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Crash Analysis of Cessna

Winter 2004

 

1.0 Introduction and Objectives

On January 10 around 4:40 pm a Cessna 208B Grand Caravan crashed shortly after take off from Pelee Island.  The single engine plane crashed into Lake Erie 1 ˝ kilometres off Pelee Island.  At the time of the crash there was snow showers and freezing rain.

The objective of this report is to reconstruct the accident and determine the possible cause of the crash and the possible factors that may have been involved. A decision will be made based on the analysis presented on whether the inherent operation characteristic of the airplane, equipment malfunction, pilot error or weather conditions were the main contribution factors in the accident.

2.0 Background Information

  •  There were ten people on board at the time of crash [2]

  • The airport runway is 3300 ft. long and paved [3]
  • The plane left Pelee Island at 4:30pm en route to Windsor [2]
  • The wreckage was located at 5:00pm, 1 ˝ km west of the island [2]
  • It wasn’t in the air very long [4]
  • There was snow showers and freezing rain at the time of takeoff [4]
  • Cessna has known since the 1990s there was a defect in the design or manufacturing of its 208B Caravan that made it prone to icing [2]
  • Tests showed the plane was prone to stall after a "minor" amount of ice got on the wings and tail [2]
  • The weather at time of crash was reported as icy rain, with drizzles of snow [3]
  • An Alaska plane crash involving the same aircraft bears striking resemblance to the Pelee Island crash. Both planes crashed moments after takeoff. Both were piloted by men who had flown four years for their airlines. Both led investigators to examine if ice built up on the wings, and if a build-up caused the crashes [3]
  • The U.S. National Transportation Safety Board concluded one year ago that undetected ice caused the Alaskan crash [2]
  • Transportation Safety Board said ice on the plane's wings is "definitely" one of the causes [2]
  • There have been other crashes of the Cessna due to ice formation.
  • Other possibilities could be mechanical or that the plane was overloaded [3]
  • Officials say the plane was designed to carry 10, but the passengers were also travelling with heavy hunting gear and two dogs were on board [3]

3.0 Cessna 208B Grand Caravan Information

 

Specifications

 

Engine

PT6A-114A

Fuel Capacity

2248 lbs

Thrust Type

3-Bladed Propeller, Constant Speed, Full Feathering, Reversible

Shaft Horse Power

675 Hp

 

 

Performance Characteristics

 

Ceiling

25000 ft/ 7620 m

Cruise Speed @ 10,000 ft

184 ktas /341 km

Range @ 10,000 ft

907 nm/ 1,596 km

Rate of Climb

975 fpm / 297 mpm

Stall Speed

31.39 m/s

Takeoff Distance

Ground Roll

50 ft. Obstruction

 

1365 ft / 416 m

2420 ft / 738 m

Landing Distance

Ground Roll

50 ft. Obstruction

 

950 ft / 290 m

1795 ft / 547 m

Maximum Useful Load

4421lb /2041kg

Maximum Weights

Ramp

Takeoff

Landing

Empty Weight

 

8785 lb / 3985 kg

8750 lb / 3969 kg

8500 lb / 3856 kg

4364 lb /1979 kg

Power Loading

13.0 lbs/hp

Standard Empty Weight

1944 kg

Wing Loading

31.3 lbs/ft2

Table 1 Specs on the Cessna 208B Grand Caravan

Figure  1 Range and payload

4.0 Analysis and Discussion

 

4.1 Airplane’s Weight

The Cessna was carrying 10 people including the pilot.  It is assumed that the average weight of each passenger is 170 lb, and each passenger was carrying 50 lb of payload [6].  Furthermore it was reported that there was two dogs which were estimated to be 75 lb each.  It is also assumed that the plane was carrying a maximum of 2000 lb of fuel. Thus the weight of the passengers and cargo at take off was 4350 lb.  The weight of the plane (4364lb) with the cargo at takeoff was 8714 lb, which is barely within the maximums takeoff weight (8750lb) as specified in Table 1.  Ice build up on the wing and the fuselage might have added extra weight, but this extra weight is assumed to be small and negligible.  Although the assumptions could have been overestimated , the plane still would be in the upper end of its capacity.  Since the plane was able to takeoff with its load and fly for 1 ˝ km the overloading of the plane alone is an unlikely cause for the crash.  If the plane was overloaded it wouldn’t have been able to take off.   

 

4.2 Stall Speed

Stall speed is the minimum speed at which the wings can produce sufficient lift for level flight.   Stall speed is calculated by the equation below.

Detailed calculations of the stall speed are detailed in Appendix A.  It was assumed that the airfoil was two dimensional and was a NACA 23000 airfoil, and had and approximates CLmax value of 2.6 for takeoff and 2.0 for steady flight [6].  It was calculated that the stall speed was 30.6 m/s for take off and 34.6 m/s for steady fight. These values are close to the stated value in Table 1.

 

4.3 Takeoff Distance and Thrust

It was found that the plane wreckage was located 1 ˝ kilometres west of the Pelee Island [2]. Thus, it is evident that the plane crash occurred while the plane was still climbing and had not reached cruising altitude or steady level flight.

