Transportation Safety Board of Canada - Aviation Safety Recommendations
DATE ISSUED: 31 January 2006
The Honourable Jean-C. Lapierre, P.C., M.P.
Minister of Transport
SUBJECT: Cessna 208 Operation into Icing Conditions
On 06 October 2005, a Cessna 208B, registration C-FEXS, operated by Morningstar Air Express as Flight MAL8060, departed Winnipeg, Manitoba, at 0537 central daylight timeFootnote 1 on an instrument flight rules (IFR) freight flight to Thunder Bay, Ontario. The aircraft, with one pilot and about 2470 pounds of cargo on board, departed Runway 36, climbed, and turned right on course. About 4.5 nautical miles (nm) southeast of the airport, the pilot requested an immediate return to the airport due to icing considerations, but did not declare an emergency. Departure control provided an initial radar vector to Runway 31, and the aircraft turned to a southwesterly heading. A second vector was provided and the pilot responded; however, the aircraft did not turn and descended below radar coverage. The aircraft departed controlled flight and crashed on railway property in the city of Winnipeg. The pilot was fatally injured. The aircraft was destroyed by impact forces and a post-impact fire. The accident occurred during hours of darkness at 0543. The investigation (A05C0187) is ongoing.
On the day before the occurrence, the accident aircraft arrived in Winnipeg on a flight from Thunder Bay. The aircraft was parked in a heated hangar overnight, and was pulled outside at about 0410. The aircraft was refuelled and taxied to Apron at the Winnipeg Airport, where it was loaded. While the aircraft was being loaded, the pilot performed a tactile inspection of the aircraft's critical surfaces. The results of that inspection were not recorded nor were they required to be recorded. No information gathered in the investigation to date indicates that the aircraft departed with ice or snow adhering to its critical surfaces.Footnote 2
The information gathered to date suggests that in-flight airframe icing was a factor in the occurrence. As well, the pilot cited aircraft icing when asking air traffic control for a clearance to return to Winnipeg. Furthermore, icing conditions were forecast, and were experienced by other aircraft operating in the Winnipeg area at the time of the occurrence.
A special weather observation taken at Winnipeg on the day of the accident at 0531 was as follows: winds 360º at 15 knots gusting to 20 knots, visibility 4 statute miles (sm) in light snow and mist, a broken cloud ceiling at 1000 feet, overcast cloud at 5300 feet. The observed weather at 0600 was as follows: winds 360º at 15 knots, visibility 6 sm in light snow and mist, scattered cloud at 700 feet, broken cloud at 1700 feet, overcast cloud at 5500 feet, temperature -1ºC, dewpoint -1ºC, altimeter 30.26 remarks, cloud stratus fractus 4, stratocumulus 2.
The Winnipeg aerodrome forecast was issued on the day of the accident at 0038 and amended at 0344 and 0504. The aerodrome forecast amendment issued at 0504 was as follows for the time period from 0500 to 0900: winds 360º at 15 knots gusting to 25 knots, visibility greater than 6 sm with light snow, scattered clouds at 800 feet, broken clouds ceiling at 3000 feet, overcast clouds at 7000 feet, with temporary conditions of visibility 3 sm with light snow and broken clouds ceiling at 600 feet.
The Graphical Area Forecast (GFA) images, valid before and after the time of the accident, show instrument meteorological (IMC) weather conditions associated with a low-pressure system over northwestern Ontario. The 0600 UTC (0100 CDT) icing panel, issued at 0531 UTC, indicates moderate mixed icing in cloud from the surface to 20 000 feet over Winnipeg. By 1200 UTC (0700 CDT), the majority of the organized precipitation was forecast to be east of Winnipeg. The 1200 UTC icing panel, issued at 0541 UTC, indicates that the back edge of an area of moderate mixed icing from the surface to 16 000 feet was forecast to be just east of Winnipeg, with nil icing forecast over Winnipeg at that time.Footnote 3
A weather analysis prepared by Environment Canada after the accident indicated that the only significant turbulence would have been found in the lower levels and would have been light-to-moderate mechanical. No supercooled water cloud droplets were forecast for southeastern Manitoba. Satellite imagery and surface weather data indicated extensive broken to overcast stratocumulus clouds over the area. The convectively unstable moist layer between 2000 and 6000 feet asl, with air temperatures in the range of -3ºC to -10ºC, was prone to light icing. With a long transit time in this airmass, such as for a shallow glide or climb out of Winnipeg, the rate of icing could have been more significant than what was reported by the pilot reports (PIREPs).
