Very localized low

[Doug Carter]

I think I flew through a very localized low pressure area,
perhaps six or eight miles in diameter and two tenths of an inch
of mercury deep.

Last Thursday flying South past Kansas City, about 1615Z, IMC,
6000 feet, Kansas City baro 30.14 on autopilot (KFC200), altitude
hold. I was vectoring East of a very active cell with dots on
the WX10 as close as the 15 mile ring; smooth air, light rain.

About this time, things got strange. I looked back at the flight
director and observed about five degrees down pitch, the
altimeter remained steady on 6000, the VSI on zero. Vacuum OK,
kicked on the standby pump for good measure... pitot heat on
(despite being 14C), alt air. All no effect. About this time
observe airspeed climbing from 145 to 160+ which on this airplane
(C82R) strongly suggests about a 700fps decent.

Altimeter still locked on 6000, VSI on zero, encoding altimeter
indication on transponder and TCAD all say 6000.

Problem is, my ears agree with the climbing airspeed indicator.

Kick off the altitude hold and bring up the nose five degrees to
bleed off airspeed; immediately indicate a climb on the VSI and
altimeters go up to about 6150. I'm about to tell ATC that I'm
going to be bouncing around for a bit when I notice the climb
stopped.

After about another minute every thing stabalized back to
normal. Back to 6000, airspeed normal. In a few minutes I note
the Chunate altimeter is 30.10. The entire episode took two or
three minutes.

All of this happened on the very downwind edge of an active cell.
Since the autopilot altitude hold is driven by the static system
I presume that it was just following an unusually steep pressure
gradiant and my true altitude dropped by about 200 feet for about
six or eight miles then returned to "normal."

Any other suggestions?

[[email protected]]

Doug Carter wrote:
> About this time, things got strange. I looked back at the flight
> director and observed about five degrees down pitch, the
> altimeter remained steady on 6000, the VSI on zero. Vacuum OK,
> kicked on the standby pump for good measure... pitot heat on
> (despite being 14C), alt air. All no effect. About this time
> observe airspeed climbing from 145 to 160+ which on this airplane
> (C82R) strongly suggests about a 700fps decent.
>
> Altimeter still locked on 6000, VSI on zero, encoding altimeter
> indication on transponder and TCAD all say 6000.
>
> Problem is, my ears agree with the climbing airspeed indicator.

It's very peculiar that your altimeter didn't agree with your ears,
since fundamentally they operate on the same principles! How could only
one of them detect the descent? My only guess is that instead the speed
increase also increased the ram pressure of air going into the cabin,
and thus the air pressure in the cabin went up, giving the sensation of
a descent.

So what about that descent? It's just a guess, but what about the
possibility of a strong updraft? In other words, the descent never
happened, and your autopilot pitched the plane down to compensate for
rising air. Airspeed increased accordingly.

My glider instructor flies 737s for a day job and got caught in
mountain wave over the Sierras a while ago. He described how in order
for the autopilot to maintain constant altitude and speed---well, he
watched the the throttle quadrant slowly move aaalllll the way back
when they caught the rising half of the wave, then when the wave
started to push downward they smoothly came all the way back. (Said the
ride was smooth, too!)

If you were caught in an updraft and you kicked off the autopilot, that
would explain why you rose upward next---and then when you flew out of
the updraft, why the climb stopped.

--Tom

[[email protected]]

Here we go, "Strong wave over Sierra Nevadas":

http://vallelywebdesign.com/cgi-bin/silent/silent-news.cgi?action=showpost&idnumber=1112217403

[Doug Carter]

In article om>,
wrote:

> It's very peculiar that your altimeter didn't agree with your ears,

Sorry, I didn't feel a pressure difference, I heard the wind speed increase.

> If you were caught in an updraft and you kicked off the autopilot, that
> would explain why you rose upward next---and then when you flew out of
> the updraft, why the climb stopped.

If I was in an updraft the altimeter would have increased.