 

The calculations and the assumptions made are presented in Appendix A.  The equations below were used to calculate the takeoff distances.

 

The table below summarizes the takeoff distance required at different thrust utilizations. 

Thrust Utilization

Take of Distance with 50 ft obstruction

100%

669 m / 2196 ft

90%

727 m / 2386 ft

75%

842 m / 2764 ft

60%

1015 m / 3332 ft

50%

1188 m / 3900 ft

Table 2 Take off distance at differnt thrust utilization

Utilizing 90% of thrust gives a take off distance that is very close to the value specified by Tble 1.  Since the actual runway length at Pelee Isaland is 3300ft, the pilot had sufficient space to take off even if he was no utilizing 100% of the available thrust.   

4.4 Weather Conditions

There was freezing rain at the time of the crash.  The amount of time the Cessna    was exposed to the freezing rain before it took off is not know, and it is also not know if the plane was de-iced before take off.  Thus it in not sure how much ice was built up in the wing.  The three main effect of ice on wings are: 

1)     Added weight to the airplane

2)     A change in lift and drag forces

3)     A change in stability characteristics

Icing occurs usually in the presence of super-cooled water impinging on aircraft wings. There are usually two types of ice formations on aircraft wings. One is rime ice, the type formed at low ambient temperatures below -15°C. All the water impinging on the aircraft wing is assumed to be converted to ice instantly. The second form of ice accretion is glaze ice. Glaze ice is formed in ambient conditions close to freezing temperatures. Since the temperatures are not very low to force freezing of all impinging water, there is a mixture of ice and water on the surface.   The conditions on the day of the crash were below freezing, so the rime ice is likely to have to have formed on the wing due to the freezing rain.

Ice build up can affect the shape of the airfoil, which can change its aerodynamic charactersitcs such as drag, lift and stability.   Ice accumulates at different rates depending on airspeed, temperatures, droplet size, moisture density, etc.  On an airfoil, ice typically builds ups on the leading edges, upper and lower surface of the wing, and the horizontal stabilizers.  Ice on the wings can cause flow separation near the leading edge which changes the drag and lift of the airfoil.

A study of the effect of ice formations on the lift and drag on an unswept NACA airfoil determined the following principal results 

1)     The detrimental aerodynamics effects as a function of the shape and size of the ice formation near he leading edge of the airfoil.  The size and shape of the ice were function of water content, temperature, angle of attack, and airspeed [7].

2)     Icing on the airfoils at angle of attack of less than 4 degrees caused an increase in the section drag coefficient (up to 350%), and reductions in section lift coefficients (up to 13%) [7].

3)     At angle of attack greater than 4 degrees the section drag coefficient actually reduced by around 45% [7].

 These facts explains why the plane crashed shortly after takeoff.  During takeoff the flaps were down to create a high angle of attack.  At this angle the ice on the wings would not have caused any increase in drag.  However slightly after take off the pilot would have brought the flaps up, and lowered the angle of attack.  At this point the airplane would have experienced a sudden significant drag increase and reduction in lift, causing the airplane to stall.  Since the airplane was loaded to capacity the pilot ability to climb and recover from the stall was probably seriously compromised since he didn’t have enough thrust.   

Furthermore it is possible that the ice formation on the wing accelerated as the velocity of the airplane increased upon takeoff, and this might have been further accelerated as the plane flew over the lake.  Generally the temperature is lower and the moisture content of the air is higher of the air over a large open body such as a lake.

There have been several crashes of Cessna Grand Caravan which are strikingly similar to the crash at Pelee Island.  Investigations have revealed that ice formation on the wings as the cause of the crash.  This fact along with reports that the design of the grand Caravan is susceptible to ice formation, leads one to believe that ice formation on the wing was the most probable cause of the crash. 

4.5 Pilot Error

Based on the background information the pilot had many years of flight experience on the same plane, and has had experience flying in various weather conditions.  However despite the experience mistakes can happen.  Because of the airplane's high-wing configuration, the pilot may have failed to notice any ice or frost on the upper surface of the wing, and failed to de-ice the plane.  It is also possible that the pilot overloaded the plane.  Although the weather conditions were primarily the cause of the crash, the accident may have been avoided if the pilot realized the detrimental effects of freezing rain and ice, and realized that the plane was fully loaded.

5.0 Conclusions

Based on the analysis performed, the most probable cause of the crash is the build up of ice on the wings.  The ice caused a sudden increase in drag force and causes the airplane to stall.  The ice build up is attributed to the freezing rain and the design flaw on the Cessna which makes it prone to ice formation.  However the fact that plane was fully loaded made the situation worse since the pilot didn’t have enough thrust to recover from the stall.  If the plane was carrying less load the pilot might have had a chance to recover.  Some responsibility must be placed on the pilot for failing to take any measures to check if there was any ice on the wings, and failing to recognise the impact of the weather conditions and payload he was flying.  An inquiry into whether Cessna knew about the design flaw and took no action must be conducted.