Weather and the impact it has on aviation

Shortly before 8:30pm Monday evening a REACT helicopter flying from Rockford Memorial Hospital to Mendota crashed in a field just south of Rochelle killing the pilot and two flight nurses.  The Lee County Sheriff's Department has turned the investigation of the crash over to the Federal Aviation Administration and The National Transportation Safety Board this morning and they will continue with their investigation throughout the day.  During a news conference held earlier this morning we now know the helicopter had encountered weather while en route and radioed back indicating that it would be turning around.  Shortly after they lost contact with the pilot only to learn the helicopter had crashed.  Witnesses around the area Monday evening stated as the helicopter flew over it just didn't sound right, like there may have been something wrong.  Whether or not this was the case is yet to be determined.  There are many factors that the investigators will be looking at to help determine the exact cause, but weather can not be ruled out.  Often times when we think of aircraft accidents we think severe weather; for example strong thunderstorms or wind.  In the winter ice is a major danger for aircraft because it can alter the air flow around the wing of a plane or rotor of a helicopter.  Before we get into the dynamics of air flow I want to talk a little bit about cloud structure.

Thousands of tiny water droplets are found in a cloud from base to top.  Many of these water droplets are known as 'super-cooled' water droplets.  This means that the water droplet is in the liquid form despite the temperature dropping well below freezing.  The reason for this is because the droplet has not yet come into contact with a surface or particle for it to freeze on.  When a plane flies into this level in the atmosphere it can usually ascend or descend to get to a safer altitude.  If not, ice can build up on the wings, or rotor, causing the lift generated by the plane/helicopter in flight to decrease.  In order to get a better idea of the weather conditions around the area we look at the observations taken from the Rochelle airport because this is the closet airport to the crash site.  In the couple hours leading up to the event skies were overcast at about 3,000 ft and light snow showers had been falling on & off, visibility varied at the surface between 7 and 10 miles.

This is the upper air sounding from the National Weather Service in Davenport, IA (the closet upper air sounding to that location).  The red line indicates the temperature profile and the green line indicates the dew point throughout the atmosphere.  Just by looking at this sounding you're able to see that the atmosphere was saturated (when the two lines are very close to each other) not that far above the surface and the temperature was below freezing.  It's possible the helicopter could have been flying at a level where these super-cooled droplets were present, but it's important to note that we don't know the exact flight plan or altitude the helicopter was flying at and we certainly don't want to make any speculations.  These are just some of the things I'm sure the FAA and NTSB will also be looking at.

Now, back to the aerodynamics of an aircraft.  Aircrafts generate their lift by creating a difference in pressure from the top of the wing to the bottom of the wing.  If a plane or helicopter flies at a level where these super-cooled water droplets are present ice can build up on either a wing or rotor and it can alter the air flow reducing the lift and force that keeps that aircraft in flight by creating more drag, or resistance.  It's not necessarily a matter of weight on the aircraft.  Ice can be very dangerous and that's why de-icing equipment are found on planes (I'm not sure if it is also present on helicopters) and many hours are spend on the ground de-icing planes before takeoff.  The chemical that is sprayed on the plane helps to lower the freezing level of water so the water droplets can't accumulate on the wing or tail.  The Chicago National Weather Service has already gathered data from the surface and above to assist with the investigation and anything official will come from the FAA and the NTSB within the next few days.  Please keep the families and friends of those lost in your thoughts and prayers as they are greatly needed at this time. 

Further updates will be provided on the WTVO Channel 17 News at 5,6 and 10pm and our sister station FOX 39 at 9pm.