One afternoon on August 2, 1985, Delta Air Lines Flight 191 (DL191) was up in the air, piloted by a veteran captain and a first officer. Flying smoothly without any problem, DL191 was about to land at Dallas/Fort Worth International Airport (DFW) in the US. This is when something strange happened.
The aircraft was suddenly accelerated 800 feet above the ground immediately before landing. The first officer fought on to lower the speed and, all of a sudden, the speed dropped rapidly and the aircraft began descending. Despite an attempt to ascend again, the aircraft lost center of lift and eventually crashed down to the ground. The DL 191 crash took the lives of 136 on board. What caused this tragic accident?
Wind shear, a sudden change in wind speed and/or direction
The DL 191 crash was caused by wind shear. Wind shear is turbulence with a sudden change in wind speed and/or direction over a short distance. It can occur in any direction—vertical and horizontal—and at any altitude. In other words, wind shear is a state of instability that can occur anytime, anywhere.
The cause of wind shear varies widely from thunderstorm and rain front to land and sea breeze. A common cause of wind shear in Korea are its topographic features. After hitting topographic features, strong wind merges together and generates a strong vortex. This vortex is wind shear. Jeju International Airport (CJU) and Narita International Airport (NRT) are notorious for wind shear because of Hallasan Mountain and Mount Fuji, respectively.
In fact, 389 flights were canceled throughout the day at Jeju International Airport due to wind shear, and a FedEx Express cargo aircraft crashed at Narita International Airport.
Why is wind shear dangerous?
Wind shear is a type of dangerous turbulence with unpredictable wind speed and/or direction. Aircrafts that encounter wind shear lose control. If flying at a high altitude, it can return to a normal state without great danger. The problem is wind shear near the ground.
Wind shear encountered during takeoff and landing in a range of 2,000 feet from the ground is the biggest threat to aircrafts. During the short span of time for takeoff and landing at this altitude, aircrafts fly at a slightly faster speed than the minimum required speed. Coming into wind shear in this state, aircrafts lose center of lift, which is the force that enables them to fly, and this causes an impact to strain the fuselage.
Wind shear is dangerous for aircrafts when taking off because it makes them difficult to lift. At landing, it causes a considerable amount of risk for aircrafts to crash, impacting objects on the ground and the runway. So, if encountering wind shear during flight, pilots decide on diversion or go-around to give up on landing and ascend again for safety.
Among various types of wind shear, the one that is considered the worst is microburst. This was actually the cause of the DL 191 crash.
Microburst is the most dangerous form of wind shear that can even pull a tree from its roots and causes significant damage to weak structures. If encountering microburst upfront, aircrafts rapidly rise up in altitude at first due to the headwind. However, as the headwind suddenly changes to tailwind, aircrafts lose center of lift and falls into an uncontrollable state. Eventually, aircrafts nosedive and crash.
So, how are we to respond to this frightening wind shear?
How to handle wind shear!
Since the DL 191 crash, the US National Transportation Safety Board (NTSB) has taken strict measures for airlines to provide education about turbulence and recommended airports to install low level wind shear alert system (LLWAS). A long time has passed since the accident and, now, major US airports are detecting the occurrence of microburst and other dangerous wind shears using high-resolution Doppler radar system.
In Korea, eight LLWAS units are installed at Incheon International Airport (ICN) and Yangyang International Airport (YNY) each. Moreover, 11 LLWAS units are installed at Jeju International Airport, where wind shear occurs frequently. The Korean Aviation Meteorological Office (AMO) issues wind shear alerts and announces information about microburst using measuring equipment, such as LLWAS. In addition to the information provided through the system, pilots, when encountering wind shear, must hold off landing and wait for change, and contact air traffic control and meteorological office at the airport so as to help in safe aircraft operation by other pilots.
Today, we discussed pilots’ dreaded wind shear and the risks it can cause. Natural phenomenon cannot be stopped. So, it is most important to effectively handle and respond to wind shear so as to prevent accidents. Fortunately, experts at the airports and pilots are working together to ensure safety using advanced technologies so that we can enjoy peace of mind.