Wake turbulence avoidance and ATC

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

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

David Rind wrote:

You are clearly right about this

Thank you for the gracious reply.

-- 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....

In this particular case (taking off right after a jet), at Vx you will reach
turning altitude in less distance and more time, both of which work in your
favour:

- less distance means that you are at a safe turning altitude further away
from the point where your climb path would intersect the jet's climb path

- more time means that the the jet's wake vortices have had more opportunity
to dissipate by the time you turn.

Or, to put it the other way, if you climb at Vy you will arrive closer to
the jet's climb path, sooner, before you reach a point when you can turn.

On the other hand, if there were a jet waiting to take off behind me and I
wanted to get out of the way as soon as possible (i.e. ATC says "right turn
to heading XXX as soon as safely able"), then Vy is the better choice, since
I want to get to turning altitude in the least time.


All the best,


David

David Rind wrote in message ...
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 [...]

I could be wrong, too, but I think even a loaded DC-9 can manage at
least 1000-1500fpm... and might have to do so for noise abatement
around the airport.

I've heard that's why cloud clearances down around us VFR planes are
1000' above and 500' below. The extra space above is needed because
an airliner is far more likely to be climbing out at high fpm... but
descends at a slower rate for passenger comfort and ILS landings.

Kev

Kevin Darling wrote:

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 [...]

I could be wrong, too, but I think even a loaded DC-9 can manage at
least 1000-1500fpm... and might have to do so for noise abatement
around the airport.

You need to know the forward speed as well. At 120 kt, 1500 fpm would be
750 ft/nm; at 180 kt, it would be only 500 ft/nm (but I think that the DC-9
can do better than that). In either case, the climb angle is not that much
greater than that of a light single.


All the best,


David

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.

Newps wrote:

Don't confuse taking off at an intersection with taking off at the end.
The three minutes applies to an intersection takeoff only.

OK, very good. That answers my question.

--
Peter










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On Thu, 18 Dec 2003 11:01:37 -0500, 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.

On the times I've been to BOS, it seems routine for tower to vector small
a/c on a different heading than the large a/c. When I've been in that
position, and visualizing the wake, I've never had a question that I would
be able to avoid the preceding a/c's wake by making my turn out as
directed.

If I had, I would have requested to delay my departure.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Peter R. wrote in message ...
snip
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?

It's been a few years since I was based at a Class B airport, but I
seem to recall that the 3 minute rule applied only if you were making
an intersection takeoff behind a departing big boy.

If you're departing from the end of the runway, ATC doesn't have to
wait 3 minutes. At that point, it's up to you to decide when to go.

A small single taking off right behind a big jet was routine at PHX.
The procedure was the same as you described. Climb hard and turn 90
degrees ASAP. The one thing I would caution you about would be the
jet-wash from the departing jet. Although the wake turbulence from
the wings doesn't start until the jet lifts off, those jet engines can
really churn up the air as it rolls down the runway. For this reason
I usually stuck to a Vy climb to give me a better margin over the
stall speed should I encounter some really churned up air. The
turbulence from the airliners was worse from those with high, fuselage
mounted engines (i.e. DC-9, 727). It was also worse when the wind was
dead calm.

John Galban=====N4BQ (PA28-180)

John Galban ) wrote:

The one thing I would caution you about would be the
jet-wash from the departing jet. Although the wake turbulence from
the wings doesn't start until the jet lifts off, those jet engines can
really churn up the air as it rolls down the runway. For this reason
I usually stuck to a Vy climb to give me a better margin over the
stall speed should I encounter some really churned up air.

Good to know. Thanks.

--
Peter












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