The International Civil Aviation Organization (ICAO) defines icing intensity in terms of ice accretion on aircraft. The following definitions are taken from the Environment Canada publication MANAIR:
Light icing - The rate of ice accretion is such that prolonged flight (over 1 hour) without using de-icing equipment may create a problem. Occasional use of de-icing/anti-icing equipment removes or prevents ice accretion. If de-icing/anti-icing equipment is used, no problem occurs.
Moderate icing - The rate of ice accretion is such that even short encounters become potentially hazardous. De-icing/anti-icing equipment must be used or a diversion is necessary.
Severe icing - The rate of ice accretion is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
Weather forecasters prepare icing forecasts of nil, trace, light, moderate or severe based on a subjective estimate of how the conditions will have an impact on aircraft operations. However, experience has shown that different aircraft types have a greatly varied tolerance for operation in icing conditions; the effects on some aircraft types could be minimal while other aircraft types operating in the same icing conditions could be significantly affected. The differences in icing tolerances between different aircraft types result from varying aerofoil shapes, aircraft power, drag, speed, operating altitude, and other considerations. For example, aircraft skin is warmed by the effects of compression and friction, which increase with the speed of the aircraft. The most significant amounts of aircraft ice accretion are usually found at temperatures just below freezing; icing potential decreases at lower temperatures. Aircraft capable of operating at high speed can reduce or avoid ice accretion in the critical temperature range just below 0ºC, which prevailed at the time of the accident, by operating at speeds where compression and friction increase aircraft skin to temperature above freezing. The Cessna 208 aircraft type operates at relatively lower airspeeds where the effects of friction and compression are reduced.
Other Aircraft Operations
A review of air traffic control records indicated that several aircraft operated in and out of Winnipeg at the time of the accident. No other incidents or control difficulties were reported. A review of NAV CANADA records indicated that five PIREPs were received by NAV CANADA within one hour of the accident. Aircraft types ranged from light (Piper PA-31) to heavy (Boeing 727). The reports indicated that pilots were encountering nil to light icing.
The Accident Pilot
At the time of the accident, the pilot held an airline transport pilot licence (ATPL) issued on 09 April 1999 with a group 3 instrument rating and validated by a category 1 medical certificate. The pilot held a current Cessna 208 pilot proficiency check (PPC). The last PPC included a satisfactory rating for icing encounter. An examination of Transport Canada files and the operator's records indicated that the pilot accumulated about 1500 hours on the Cessna 208 aircraft and a total of about 4570 flight hours.
The Accident Aircraft Type
The Cessna 208 aircraft type is a high-wing monoplane with fixed landing gear and wing struts. The aircraft is certified for single-pilot operations into instrument meteorological conditions (IMC). Many Cessna 208 aircraft have been fitted with a belly- mounted pod to incorporate extra cargo volume. The Cessna 208 aircraft type was certified in 1985, and was followed by the Cessna 208A and the Cessna 208B (Grand Caravan).Footnote 4 All three variants are included under the Cessna 208 model certification. The Cessna 208B's certificated maximum gross weight is 8750 pounds and the maximum weight for flight into known icing conditions with a cargo pod installed is 8550 pounds. Information gathered to date indicates that the aircraft's gross weight at take-off was about 9070 pounds, 520 pounds over maximum gross weight for flight into known icing conditions.
