Wake turbulence avoidance and ATC

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

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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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

David Rind wrote:

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.

It was VMC but the tall control tower was close to my turnout heading
and it was night time. We were departing 22 Right with a turnout to the
west.



--
Peter










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Peter R. wrote in message ...
It was VMC but the tall control tower was close to my turnout heading
and it was night time. We were departing 22 Right with a turnout to the
west.

When you get a legitimate opportunity to buzz the tower, take it!

In article ,
Peter R. wrote:

David Rind wrote:

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.

It was VMC but the tall control tower was close to my turnout heading
and it was night time. We were departing 22 Right with a turnout to the
west.

It would be in character with Logan for them to assume that you had
waved the wake caution unless you objected.

Noting the 9's rotation point and positioning yourself above the wake
until the turn is actually much better than delaying the 3 minutes.

Any crosswind at all would clear your takeoff run of his jet blast.

As we used to say, "You did good."

--
Ron

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