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Wake Turbulence

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					                             Caution - Wake Turbulence

Often on approach, and occasionally enroute, ATC will call out traffic and advise
“Caution, wake turbulence.” This is definitely important information. An encounter with
wake turbulence can mean anything from a nasty thump to an upset and, too often, a
crash. Without hesitation, when we get such an advisory from ATC, we should
implement our wake turbulence avoidance plan. Some pilots are rather low key about
dealing with the threat of wake turbulence. I have to believe they have never experienced
a wake turbulence encounter. Anyone who has been slapped around by a wake turbulence
encounter will tell you that it is not something you take lightly. The risk is very real and
the consequences can be life threatening.

We’ve all been taught what to do in the takeoff and landing situations, but what about
when you are on the approach or enroute? The purpose of this paper is to help pilots of
small aircraft develop wake turbulence avoidance plans for each segment of flight.

Rules of the Game
The FAA has very specific rules for dealing with wake turbulence. The most important
rule is “Pilots are expected to adjust their operations and flight path(s) as necessary
to preclude serious wake encounters .1 ” This makes the pilot the responsibility and
gives him the authority to do whatever is necessary to avoid an encounter. This also says
we cannot blame ATC if we do encounter wake turbulence. Once the hazard has been
pointed out, the monkey is on us, not ATC. It may seem like an unfortunate outcome of
the regulations, but that’s the way it is. The help we get from ATC for avoiding wake
turbulence is limited and we need to remember that when planning out action. Here is
what you can expect from ATC.2

Takeoff – A 2 to 3-minute interval (the equivalent of 4 to 5 miles of radar separation)
will be provided when a small aircraft takes off behind a departing large, B757 or heavy
jet aircraft. If it seems like the tower is cutting the time short, remember that there is
some anticipation going on as to how the relative flight paths of the small and large
aircraft will play out.

Landing – Based on the time of arrival over the landing threshold, ATC will provide 4, 5
or 6 miles separation for small aircraft behind large, B757, and heavy jet aircraft
respectively.

Enroute – For small aircraft operating at the same altitude or less than 1000 feet below,
separation of 5 miles from large, B757, or heavy aircraft is provided by ATC.

A critical modifier is that once we report the traffic in sight, ATC assumes we will
maintain separation for wake turbulence purposes. At this point, ATC has done its part
and will give no further assistance in that regard.
1
    AIM 7-3-6 (a)
2
    AIM 7-3-9


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Turbulence Avoidance Planning
Take charge. The first step is to adopt the attitude that you alone, the pilot, are
responsible for wake turbulence avoidance and that any assistance received from ATC is
welcome but unexpected. Doing so arms the pilot to be proactive in dealing with the
threat rather than being overly trusting of someone else to ensure their safety. Time is
critical in dealing with wake turbulence risk. This keeps our defensive skills always at the
ready so we can take immediate action when the threat pops up.

In every case, tighten your seat belt. Make it really tight on your waist, not just snug.
You are guaranteed to hit your head hard on the roof of the cabin unless you, and your
passengers, are firmly strapped in. You will have your hands full controlling the aircraft.
You don’t need a sever jolt to the head in addition. Tighten that seat belt now. Loosen it
to a more comfortable tension later after the risk is passed.

Takeoff – Departing behind a large jet plan to lift off before the point where the jet
rotates and then stay above its flight path. Lifting off before of a jet’s rotation point is
easy to do, but you can forget about staying above the flight path. Small aircraft don’t
have anywhere near the rate of climb that jets have. This leaves two options: either track
upwind of the jet’s departure path immediately after takeoff, or delay the takeoff so the
jet’s wake turbulence has time to dissipate. Make your request to the tower as early as
possible, preferably before you pull onto the runway. You may have to pull to the side to
let other traffic depart and your delay may be much longer than the two minutes you feel
you need. Just remind yourself that if you run into wake turbulence, it’s your fault and
your life, not the controller’s. Don’t assume they know for sure there won’t be a problem
using the minimum separation requirements. It’s a guessing game and you should assume
your guess is just as good as theirs.

If operations on parallel runways are involved pay attention to how the surface wind is
pushing the jet’s vortex. The same rules apply.

Landing – Coming in behind a landing jet, stay at or above the jet’s flight path and plan
on landing long. You want to land beyond the jet’s touchdown point. Judging another
plane’s flight path can be difficult. So rather than trust your ability to judge your relative
flight paths, use the VASI. If there is an ILS for that runway, tune it in and stay above the
glide slope. Most jets, especially the heavy air carrier ones, will be on the glide slope
even in visual conditions so this is a good gauge of where you don’t want to be.

When landing behind a departing jet, plan to land prior to the departing jet’s rotation
point. Shoot for the numbers.

Approach – Avoiding wake turbulence in the approach phase is complicated but the fact
that it is very difficult to judge distance from other aircraft unless you have TCAS,
TCAD, or a Mode S transponder with traffic display. Therefore, think twice about



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acknowledging traffic in sight because once you do, you assume separation
responsibility. Unless you are certain you can maintain separation from the aircraft’s
wake turbulence, seriously consider not acknowledging the traffic. Doing so will keep the
approach controller working the separation issue. You may get a delay turn as a result,
but that’s ok. It was probably needed anyway.

Once on the approach path, stay two dots above the glide slope. This is not the time for
an autopilot-coupled approach. That’s probably what the jet is doing, and if you do too,
you have now put yourself in the high-risk zone. Some people also recommend flying
two dots off to the upwind side of the localizer. Stay centered on the localizer. Being off
the localizer centerline puts you closer to potential obstacles. In addition, it’s one more
thing to do that is non-standard and, unless you are very proficient, could lead to a
botched approach. My recommendation is to stay on the localizer but high on the glide
slope.

An additional option is to slow down. Put some distance between you and the traffic
ahead. Jets will be doing about 150 KIAS on the approach. Small aircraft can
comfortably fly approaches at 90-100 KIAS. The problem with slowing down is that, at
airports where you encounter large carrier aircraft, ATC usually wants you to keep your
speed up so as not to delay traffic behind you. If this is the case, and you are concerned
about wake turbulence, ask for vectors or even a 360 to add separation. If you’re thinking
this might be being a bit chicken hearted, remember it’s your life that’s at stake, not the
controller’s.

Enroute – Like the approach phase, the challenge enroute is reliably judging the distance
from other traffic. Without some electronic traffic display system, you are at a serious
disadvantage. As in the approach phase, not acknowledging visual contact at least keeps
ATC engaged in the problem. Beyond that, your only option, and it’s a poor one, is to
change your heading to try and keep a safe distance. Since you’re dependant on ATC to
call the traffic and they likely won’t advise you until the traffic is within about 5 miles,
making such a call is of marginal value. You’re best bet is to tighten your seat belt and
trust that ATC will keep you out of harm’s way. Do look for the traffic, but don’t
acknowledge it, even in the unlikely event you do spot it.


Summary
An encounter with wake turbulence can have very serious consequences. Take the
situation seriously and be proactive. In most cases your avoidance plan will cause little
inconvenience to all parties and you will get to fly another day.



Peter Cassidy
Sept. 23, 2003
www.cassidys.ca




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