG 7-3-3
Wake Ends/Wake Begins
FIG 7-3-5
Vortex Movement Near Ground - No Wind
FIG 7-3-6
Vortex Movement Near Ground - with Cross Winds
5. There is a small segment of the aviation
community that have become convinced that wake
vortices may "bounce" up to twice their nominal
steady state height. With a 200-foot span aircraft, the
"bounce" height could reach approximately 200 feet
AGL. This conviction is based on a single
unsubstantiated report of an apparent coherent
vortical flow that was seen in the volume scan of a
research sensor. No one can say what conditions
cause vortex bouncing, how high they bounce, at
what angle they bounce, or how many times a vortex
may bounce. On the other hand, no one can say for
certain that vortices never "bounce." Test data have
shown that vortices can rise with the air mass in which
they are embedded. Wind shear, particularly, can
cause vortex flow field "tilting." Also, ambient
thermal lifting and orographic effects (rising terrain
or tree lines) can cause a vortex flow field to rise.
Notwithstanding the foregoing, pilots are reminded
that they should be alert at all times for possible wake
vortex encounters when conducting approach and
landing operations. The pilot has the ultimate
responsibility for ensuring appropriate separations
and positioning of the aircraft in the terminal area to
avoid the wake turbulence created by a preceding
aircraft.
b. A crosswind will decrease the lateral movement
of the upwind vortex and increase the movement of
the downwind vortex. Thus a light wind with a cross
runway component of 1 to 5 knots could result in the
upwind vortex remaining in the touchdown zone for
a period of time and hasten the drift of the downwind
vortex toward another runway. (See
FIG 7-3-6.)
Similarly, a tailwind condition can move the vortices
of the preceding aircraft forward into the touchdown
zone. THE LIGHT QUARTERING TAILWIND
REQUIRES MAXIMUM CAUTION. Pilots should
be alert to large aircraft upwind from their approach
and takeoff flight paths. (See
FIG 7-3-7.)
FIG 7-3-7
Vortex Movement in Ground Effect - Tailwind
7-3-5. Operations Problem Areas
a. A wake encounter can be catastrophic. In 1972
at Fort Worth a DC-9 got too close to a DC-10
(two miles back), rolled, caught a wingtip, and
cartwheeled coming to rest in an inverted position on
the runway. All aboard were killed. Serious and even
fatal GA accidents induced by wake vortices are not
uncommon. However, a wake encounter is not
necessarily hazardous. It can be one or more jolts with
varying severity depending upon the direction of the
encounter, weight of the generating aircraft, size of
the encountering aircraft, distance from the generating aircraft, and point of vortex encounter. The
probability of induced roll increases when the
encountering aircraft's heading is generally aligned
with the flight path of the generating aircraft.
b. AVOID THE AREA BELOW AND BEHIND
THE GENERATING AIRCRAFT, ESPECIALLY
AT LOW ALTITUDE WHERE EVEN A
MOMENTARY WAKE ENCOUNTER COULD BE
HAZARDOUS. This is not easy to do. Some
accidents have occurred even though the pilot of the
trailing aircraft had carefully noted that the aircraft in
front was at a considerably lower altitude. Unfortunately, this does not ensure that the flight path of the
lead aircraft will be below that of the trailing aircraft.
c. Pilots should be particularly alert in calm wind
conditions and situations where the vortices could:
1. Remain in the touchdown area.
2. Drift from aircraft operating on a nearby
runway.
3. Sink into the takeoff or landing path from a
crossing runway.
4. Sink into the traffic pattern from other airport
operations.
5. Sink into the flight path of VFR aircraft
operating on the hemispheric altitude 500 feet below.
d. Pilots of all aircraft should visualize the
location of the vortex trail behind larger aircraft and
use proper vortex avoidance procedures to achieve
safe operation. It is equally important that pilots of
larger aircraft plan or adjust their flight paths to
minimize vortex exposure to other aircraft.
7-3-6. Vortex Avoidance Procedures
a. Under certain conditions, airport traffic controllers apply procedures for separating IFR aircraft. If a
pilot accepts a clearance to visually follow a
preceding aircraft, the pilot accepts responsibility for
separation and wake turbulence avoidance. The
controllers will also provide to VFR aircraft, with
whom they are in communication and which in the
tower's opinion may be adversely affected by wake
turbulence from a larger aircraft, the position, altitude
and direction of flight of larger aircraft followed by
the phrase "CAUTION - WAKE TURBULENCE."
