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Wake turbulence avoidance and ATC



 
 
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  #1  
Old December 18th 03, 05:01 PM
Peter R.
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Default Wake turbulence avoidance and ATC

A couple of days ago I flew into Logan Airport (Boston, MA, USA) in a C172
for an Angel Flight. Taxing to the departing runway, we were behind a B767
and a DC9 (in that order), with several other large airliners behind us.

I noticed that there seemed to be no wake turbulence delay for the DC9
behind the B767, as he was cleared for TO less than a minute after the B767
departed.

Tower then positioned me on the runway, and again, less than a minute later
(after awaiting a crossing runway landing), gave me a 90 degree right turn
after takeoff heading, cautioned wake turbulence, then cleared me to go.

My question has to do with the ATC's wake turbulence procedures. At the
class C airport where I am based, I constantly hear about the 3 minute rule
from ATC. In other words, if I am departing from an intersection mid-
field, tower will say that they are required to make me wait three minutes
for wake turbulence avoidance (unless I wave it, which I normally do not).

In the case of Boston's tower, did her "wake turbulence caution" and/or
right turn heading allow her to clear me sooner than the three minutes?

BTW, the DC9 ahead of me took at least three quarters of the runway to lift
off, then turned left. When I departed, I dropped a notch of flaps to
lift off very quickly, climbed a few hundred at Vx as per the obstacle DP,
then turned the 90 degrees right as per the instruction to be well away
from the previous two aircrafts' wake turbulence.


--
Peter












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  #2  
Old December 18th 03, 05:58 PM
David Rind
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Peter R. wrote:
BTW, the DC9 ahead of me took at least three quarters of the runway to lift
off, then turned left. When I departed, I dropped a notch of flaps to
lift off very quickly, climbed a few hundred at Vx as per the obstacle DP,
then turned the 90 degrees right as per the instruction to be well away
from the previous two aircrafts' wake turbulence.


Not answering your actual question about the clearance, but for
what it's worth, I would not have tried to climb out at Vx. You
have no hope of outclimbing a jet, and might want some additional
airspeed if you hit a wake. Instead, I would have requested an
early turnout and started my turn when I was 100' or 150' up. If
they couldn't give me the early turnout, I would have refused
the takeoff clearance. I'd be interested to know what others
would do.

--
David Rind


  #3  
Old December 18th 03, 06:03 PM
PS2727
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I think you will find that, as usual, it's up to you to get the spacing you
need. Simply tell the tower you need x minutes for wake turbulence. You cannot
be forced to takeoff on their schedule. They will accomodate you if you let
them know what your needs are.
  #4  
Old December 18th 03, 07:05 PM
Maule Driver
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I'm not currently familiar with the 3min rule but it helps to have a clear
picture of where the turbulence is and what you need to avoid it independent
from ATC. Whatever ATC does or says, you are the one that has to decide
what to do. And you have to know what to do when ATC isn't there.

Taking off from the same point as a preceding 'heavy', your 172 is always
able to get off before the heavy can start generating turbulence (at
rotation), so time doesn't really factor in. But the turn away from the
heavy's flight path is important because you will fly thru his path during
climb if you don't.

Intersection takeoffs are another matter. Here you need to make sure you
can get off before their rotation point or that you wait '3 mins'.

You also want to stay aware of wind drift since the wake gets blown.\

On approach and landing, you have even more responsibility because you have
to choose to stay above their glide path and land beyond their touchdown
point. Easy to do but all ATC can consistently do is spread out the traffic
and tell you to be aware.

"Peter R."
My question has to do with the ATC's wake turbulence procedures. At the
class C airport where I am based, I constantly hear about the 3 minute

rule
from ATC. In other words, if I am departing from an intersection mid-
field, tower will say that they are required to make me wait three minutes
for wake turbulence avoidance (unless I wave it, which I normally do not).

In the case of Boston's tower, did her "wake turbulence caution" and/or
right turn heading allow her to clear me sooner than the three minutes?

BTW, the DC9 ahead of me took at least three quarters of the runway to

lift
off, then turned left. When I departed, I dropped a notch of flaps to
lift off very quickly, climbed a few hundred at Vx as per the obstacle DP,
then turned the 90 degrees right as per the instruction to be well away
from the previous two aircrafts' wake turbulence.


--
Peter












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  #5  
Old December 18th 03, 07:38 PM
Peter R.
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David Rind ) wrote:

You
have no hope of outclimbing a jet, and might want some additional
airspeed if you hit a wake. Instead, I would have requested an
early turnout and started my turn when I was 100' or 150' up. If
they couldn't give me the early turnout, I would have refused
the takeoff clearance. I'd be interested to know what others
would do.


My point of climbing at Vx was not to outclimb the DC9, which rotated about
7,000 feet down the runway. My point was to get above the obstacle DP
altitude well before the DC9s rotation point, then turn the 90 degrees
right that the tower had already approved.

It's doubtful that the extra 10-15 knots of the C172's climb-out speed
would make a bit of difference in an actual wake turbulence encounter. My
plan was not to inadvertently encounter the wake, but rather to avoid it
entirely. Hence, the plan to climb steeply then immediately turn away.