The Cessna 208 aircraft type was certified to conform to the ice protection requirements of FAR 23.1419 when the ice protection equipment is installed in accordance with the aircraft equipment list. The details of the equipment and procedures for the aircraft's approval for flight into icing conditions are contained in Supplement S1 to the aircraft flight manual (AFM). Revision 7 of Supplement S1 was in effect at the time of the accident. The accident aircraft was equipped with a Cessna 208 ice protection equipment package. Part of this package includes pneumatic de-icing boots on the wings, wing struts, main landing gear struts, cargo pod nose cap, and the horizontal and vertical stabilizer leading edges. Also included are electrically heated propeller blade anti-ice boots and a detachable electric windshield anti-ice panel. The ice protection certification allows flight in icing conditions as defined by FAR 25 Appendix C envelopes for continuous maximum and intermittent maximum icing when the aircraft is operated in accordance with the Pilot's Operating Handbook (POH) and the Federal Aviation Administration-approved AFM. Supplement S1 provides that "These conditions do not include all icing conditions that may be encountered (e.g. freezing rain, freezing drizzle, mixed conditions or conditions defined as severe)." Flight into known moderate icing conditions is permitted.
Supplement S1 advises pilots that "whenever icing conditions are encountered, immediate action should be taken to leave these conditions before airplane performance is degraded to a point where a climb, which is normally the best action to take, may not be achievable due to the residual ice buildup." It also requires that the pilot must maintain a minimum of 105 knots indicated airspeed (KIAS) in icing conditions, request priority handling from air traffic control for a route or altitude change to exit icing conditions, and request priority handling from air traffic control if altitude cannot be maintained at 120 knots. On encountering icing conditions, pilots are required to:
- select windshield heat,
- select propeller anti-ice,
- switch on the ice detector light,
- monitor ice buildup until 1/4 to 3/4 inch of ice accumulates on the wing leading edge,
- select the boot switch to auto to activate the airframe de-icing boots for each cycle.
Supplement S1 states that ice accumulations on the airframe may result in a stall speed increase of 20 knots or more, and a decrease in climb rate of 500 feet per minute (fpm) or more. With significant ice accretions, the aural stall warning may be concurrent with the pre-stall airframe buffet. Supplement S1 warns: "Do not cycle the boots during landing (below approximately 500 ft agl) because boot inflation may increase stall speed by as much as 10 knots."
Supplement S1 provides that "optimum performance of the deice and anti-ice boots is dependant on keeping the boots clean and coated with an ice adhesion depressant such as ICEX II." The operator's records indicated that, on the day before the accident, the windshield de-ice panel was installed and Ice-X was applied to all boots in accordance with the applicable maintenance manual.
Cessna has produced an information package entitled the "Caravan Safety Awareness Program." Information in the program indicates that the aircraft can sustain a reduction in airspeed from 165 to 105 knots after repeated operation of the de-icing boots in icing conditions, accompanied by a significant increase in the aircraft's stall speed from 78 to 92 knots. As well, the aircraft's ability to sustain a climb or maintain altitude is reduced, from an initial climb of 800 fpm to a descent of 100 fpm.Footnote 5
TSB investigators reviewed 19 occurrences from 1990 to 2005 involving Cessna 208A and 208B aircraft operating in airborne icing conditions. These occurrences resulted in 42 fatalities and 4 serious injuries. In 14 occurrences, the aircraft was destroyed, in 4, the aircraft sustained substantial damage, and in the other occurrence, there was minor or no damage. Thirteen occurrences resulted in total loss of aircraft control, and in another two occurrences, the pilot maintained aircraft control but was unable to maintain altitude.
On 27 February 1990 at Denver, Colorado, a Cessna 208A was destroyed with one fatality. The National Transportation Safety Board (NTSB) factual report (DEN90FA068) indicates that moderate-to-heavy icing conditions were forecast for the Denver area, freezing drizzle was observed, and the aircraft encountered icing conditions on final approach. The NTSB determined the probable cause of the accident to be "the accumulation of structural ice and subsequent stalling of the aircraft."
On 30 November 1995 at Ardmore, Oklahoma, a Cessna 208B sustained substantial damage and the pilot sustained minor injuries. The NTSB factual report (FTW95FA129) indicates that freezing drizzle was reported at the airport. The aircraft rapidly accumulated ice during the approach, resulting in an inability to maintain altitude and a controlled impact with terrain during an off-airport forced landing. The NTSB determined that "icing conditions prevailing at the destination airport" were a factor in the accident.