After issuing the caution for wake turbulence, the
airport traffic controllers generally do not provide
additional information to the following aircraft
unless the airport traffic controllers know the
following aircraft is overtaking the preceding
aircraft. WHETHER OR NOT A WARNING OR
INFORMATION HAS BEEN GIVEN, HOWEVER,
THE PILOT IS EXPECTED TO ADJUST AIRCRAFT OPERATIONS AND FLIGHT PATH AS
NECESSARY TO PRECLUDE SERIOUS WAKE
ENCOUNTERS. When any doubt exists about
maintaining safe separation distances between
aircraft during approaches, pilots should ask the
control tower for updates on separation distance and
aircraft groundspeed.
b. The following vortex avoidance procedures are
recommended for the various situations:
1. Landing behind a larger aircraft- same
runway. Stay at or above the larger aircraft's final
approach flight path-note its touchdown point-land
beyond it.
2. Landing behind a larger aircraft- when
parallel runway is closer than 2,500 feet. Consider
possible drift to your runway. Stay at or above the
larger aircraft's final approach flight path- note its
touchdown point.
3. Landing behind a larger aircraft- crossing
runway. Cross above the larger aircraft's flight path.
4. Landing behind a departing larger aircraft- same runway. Note the larger aircraft's
rotation point- land well prior to rotation point.
5. Landing behind a departing larger aircraft- crossing runway. Note the larger aircraft's
rotation point- if past the intersection- continue the
approach- land prior to the intersection. If larger
aircraft rotates prior to the intersection, avoid flight
below the larger aircraft's flight path. Abandon the
approach unless a landing is ensured well before
reaching the intersection.
6. Departing behind a larger aircraft. Note
the larger aircraft's rotation point and rotate prior to
the larger aircraft's rotation point. Continue climbing
above the larger aircraft's climb path until turning
clear of the larger aircraft's wake. Avoid subsequent
headings which will cross below and behind a larger
aircraft. Be alert for any critical takeoff situation
which could lead to a vortex encounter.
7. Intersection takeoffs- same runway. Be
alert to adjacent larger aircraft operations, particularly upwind of your runway. If intersection takeoff
clearance is received, avoid subsequent heading
which will cross below a larger aircraft's path.
8. Departing or landing after a larger
aircraft executing a low approach, missed
approach, or touch-and-go landing. Because
vortices settle and move laterally near the ground, the
vortex hazard may exist along the runway and in your
flight path after a larger aircraft has executed a low
approach, missed approach, or a touch-and-go
landing, particular in light quartering wind conditions. You should ensure that an interval of at least
2 minutes has elapsed before your takeoff or landing.
9. En route VFR (thousand-foot altitude plus
500 feet). Avoid flight below and behind a large
aircraft's path. If a larger aircraft is observed above on
the same track (meeting or overtaking) adjust your
position laterally, preferably upwind.
7-3-7. Helicopters
In a slow hover taxi or stationary hover near the
surface, helicopter main rotor(s) generate downwash
producing high velocity outwash vortices to a
distance approximately three times the diameter of
the rotor. When rotor downwash hits the surface, the
resulting outwash vortices have behavioral characteristics similar to wing tip vortices produced by fixed
wing aircraft. However, the vortex circulation is
outward, upward, around, and away from the main
rotor(s) in all directions. Pilots of small aircraft
should avoid operating within three rotor diameters
of any helicopter in a slow hover taxi or stationary
hover. In forward flight, departing or landing
helicopters produce a pair of strong, high-speed
trailing vortices similar to wing tip vortices of larger
fixed wing aircraft. Pilots of small aircraft should use
caution when operating behind or crossing behind
landing and departing helicopters.
7-3-8. Pilot Responsibility
a. Government and industry groups are making
concerted efforts to minimize or eliminate the
hazards of trailing vortices. However, the flight
disciplines necessary to ensure vortex avoidance
during VFR operations must be exercised by the pilot.
Vortex visualization and avoidance procedures
should be exercised by the pilot using the same degree
of concern as in collision avoidance.
b. Wake turbulence may be encountered by
aircraft in flight as well as when operating on the
airport movement area.
REFERENCE-
Pilot/Controller Glossary Term- Wake Turbulence.
c. Pilots are reminded that in operations conducted
behind all aircraft, acceptance of instructions from
ATC in the following situations is an acknowledgment that the pilot will ensure safe takeoff and
landing intervals and accepts the responsibility for
providing wake turbulence separation.
1. Traffic information.
2. Instructions to follow an aircraft; and
3. The acceptance of a visual approach
clearance.
d. For operations conducted behind heavy aircraft, ATC will specify the word "heavy" when this
information is known. Pilots of heavy aircraft should
always use the word "heavy" in radio communications.
e. Heavy and large jet aircraft operators should use
the following procedures during an approach to
landing. These procedures establish a dependable
baseline from which pilots of in-trail, lighter aircraft
may reasonably expect to make effective flight path
adjustments to avoid serious wake vortex turbulence.
1. Pilots of aircraft that produce strong wake
vortices should make every attempt to fly on the
established glidepath, not above it; or, if glidepath
guidance is not available, to fly as closely as possible
to a "3-1" glidepath, not above it.
