FOLLOWING IS THE EXTRACT FROM 'DRAFT, ICAO "Runway Surface Condition Assessment, Measurement and Reporting"
When the runway is covered by a loose contaminant (e.g., standing water, slush, non compacted snow), there are additional drag forces resulting from the displacement or compression of the contaminant by the wheel. The driving factors of these displacement drag forces are: aircraft speed and weight, tire size and deflection characteristics, and contaminant depth and density. Their magnitude can significantly impair the acceleration capability of the aircraft during take-off.
A second effect of these displaceable contaminants (slush, wet snow, and standing water) is the impingement drag, whereby the plume of sprayed contaminant creates a retardation force when impacting the aircraft structure. The combination of the displacement retardation force and impingement retardation force can be as high as 8 to 12 per cent of the aircraft weight for a typical small/mid size passenger aircraft. This force can be large enough that in the event of an engine failure the aircraft may not be able to continue accelerating.
Loose contaminants:
• acceleration capability reduced by displacement and impingement drag (slush, wet snow, and standing water) or the force required to compress the contaminant (dry snow); and
• deceleration capability reduced by lower friction, aquaplaning at high speeds, partially compensated by displacement and impingement drag
As a result,
• take off distance is longer (worse when contaminant is deeper);
• accelerate-stop distance is longer (less so when contaminant is deeper because of higher displacement and impingement drag);
Compact snow:
• the acceleration and continued takeoff is not affected;
• the accelerate-stop distance is increased by 30-60 per cent, reduced to 20-30 per cent with the use of thrust reversers (one-engine-inoperative)
The DOC also mentions about Wet ice, and other things very nicely. Can google and download it.
As to your question - Any idea why Conpacted Snow would require less thrust and flaps than the Dry Snow, I hope this helps.
Also you mentioned about DEICE and dispatch suggesting a derate later, our policy is to have no denates after DEICE/ANTIICE
Lastly you mentioned "This leads me to think the acceleration somehow benefits third stage climb, but I am missing a piece"
I am not too sure if I would relate the acceleration to third stage, if anyone has any explanation, more than welcome.
