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Transcript

A difference in wind speed and direction over a relatively

short distance in the atmosphere

Accident Highlight

Delta Air Lines Flight 191 was a regularly scheduled service from Fort Lauderdale-Hollywood International Airport, Florida to Los Angeles International Airport, California, by way of Dallas-Fort Worth International Airport. On the afternoon of August 2, 1985, the plane crashed when it hit a microburst while landing at DFW, killing 8 of the 11 crew members and 126 of the 152 passengers on board, and one person on the ground. Two people later died 30 days or more after the crash, bringing the total fatalities to 137.

http://www.weather.com/video/multiple-crashes-in-a-microburst-10008

Causes of Wind Shear:

  • Microbursts
  • Fronts and shallow lows
  • Jet streams

Microbursts:

•Description: A microburst is a very localized column of sinking air caused by a small and intense downdraft within a thunderstorm. There are two types of microbursts: wet microbursts and dry microbursts. They go through three stages in their life cycle: the downburst, outburst, and cushion stages.

•Dangers of it: The scale and suddenness of a microburst makes it a great danger to aircraft due to the low-level wind shear caused by its gust front, with several fatal crashes having been attributed to the phenomenon over the past several decades.

•How to minimize or avoid:

Pilot Training -

• Incorporated into the training syllabus were means to recognize the characteristics of microbursts

• Training in simulators to deal with microbursts in the proper manner

• Pilots should also be trained to avoid microbursts if possible and divert/ carry out missed approach if necessary

Also, the majority of all major airports have LLWAS (low level wind alert system).

Fronts and Shallow Lows:

Description of what it is: A front is the defined zone between two air masses. There are four different types of fronts cold, warm, stationary front, and occluded

•Dangers of it: Thunder storms and low level wind shear are coming present in these phenomenon's. For warm fronts the most critical time for LLWS is before the front passes.

•How to minimize or avoid: It is the pilot’s responsibility to obtain a pre-flight weather briefing. Any ATC reported weather information, along with periodic contacts with Flight Watch while airborne, will supplement what was learned during the pre-flight briefing.

Airmass Wind Shear:

Description of what it is: Air mass wind shear - This type of wind shear occurs at night under fair weather conditions in the absence of strong fronts and/or strong surface pressure gradients. We know with radiational cooling, the ground will cool faster than the overlying air mass. If the cooling is strong enough, a nocturnal inversion results.

•Dangers of it: low level wind shear

•How to minimize or avoid: It is the pilot’s responsibility to obtain a pre-flight weather briefing. Any ATC reported weather information, along with periodic contacts with Flight Watch

while airborne, will supplement what was learned during the pre-flight briefing.

Jet Streams

Description of what it is: Jet streams are fast flowing, narrow air currents

•Dangers of it: Associated with jet streams is a phenomenon known as clear-air

turbulence (CAT), caused by vertical and horizontal wind shear connected to the jet

streams.The CAT is strongest on the cold air side of the jet stream, next to and just underneath

the axis of the jet stream. Clear-air turbulence can cause aircraft to plunge and so present a

passenger safety hazard that has caused fatal accidents.

•How to minimize or avoid: It is the pilot’s responsibility to obtain a pre-flight weather

briefing. Any ATC reported weather information, along with periodic contacts with Flight Watch

while airborne, will supplement what was learned during the pre-flight briefing.

Jet Stream Fatality: On December 28, 1997, United Airlines Flight 826 was a Boeing

747-100 flying from Narita International Airport, Japan to Honolulu International Airport in

Hawaii. Two hours into the flight, at 9400 meters (31,000 ft), the plane received reports of

severe clear-air turbulence in the area and the seat belt sign was turned on. Moments later, the

aircraft suddenly dropped around 30 meters (100 ft), seriously injuring 15 passengers and 3 crew members, although there was no structural damage to the aircraft. The plane turned

around and landed safely back in Tokyo, but it was not put back in service since it had been

scheduled to be retired in early 1998. One passenger died of her injuries after landing in Tokyo.

Informational Websites

Introduction to Wind Shear

  • Terminal forecast
  • Hourly sequence reports
  • Severe weather watch reports
  • LLWAS reports
  • SIGMET's and convective SIGMET's
  • PIREPS
  • METARS, TAF

Description of what it is: Jet streams are fast flowing, narrow air currents

•Dangers of it: Associated with jet streams is a phenomenon known as clear-air

turbulence (CAT), caused by vertical and horizontal wind shear connected to the jet

streams.The CAT is strongest on the cold air side of the jet stream, next to and just underneath

the axis of the jet stream. Clear-air turbulence can cause aircraft to plunge and so present a

passenger safety hazard that has caused fatal accidents.

•How to minimize or avoid: It is the pilot’s responsibility to obtain a pre-flight weather

briefing. Any ATC reported weather information, along with periodic contacts with Flight Watch

while airborne, will supplement what was learned during the pre-flight briefing.

Jet Stream Fatality: On December 28, 1997, United Airlines Flight 826 was a Boeing

747-100 flying from Narita International Airport, Japan to Honolulu International Airport in

Hawaii. Two hours into the flight, at 9400 meters (31,000 ft), the plane received reports of

severe clear-air turbulence in the area and the seat belt sign was turned on. Moments later, the

aircraft suddenly dropped around 30 meters (100 ft), seriously injuring 15 passengers and 3 crew members, although there was no structural damage to the aircraft. The plane turned

around and landed safely back in Tokyo, but it was not put back in service since it had been

scheduled to be retired in early 1998. One passenger died of her injuries after landing in Tokyo.

Definition of wind shear:

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