Air Canada Regional DHC-8-100 Engine Flameouts Due to Inlet Ice

Incident Overview and Engine Flameouts On 03 April 2001, an Air Canada Regional Airlines DHC-8-100 aircraft, carrying 35 passengers and a crew of three, departed Sydney, Nova Scotia, for Halifax, Nova Scotia, at 1547 Atlantic daylight time. During the climb-out at approximately 6,000 feet, the first officer observed ice in the right engine (Pratt & Whitney 120A) air inlet duct. Approximately five seconds later, the right engine flamed out. The aircraft was operating with engine ignition on, and the engine recovered almost immediately. About two minutes after the recovery, the right engine flamed out and recovered again. After completing checklist procedures, the first officer observed that the ice in the right inlet duct had disappeared. The captain then checked the left engine and found ice present in the left air inlet duct. A few minutes later, as the aircraft reached its cruise altitude of 14,000 feet, the left engine experienced a similar flame-out and recovery sequence. The aircraft continued to its destination without further incident. The engine flameouts were caused by ice in the engine air inlet ducts lifting up as a solid sheet, interrupting the airflow to the engines. It could not be determined conclusively how the ice formed in the inlet ducts. Two possible scenarios were established for the ice buildup. The first scenario postulated that water was present in the engine air inlet ducts when the aircraft was removed from an unheated hangar in Sydney. This water went undetected during inlet inspections and then froze into a solid sheet. The second scenario postulated that the inlet ducts were clear of water and ice prior to engine start, and that ice developed in the inlet ducts after the engines were running. The aircraft had arrived in Sydney the previous evening. On approach, there were traces of light rime icing with no appreciable accumulation, and aircraft anti-ice and de-ice systems were used. The wing and tail leading edges were clean on arrival at the ramp. The Sydney weather was blowing snow with a temperature of minus 1°C and a dew point of minus 4°C. A short time after arrival, the aircraft was placed in an unheated hangar where the temperature was slightly above freezing. Due to a snowstorm, all morning flights out of Sydney were canceled. The occurrence aircraft was removed from the hangar and positioned on the ramp, into the wind, at 1450 Atlantic daylight time. The weather was reported as wind 340° true at 22 knots, gusting to 31 knots, visibility 1 mile in light snow and blowing snow, temperature −1°C, and dew point −5°C. Thirty minutes after the aircraft was removed from the hangar, the right engine was started and run for five minutes to heat the aircraft. Both engines were started 46 minutes after the aircraft was out of the hangar. Of the total period outside the hangar before both engines were started, the inlet plugs were estimated to have been in place for 20 minutes, leaving the inlets exposed to the wind and blowing snow for 26 minutes, with the right engine running for five minutes of that. Heat transfer analysis concluded that, in conditions similar to those on the ramp (temperature −1°C, wind 10 knots), ½ inch of water could freeze in 30 minutes. The analysis also concluded that the conditions on 03 April 2001, after engine start, were not conducive to inlet duct icing, and consequently, the inlet icing could not have occurred as described by the crew unless there was a pre-existing, ground-accumulated ice sheet. The second scenario relies on statements from Air Canada Regional personnel that no ice was present before engine start and that proper ground and flight procedures were followed. While it is not possible to determine conclusively which scenario is accurate, the implications of either possibility are serious. To address the first scenario, the manufacturer introduced additional ground handling safety defenses, which have been implemented by Air Canada Regional. The second scenario, that the multiple in-flight engine flameouts may have been caused by ice accumulation after the engines were started, cannot be discounted as a possibility. Appropriate follow-up action is required to ensure that the risk of significant in-flight ice accumulation causing flameouts is adequately assessed. Three of the four drain holes in the right engine inlet duct were completely blocked and the fourth was partially blocked, which increased the risk that water could pool and freeze in the duct. Bombardier Aerospace published a revised ground procedure training guide in September 2001, containing a more detailed description of the areas to be inspected and cleaned, and suggesting tools and methods for carrying out the inspections and cleaning procedures. Bombardier Aerospace also provided instructions, Customer Special Installation (CSI) 826930, on enlarging the drain holes in the engine air inlet ducts. Air Canada Regional has incorporated the revised procedures into their training program and standard operating procedures. In addition, Air Canada Regional has developed an “Engine Intake Ice Survey” form for data collection purposes, which flight crews complete anytime ice is detected in the engine air inlets. In conjunction with this program, Air Canada Regional and Environment Canada have entered into a program providing real-time monitoring of in-flight atmospheric conditions. Data from this program will be correlated with data from the ice surveys in an attempt to understand the conditions which lead to engine air inlet duct ice formation in order to develop appropriate icing avoidance procedures. As of 31 December 2001, the operator had received several completed “Ice Survey” forms. Air Canada Regional has commenced the installation of splitter angles, designed by Bombardier Aerospace at the request of Air Canada Regional and provided to Air Canada Regional as CSI 44022, in the engine nacelle lower cowl. Air Canada Regional has installed this device in all of their DHC-8 aircraft. The purpose of the splitter angles is to prevent a single, solid sheet of ice from forming in the engine lower cowl. The company has also completed a program to enlarge the drain holes in the engine inlet ducts in accordance with CSI 826930. A Transport Safety Board of Canada Aviation Safety Advisory was sent to Transport Canada on 17 August 2001, suggesting that this and previous occurrences involving DHC-8-100 engine flameouts be reviewed to validate that the aircraft and engines (Pratt & Whitney 120A) were performing acceptably under conditions for which they are certified. On 23 October 2001, Transport Canada responded to the safety advisory, stating that Bombardier has developed extensive ground procedures for the upcoming winter 2001/2002 and will assist Air Canada Regional to implement them. Bombardier will station a Field Service Representative in the Atlantic region this winter to ensure that the procedures are understood and to collect data in the Air Canada Regional operating environment. Transport Canada Civil Aviation staff members are satisfied that Air Canada Regional, Bombardier, and Pratt and Whitney Canada are working collaboratively to ensure that there is not a recurrence of the event experienced by Air Canada Regional on 03 April 2001. Transport Canada is satisfied with the progress to date and will continue to monitor and support these efforts until the issue is resolved. This report concludes the Transport Safety Board of Canada’s investigation into this occurrence. Consequently, the Board authorized the release of this report on 03 June 2003.