Just because a runway is covered with contamination, it doesn't mean necessarily that braking and steerage is non existent. In a Classic 747 we were on final approach to a JFK completely covered in snow with heavy deposits on the runway. A preceeding landing DC-9 was asked for BA and his reply was 'nil'. The Captain and I looked at each other in horror and switched the autobrakes to Max. When we touched down, there was a pause while the wheels spun up, and then the brakes came on. It felt like we were going to go through the windscreen it was so strong- thrown forward like those astronauts in Apollo 13 when the main engine cut out. That is why peoples' assessments are not reliable and the only reliable way in the absence of a Mu reading is to get out there and see what there is and measure it.