The US FAA today proposed new regulations that would require aircraft and engine manufacturers to demonstrate safe operations in a broader range of icing conditions.
The rule will require airframers seeking flight into known icing certification of new Part 25 aircraft with a maximum takeoff weight of less than 27,216kg (60,000lb) or those with reversible flight controls (ailerons, for example) to demonstrate safe operations in the presence of so-called super cooled large droplets (SLDs).
The FAA says SLDs, which can come in the form of freezing rain or freezing drizzle, may impinge aft of existing icing protection systems on an airfoil "negatively affecting aircraft performance and handling qualities".
Those factors were determined to be the primary cause a fatal 1994 ATR 72 accident near Roselawn, Indiana. "These investigations led to the conclusion that freezing drizzle conditions created a ridge of ice on the wing's upper surface aft of the deicing boots and forward of the ailerons," says the FAA of the US National Transportation Safety Board's (NTSB) investigation of the accident. "It was further concluded that this ridge of ice contributed to an uncommanded roll of the airplane."
As a result of NTSB recommendations, the FAA tasked an ice protection working group as part of an aviation rulemaking advisory committee to review certification requirements.
Along with airframes, the group also studied the effects of potential icing certification requirements on windscreens, airspeed indicating systems and angle-of-attack sensors. The proposed rule, based in large part on the ARAC's review, calls for verification of engine performance in the presence of SLD or ice crystals, as well as for airframes and airspeed and angle-of-attack systems. The ARAC found that there had been 14 documented cases of ice crystal and mixed phase engine power loss events between 1988 and 2009, two of which resulted in diversions.
The FAA to date has addressed SLD and icing issues with directed airworthiness directives for certain aircraft as well as a recent amendment to certification standards requiring transport category aircraft manufacturers to either have icing protection systems that automatically activate or provide a method to alert pilots that the system should be activated.
Alternatives the FAA considered to changing the certification rules included using terminal area radar and ground-based radar or weather diagnostic warning tools to alert pilots to the location of SLD areas. Both methods were determined to be inadequate however, in part due to immature technologies.
On an annualized basis, the FAA estimates the industry will spend $3.8 million per year from 2012 - 2064 for the new rules while saving $7 million a year.