Some interesting info on actual in flight tests. Their procedure for not having all engines affected similarly.....keep the power settings all slightly different. Actual experience of ICI included the smell of ozone which is listed in out QRH. Didn't mention the smell of sulphur which is also in our QRH. According to Boeing, their listed indications of ICI when it comes to smells is based on pilot interviews.
Putting The Chill On Engine Core Icing Flight Tests | Technology content from Aviation Week
"Putting The Chill On Engine Core Icing Flight Tests
NASA pilots develop test techniques for hazardous engine icing conditions
Flight testing for engine core icing by necessity involves deliberately flying a large research aircraft into atmospheric conditions that are known to cause turbofans to lose thrust, flame out and even sustain damage.
So how do you fly such a test program efficiently and gather useful data without endangering the aircraft, its engines and the crew? Although the CFM56-2 engines powering NASA’s DC-8 have no history of uncommanded power reductions, or “roll back,” test planners were aware that very few engines of any make have been purposely subjected to sustained and repeated exposure to high ice water content (HIWC) conditions.
“We need to make sure we don’t flame our engines out, so we worked with General Electric, Boeing icing experts, NASA Glenn and Langley,” says Wayne Ringelberg, lead HIWC project pilot for NASA. A former U.S. Air Force test pilot with heavy, multi-engine aircraft experience, Ringelberg worked with NASA Armstrong Flight Research Center chief pilot Nils Larson to develop an appropriate piloting approach.
“We ran a systems-safety working group just on the icing hazard mitigation to see what we thought the hazards are, and what we think we can do. It turns out a lot of it was ‘we don’t know for sure’ because the phenomenon is quite unknown,” says Ringelberg. “We had a sense we were probably not highly susceptible, but we just didn’t know,” he adds.
The agreed mitigation procedure involves staggering the four throttles as soon as the ice particle instruments indicate the aircraft has entered air with high ice water content above 0.5 grams/cu. meter. “Once we hit that level we are in it,” says Larson, who is also a former Air Force test pilot. “Every five minutes or so, you tweak them,” he adds. “We are flying on autopilot and the throttles are only staggered in the 2-5% range. We also only move one throttle at a time.” The slight variation in N1 (fan speed) is expected to be enough that if icing were to strike, only one engine at a time would be vulnerable. But there are no guarantees, says Larson. “Everyone’s guessing here, there are a lot of unknown unknowns.”
NASA flight engineer Tim Sandon continuously ran both anti-icing ignitor loops on each engine as a precaution. “We leave them on 20 minutes after exiting the ICI [ice crystal icing] area and leave them on prior to descent, as that’s a risk area,” says Ringelberg.
The campaign also is aimed at educating crews for “seat of the pants” HIWC warning signs. Even though nothing might be showing up on the weather radar, there are plenty of clues to be had, says Larson. “Other crews didn’t think they were in icing because sometimes on the windscreen it looks like water. That’s because the particles splatter when they hit the windscreen and melt. Sometimes there’s St Elmo’s fire and sometimes there’s a sound like a ‘whoosh’—similar to the sound of an emptying drain.” Descriptions of this rain-like effect baffled scientists for years, helping to compound the puzzle over HIWC. Pilots have also noted speckling on the windscreen, humidity changes, an ozone-like smell, crackling on the radios and a sound like rain on the cockpit roof."