EXAMPLE-
Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5
miles, 1,200 feet at 4 miles, and so on to touchdown.
2. Pilots of aircraft that produce strong wake
vortices should fly as closely as possible to the
approach course centerline or to the extended
centerline of the runway of intended landing as
appropriate to conditions.
f. Pilots operating lighter aircraft on visual
approaches in-trail to aircraft producing strong wake
vortices should use the following procedures to assist
in avoiding wake turbulence. These procedures apply
only to those aircraft that are on visual approaches.
1. Pilots of lighter aircraft should fly on or
above the glidepath. Glidepath reference may be
furnished by an ILS, by a visual approach slope
system, by other ground-based approach slope
guidance systems, or by other means. In the absence
of visible glidepath guidance, pilots may very nearly
duplicate a 3-degree glideslope by adhering to the
"3 to 1" glidepath principle.
EXAMPLE-
Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at
5 miles, 1,200 feet at 4 miles, and so on to touchdown.
2. If the pilot of the lighter following aircraft has
visual contact with the preceding heavier aircraft and
also with the runway, the pilot may further adjust for
possible wake vortex turbulence by the following
practices:
(a) Pick a point of landing no less than
1,000 feet from the arrival end of the runway.
(b) Establish a line-of-sight to that landing
point that is above and in front of the heavier
preceding aircraft.
(c) When possible, note the point of landing
of the heavier preceding aircraft and adjust point of
intended landing as necessary.
EXAMPLE-
A puff of smoke may appear at the 1,000-foot markings of
the runway, showing that touchdown was that point;
therefore, adjust point of intended landing to the
1,500-foot markings.
(d) Maintain the line-of-sight to the point of
intended landing above and ahead of the heavier
preceding aircraft; maintain it to touchdown.
(e) Land beyond the point of landing of the
preceding heavier aircraft.
3. During visual approaches pilots may ask ATC
for updates on separation and groundspeed with
respect to heavier preceding aircraft, especially when
there is any question of safe separation from wake
turbulence.
7-3-9. Air Traffic Wake Turbulence
Separations
a. Because of the possible effects of wake
turbulence, controllers are required to apply no less
than specified minimum separation for aircraft
operating behind a heavy jet and, in certain instances,
behind large nonheavy aircraft (i.e., B757 aircraft).
1. Separation is applied to aircraft operating
directly behind a heavy/B757 jet at the same altitude
or less than 1,000 feet below:
(a) Heavy jet behind heavy jet-4 miles.
(b) Large/heavy behind B757 - 4 miles.
(c) Small behind B757 - 5 miles.
(d) Small/large aircraft behind heavy jet -
5 miles.
2. Also, separation, measured at the time the
preceding aircraft is over the landing threshold, is
provided to small aircraft:
(a) Small aircraft landing behind heavy jet -
6 miles.
(b) Small aircraft landing behind B757 -
5 miles.
(c) Small aircraft landing behind large
aircraft- 4 miles.
REFERENCE-
Pilot/Controller Glossary Term- Aircraft Classes.
3. Additionally, appropriate time or distance
intervals are provided to departing aircraft:
(a) Two minutes or the appropriate 4 or 5 mile
radar separation when takeoff behind a heavy/B757
jet will be:
(1) From the same threshold.
(2) On a crossing runway and projected
flight paths will cross.
(3) From the threshold of a parallel runway
when staggered ahead of that of the adjacent runway
by less than 500 feet and when the runways are
separated by less than 2,500 feet.
NOTE-
Controllers may not reduce or waive these intervals.
b. A 3-minute interval will be provided when a
small aircraft will takeoff:
1. From an intersection on the same runway
(same or opposite direction) behind a departing large
aircraft,
2. In the opposite direction on the same runway
behind a large aircraft takeoff or low/missed
approach.
NOTE-
This 3-minute interval may be waived upon specific pilot
request.
c. A 3-minute interval will be provided for all
aircraft taking off when the operations are as
described in subparagraph
b1 and
2 above, the
preceding aircraft is a heavy/B757 jet, and the
operations are on either the same runway or parallel
runways separated by less than 2,500 feet.
Controllers may not reduce or waive this interval.
d. Pilots may request additional separation i.e.,
2 minutes instead of 4 or 5 miles for wake turbulence
avoidance. This request should be made as soon as
practical on ground control and at least before taxiing
onto the runway.
NOTE-
14 CFR Section 91.3(a) states: "The pilot-in-command of
an aircraft is directly responsible for and is the final
authority as to the operation of that aircraft."
e. Controllers may anticipate separation and need
not withhold a takeoff clearance for an aircraft
departing behind a large/heavy aircraft if there is
reasonable assurance the required separation will
exist when the departing aircraft starts takeoff roll.
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