--
Peter












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  #6  
Old December 18th 03, 07:49 PM
David Megginson
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David Rind wrote:

Not answering your actual question about the clearance, but for
what it's worth, I would not have tried to climb out at Vx. You
have no hope of outclimbing a jet,


It's not that far off -- I think (but am not certain) that a fully-loaded
DC-9 has a best climb angle of around 650 ft/nm, while a small single-engine
plane will manage something like 400-600 ft/nm at Vx depending on horsepower
and load. Of course, the DC-9 has a much better climb *rate*, but that's
not the concern here (also, the DC-9 is designed for short fields; other
transport jets may have worse climb angles).

More importantly, a Vx climb will probably put you a couple of hundred feet
up and another 30 seconds behind by the time you arrive above the point
where the DC-9 lifted off -- that gives you lots of room to make a turn
before you intersect its path. If you took off at a higher speed, you'd
have less space for your turn because your climb angle would be lower (even
though the rate was higher). Even if you stay straight ahead, at VX you
probably won't intersect the DC-9's climb path until the vortices are
well-dissipated. A slow forward speed is your friend in this situation,
either way.


All the best,


David

  #7  
Old December 18th 03, 08:40 PM
David Rind
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Default

Peter R. wrote:
My point of climbing at Vx was not to outclimb the DC9, which rotated about
7,000 feet down the runway. My point was to get above the obstacle DP
altitude well before the DC9s rotation point, then turn the 90 degrees
right that the tower had already approved.


Were you in IMC such that the DP altitude mattered to you?
I was picturing this happening in visual conditions where you
could start maneuvering (as long as Logan permitted) much
sooner.

--
David Rind


  #8  
Old December 18th 03, 08:43 PM
David Rind
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David Megginson wrote:
David Rind wrote:

Not answering your actual question about the clearance, but for
what it's worth, I would not have tried to climb out at Vx. You
have no hope of outclimbing a jet,



It's not that far off -- I think (but am not certain) that a
fully-loaded DC-9 has a best climb angle of around 650 ft/nm, while a
small single-engine plane will manage something like 400-600 ft/nm at Vx
depending on horsepower and load. Of course, the DC-9 has a much better
climb *rate*, but that's not the concern here (also, the DC-9 is
designed for short fields; other transport jets may have worse climb
angles).

More importantly, a Vx climb will probably put you a couple of hundred
feet up and another 30 seconds behind by the time you arrive above the
point where the DC-9 lifted off -- that gives you lots of room to make a
turn before you intersect its path. If you took off at a higher speed,
you'd have less space for your turn because your climb angle would be
lower (even though the rate was higher). Even if you stay straight
ahead, at VX you probably won't intersect the DC-9's climb path until
the vortices are well-dissipated. A slow forward speed is your friend
in this situation, either way.


You are clearly right about this -- I was thinking in terms
of rate of climb, not angle of climb. That said, I would
still be more interested in making an early turn than in trying
to climb quickly and would always ask for an early turnout
in this situation....

--
David Rind


  #9  
Old December 18th 03, 09:08 PM
Newps
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Default

Don't confuse taking off at an intersection with taking off at the end.
The three minutes applies to an intersection takeoff only. The rule
for taking off behind a heavy is two minutes after he starts his takeoff
roll, it has nothing to do with where or when he gets airborne. There
is no delay for you taking off behind a DC9. A controller may also use
radar separation instead of time.

Peter R. wrote:

A couple of days ago I flew into Logan Airport (Boston, MA, USA) in a C172
for an Angel Flight. Taxing to the departing runway, we were behind a B767
and a DC9 (in that order), with several other large airliners behind us.

I noticed that there seemed to be no wake turbulence delay for the DC9
behind the B767, as he was cleared for TO less than a minute after the B767
departed.

Tower then positioned me on the runway, and again, less than a minute later
(after awaiting a crossing runway landing), gave me a 90 degree right turn
after takeoff heading, cautioned wake turbulence, then cleared me to go.

My question has to do with the ATC's wake turbulence procedures. At the
class C airport where I am based, I constantly hear about the 3 minute rule
from ATC. In other words, if I am departing from an intersection mid-
field, tower will say that they are required to make me wait three minutes
for wake turbulence avoidance (unless I wave it, which I normally do not).

In the case of Boston's tower, did her "wake turbulence caution" and/or
right turn heading allow her to clear me sooner than the three minutes?

BTW, the DC9 ahead of me took at least three quarters of the runway to lift
off, then turned left. When I departed, I dropped a notch of flaps to
lift off very quickly, climbed a few hundred at Vx as per the obstacle DP,
then turned the 90 degrees right as per the instruction to be well away
from the previous two aircrafts' wake turbulence.



  #10  
Old December 18th 03, 09:10 PM
Newps
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Default

He flew into Logan airport. If you pull that I need three minutes crap
at an airport of that size then you shouldn't have landed there in the
first place. The controller would most likely taxi you back off the
runway and send you to the end of the line or some other runway. When
there are 42 jets lined up on the taxiway and your spamcan in the middle
if you can't play by the big boys rules then you need to get to a
smaller sandbox.

PS2727 wrote:

I think you will find that, as usual, it's up to you to get the spacing you
need. Simply tell the tower you need x minutes for wake turbulence. You cannot
be forced to takeoff on their schedule. They will accomodate you if you let
them know what your needs are.


 




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