On 04 March 1997 at Barrie, Ontario, a Cessna 208B was destroyed with one fatality. The TSB occurrence report (A97O0032) indicates that moderate rime icing and moderate mixed icing in light freezing drizzle were forecast, and freezing drizzle was reported in the vicinity of the airport. The TSB made a finding that "ice accumulation on the unprotected leading edge surfaces of the aircraft degraded aircraft performance . . .," and that "the aircraft struck the ground for undetermined reasons."
On 25 November 1997 at North Bay, Ontario, a Cessna 208B sustained substantial damage and two people sustained serious injuries. The TSB occurrence report (A97O0247) indicates that light-to-moderate rime icing in cloud and moderate-to-severe mixed icing in freezing drizzle were forecast. The aircraft stalled on final approach. The TSB found that "ice accumulation on the unprotected leading edge surfaces and underside wing surfaces degraded aircraft performance, increasing the stall speed."
On 20 January 1998 at Grand Island, Nebraska, a Cessna 208B sustained substantial damage and the pilot was not injured. The NTSB factual report (CHI98LA084) indicates that the aircraft encountered icing conditions en route. When the pilot reduced power for landing, the aircraft stalled and landed hard. The NTSB determined the probable cause of the accident to be "ice build-up on the airplane's wings and empennage which led to an inadvertent stall and hard landing."
On 05 March 1998 at Clarksville, Tennessee, a Cessna 208B was destroyed with one fatality. The NTSB factual report (MIA98FA091) indicates that light occasional moderate rime icing in cloud was forecast. The aircraft encountered icing conditions, resulting in degraded performance, loss of control, and impact with the terrain. The NTSB determined the probable cause of the accident to be that "the pilot did not maintain control of the airplane due to undetected airframe ice, resulting in an inadvertent stall, and subsequent impact with the ground."
On 07 March 1998 at Bismarck, North Dakota, a Cessna 208B was destroyed with one fatality. The NTSB factual report (CHI98FA119) indicates that the aircraft encountered icing conditions en route, and that a loss of control occurred during final approach, resulting in impact with terrain. The NTSB determined that "factors associated with the accident were the icing conditions."
On 28 April 2001 at Roque Perez, Argentina, a Cessna 208B was destroyed with 10 fatalities. The NTSB preliminary report (MIA01WA133), based on information from the Argentine investigation authority, indicates that conditions aloft were favourable for the formation of airframe icing, and that the pilot requested permission to descend to a lower altitude due to ice formation on the wings.
On 05 May 2001 at Steamboat Springs, Colorado, a Cessna 208B was destroyed with one fatality. The NTSB factual report (DEN01FA094) indicates that occasional moderate rime and mixed icing in clouds were forecast. The aircraft stalled on final approach, resulting in impact with terrain. The NTSB determined the probable cause of the accident to be "an inadvertent stall during an instrument approach, which resulted in a loss of control. Contributing factors were . . . conditions conducive to airframe icing . . . ."
On 06 March 2002 at Barrow, Alaska, a Cessna 208B sustained substantial damage and the pilot and four passengers were not injured. The NTSB factual report (ANC02FA020) indicates that severe icing was forecast and that the aircraft stalled on approach. The NTSB determined the probable cause of the accident to be "the pilot's continued flight into adverse weather conditions, and an inadvertent stall. Factors associated with the accident are . . . icing conditions."
On 15 March 2002 at Alma, Wisconsin, a Cessna 208B was destroyed with one fatality. The NTSB factual report (CHI02FA093) indicates that severe mixed and clear icing conditions in clouds and in precipitation were forecast. The pilot encountered icing inbound to an en route stop. The pilot did not have the aircraft de-iced, choosing instead to chip the ice off the aircraft before departure. "The pilot departed with the airplane contaminated with ice, into known severe icing conditions, and was unable to maintain altitude, subsequently impacting trees and terrain." The NTSB determined that one of the factors to the accident included the icing conditions.
