Microbursts are columns of air downdrafts that are localized and create a mixture of straight line and divergent winds near the surface. They may last for a few seconds but they have immense power and can bring down an aircraft and make it crash. In the microburst, air dashes to the earth from the top in a downward directed funnel and then spreads outwards. In a tornado the opposite happens and air from the bottom rises to the top in an upward directed funnel (Fujita, 1985). This paper would discuss the effects of a microburst on an airborne aircraft.
The paper would then examine the incident when a microburst hit the Delta Air Lines Flight 191 killing 8 crew and 126 passengers on August 2, 1985. 2. EFFECT OF MICROBURST ON AIRCRAFT PERFORMANCE A brief discussion of the types of microburst’s and their effects on airborne aircraft are given in this section. 2. 1. Types of Microburst’s It is suggested that microburst’s would affect a circle with a diameter of 4 kilometers while macro bursts would affect areas greater than 4 kilometers. There are two types of, wet and dry microburst’s and in both, wind speeds can be more than 270 kilometers per hour (Fujita, 1985).
An illustration of a microburst is as shown in the following figure. Dry Microburst’s are caused when there is a rainfall that mixes with dry air. The dry air causes the rain drops to vaporize and thus brings the air temperature down. The heaver cold air starts moving down like a funnel, very fast with increasing acceleration as it nears the ground. When it hits the ground the air fans out along the center. Such microbursts are triggered by high thunderstorms and there is very little rain. Following figure illustrates the formation of a dry microburst. Figure 2. 1.
Microburst formation (Psiaki, 1991) Wet Microburst’s type is formed along with a lot of precipitation near the surface of the earth that may be at higher temperatures. In some cases hail stones and melting ice help in the formation of the downburst. At the lower levels, the atmosphere is more humid and warm while in the upper levels it is dry. Therefore, when thunderstorms are formed it results in heavy rain. However, the dry air at the top makes some rain to evaporate. This causes the air at the top to cool down suddenly and makes it heavy, causing it to rush down in a funnel at great speed.
There is usually heavy rain and high winds (Fujita, 1985). 2. 2. How microburst’s cause aircrafts to crash According to Gavaghanm (15 August 1985), between 1964 and 1982, 19 aircraft crashes and accidents were due to wind shear that are a result of microburst’s. When a microburst occurs, the plane would initially face a head wind that would give an up thrust and lift the airplane over the correct altitude. The pilot would then correct and move the aircraft down by decreasing the engine thrust. In just 10 seconds, the airplane would then be struck by a tail wind which forces the airplane below the flight path.
The pilot would then increase the thrust to take the plane up but the engines require a few seconds to respond. When the required thrust does not happen, the aircraft starts plunging down. At the same time, the wind shear would try to force the aircraft to flip and the pilot would again increase thrust. In this type of a fight, the microburst usually wins and the aircraft quickly drops. Following figure shows how the aircraft is forced off the flight path. Figure 2. 2. Illustration of wind shear in a microburst (Gavaghanm, 15 August 1985)
The problem with wind shear is that while landing or taking off when the aircraft does not have sufficient altitude, it does not have the required height for the aircraft to be maneuvered. The pilot lacks the ceiling to compensate for the change in wind speed that the manner in which the aircraft is moved off the flight path. A microburst would be alive for about 15 minutes and have a speed of 270 kilometers per hour and it would be about 2. 5 kilometers diameter. If a jet aircraft is flying through this wind shear it will be brought down by 230 meters in forty seconds.
Given below is an illustration of an aircraft that is attempting to take off during a microburst (Houze, 1994). Figure 2. 3. Illustration of an airplane taking off during a cloudburst (Zhenxing, 2009) As seen in the aircraft, after it lifts off, it faces a strong headwind at the position 1. As the aircraft rises, the headwind reduces and the plane enters the zone of downdraft as shown in 2. There is more headwind loss at position 3 and at 4, there is an increasing tailwind. The tail wind forces the aircraft down and after 5, the aircraft would crash since the fast descent rate cannot be stopped (Zhenxing, 2009).
2. 3. How technology can help As per the current norms of various governments, all aircrafts need to have instruments that quickly measure fall in temperature and pressure and thus they are able to detect a microburst. A pilot can thus be prepared for such an eventuality. If the problem occurs at higher altitudes, then the pilot can maneuver away to a safe zone. If the problem occurs during landing, the pilot is warned in advance and he can avoid landing (Zhenxing, 2009).
3. CASE STUDY OF DELTA AIR LINES FLIGHT According to NTSB (August 1985), the Delta Air Lines Flight 191 crashed on 2 August 1985 when it was on a routine flight from Fort Lauderdale to LA. A total of 135 people died in the crash. Among the reasons stated are pilot error combined with wind shear and Microburst. The aircraft flown was a Lockheed L-1011-385-1 TriStar, a very reliable and long flying aircraft. When the airplane flew come to Louisiana, there was a thunderstorm and when it began descent. The pilots noticed the thunderstorm but decided to ignore it, unaware that it was a deadly microburst.
The plane was to land at 276 kmph but suddenly the speed increased to 320 kmph. Suddenly the speed fell to 220 kmph. There was a fierce tailwind and a shear wind and started to plummet at 5000 feet per minute and it was just 250 meters above the ground. Though the pilot tried to climb, the wind shear and the microburst caused the aircraft to crash (NTSB, August 1985). 4. CONCLUSIONS The paper has examined the phenomenon of microburst that are localized downwards air movements at very high speeds. The paper also discussed wet and dry types of microbursts and also examined how aircrafts are affected by the microbursts.
It was seen that during take off and landing when the altitude is low, the microburst applies a wind shear that forces the aircraft down and causes it to crash. An example of a flight that crashed due to microburst was briefly examined. REFERENCES Fujita, T. T. , (1985). The Downburst, microburst and macroburst. SMRP Research Paper 210, University of Chicago Gavaghanm H. , (15 August 1985). The wind shear factor. New Scientist, p. 25-26 Houze, R. A. , (1994). Cloud Dynamics. NY: Academic Press NTSB, (August 1985).
Aircraft Accident Report: Delta Air lines, Lockheed L-1011-385-1, N726DA, Dallas/Fort Worth International Airport, Texas. Retrieved from http://www. ntsb. gov/publictn/1986/AAR8605. htm Psiaki, M. , (1991). Optimal aircraft performance during microburst encounter: NASA Technical Reports Server. Retrieved from http://ntrs. nasa. gov/search. jsp? Ns=HarvestDate%7c1&N=277&Ntk=Keywords&Ntx=mode%20matchall&Ntt=Microburst Zhenxing, G. , (October 2009). Real-time Simulation of Large Aircraft Flying Through Microburst Wind Field, Chinese Journal of Aeronautics, Volume 22, Issue 5, PP. 459-466