Professional Pilot, October 2016

Flying tropical weather However the warm air and gentle breezes that dominate much of the tropics do not mean that pilots will experience benign weather throughout the region Although phenomena such as strong windshear and severe turbulence are relatively uncommon in the tropics outside of the area around tropical cyclones and major mountain ranges other adverse aviation weather conditions are frequently encountered including thunderstorms and icing Much of this article has been about water in the tropical atmosphere from clouds to rainfall to thunderstorms of various sorts Because of the endless supply of water hydrometeorological conditions present some of the most serious threats to tropical flying For example many places in the tropics are dominated by IMC days Thick cumulus decks form in these regions on a daily basis Due to the high humidity the bases of the clouds may be well below 1000 ft AGL and tops may extend well past 10000 ft Additionally these decks often also produce widespread rain showers that can greatly reduce visibility beneath the cloud deck and reduce braking action on wet runways or enhance hydroplaning danger upon landing In complex terrain such cloud decks and precipitation can easily obscure ridges mountaintops or other obstacles making low level operation such as arrivals and departures dicey propositions Similarly given the high humidity frequent and often dense fog can be an issue especially when rainfall serves to saturate the surface layer Fog is most likely to form inland or 38 PROFESSIONAL PILOT October 2016 in mountain locations in the vicinity of the ITCZ and near the Equator Fog may also form any time the winds drop below about 5 kts but are not completely calm Unfortunately unlike drier regions mixing to higher levels may not easily dissipate the fog as higher altitudes may be near saturation as well Tropical fogs normally dissipate when the winds freshen Thunderstorm icing As the pilots in the opening story discovered thunderstorm flying is one of the more dangerous aspects of tropical weather flying Although conditions at the surface may be quite warm like other parts of the world the atmosphere still cools as we ascend If we consider that the surface air temperature is 34 C 93 F and the normal environmental lapse rate is 198 C 356 F per 1000 ft that means the atmosphere cools by 81 C 146 F by the time we reach FL410 Our pilots were flying through approximately 47 C 53 F air Using these numbers even in the tropics the freezing level would be at approximately 18000 ft MSL Inside the cumulonimbus the release of latent heat by condensation of water vapor would have kept things a bit warmer about 29 C Since liquid water droplets can and do exist inside clouds to about 40 C 40 F our pilots would have been flying through a region where icing was a distinct possibility In fact a number of incidents have been reported where pilots failed to activate pitot heat in such situations and their pitot static system became blocked by ice leading to erroneous altitude and airspeed readings In one case the flight computer of an Airbus A330 203 experienced conflicting airspeed readings due to an ice blocked pitot tube and disengaged the autopilot The pilots startled at the sudden change in attitude put the nose up until the aircraft stalled a configuration that the pilots maintained until it crashed into the Atlantic ocean beneath the ITCZ storms through which they had been flying While it can be a relatively simple maneuver to deviate around a single airmass storm and avoid a possible icing or turbulence encounter the ITCZ presents a unique challenge The ITCZ wraps around the entire planet and may contain sections that can be hundreds of miles in length which are solidly occupied by towering cumuli reaching heights of 60000 ft or so Whats more these storms are not moving anywhere anytime soon There is no practical way of flying over or around the only option is to fly through them using a radar to pick the most benign route In this situation pitot heat and other anti icing measures should be switched on all the way through autopilot should be disengaged the aircraft configured for turbulent air penetration and all flightcrew members should be vigilant Usually a jet aircraft flying at cruise speed should be able to traverse the ITCZ in a matter of 15 to 20 minutes after which the flight can likely return to normal Home to vacation destinations and major population and industrial centers the tropics are a highly trafficked region of the world Like any other region the tropics have their share of favorable and adverse weather Pilots are advised to follow the same weather safety protocols here as they would anywhere else This includes a good preflight weather briefing maintaining minimum safe altitudes respecting any weather you may encounter and of course filing a pirep to keep other pilots informed whenever you have a chance Karsten Shein is a climatologist with NOAA in Asheville NC He formerly served as an assistant professor at Shippensburg University Shein holds a commercial license with instrument rating Hurricane Ivan prepares to enter the Gulf of Mexico in Sep 2004 Tropical cyclones can cause widespread damage and disruption to aviation operations in the tropics and beyond but their development and movement is heavily monitored and forecast and can therefore be anticipated by pilots Photo courtesy NOAA