A Chinook with an orange boom spraying a light mix of water on a trailing helicopter is a familiar sight in the skies of the Upper Peninsula of Michigan during the winter months.

Since the early 1970's, airborne artificial and natural icing testing has been conducted by the U.S. Army in Fairbanks, Arkansas; Moses Lake, Seattle, and Spokane, Washington; and Edwards Air Force Base, California; and in Duluth, Minnesota from 1983 to 2014, until the hangar facility was no longer available. Testing has been conducted in Marquette, Michigan at K.I. Sawyer International Airport since 2015.

Training for those tests happens here in North Alabama by highly-skilled experimental test pilots, flight test engineers, and flight engineers from the U.S. Army Redstone Test Center's (RTC) Aviation Flight Test Directorate. But this year, the training underway is a little different. In fact, it's history in the making.

The icing system -- the 'orange boom' -- known as the Helicopter Icing Spray System, or HISS -- is the same one that's been used for years. Eagle-eyed observers will notice it's the bird in the sky that's different this year.

RTC is moving this system from the legacy CH-47D helicopter to the current generation F-model platform.

The flight training underway over Huntsville and Decatur is designed to discover and document any changes to flight characteristics between the CH-47D and CH-47F while flying with the HISS installed.

"We need to verify that the airworthiness of the F [CH-47F] is the same, or better than the D-model [CH-47D], while flying with the HISS boom installed," explained RTC Flight Test Engineer Kimberly Hanks.

"The thing we figured would cause us the biggest problem was the Automatic Flight Control System (AFCS) -- in a D model, and in prior models, this system was operated by an analog computer. Boeing designed and built a digital AFCS, which we qualified here. In switching from analog to digital in the past, we've run into problems. That's one of the things we are watching," explained Lynn Hanks, experimental test pilot at RTC.

The HISS is a system designed to provide artificial icing conditions and is used as part of icing certification for military and civilian aircraft. The HISS creates a controlled-cloud environment through which the test aircraft flies. The effects of ice accumulation and shedding are then documented with onboard instrumentation and photography.

"This transition to the new platform has been a major undertaking and the result of years of planning and preparation," said Lt. Col. Cornelius Allen, Jr., commander of RTC's Aviation Flight Test Directorate. "RTC not only has the only HISS aircraft in the world, but more importantly, we have the world's leading experts in maintaining the aircraft and conducting icing testing. Without our experts here, this transition would not have been possible. Their hard work has been critical to the success of this mission."

The HISS system on the new Chinook will be ready for testing this October in Marquette, Michigan.