The rubber de-icing boots bridging myth?
https://airfactsjournal.com/2020/11/ice ... on%27t+DieIce bridging is the idea that if you operate the boots too early, you will stretch the ice but not fracture it. When the boot deflates following the cycle, the stretched ice will remain, with more ice building on top of it. Ergo, the next cycle of the boot will be useless, because the boot will simply expand into the cavity underneath the previously stretched ice. It can be traced back to at least the 1943 edition of Civil Aeronautics Bulletin No. 25, Meteorology for Pilots, which stated that, “If aircraft is equipped with leading edge boot type de-icers, allow ice to form to inch thick, then crack off by inflating de-icers. Use de-icers periodically as ice forms. This prevents ice from forming a pocket in which the boot expands…” The identical phrase is found in Zweng’s Airline Transport Rating textbook from 1947, so it is difficult to know who had actually thought about the idea, and who was copy-and-pasting.
It was enshrined in the FAA Aircraft Icing Handbook as late as 1991, which stated that, “Bridging is the formation of an arch of ice over the boot which is not removed by boot inflation. This can occur if the system is activated too early or too frequently, especially in glaze icing conditions.” The Handbook went on to opine that, “A certain degree of pilot skill is required for safe and effective pneumatic boot operation. Actuation when accreted ice is too thin may result in ‘bridging’ where the formation of ice over the boot is not cracked by boot inflation. Thus, attention is required to judge whether the cycle time continues to be correct as icing conditions change.”
Ice on wing of turboprop
Should you wait to pop the boots until “enough” ice has formed?
Yet there is not a single test conducted in anyone’s icing research wind tunnel that has been able to replicate ice bridging, nor are there any accidents that document ice bridging as a cause or contributory factor. There are precisely two reports in forty years of NTSB data that use the term. To be fair, there are plenty of accidents for which we do not have any specific cause aside from a generalization, or even any pilot accounts, so we don’t know what many of those pilots saw; that said, there are also precisely two reports in the entire NASA Aviation Safety Reporting System that use the term ice bridging. In the FAA Accident and Incident Data System, the term does not appear. Most interestingly, the term is never used in the Transportation Safety Board of Canada’s accident data, nor does it appear in Canada’s Civil Aviation Daily Occurrence Reporting System (CADORS).