DENVER, CO, USA
CFBAO
British Aerospace BAE 146-200
Power on the no. 1 engine 'rolled back' (dropped to flight idle) while the airplane was in IMC at FL 290. The operator reported considerable convective cloud and thunderstorm activity in the area, with cloud tops reaching FL500. When the flight descended to FL 260, power on nos. 2 and 4 engines also rolled back. As the airplane passed 12,500 feet msl, power returned to the three idling engines. The flight landed without further incident. The airplane was equipped with four unmodified Honeywell ALF502R engines. In certain temperature and moisture conditions, the engines become susceptible to ice accumulation on the supercharger exit guide vanes. This reduces core engine airflow, resulting in a loss of power. Early airworthiness directives prohibited flight in icing conditions above 26,000 feet and at an outside air temperature above -40 degrees C. SAT within 30 nm of thunderstorms. Some operators were granted waivers from these ADs. Recent airworthiness directives mandate certain engine modifications be made by December 2002. Newer models of the airplane feature redesigned engines.
HISTORY OF FLIGHT On April 30, 2000, approximately 1840 mountain daylight time, a British Aerospace BAe 146-200, C-FBAO (Canadian registered), operated by Air BC as flight 817 (ABL817), lost power on three of its four engines while in cruise flight about 130 miles north of Denver, Colorado. There were no injuries the airline transport certificated captain and first officer, 3 flight attendants, and 15 passengers. There was no damage to the airplane. At the time of the incident, the airplane was flying in instrument meteorological conditions. An IFR flight plan had been filed for the scheduled international passenger flight that originated in Winnipeg, Manitoba, approximately 1730 central daylight time. According to Air BC, power dropped to flight idle on the no. 1 engine while the airplane was in instrument meteorological conditions at FL (flight level) 290. Denver ARTCC (Air Route Traffic Control Center) cleared the flight to descend to FL260, at which point power on nos. 2 and 4 engines also dropped to flight idle. An emergency was declared. As the airplane passed 12,500 feet msl (above mean sea level), power was restored to the three idling engines. The emergency was cancelled, and the flight proceeded to Denver and landed without further incident. PERSONNEL INFORMATION The captain and first officer of flight 817 were interviewed on May 23, 2000, at Air BC's facilities in Vancouver, British Columbia. Both pilots said that they were aware of the engine power "rollback" phenomena, that it was well-known throughout the industry, and that Air BC had trained its pilots on the emergency procedures to follow, and had published an emergency procedures checklist. According to Air BC's BAe 146 Aircraft Operating Manual, "The phenomenon of uncommanded thrust reduction (rollback) is known to occur at altitudes above 26,000 feet in temperature conditions of ISA +9 degrees C. or greater and in the vicinity of thunderstorm activity. . .Except for descent, flight in icing conditions above 26,000 feet and at an outside air temperature above -40 degrees C. SAT is prohibited within thirty nautical miles of thunderstorms." AIRCRAFT INFORMATION C-FBAO was equipped with four Honeywell (formerly Allied Signal) ALF502R engines, each rated at 6,970 pounds of thrust. According to Honeywell, when operating at or above FL280 and in certain temperature and moisture conditions, the engines become susceptible to accumulating ice on the supercharger exit guide vanes (EGV). This reduces core engine airflow, resulting in a loss of power. To protect the engines from ice accumulation, both Honeywell and BAE SYSTEMS have issued service bulletins that recommend: (1) reducing the length of the core-flow/fan-flow splitter lip (cut-back splitter) "to reduce ice crystal/water ingestion to the core"; (2) insulating the splitter lip baffle "to reduce heat loss"; (3) substituting a heated, single row of 71 EGVs in lieu of the unheated, double row of 88 EGVs in the engine supercharger (compressor) "to prevent ice build up," and relocating the engine anti-ice air source to the combustor bleed plenum "to reduce system heat loss"; (4) relocating the anti-ice valve (this necessitates the installation of a dished section on the inner surface of the left rear cowling door); and (5) installing insulated plumbing. The engines on C-FBAO had not been modified in accordance with these service bulletins. Honeywell has also developed a redesigned engine, the ALS507R, that will be installed in all new BAe 146s. METEOROLOGICAL INFORMATION According to the operator, "considerable convective cloud and thunderstorm activity [was] in the area. [Cloud] tops [reached upwards] to FL500." NTSB's Operational Factors Division was asked to conduct a weather study in support of this investigation. Upper air data, NTAP (National Track Analysis Program) data, doppler weather radar images, and GOES-10 (Geostationary Operational Environmental Satellite) photographs were retrieved and reviewed. According to the NTSB meteorologist's factual report, the engine rollbacks occurred "in convective clouds in weak weather echoes. Very strong to intense weather echoes were located along the track of Air BC. However, these echoes extended up to an altitude of about 10,000 feet and depending on the setup of airborne weather radar, such as radar tilt and range, may not have been displayed as significant echoes. Very strong to intense weather echoes up to 38,000 feet were seen about 50 nautical miles to the east of the Air BC track. The crew stated that they were anticipating rough air from the towering cumulus (TCUs) 40 to 45 miles to their left (east). These very strong to intense weather echoes should have been displayed as significant returns on airborne weather radar. Therefore, any operational procedure based on distances from significant convection (thunderstorms), using information from airborne weather radar, may be misleading and should be reevaluated." Upper air