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.
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.
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
Wake Vortex Generation
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
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
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
The wake of larger aircraft requires
the respect of all pilots.
Wake Encounter Counter
Top of page
Trailing vortices have certain behavioral
characteristics which can help a pilot visualize the
wake location and thereby take avoidance precautions.
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.)
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.
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
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.)
Wake Ends/Wake Begins
Vortex Flow Field
Vortex Movement Near
Ground - No Wind
Vortex Movement Near
Ground - with Cross Winds
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.
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
Vortex Movement in
Ground Effect - Tailwind
Operations Problem Areas
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
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.
Pilots should be particularly alert in
calm wind conditions and situations where the vortices
Remain in the touchdown area.
Drift from aircraft operating on a
Sink into the takeoff or landing path
from a crossing runway.
Sink into the traffic pattern from
other airport operations.
Sink into the flight path of VFR
aircraft operating on the hemispheric altitude 500
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.
Vortex Avoidance Procedures
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.
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
3. Landing behind a
larger aircraft- crossing runway.
Cross above the larger aircraft's
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.
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.
takeoffs- same runway.
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
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.
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.
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.
Wake turbulence may be encountered by
aircraft in flight as well as when operating on the
airport movement area.
Pilot/Controller Glossary Term- Wake Turbulence.
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.
Instructions to follow an aircraft; and
The acceptance of a visual approach
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
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.
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.
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
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.
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
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"
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
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:
Pick a point of landing no less than
1,000 feet from the arrival end of the runway.
Establish a line-of-sight to that
landing point that is above and in front of the
heavier preceding aircraft.
When possible, note the point of
landing of the heavier preceding aircraft and adjust
point of intended landing as necessary.
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.
Maintain the line-of-sight to the
point of intended landing above and ahead of the
heavier preceding aircraft; maintain it to
Land beyond the point of landing of
the preceding heavier aircraft.
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
Air Traffic Wake Turbulence Separations
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
Separation is applied to aircraft
operating directly behind a heavy/B757 jet at
the same altitude or less than 1,000 feet below:
jet behind heavy jet-4 miles.
behind B757 - 4 miles.
behind B757 - 5 miles.
aircraft behind heavy jet - 5 miles.
Also, separation, measured at the time
the preceding aircraft is over the landing threshold,
is provided to small aircraft:
aircraft landing behind heavy jet - 6 miles.
aircraft landing behind B757 - 5 miles.
aircraft landing behind large aircraft- 4
Pilot/Controller Glossary Term- Aircraft Classes.
Additionally, appropriate time or
distance intervals are provided to departing aircraft:
Two minutes or the appropriate 4 or 5
mile radar separation when takeoff behind a
heavy/B757 jet will be:
From the same threshold.
On a crossing runway and projected
flight paths will cross.
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.
Controllers may not reduce or waive these
A 3-minute interval will be provided when
a small aircraft will takeoff:
From an intersection on the
same runway (same or opposite direction) behind a
departing large aircraft,
In the opposite direction on
the same runway behind a large aircraft takeoff or
This 3-minute interval may be waived upon specific
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.
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.
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."
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.