On 4 August 2008, a Cessna 500 (N113SH) with the appearance of being operated by InterState Helicopters on a business charter flight from Wiley Post, Oklahoma City to Mankato MN encountered a big flock of very large birds in day VMC shortly after take-off and impacted some of them. Wing impact damage to the left wing led to immediate loss of control and terrain impact some 4 miles south of the departure airport. The aircraft was destroyed by terrain impact forces and the effects of a post crash fire, and all 5 occupants were killed.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). The aircraft was not fitted with an FDR nor required to be. The 30 minute CVR was recovered but found to have been unserviceable prior to the crash and so contained no useful data. However, it was possible to recreate the flight path followed using recorded radar data.
The flight was a single pilot operation with a second pilot carried at the request of the charterer. The 44 year-old Captain had an estimated 6,100 hours total flying experience which included 668 on turbine powered types. There was no evidence that the 40 year-old second pilot was qualified to serve as second-in-command of an aircraft type-certificated for and operated by two pilots and most of his flying experience had been acquired on single engine piston aircraft. He was present in the unofficial role of a 'pilot's assistant'. Both pilots were found to be self-employed and working for the de facto aircraft operator, InterState Helicopters on a freelance on-call basis.
The prevailing weather conditions were good and played no part in the accident. It was found that after taking off from runway 17L and reaching an altitude of 3000 feet on a standard radar heading of 200º, ATC Radar had, approximately two minutes after departure, cleared the aircraft to turn right onto a heading of 290º and continue the climb. There was no response to this clearance and an eyewitness subsequently reported that at about this time, they had seen an aircraft "crash and explode". A security camera located about half a mile southwest of the accident site had captured images of the accident aircraft descending steeply nose down towards the ground. This video also showed that impact had been followed by "a large fireball". Witnesses "predominantly reported" that the aircraft had "rolled left and spiralled or spun" into the ground. Radar data used to replicate the aircraft flight path showed that the "steep, rapid descent included a left roll through the inverted position" by the time that the aircraft hit the ground. The wreckage was located in a wooded area about 4 miles south of the departure airport and was "severely fragmented and fire damaged due to the impact and post crash fire". There was no evidence that any pieces of the aircraft had separated in flight.
A plot of primary and secondary radar returns used to reconstruct the flight path (reproduced from the Official Report)
The overlaid aerial picture above shows the aircraft track and altitude based on recordings from the Oklahoma City Airport (OKC) surveillance radar. The green primary returns are from birds - assessed visually as likely to have been Pelicans - which were calculated to have been flying at an average speed of 44 knots over Lake Overholser to the south west of and close to the departure airport.
It was found that both engines had ingested birds but that only the right engine had, as a result, stopped operating before impact. The left engine was assessed as having been capable of producing sufficient thrust to sustain flight to the extent that "the loss of power in one engine would not alone" have been the cause of loss of control of the aircraft.
There was no evidence that an in-flight fire had occurred or of any bird strike to the empennage, but remains of "splatter" attributable to bird impact was found on the right horizontal stabiliser and on the right hand side of the vertical stabiliser. Examination of flight deck window framework and the bulkhead between the flight deck and the baggage compartment found no evidence to indicate that a bird entered the flight deck and it was therefore considered "unlikely that a bird strike incapacitated either pilot" or interfered with their ability to control the aircraft trajectory from the flight deck. Only a few sections of the aircraft wings were recovered and none of these showed any evidence of bird impact. However, it was noted that witnesses had reported and video evidence confirmed that as the aircraft had descended, it had emitted "a (light coloured) visible trail from its left side" which in the absence of any evidence that there had been an in-flight fire, "likely resulted from a bird strike on at least the left wing’s leading edge structure" – which would have breached the integral wet-wing fuel tank and created a visible fuel or fuel-vapour trail.
The bird species involved was subsequently identified from DNA taken from remains found on the wreckage as American white pelican, which it was noted can reach a weight of about 20 lbs (9kg). A witness to the event “commented that a flock of pelicans had been in the area for about 2 weeks before the accident" and another witness had observed (and subsequently recovered) bird debris identifiable to this species from the vicinity of the aircraft at the point where its flight path became irregular.