[Paul Tomblin]

In a previous article, said:
>All of this happened on the very downwind edge of an active cell.
>Since the autopilot altitude hold is driven by the static system
>I presume that it was just following an unusually steep pressure
>gradiant and my true altitude dropped by about 200 feet for about
>six or eight miles then returned to "normal."
>
>Any other suggestions?

Mountain wave? Either from actual geography, or from air flowing over
other weather?


--
Paul Tomblin > http://xcski.com/blogs/pt/
They thought that faxing one's butt was bad - just wait till they hear
about blurry, pixilated, mpeg artifacted live porn by phone!
-- Geoff Lane contemplates the 3G future

[Ron Garret]

In article

et>,
Doug Carter > wrote:

> In article om>,
> wrote:
>
> > It's very peculiar that your altimeter didn't agree with your ears,
>
> Sorry, I didn't feel a pressure difference, I heard the wind speed increase.
>
> > If you were caught in an updraft and you kicked off the autopilot, that
> > would explain why you rose upward next---and then when you flew out of
> > the updraft, why the climb stopped.
>
> If I was in an updraft the altimeter would have increased.

But you said it did -- as soon as you turned off the autopilot.

My money is on an updraft too.

rg

[Doug Carter]

In article >, Ron
Garret wrote:

> My money is on an updraft too.

OK, I can buy that, I should have made a power reduction to mitigate the
airspeed rather than bringing the nose up. Too much time flying
aerobatics rather than IFR perhaps :)

[Guy Elden Jr]

I've had about the same thing happen to me flying a C-172SP over
western Vigrinia at 10000, the foothills east of the Appalachians.
Autopilot was on, level, smooth as can be, and within a minute or two I
notice the RPM indicator dropping by about 150RPM, and airspeed going
from about 110 IAS to near 80. Altimeter and VSI both reported level,
and I couldn't detect much of a climb on the attitude indicator either.
Throttle was full in as well. I chalked it up to a downdraft.

--
Guy

[Brad Zeigler]

Simple. It was an updraft. The autopilot was adjusting pitch to maintain
altitude. The nose-down pitch caused an increase in airspeed. Did the
opposite happen some time later? If so, you caught a downdraft as well.

"Doug Carter" > wrote in message
...
>I think I flew through a very localized low pressure area,
> perhaps six or eight miles in diameter and two tenths of an inch
> of mercury deep.
>
> Last Thursday flying South past Kansas City, about 1615Z, IMC,
> 6000 feet, Kansas City baro 30.14 on autopilot (KFC200), altitude
> hold. I was vectoring East of a very active cell with dots on
> the WX10 as close as the 15 mile ring; smooth air, light rain.
>
> About this time, things got strange. I looked back at the flight
> director and observed about five degrees down pitch, the
> altimeter remained steady on 6000, the VSI on zero. Vacuum OK,
> kicked on the standby pump for good measure... pitot heat on
> (despite being 14C), alt air. All no effect. About this time
> observe airspeed climbing from 145 to 160+ which on this airplane
> (C82R) strongly suggests about a 700fps decent.
>
> Altimeter still locked on 6000, VSI on zero, encoding altimeter
> indication on transponder and TCAD all say 6000.
>
> Problem is, my ears agree with the climbing airspeed indicator.
>
> Kick off the altitude hold and bring up the nose five degrees to
> bleed off airspeed; immediately indicate a climb on the VSI and
> altimeters go up to about 6150. I'm about to tell ATC that I'm
> going to be bouncing around for a bit when I notice the climb
> stopped.
>
> After about another minute every thing stabalized back to
> normal. Back to 6000, airspeed normal. In a few minutes I note
> the Chunate altimeter is 30.10. The entire episode took two or
> three minutes.
>
> All of this happened on the very downwind edge of an active cell.
> Since the autopilot altitude hold is driven by the static system
> I presume that it was just following an unusually steep pressure
> gradiant and my true altitude dropped by about 200 feet for about
> six or eight miles then returned to "normal."
>
> Any other suggestions?
>