Around 8:30pm Monday night a REACT medical helicopter, flying from Rockford Memorial Hospital, crashed in a field south of Rochelle, Illinois killing the pilot and two flight nurses. While the Federal Aviation Administration and The National Transportation Safety Board are investigating the cause of the crash, weather is a suspect not worth overlooking.
Normally, we think of wind shear and severe thunderstorms as the main causes for air disasters. In this case, the helicopter was likely flying through super-cooled water droplets. These unfrozen droplets can exist in the atmosphere in liquid form, even though the temperature of the atmosphere is below freezing. As an aircraft flies into these droplets, the cold fuselage comes in contact with the cold water, causing ice to form on the aircraft and wings (or in the case of helicopters, rotors). The accumulation of ice may not be noticed by pilots and passengers during night flying and if it is, usually the pilot can ascend or descend to a different flight level where the ice won’t accumulate or will melt. It is not immediately known whether the helicopter involved in Monday’s crash had deicing equipment.
Icing is dangerous because of a few reasons. 1.) It can change the aerodynamics of the aircraft; proper aerodynamics are needed to ensure there is enough lift to keep the vessel in the air. 2.) Ice can produce a significant amount of weight, causing the rotors to spin too slow to maintain flight. 3.) Ice can cause blockages of pitot tubes and vents. This can cause errors in instruments such as altimeters and airspeed indicators. 4.) Ice formation on unheated parts of the aircraft can affect the ability of radio transmission. 5.) Ice accumulation that falls off of aircraft may damage necessary pieces of equipment for flight.
Conditions at Rochelle, the nearest airport to the crash site, were cloudy with intermittent snow showers and flurries. At flight level, it is certainly possible that the crew ran into low clouds and super-cooled water droplets, which could’ve caused icing. Shortly after we confirmed the crash, I spoke with Casey Sullivan, a Meteorologist with the National Weather Service in Romeoville, Illinois. He could not confirm whether icing was a factor. “We did not receive any reports of any icing in that area from aircraft, but that’s not to say it wasn’t possible.”
Since there were no survivors or pilot reports in the area, we must look to the profile of the atmosphere from this evening’s balloon launch at the Davenport, Iowa National Weather Service Office. To the left is the vertical profile of the atmosphere around Northern Illinois this evening. Meteorologists and pilots refer to these charts as “Skew-T Log P” diagrams. What’s important to know is the red line represents the temperature (slightly below freezing at the ground (bottom)), with a steady decrease as you ascend into the atmosphere (top). The green line is the dewpoint. The air is completely saturated where the green and red lines are very close together. This occurred from near the surface, up to about 5,955 feet. This is the most likely area for super-cooled water droplets to be present within the atmosphere. Unfortunately, most medical helicopters travel at a level below 5,955 feet…making the threat of aircraft icing a real and present danger if the aircraft did not have deicing equipment.
Stay with 13News as we continue to cover this air crash. -Eric
This post was written by Eric Sorensen on December 11, 2012