On 08 November 2002 at Parks, Arizona, a Cessna 208B was destroyed with four fatalities. The NTSB factual report (DEN03FA012) indicates that moderate mixed and rime icing was forecast. The flight encountered icing en route at 15 000 feet and a loss of control and impact with terrain occurred.
On 24 January 2003 at San Angelo, Texas, a Cessna 208B was destroyed with two serious injuries. The NTSB preliminary report (FTW03FA089) indicates that witnesses "observed between 1/4 and 1-inch of ice on the various protected and unprotected surfaces of the aircraft." This investigation is ongoing and no final report is available.
On 29 October 2003 at Cody, Wyoming, a Cessna 208B was destroyed with one fatality. The NTSB factual report (DEN04MA015) indicates that the pilot reported light rime icing at 12 000 feet. A loss of control and impact with terrain occurred. The NTSB determined that an inadvertent stall and the snow and icing conditions were contributing factors.
On 04 November 2003 at Bangor, Maine, a Cessna 208B sustained minor damage and the pilot was not injured. The NTSB factual report (NYC04IA023) indicates that occasional moderate rime and mixed icing in precipitation and clouds were forecast. The flight encountered freezing rain en route, and the pilot requested descent and diversion to Bangor. The pilot "had no forward visibility through the windshield" and the aircraft landed hard on the runway without any landing flare. The NTSB determined that the probable cause of the incident was "the pilot's inability to see through the windshield, which was obscured due to icing conditions. . . . A factor contributing to the accident was the wing icing."
On 17 January 2004 at Pelee Island, Ontario, a Cessna 208B was destroyed with 10 fatalities. The TSB investigation (A04H0001) is ongoing. Freezing precipitation was observed at the departure airport. The aircraft was observed to be contaminated with ice before the departure, but the aircraft was not de-iced. A loss of control and impact with the lake ice surface occurred during the departure.
On 06 December 2004 at Bellevue, Idaho, a Cessna 208B was destroyed with two fatalities. The NTSB factual report (SEA05FA025) indicates that occasional moderate rime or mixed icing in clouds and precipitation was forecast, and that an aircraft on approach ahead of the Cessna 208B encountered light-to-moderate rime ice. The NTSB determined that the probable cause of the accident was "the pilot's failure to maintain aircraft control while on approach for landing in icing conditions."
On 06 October 2005 at Winnipeg, Manitoba, a Cessna 208B was destroyed with one fatality. The TSB investigation (A05C0187) is ongoing. Moderate mixed icing conditions were forecast. About three minutes after take-off, the pilot reported encountering icing conditions and requested an immediate return to the airport. A loss of control and impact with terrain occurred. On the inbound flight on the day before the occurrence, another company pilot indicated having difficulty maintaining altitude in icing conditions on descent into Winnipeg.
On 19 November 2005 at Moscow, Russia, a Cessna 208B was destroyed with eight fatalities. The Russian investigation is ongoing. Information from the Russian investigative authorities indicates that the aircraft was in level flight in icing conditions and experiencing a loss of airspeed. A loss of control and impact with terrain occurred.
Unsafe Condition and Underlying Factors
In the Winnipeg occurrence, the accident flight after departure attained a rate of climb of over 700 fpm during the first two minutes of the flight. However, as the flightFootnote 6 progressed, the rate of climb deteriorated and became an increasing rate of descent. The pilot requested a diversion back to Winnipeg about four minutes into the flight. Examination of the wreckage indicated significant engine power at the time of impact. Although the aircraft was being operated at a weight above the maximum certificated gross weight, the aircraft departed Winnipeg and climbed out without apparent difficulty. No mechanical aircraft malfunctions have been identified to date; the deterioration in aircraft performance is considered to have resulted from in-flight aircraft icing. The accident flight lasted just five minutes. The pilot's request for a diversion back to Winnipeg conforms to the procedures set out in Supplement S1. However, the aircraft's deteriorating performance and the pilot's inability to maintain control of the aircraft indicate that the procedures identified in Supplement S1 are not adequate to ensure the safe operation of the aircraft in the forecast moderate icing conditions, which prevailed at the time of the accident.
