Number 273 May 2002
CAUTION: Wake Turbulence
Respect Rotary Wing Wake
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), as discovered
by this General Aviation fixed wing pilot:
While on downwind leg [to uncontrolled airport], a light
helicopter entered final approach from the north. When
turning from base to final, I had him in sight near the
runway threshold, slightly right of the runway. He
proceeded to move right to the parallel taxiway. I continued
the approach, not considering rotor wash to be a factor as
he was now at the taxiway and standard glide path could
have me above the area he had been near the runway.
Approximately 200-300 feet past the runway threshold, the
aircraft suddenly rolled right, yawed right, and sank.
Opposite control input failed to arrest the roll or sink,
however it did seem to slow the yaw. [The aircraft]
impacted the ground right wing low, yawed slightly right
and nose high. The aircraft became airborne again and I
was able to maintain control and land in the grass parallel
to the runway. Damage included a collapsed nose gear
assembly, prop strike, gear doors and lower cowl.
This event was classified as an incident rather than an
accident. Rotor vortices circulate outward, upward,
around, and away from the main rotor(s) in all directions.
Pilots of small aircraft should operate three or more rotor
diameters away from any helicopter in a slow hover taxi,
or stationary hover.
All pilots are taught to be wary of wake turbulence, the
“horizontal tornadoes” that trail behind the wingtips of
any aircraft in flight, especially larger and heavier
aircraft. The greatest hazard from wake turbulence is
induced roll and yaw, particularly during the takeoff and
landing phases where there is little altitude for recovery.
A Captain’s recent report to ASRS offers a graphic
example of wake turbulence hazard:
[My] PA31-350 was on visual approach to Runway 18,
following a B727. The 727 made a normal, uneventful
landing and was in the process of taxiing clear of the active
runway when the Tower controller requested [that we] “go
around and maintain runway heading” due to traffic still
on the runway. I complied with the Tower controller’s
request and applied power and began a go-around from 200
feet, when my aircraft was “rocked” by the wing tip vortices
of the already landed B727. Wind at the time was reported
by the ATIS as “calm.”
My PA31-350 aircraft entered an uncommanded right bank
of 60 degrees... I confirmed that both engines were making
full power. I applied opposite aileron input to counteract the
uncommanded bank. I was unable to escape the effects of
the wing tip vortices. I exited the vortex approximately 800
horizontal feet later... The airplane wings leveled, and then
the aircraft entered a second uncommanded bank of 20-30
degrees, this time to the left. At approximately 1/4 the way
down the runway, I regained complete control...
The Tower controller…requested that I join left downwind,
cleared to land. I followed the request and landed safely.
Upon taxiing clear of the active runway, the Tower/Ground
controller asked, “Is everyone OK?” Upon reaching the
parking area…I discovered that there were no injuries.
When the Tower gave me clearance to land, I looked to see
where the 727 was on the approach. At the time I felt I was
far enough behind the 727 to limit my exposure to his wake
turbulence. Obviously, this was incorrect…
The Aeronautical Information Manual (Chapter 7,
Section 3) and Order 7110.65N, Air Traffic Control
(Paragraphs 2-1-19 and 2-1-20) explain the respective
pilot and controller responsibilities for wake turbulence
avoidance. ATC is responsible in both VFR and IFR
conditions for issuing wake turbulence advisories. An
airport’s air traffic controllers will provide an advisory to
any VFR aircraft on its frequency that may, in the
controller’s opinion, be adversely affected by wake
turbulence from a larger aircraft. This advisory will give
the position, altitude (if known) and direction of flight of
the larger aircraft, followed by the phrase, “CAUTION –
WAKE TURBULENCE.”
However, if a pilot accepts a visual approach clearance to
follow an aircraft, the pilot accepts responsibility for
separation and wake turbulence avoidance. Because wake
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