data from Denver, Colorado, for April 30, 2000, at 0000Z (1800 mdt), indicated a temperature of -27.1 degrees C. (ISA +4.6 degrees C.) at about 24,100 feet, and a temperature of -44.5 degrees C. (ISA +0 degrees C.) at about 30,700 feet. FLIGHT RECORDERS No useful data were recovered from the cockpit voice recorder. The digital flight data recorder was sent to NTSB's laboratory in Washington, DC, for readout. The plots are attached as exhibits to the docket. Examination of plots -5 and -4 indicate all four engines operating approximately 87% N1, with the airplane cruising at FL310. Airspeed was 250 knots, the heading was approximately 216 degrees, and the outside air temperature (OAT) was -22 degrees C. At frame 12059, power was reduced on all four engines to 79% N1, airspeed remained constant, and a descent was made to FL290 (plots -4 and -3), at which point power was increased to 81% N1. At frame 12615, power was further increased to 84% N1, followed shortly thereafter by a heading change to 180 degrees (plot -2). During this heading change, there was a vertical velocity spike to +1.25 Gs. Plot -1 showed a slight decrease --- then increase --- in power. Vertical velocity continued to vary between +1.25 and -0.25 Gs, and airspeed decreased to 220 knots. Altitude remained constant, and OAT decreased to -23 degrees C. Plots 1 and 2 depict power rollbacks on engines 1, 2, and 4, that appear to be almost simultaneous, dropping from 81%, 83%, and 81% N1, and eventually stabilizing at 30%, 26%, and 30% N1, respectively. Power on #2 engine increased from 85% N1 to 90% N1. At frame 13147, the airplane began a descent from FL290 and eventually leveled off at 12,000 feet msl. The airplane began a left turn to 060 degrees, and power returned to engines 1, 2, and 4 shortly thereafter. TESTS AND RESEARCH Since 1988, in addition to this incident, there had been 13 uncommanded thrust reductions (rollbacks) involving the ALF502R (see exhibits in docket). The United States' Federal Aviation Administration (FAA), Britain's Civil Aviation Authority (CAA), Australia's Civil Aviation Safety Authority (CASA), and Canada's Transport Canada (TC) have all issued airworthiness directives applicable to the BAe 146 (attached, see docket). The following is a brief history: A.D. 94-07-09 CAA issued A.D. 004-02-94, and Transport Canada issued A.D. AC-002-94. These two A.D.s imposed altitude and operational restrictions if the aircraft was operated in icing conditions. Following the United Kingdom and Canada's lead, FAA issued A.D. 94-07-09 on March 24, 1994, requiring (1) the installation of a placard that prescribed special procedures to be followed when operating at certain flight levels with the engine and airframe anti-ice switch ON, (2) modification of the air brake auto-retract function, and (3) revisions to the AFM prescribing certain altitude and operating limitations and procedures when operating in certain icing conditions. Three Canadian operators --- Air Atlantic, Air Nova, and Air BC --- petitioned Transport Canada for an alleviation (waiver), seeking an alternative means of compliance (AMOC) with A.D. AC-002-94. After discussions with Honeywell, Transport Canada granted the alleviation (AARDG/94A35) on May 9, 1994, but a thunderstorm caveat was added: "Except for descent, flight in icing conditions above 26,000 feet and at an outside air temperature above -40 degrees C. SAT is prohibited within thirty nautical miles of thunderstorms." A.D. 96-14-09 British Aerospace issued Temporary Revisions (TRs) to the Airplane Flight Manual (AFM), prohibiting flight into known or forecast icing conditions above FL260. In July 1996, the company issued Temporary Revisions (TRs) 32, 44, and 25 to the AFM. These TRs prescribed certain altitude and operating limitations, and prohibited flight into known or forecast icing conditions above an altitude of 26,000 feet, and defined procedures to be followed when icing conditions were inadvertently encountered above 26,000 feet. CAA approved these AFM revisions and, on July 1, 1996, issued A.D. 003-06-096 to mandate these limitations and procedures to assure the continued airworthiness of the airplane. Since the airplane was type certificated for operation in the United States (and pursuant to the bilateral airworthiness agreement with the United Kingdom), FAA issued A.D. 96-14-09 (superceding A.D. 94-07-09). Issuance was also prompted as a result of NTSB's investigation into an Air Wisconsin BAe 146 incident that occurred on May 26, 1996 (CHI 96-I-A131). It required an additional revision to the AFM relative to altitude and operating limitations associated with flight in icing conditions above 26,000 feet, and defined procedures to follow if icing conditions were inadvertently encountered above 26,000 feet. A.D. 99-15-06: On July 19, 1999, FAA issued AD 99-15-06, requiring the "incorporation of an improved fan core inlet anti-ice system. . .to prevent ice accretion on the fan core inlet stator vane surfaces, which can result in engine rollback and loss of thrust control in icing conditions." According to FAA, these modifications would cost $75,000 per engine to accomplish and must be completed by December 31, 2002. Again, on August 13, 1999, Air BC petitioned Transport Canada for an AMOC to A.D. 99-15-06, and it was granted on September 16, 1999 (AARDG/99A124). However, after the Air BC incident on April 30, 2000, Transport Canada withdrew this alleviation, and issued A.D. CF-2000-12 on July 19, 1999, prohibiting flight of unmodified engines into known or forecast icing conditions above 26,000 feet. ADDITIONAL INFORMATION Verbal permission was given to Air BC to return the airplane to service on April 30, 2000. In addition to the Federal Aviation Administration, parties to the investigation included Air BC, BAE SYSTEMS, Honeywell, the Air Line Pilots Association, and Teamsters Canada.
The accumulation of ice on the supercharger exit guide vanes that reduced core engine airflow, resulting in a loss of power. Factors were the clouds and icing conditions.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