A review of aircraft performance data and the occurrences involving the Cessna 208 aircraft type indicates that it is more significantly affected by atmospheric icing than some other types of Cessna turbopropeller aircraft certified for flight into known icing conditions. The aircraft's speed does not provide it with much benefit from the effects of friction and compression. In addition to the aircraft's fuselage and empennage, the aircraft design incorporates fixed landing gear, wing struts and a cargo pod. The manufacturer's data indicate that, with residual icing on exposed aircraft surfaces, the aircraft's cruising speed decreases to a point approaching that of the stall speed. Moderate icing conditions require aircraft ice protection systems to operate with a degree of efficiency that allows the aircraft to either operate in those conditions, or to maintain altitude and a safe airspeed for sufficient time that a diversion out of those conditions can be effected. Light icing conditions reduce the demands on ice protection systems and increase the time available for diversions into more benign conditions.
The manufacturer's data (Appendix 3) and historical data from the reviewed occurrences above indicate that, in icing conditions, the aircraft's stall speed can increase substantially from 78 to 92 knots due to residual ice on the aircraft. As well, the manufacturer's data indicate that the operation of the de-icing equipment can increase the stall speed of the aircraft by 10 knots, resulting in a possible stall speed of over 100 knots in icing conditions while the de-icing equipment is operating. The manufacturer has set a minimum operating airspeed of 105 knots in icing conditions, which provides little threshold above an impending stall. In addition, the operation of the aircraft's stall warning system in icing conditions may not be reliable due to the effects of residual ice. This further reduces the pilot's ability to safely operate the aircraft in icing conditions. Some operators indicate that they have adopted the practice of maintaining 120 knots in icing conditions. The Cessna POH Supplement S1, Revision 7, dated 27 June 2005, recommends exiting icing conditions when the airspeed falls below 120 knots; however, it does not specify 120 knots as the minimum airspeed in icing conditions.
Previous Safety Recommendations
The United States NTSB initiated an in-depth assessment in late 2003 of 26 icing-related occurrences involving Cessna 208 series aircraft from 1987 to 2003. As a result of this assessment, on 15 December 2004, the NTSB issued a number of recommendations. Three of these recommendations related to:
- requiring all pilots and operators of Cessna 208 series airplanes equipped for flight into known icing conditions to undergo seasonal training for ground deicing and flight into icing conditions on an annual basis (A-04-64)
- requiring Cessna Aircraft Company, working with Cessna 208 operators, to develop effective operational strategies and related guidance materials to minimize the chance of Cessna 208 ground and in-flight icing accidents or incidents (A-04-65)
- requiring all pilots and operators of Cessna 208 series airplanes to conduct a visual and tactile examination of the wing and horizontal stabilizer leading edges and upper surfaces to ensure that those surfaces are free of ice and/or snow contamination before any flight from a location at which the temperatures are conducive to frost or ground icing (A-04-66)
The United States Federal Aviation Administration (FAA) has issued an airworthiness directive (AD) relating to the Cessna 208 aircraft type, requiring a revision to the POH icing supplement to expand guidance regarding the minimum icing airspeeds and stating that, if the pilots are unable to maintain 120 knots, they are to exit icing conditions immediately and sacrifice altitude to maintain a minimum of 105 knots. In April 2005, the FAA asked Cessna to conduct flight tests with ice shapes representing critical ice accretions (AD 2005-07-01).
The NTSB has also issued other icing-related recommendations not specific to Cessna 208 series aircraft.
