Meteorological aircraft icing in two case studies, (Tehran-Mehrabad Airport)

Ice can form on aircraft surface at 0° C or colder temperatures when liquid water is present. This can happen both on the ground and at the flight levels of the aircraft. Aircraft icing has not only an impact on flight safety but has also an economic impact. According to the report of research department of the FAA (Federal Aviation Administration)‎, from 1982 to 2001, 13% of accidents due to aircraft icing resulted in loss of human life, about 30 cases in 20 years. This research refers to case studies in Mehrabad airport during winter in order to determine the threshold values of some important indices in aircraft icing forecast. In the first case study, aircraft engine failed and emergency landing occurred due to aircraft icing. Second case study is considered because of domination of a very cold outbreak in northern parts of Iran and all Mehrabad flights were cancelled. In addition to study the synoptic and thermodynamic weather patterns, the relevant MM5 outputs including vertical velocity, liquid water content (LWC)‎ and relative humidity are surveyed and their thresholds are compared with other studies in this area including Ellrod and Bailey in 2007 and Henderson in 2006. Synoptic conditions are analyzed using GFS data with 1° resolutions in latitude and longitude directions (http://nomads.ncdc.noaa.gov/data.php)‎. The initial and boundary conditions come from the operational 12Z runs of global forecasting system (GFS)‎ of NCEP (National Center for Environmental Prediction)‎ for MM5 (Meso scale Model version 5) model(Dudhia 1993; Grell et al. 1994) to survey the NWP model outputs. Satellite images as the other reference in this study, are taken from METEOSAT-7 which provides one image every 30minutes over Iran. Thermodynamic charts are drawn by RAOB software using Mehrabad station upper air sounding data to determine the threshold values of instability indices such as LI, KI, SI, and SWEAT. The most important results of this research show that large scale patterns are not a suitable factor to determine the type and severity of icing. Relative humidity and vertical velocity as the outputs of the MM5 regional numerical weather prediction model does have a rather good consistence with the observations and their thresholds are quite similar with other thresholds gained from the other parts of the world and the small differences can be due to topographical and boundary conditions in the regional model. The liquid water content values do not match the available thresholds and require more research. Thermodynamic charts show temperature and dew point deficits values can show the aircraft icing conditions but the thermodynamic indices such as LI, KI, SI, and SWEAT are not suitable and did not satisfy icing conditions. It can be said that due to shallow stratiform clouds which are reported in Mehrabad station observations provides threshold values so that the dew point deficit is less than 3°C, temperature between zero and -10°C, vertical velocity -0.2 hPa/s, relative humidity more than 75% in 700 hPa level are good diagnosis in the first guess in aircraft icing and they make easy icing forecast for the Mehrabad airport area.