It was noted that the Airframe Certification Standard applicable to the Cessna 500 required that the wing structure is certificated to withstand a 4 lb bird impact at a cruise speed of 287 knots such that continued safe flight and landing remains possible. The Investigation "performed a bird-strike energy study that determined that the kinetic energy of such a strike […] is 14,586 foot-lbs”. Based on the estimated speed of the accident aircraft of almost 200 KTAS at the apparent impact position, and a bird weight of as much as 20 lbs, it was noted that the kinetic energy of the collision could have been in excess of 35,000 ft lbs for a single bird, far in excess of certification requirements. It was noted that although not a factor in this accident, certification requirements for the empennage on the aircraft type involved required that it be able to withstand an 8 lb bird strike.
From all the assembled evidence, it was concluded that the loss of control that had occurred was likely to have been a direct consequence of damage to the left wing, sustained as a result of impact with at least one very large bird of the flock which had been crossing the flight path in a loose formation.
It was accepted that Controllers are required to provide bird activity reports to pilots when such information is available, but noted that in this instance, there was "no evidence that controllers had any information about any known bird activity that could have been used to alert the flight crew of a heightened bird hazard".
It was, however, noted that Wiley Post Airport had not complied to the requirement in FAA AC 150/5200-33B that "operators of airports surrounded by woodlands, water, or wetlands should provide for a wildlife hazard assessment (which includes birds) conducted by a wildlife damage management biologist" and also to the requirement that "airport operators should establish a distance of 5 miles between the farthest edge of the airport operations area and any wildlife attractant that could cause hazardous wildlife movement into or across the approach or departure airspace”. The Investigation was concerned that the FAA had not detected that the airport had failed to conduct a wildlife hazard assessment, especially given that it was "surrounded by multiple attractants, some of which (including Lake Overholser) were known to the FAA" because they were detailed in the FAA-approved wildlife management plan of the nearby but much larger Oklahoma City airport.
The Investigation noted that pilot awareness of the role of airspeed in reducing the risk of damaging bird impact could be heightened by taking more account of reference charts that show the relationship between aircraft speed and bird weight for equivalent bird-strike energy. To that end, the Investigation included an example of such a chart applicable to a Cessna 500 in its Final Report. This is reproduced below and shows two bird-strike energy speed curves, a blue line corresponding to the 4 lb bird at 287 knots and a red line corresponding to the 8 lb bird at the same speed. It also shows the minimum safe airspeed (1.3 times the stall speed) as a function of weight and in various flap configurations - shown as solid, dashed or dotted black lines.
An example of a reference chart showing airplane speed and bird-weight relationships for equivalent bird-strike energy (reproduced from the Official Report)
Entirely separate from the Investigation into direct cause, a substantial part of the Investigation also examined the basis on which the accident flight had been operated. Representatives from the organisation which "facilitated" the flight -InterState Helicopters -, from the aircraft owner Southwest Orthopedic and from the charterer - United Engines - "all denied responsibility for operating the flight" and "no documented aircraft lease, time-sharing, pilot services, or other agreements were discovered to help determine the identity of the operator or the nature of the flight". Whilst "the accident pilot was certificated, trained, and qualified" to fly the accident aircraft in non-commercial operations as a single pilot, the second pilot was "not trained, qualified, or current" to fly it, although it was noted that "because the pilot was authorised to fly [...] as a single pilot, the second pilot could occupy a cockpit seat and assist the pilot as directed".
The Investigation considered that "given the apparent decades-long existence of operational issues similar to those which were observed in this accident, a combination of measures must be taken to ensure that all participants in each facet of a flight operation - operators, pilots, aircraft owners and managers, aircraft lessees and charter customers - are aware of and take responsibility for their respective roles". It was concluded that a solution to the problem would depend on better "lease disclosure, pilot reporting, and carriage disclosure requirements, as well as improved customer awareness resources and Federal oversight and surveillance". In respect of the latter and Interstate Helicopters, it was concluded that it had been ineffective over a significant period prior to the accident.
The key formally-stated Conclusions of the Investigation in respect of the direct cause of the loss of control of the aircraft included the following:
- The airplane’s departure from controlled flight likely resulted from wing structure damage sustained during an in-flight collision with a flock of American white pelicans, which far exceeded the airframe’s design certification limit. This damage would have made the aircraft un-flyable.
