Investigators discover numerous incidents in which de-icing fluid residue build-up has jeopardised handling
UK accident investigators are calling on the European Aviation Safety Agency (EASA) to address the persistent problem of de-icing fluid residue causing partial loss of control of regional aircraft.
Between March and June 2005 the UK Air Accidents Investigation Branch (AAIB) recorded in-flight control restriction events affecting 32 BAe 146/Avro RJ-series aircraft, two Embraer ERJ-145s and three Bombardier Dash 8s, all caused by de-icing fluid residue accumulating over time in control surfaces or their hinges and freezing at high altitude.
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The common factor among the aircraft types affected is that they do not have powered flight controls. The AAIB says that the risk is highest when a period of cold weather in which de-icing has been regularly carried out is followed by warmer weather that dries any accumulated fluid, leaving a residue. Then if the residue is exposed to moisture it can re-hydrate and freeze at high altitude. This causes control surface restrictions that affect flight both with autopilot engaged and when flown manually.
Pilots have found that aircraft have entered uncommanded turns, departed from their cleared level, entered a phugoid pitching regime, and in some cases could be controlled only by use of power because the controls were completely jammed. Normally the restriction eases as the aircraft descends into warmer air, says the AAIB. Affected controls can include elevators, ailerons and trim tabs, and the fluid residue can clog hinges, gaps or even penetrate the control surface itself.
Regular inspection and cleaning away residue is the remedy, says the AAIB, and the UK Civil Aviation Authority has acted to ensure UK operators are familiar with the problem. But this time the AAIB has recommended action by EASA, partly because seven of the 32 aircraft affected in the four-month period studied last year were not UK registered.
The AAIB wants EASA to set standards for de-/anti-icing fluids, being concerned that operators tend to choose thickened rather than Type 1 fluids because they give a longer hold period between de-ice and take-off, although it is the thick fluids that cause residue build-up. It also wants EASA to require operators that use thickened fluid to adopt a maintenance programme that deals with the residue accumulation; and finally – when EASA takes over its operations remit fully – the AAIB wants it to consider licensing de-icing operators to ensure they train their staff to apply fluid in ways that will minimise its penetration into flight control mechanisms and control surfaces.
DAVID LEARMOUNT / LONDON
Source: Flight International
