Re-Thinking the Operation of De-Icing Boots

America’s National Transportation Safety Board (NTSB) has issued a Safety Alert in relation to the operation of pneumatic de-icing boots on aircraft.

Historically, pilots have been taught to delay activating de-icing boots until such time as a reasonable layer of ice has accumulated, in order to avoid the (theoretical) condition of “ice bridging”.

The theory behind “ice bridging” is that premature activation of pneumatic boots could push the ice into a frozen shape around the expanded boot, thus rendering the boots ineffective at breaking and displacing any further accumulation.

Following extensive research, the NTSB says it does not know of any cases where “ice bridging” has resulted in an accident or incident. Its conclusion is that “ice bridging” is extremely rare—if it exists at all.

However, the organisation has investigated numerous accidents and incidents involving delayed activation of de-icing boots. It says that as little as quarter of an inch of ice can increase stall speed by 25–40 knots and because some such minor accumulations have minimum impact, pilots become over-confident. The safety alert says that sudden departure from controlled flight is possible with only quarter of an inch of leading edge ice accumulation at normal approach speeds.

The NTSB’s Safety Alert advises early activation of de-icing boots—as soon as icing is encountered (unless the aircraft flight manual or pilot operating handbook specifically directs otherwise)—in order to limit the effects of leading-edge ice and improve operational safety margins.

Even if an aircraft flight manual or pilot operating handbook specifically directs delaying the activation of de-icing boots, the Safety Alert advises pilots to “maintain extremely careful vigilance of airspeed and any unusual handling qualities”.

The Alert also advises pilots to disconnect or limit the use of autopilots in icing conditions, in order to “feel” changes in the handling qualities of the aeroplane better.

The NTSB’s Safety Alert (report identification number NYC07LA081) is available online at the NTSB website, www.ntsb.gov, and an article on the subject by an NTSB investigator is available on the Professional Pilot website: http://www.propilotmag.com/archives/2008/Dec08/A2_Icing_p1.html