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 U.S.
Department of Transportation
Federal Aviation Administration |
CAUTION
Wake Turbulence
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- Wake Turbulence - The
Problem
- All Pilots are taught to be aware of wake
turbulence. However, recent incidents indicate that pilots need to keep in
mind how severe wake turbulence can be. In any event, wake turbulence is
still out there and it can put a pilot and the aircraft at risk. This page
was prepared as a reminder to pilots, to make them aware of wake
turbulence and how to best avoid it.
Remember, the best defense
against wake turbulence is to know and avoid areas where it occurs.
- What is Wake Turbulence
- All Aircraft produce wake turbulence. Wake
vortices are formed any time an airfoil is producing lift. Lift is
generated by the creation of a pressure differential over the wing
surfaces. The lowest pressure occurs over the upper surface and the
highest pressure under the wing. This pressure differential triggers the
rollup of the airflow aft of the wing resulting in swirling air masses
trailing downstream of the wingtips. Viewed from behind the generating
aircraft, the left vortex rotates clockwise and the right vortex rotates
counterclockwise.
 The intensity
or strength of the vortex is primarily a function of aircraft weight and
configuration (flap setting etc.). The strongest vortices are produced by
heavy aircraft, flying slowly, in a clean
configuration. For example, a large or heavy aircraft that must reduce its
speed to 250 knots below 10,000 feet and is flying in a clean
configuration while descending, produces very strong wake. Extra caution
is needed when flying below and behind such aircraft.
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- Induced Roll - The
Greatest Hazard
- While there have been rare instances where wake turbulence caused
structural damage, the greatest hazard is induced roll and yaw. This is
especially dangerous during takeoff and landing when there is little
altitude for recovery.
Short wing span aircraft are most susceptible to wake turbulence. The
wake turbulence-induced roll rates can be extreme. Countering roll rates
may be difficult or impossible even in a high performance aircraft with
excellent roll control authority.
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- Parallel or crossing
Runways - Stay Heads Up for the Wake
- During takeoff and landing, the vortices
sink toward the ground and move laterally away from the runway when the
wind is calm. A 3 to 5 knot crosswind
 will
tend to keep the upwind vortex in the runway area and may cause the
downwind vortex to drift toward another runway.
At altitude, vortices sink at a rate of 300 to 500 feet per minute and
stabilize about 500 to 900 feet below the flight level of the generating
aircraft.
- Helicopter Wake
-
 Helicopters
also produce wake turbulence. Helicopter wakes may be of significantly
greater strength than those from a fixed wing aircraft of the same weight.
The strongest wake can occur when the helicopter is operating at lower
speeds (20 - 50 knots). Some mid-size or executive class helicopters
produce wake as strong as that of heavier helicopters This is because two
blade main rotor systems, typical of lighter helicopters, produce stronger
wake than rotor systems with more blades.
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- Stay On or Above
Leader's Glide Path
- Incident data shows that the greatest
potential for a wake vortex incident occurs when a light aircraft is
turning from base to final behind a heavy aircraft flying a straight-in
approach.
 Use
extreme caution to intercept final above or well behind the heavier
aircraft. When a visual approach is issued and accepted to visually follow
a preceding aircraft, the pilot is required to establish a safe landing
interval behind the aircraft s/he was instructed to follow. The pilot is
responsible for wake turbulence separation. Pilots must not decrease the
separation that existed when the visual approach was issued unless they
can remain on or above the flight path of the preceding aircraft.
- Warning Signs
- Any uncommanded aircraft movements (i.e.,
wing rocking) may be caused by wake. This is why maintaining situational
awareness is so critical. Ordinary turbulence is not unusual, particularly
in the approach phase. A pilot who suspects wake turbulence is affecting
his or her aircraft should get away from the wake, execute a missed
approach or go-around and be prepared for a stronger wake encounter. The
onset of wake can be insidious and even surprisingly gentle. There have
been serious accidents where pilots have attempted to salvage a landing
after encountering moderate wake only to encounter severe wake vortices.
Pilots should not depend on any aerodynamic warning, but if the onset of
wake is occurring, immediate evasive action is a MUST!
- How to Avoid Wake
Turbulence
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- Takeoff
If you think wake turbulence from the preceding aircraft
may be a factor, wait at least 2 or 3 minutes before taking off. (See
AIM para. 7-58 b & c). Before taking the runway, tell the tower that you
want to wait. Plan your takeoff to liftoff before the rotation point of
the preceding aircraft.
- Climb
If you can, climb above the preceding aircraft's flight
path. If you can't out climb it, deviate slightly upwind, and climb
parallel to the preceding aircraft's course. Avoid headings that cause
you to cross behind and below the preceding aircraft.
