AOPA Pilot Magazine - February 2017 - 86

P&E

WX WATCH

Every fatal accident report carries
its own burden of tragedy and what-ifs,
as well as opportunities for often stern,
arm's-length judgment. The airplane was
not certified for flight into known icing.
There was no record of the pilot's receiving a preflight weather briefing or looking
at weather information before the flight.
He had only flown 23 hours in the previous year, with one instrument approach
into Fresno. In the end, the NTSB's ruling
on the accident's probable cause was "An
in-flight encounter with icing conditions
during descent, which resulted in the airplane exceeding its critical angle of attack
and experiencing an aerodynamic stall followed by an in-flight breakup."
Accidents like these can serve as valuable instruction in the dynamics of adverse
weather. The Bonanza crash's accident
report is somewhat unusual in that it
delves into specific meteorological aspects
affecting the flight. Upper air soundings
are mentioned, along with satellite and
Doppler radar observations. The National
Climatic Data Center (now the National
Center for Environmental Information)
stated that the accident took place "in a
very favorable location for vertical motion,
clouds, and precipitation given the low-,
mid-, and upper-level environment."
So what would a location like this look
like on weather graphics you can check
during your preflight weather information searches? As an example, let's look at
a few of the graphics from this accident's
NTSB report.
First, it helps to remember that surface
low-pressure systems and their fronts are
created from the top down. Looking at the
500-millibar constant pressure chart on
page 90, you can see there is a pronounced
trough aloft over California, complete with
its own, closed low-pressure center-right
over the accident site. The contours on the
chart show where the 500-millibar pressure levels (about 18,000 feet msl) are at
the same height, which is why they're
called height contours. The converging winds circulating around that closed
low impart weak lifting to the air below
it, and so do the cold, minus 25 degree
Celsius temperatures at altitude. (Surface
temperatures in the area were in the low
50-degree Fahrenheit range.) By the way,
86 | AOPA PILOT February 2017

this upper-level trough persisted from the
850-millibar (5,000-foot) all the way to
the 300-millibar (30,000-foot) level. And
although the trough and closed low was
deep, winds around them were light. With
a situation like this, the low pressure system and its clouds and precipitation won't
be moving away any time soon.
What about conditions at the surface?
The surface analysis chart on page 91 (top)
for the time near the crash shows a low
over the Las Vegas area, with an occluded
front running to the northwest, a cold
front extending to the south, and a dying
stationary front curving to the east. Like
the winds aloft, winds at the surface were
weak. At the nearby Porterville (California)
Municipal Airport, winds were calm, visibility was 10 miles, and the sky was 3,800
scattered, 5,500 overcast. Temperature and
dew point were 9 and 6 degrees Celsius,
respectively.
A look at the Skew-T/log P (sounding)
chart on page 91 (bottom) near the time of
the crash shows several items of interest
in the atmosphere's vertical profile. The
upward slanting line on the right is temperature; the one on the left is dew point.
Based on the closer temperature/dew point
spreads, dense cloud was analyzed from
the surface to 10,000 feet. Investigators
looked at radar imagery and analyzed likely
icing conditions from 5,000 to 7,000 feet.
Bottom line? This is a deep, slow-moving low-pressure system bringing moist
oceanic air inland, and lifting it into progressively colder-than-standard air aloft.
It's certainly not a convective system, but
one that's very conducive to IMC and lowaltitude icing. West Coast pilots: If you see
a winter pattern like this, you might want
to delay your flight. Come to think of it, no
matter where you're flying, any closed lows
showing up at 500 millibars or higher can
be trouble.
AOPA
EMAIL tom.horne@aopa.org

WEB

Constant pressure charts: http://weather.
rap.ucar.edu/upper/
Soundings: www.twisterdata.com (then
click anywhere on the U.S. map)
Surface analysis charts: http://aviationweather.gov/progchart/sfc
http://weather.rap.ucar.edu/upper/ http://www.twisterdata.com http://aviationweather.gov/progchart/sfc http://aviationweather.gov/progchart/sfc http://www.yinglingaviation.com
Table of Contents for the Digital Edition of AOPA Pilot Magazine - February 2017