The NTSB issued a number of recommendations after an accident involving an ATR 72 aircraft at Roselawn, Indiana, on 31 October 1994 (NTSB occurrence DCA95MA001). Some of the recommendations related to:
- revision of existing aircraft icing intensity reporting criteria (A-96-51)
- sponsorship of the development of methods to produce weather forecasts that both define specific locations of atmospheric icing conditions and produce short-range forecasts that identify icing conditions for a specific geographic area with a valid time of 2 hours or less (A-96-53)
- revision of the icing criteria published in FAR parts 23 and 25, and expansion of the Appendix C icing certification envelope (A-96-54)
- revision of the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate (A-96-56)
The NTSB issued a number of recommendations after an accident involving an Embraer EMB 120RT at Monroe, Michigan, on 09 January 1997 (NTSB occurrence DCA97MA017). Some of the recommendations related to:
- requiring manufacturers of all turbine-engine driven airplanes to provide minimum maneuvering airspeed information for all airplane configurations, phases, and conditions of flight (icing and nonicing conditions) (A-98-94)
- expediting the research, development, and implementation of revisions to the icing certification testing regulations to ensure that airplanes are adequately tested for the conditions in which they are certificated to operate (A-98-99)
- reviewing turbopropeller-driven airplane manufacturers' airplane flight manuals and air carrier flightcrew operating manuals to ensure that these manuals provide operational procedures for flight in icing conditions, including the activation of leading edge deicing boots, the use of increased airspeeds, and disengagement of autopilot systems before entering icing conditions (A98-101)
- reiteration of safety recommendations A-96-54 and A-96-56
On 17 January 2006, as a result of its participation in the accident investigations being conducted by the Interstate Aviation Commission of Russia and the TSB, and its assessment of the circumstances of these accidents, the NTSB issued the following recommendations:
- requiring all operators of Cessna 208 series airplanes to maintain a minimum operating airspeed of 120 knots during flight in icing conditions, even if a descent is required to do so (A-06-01) Urgent
- prohibiting all operators of Cessna 208 series airplanes from conducting flight into any icing conditions determined to be more than light icing (A-06-02) Urgent
- requiring all operators of Cessna 208 series airplanes to disengage the autopilot and fly the airplane manually when operating in icing conditions (A-06-03) Urgent
Although the manufacturer has taken action to provide procedures for the operation of the Cessna 208 aircraft type in icing conditions, pilots continue to experience difficulty in maintaining control of the aircraft and exiting those conditions as specified in the aircraft flight manual (AFM). Although the aircraft is approved for flight into moderate icing conditions, continuing occurrence experience and the manufacturer's data indicate that the aircraft may not be able to safely operate in those conditions or to safely exit those conditions as specified in the AFM. Therefore, the Board recommends that:
The Department of Transport take action to restrict the dispatch of Canadian Cessna 208, 208A, and 208B aircraft into forecast icing meteorological conditions exceeding "light," and prohibit the continued operation in these conditions, until the airworthiness of the aircraft to operate in such conditions is demonstrated.
Assessment/Reassessment Rating: Fully Satisfactory
The manufacturer's data (Appendix 4) and historical data from the reviewed occurrences above indicate that the aircraft's stall speed can increase substantially in icing conditions due to residual ice on the aircraft, combined with the effects of the operation of the de-icing equipment. Although the manufacturer has set a minimum operating airspeed in icing conditions, the Board is concerned that the recommended 105 knots is not sufficient to provide an adequate stall warning threshold. Although the Cessna POH Supplement S1, Revision 7, dated 27 June 2005, recommends exiting icing conditions when the airspeed falls below 120 knots, it does not specify 120 knots as the minimum airspeed in icing conditions. Therefore, the Board recommends that:
The Department of Transport require that Canadian Cessna 208 operators maintain a minimum operating airspeed of 120 knots during icing conditions and exit icing conditions as soon as performance degradations prevent the aircraft from maintaining 120 knots.
Assessment Rating: Fully Satisfactory
As the investigation continues, the Board may make further safety recommendations should additional safety deficiencies be identified.
Wendy A. Tadros
on behalf of the Board
Icing and Turbulence, Winnipeg area (valid 06 October at 0600 UTC)
|Total Occurrences Involving Cessna 208||37||57||51||45||85||54|
|Total Occurrences Involving Cessna 208
Where Icing was an Event
|Number of Cessna 208 Registered as of
|Cessna 208 Accident Rates Based on
|Hours Flown - Cessna 208||52 004||57 045||57 507||55 531||n/a||n/a|
|Cessna Accident Rates Based on
Cessna 208 Hours Flown per 10 000
* as reported by Transport Canada