- The current airframe certification standards for bird strikes are insufficient because they are not based on bird-strike risks to aircraft derived from analysis of current bird-strike and bird-population data and trends, and because they allow for lower levels of bird-strike protection for some structures on the same airplane.
- The accident airplane’s left engine was capable of producing sufficient thrust for the airplane to continue to fly with the right engine inoperative, and the loss of power in one engine would not alone result in a loss of control of the airplane.
- Because an airport’s wildlife hazard management plan is based on a wildlife hazard assessment, Wiley Post Airport’s failure to perform such an assessment as required prevented the determination of what mitigation measures, if any, could have been implemented to reduce the risk of an in-flight collision with American white pelicans.
- Reference charts that depict both the airspeeds at which the airframe can sustain strikes from various-sized birds without exceeding certification standards and minimum safe airspeeds could help pilots devise precautionary operational strategies for minimizing potential airframe bird-strike damage in high-risk areas for bird strikes.
- Reliable information about the mass, numbers, and activity of birds likely to be encountered near the airports of operation is necessary for pilots who wish to devise precautionary operational strategies for minimizing potential airframe bird-strike damage.
It was formally concluded that the Probable Cause of the accident "was airplane wing-structure damage sustained during impact with one or more large birds (American white pelicans), which resulted in a loss of control of the airplane".
Ten Safety Recommendations were made as a result of the Investigation as follows:
- that the FAA revise the bird-strike certification requirements for 14 Code of Federal Regulations Part 25 airplanes so that protection from in-flight impact with birds is consistent across all airframe structures (and) consider the most current military and civilian bird-strike database information and trends in bird populations in drafting this revision. [A-09-72]
- that the FAA verify all federally obligated general aviation airports that are located near woodlands, water, wetlands, or other wildlife attractants are complying with the requirements to perform wildlife hazard assessments as specified in Federal Aviation Administration Advisory Circular 150/5200-33B: Hazardous Wildlife Attractants On or Near Airports. [A-09-73]
- that the FAA require aircraft manufacturers to develop aircraft-specific guidance information that will assist pilots in devising precautionary aircraft operational strategies for minimizing the severity of aircraft damage sustained during a bird strike, should one occur, when operating in areas of known bird activity. This guidance information can include, but is not limited to, airspeed charts that depict minimum safe airspeeds for various aircraft gross weights, flap configurations, and power settings; and maximum airspeeds, defined as a function of bird masses, that are based on the aircraft’s demonstrated bird-strike energy. [A-09-74]
- that the FAA require all 14 Code of Federal Regulations (CFR) Part 139 airports and 14 CFR Part 121, Part 135, and Part 91 Subpart K aircraft operators to report all wildlife strikes, including, if possible, species identification, to the Federal Aviation Administration National Wildlife Strike Database. [A-09-75]
- that the FAA revise 14 Code of Federal Regulations 91.23 truth-in-leasing regulations to include all turbine-powered airplanes. [A-09-76]
- that the FAA require that Federal Aviation Administration flight plans include a block for the pilot to identify the operator and a block to specify the operating rules under which the flight is being conducted. [A-09-77]
- that the FAA require 14 Code of Federal Regulations Part 135 on-demand operators to provide their customers with a written document, correspondence, or ticket that expressly describes the terms of carriage, including the regulatory part under which the flight is operated. [A-09-78]
- that the FAA update and keep current the aircraft charter guide on the Federal Aviation Administration’s website to include reliable information on the certification status of on-demand commercial operators and the aircraft that they are authorized to operate. [A-09-79]
- that the FAA explore and implement strategies to improve on-site inspector surveillance activities at airports and of flight operations to detect and deter improper charter operations. [A-09-80]
- that the FAA assess why existing policies, procedures, and practices resulted in a failure to detect the noncompliant actions of Interstate Helicopters and develop additional methods, measures, or procedures for performing inspections of and following up on complaints about 14 Code of Federal Regulations Part 135 on-demand operators that can successfully detect noncompliant charter operations. [A-09-81]
One Previously issued Safety Recommendation concerning the functionality of CVRs made three years previously was also reiterated and fully restated.
The Final Report of the Investigation was adopted by the NTSB on 28 July 2009 and subsequently published.