- Crossing
If you must cross behind the preceding aircraft, try to
cross above its flight path or (terrain permitting) at least 1,000 feet
below.
- Following
Stay either on or above the preceding aircraft's
flight path, upwind, or at least 1,000 feet below.
- Approach
Maintain a position on or above the preceding
aircraft's flight path with adequate lateral separation.
- Landing
Ensure that your touchdown point is beyond the preceding
aircraft's touchdown point, or land well before a departing aircraft's
rotation point.
- Crossing Approaches
When landing behind another aircraft on
crossing approaches, cross above the other aircraft's flight path.
- Crosswinds
Remember crosswinds may affect the position of the
vortices. Adjust takeoff and landing points accordingly.
- Helicopters
Helicopter wake vortices may be of significantly
greater strength than fixed wing aircraft of the same weight. Avoid
flying beneath the flight paths of helicopters.
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- Tell Your Story to ASRS
- You can help support an FAA effort to make a safe system even safer.
If you have recently experienced wake turbulence, tell the
Aviation Safe Reporting System (ASRS) about it. The ASRS is one of the
ways the industry learns about safety issues. You can also help support an
FAA effort to reduce the frequency and danger of wake vortex events.
Report participation is voluntary and all identifying information will
be removed before the ASRS research data are given to the FAA. You can
report a wake turbulence incident to the ASRS by completing a NASA/ASRS
form obtained from your company, a Flight Service Station, or directly
from NASA. Write to NASA/ASRS at 625 Ellis Street, Suite 305, Mountain
View, California 94043, phone (415) 969-3969 or fax (415) 967-4170.
The people at ASRS may call you and request to interview you about the
wake vortex incident. Participation in the callback program is also
voluntary, and confidential Your information will help improve the safe of
the National Airspace System.
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"It felt as though we had hit a twenty-foot
thick concrete wall."
Statement from a corporate pilot after hitting the wake
from a large aircraft in a slow descent across his flight path.
"We were cleared for a visual approach to
the right runway and to maintain visual separation on a wide-body
cleared to land on the left. Our yaw damper was MEL'ed inoperative and
the aircraft kind of wallowed in the landing configuration. At 2,200
feet MSL. on about a 7 mile final, the aircraft began an abrupt roll to
the left and the nose pitched down. Full scale deflection of the yoke to
the right did not arrest the left roll and for a moment, it appeared the
aircraft was going to roll over onto its back. Suddenly, the aircraft
began to recover as the ailerons regained effectiveness. The rest of the
approach and landing were normal. The winds were from the left and must
have caused the wake vortices from the wide-body to drift toward the
right runway final."
Statement from pilot flying a regional turboprop.
"While holding for an IFR release, a large
military transport was cleared for a low approach and to remain in the
pattern. Approximately 2 to 3 minutes later, I was cleared for takeoff.
After takeoff, I cleaned the aircraft up and climbed about 20 knots
faster than normal. Going through 1,200 feet MSL, I lowered the nose
slightly because I thought I had cleared the crosswind path of the
military transport. About that time, the aircraft began a smooth roll to
the left and felt like it was trying to pitch up. I applied nose down
pressure and then heard the engines cavitate and we pitched down rather
abruptly. The aircraft then felt solid, so I throttled back and
gradually pulled up and resumed the climb. The incident shook up some of
the passengers, so I explained to them what I thought had happened and
that I was going to return to the airport. In all my years of flying,
this is the first time I have encountered the wake from a large
aircraft."
Statement from a light transport pilot.
The above are actual excerpts from reports to the Aviation Safety
Report System (ASRS).
- For Additional
Information
- Order the free Advisory Circular,
AC-90-23E from:
- DOT, M-443.2
- General Services Section
- Washington, D.C. 20590
A self addressed mailing label will speed processing and delivery
A wake turbulence Training Aid has been developed by the FAA and
industry. The training aid includes a report, a videotape, and a CD-ROM.
Copies of the training aid are available for a fee from:
- National Technical Information Service
- U.S. Department of Commerce
- 5285 Port Royal Road
- Springfield, VA 22161
or call: 703.487.4650. Request the following NTIS Accession Numbers:
- Report PB95780136
- Videotape AVA19661-VNB1
- CD-ROM PB95502613
Also, there is a section on wake turbulence in the current Aeronautical
Information Manual (AIM).
To receive a printed copy of publication No: ASY-20 95/003
"CAUTION Wake Turbulence" brochure from which these pages are based,
write:
- FAA/ASY-20
- 400 7th Street, S.W.
- Washington, D.C. 20590
You may also leave your request with an automated telephone answering
service by dialing: (202) 267-7770
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