From the year 1985-89 Mumbai Airport received 329 wind shear reports. It happens every year, yet there is no concerted action taken to prevent incidents at Mumbai airport, with the onset of monsoons and rains. The one action which is taken by the regulator is to suspend pilots. They may have a role to play but what about the authorities who have been ignoring data on threats associated with rain and thunderstorm peculiar to Mumbai?
The threat
A series of fatal commercial aviation accidents starting in the 1970s led to the identification of thunderstorm-related wind shear as a critical hazard to aircraft takeoffs and landings. In aggregate, these accidents resulted in over 400 fatalities and pressured the Federal Aviation Administration (FAA) to develop effective warning technologies. In response, the aviation community invested in and deployed wind-shear protection systems, ranging from pilot training for avoidance and recovery to sophisticated Doppler-radar detection algorithms for wind-shear warnings and detections.
Wind shear: an invisible enemy to pilots?
Weather plays a significant role in aviation safety and is regularly cited as a contributing factor in accidents or major incidents. Wind shear in the form of microbursts particularly, can be a severe hazard to aircraft during take-off, approach and landing.(Airbus)
Why Mumbai?
The monsoon season has the highest frequency of occurrence of low level wind shear (LLWS) mainly due to rain and strong gusty winds. Other than monsoon season the occurrence is related to sea and land breeze and nocturnal increase of surface temperature. The estimated time of occurrence is 0000 to 0600 IST & 1800 to 2400 IST(1985-89 data). The simultaneous occurrence of strong & severe LLWS, low ceiling, poor visibility has an adverse effect on aircraft operations.
What is LLWS?
The sudden change in wind speed and direction that the aircraft encountered is called wind shear. Wind shear can occur at many different levels of the atmosphere, however it is most dangerous at the low levels, as a sudden loss of airspeed and altitude can occur. Plenty of altitude is normally needed to recover from the stall produced by the abrupt change in wind speed and direction, which is why pilots need to be aware of the hazards and mitigation of low-level wind shear.
Data on wind shear
Enough data is available from two studies. Data range is from 1985-89 and 2005-2017.
| Ground to 60m | 60-150m | 150-300m | 300-600m | 600-900m | Final approach Runway 27 | |
| No. of LLWS reported by aircraft | 21 | 54 | 46 | 17 | 06 | 185 |
| Light | 0-4 kts per 30m |
| Moderate | 5-8 kts per 30m |
| Strong | 9-12 kts per 30m |
| Severe | above 12 kts per 30m |
| Season | Light | Moderate | Strong | Severe | Total |
| Winter Jan-Feb | 13 | 4 | 15 | 2 | 34 |
| Summer Mar-May | 27 | 7 | 15 | 10 | 59 |
| Monsoon Jun-Sep | 85 | 15 | 35 | 23 | 134 |
| Post-Monsoon Oct-Dec | 34 | 12 | 28 | 4 | 78 |
- Maximum number of low level wind shear (LLWS) cases have been reported over CSI Airport , Mumbai airport during SW monsoon months of June ,July ,August , September with maximum frequency observed in July with favourable time between 0000 to 0300 UTC and 1500 to 1800 UTC. Minimum frequency of LLWS was reported during May followed by December November October and January.
- In contrast to the general belief that wind shear is a short lived phenomenon, there are cases at which LLWS was active for more than four to five hrs over CSI Airport Mumbai .
Cost of an accident
The National Transportation Safety Board (NTSB) USA, is an independent agency responsible for the investigation of aviation accidents. The NTSB attempts to determine the cause of each accident logged, and detailed investigations are required for accidents involving serious or fatal injuries and/or major damage to aircraft. In the years 1975 to 2006, the NTSB attributed wind shear as the cause of 20 aircraft accidents, with a total of over 500 fatalities.
The estimated average wind-shear-related accident costs are based on FAA guidelines that assign values to people (deaths and injuries) and aircraft (damaged or destroyed).
Mitigation
A comparison of measured and mitigation-adjusted accident rates permits the “filling out” of the entire chart. The bars with hatching are the measured accident rates. Modeling the other protection conditions from the measured data provides estimates of accident rates for all possible mitigations in each time period.
The picture above show the location of sensors to be outside the airport perimeter. ICAO Annex six requires that the sensors be located 10m above the highest structure to prevent errors in measuring the wind. It is not certain if this requirement is met.
To predict wind shears over the airport, a combination of TDWR predictions and observations is necessary. As on today with non availability of Terminal Doppler Weather Radar (TDWR) or LLWS systems or profilers , whenever CB is observed towards E or NE side and wind speed starts rising more than 17 knots , taking guidance from DWR situated at Colaba , Mumbai which is nearly 15 to 20 kms away , WS warnings can be issued as a precautionary measure.
mindFly analysis
Data from 1985 strongly indicate that the threat to aircraft from wind shear is high at Mumbai airport. Yet, the authorities have not paid much attention to the reports. While huge sums are being invested in developing new terminal buildings at the airport, the air side infrastructure is highly unsafe.
The wind reporting is inaccurate and wind shear is not measured. Even though Doppler weather radar systems are available and indigenously manufactured, they are not being deployed at the airport.
Spate if incidents during monsoon period have led to runway closure, thereby affecting hundreds of flights, passengers and fuel burn. Safety is being blatantly ignored once again. The losses on account of closures could have easily financed better equipment.
ICAO Annex 3 requirement Wind shear warnings and alerts
Wind shear alerts are expected to complement wind shear warnings and together are intended to enhance situational awareness of wind shear.
7.4.1 Wind shear warnings shall be prepared by the aerodrome meteorological office designated by the meteorological authority concerned for aerodromes where wind shear is considered a factor, in accordance with local arrangements with the appropriate air traffic services unit and operators concerned. Wind shear warnings shall give concise information on the observed or expected existence of wind shear which could adversely affect aircraft on the approach path or take-off path or during circling approach between runway level and 500 m (1 600 ft) above that level and aircraft on the runway during the landing roll or take-off run. Where local topography has been shown to produce significant wind shears at heights in excess of 500 m (1 600 ft) above runway level, then 500 m (1 600 ft) shall not be considered restrictive.
Sources:
- Low level wind shear over Bombay airport Surendra Kumar, PV Patkar 1990
- Wind-Shear System Cost-Benefit AnalysisRobert G. Hallowell and John Y. N. Cho 2010
- Wind shears over the CSI Airport, Mumbai: Case study R B S NARAYANA 2017
- ICAO Annex 3
