System To Be Tested At DIA This Winter
Halloween weather has
tricked, not treated, airport meteorologists the past two years in
Denver. Heavy freezing drizzle -- appearing to be harmless light
drizzle -- has cost airlines as much as $2 million in engine damage
in a single storm as jets have waited for takeoff. Now, Roy
Rasmussen of the National Center for Atmospheric Research (NCAR)
has developed a new system to identify the drizzle accurately.
His research has enabled airlines to revise pilot training and
on-ground procedures to avoid future damage. The new detection
system will be installed this winter at Denver International
Airport (DIA).
Rasmussen studied two cases of heavy freezing drizzle at DIA on
October 31, 2002, and the same date in 2003. The two storms wreaked
a total of $2.85 million in damage to 18 jet engines on United
Airlines 737 aircraft. Trained meteorologists were on site
throughout both events, but the freezing drizzle conditions were
not accurately noted. In about half of all cases of freezing
drizzle, the intensity is under reported, according to
Rasmussen.
"Freezing drizzle is hard to see and its intensity is hard to
estimate visually," says Rasmussen. "Often it goes undetected
because the droplets are so small." The typical droplet diameter is
about half of a millimeter, or half the thickness of a compact
disk.
Rasmussen has worked with United Airlines to alert pilots of 737
aircraft on the hazard, and the airline has changed its procedures
as a result of his research. Formerly, if an airport meteorologist
observed heavy freezing drizzle, engines were revved close to
flying speed (called an engine run-up) every 30 minutes to throw
off ice.
"Now, if anyone says 'freezing drizzle,' they do engine run-ups
every ten minutes," says Rasmussen. "Airline people are sensitized
to the possibility that freezing drizzle can cause engine
damage."
The real-time
freezing-drizzle detection system developed by Rasmussen and
colleagues will be part of Weather Support for Decision Making
(WSDM), a system now at DIA that offers minute-by-minute weather
reports tailored to aviation users. WSDM data are displayed in a
color-coded, user-friendly format that can be easily read by pilots
and other non-meteorologists. WSDM also provides data on snow and
unfrozen rain.
Freezing rain falls right past an idling jet engine, Rasmussen
explains, but freezing drizzle falls at a much slower rate, so it
gets sucked into the engine. The droplets freeze on contact, and
the resulting ice builds up on the engine's hub, or spinner. When
the engine is revved up to takeoff speed, ice shards are thrown off
the spinner into the rest of the engine.
In the two Denver storms, the major damage was to the delicate
tips of the fan blades. These blades generate the lift that
produces thrust, Rasmussen says. If they're damaged, the plane
loses thrust, because the blades are not at the correct angle to
produce the maximum thrust.
"It's not particularly dangerous," says Rasmussen, "but they
have to repair the damage, and that's very costly."