Vulture Disaster: Bad Luck, Poor Planning or Fate?

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A final report published earlier this year by the Spanish civil aviation accident and incident investigation commission lays out the heart-wrenching end to a family of four out for an adventure in their four-seat general aviation aircraft. 

The report is not new – it came out in March – but not much has been reported on its results.

Was their fate avoidable? 

A basic tenet of pre-flight planning is to “become familiar with all available information concerning that flight….” That’s a tall order given the ever-growing glut of information available, and one that I have to admit I do not always carry out 100%, particularly for a pleasure flight on a known route.

But in the case of the family in a Socata TB-20 (EC-ESK) flying home to Leon, Spain, the afternoon of January 16, 2016, after a Saturday in Alicante on the Mediterranean coast, did it prove to be fatal?

Two key pieces of pre-flight information or knowledge about the flight over the beautiful Serrania de Cuenca Natural Park would have been critical, both of which are relatively obscure. 

- The Spanish aeronautical information publication (AIP) showed a “bird gathering area” along the route. 

- The impact energy when hitting one of those birds goes up with the square of the aircraft’s velocity.

Radar tracking of the aircraft showed it to be flying at a ground speed of 140 kt. and 2,000 ft. when it struck a griffin vulture.

Given the winds at the time, the aircraft likely had a headwind, so actual airspeed would have been 140 kt. or greater. The adult bird, which most likely had a wingspan of 7.5-8.7 ft. and weighed from 13-20 lb., hit the leading edge of the left wing, which then detached from the aircraft, falling to the ground along with the bird. The aircraft went out of control and crashed.

Data from the radar track was overlaid on the bird concentration map published in the AIP, and the accident location coincided with “an area identified as having one of main vulture breeding colonies,” according to the final report. Griffon vultures glide between 6000 ft. – 11,500 ft., but sometimes climb to as high as 20,000 ft. 

The low-time pilot (334 hr. flight time) appeared to be conscientious, filing a flight plan and talking with air traffic control, but was he aware of the AIP and the griffon’s? Investigators talked to his friend and partner, who said he flew the route often. 

What’s an AIP anyway ? It’s an ICAO-recommended document published by a country’s air navigation service provider (in our case, the FAA) every six months with updates in-between covered by Notams. To be honest, I’d never looked at one until reporting this story. I do however regularly look at the FAA’s Airport/Facility Directory, which has much of the same information, but the AIP is a bit broader and includes, among other information, some generic wisdom about birds, including:

“Avoid overflight of known areas of bird concentration and flying low altitudes during bird migration. Charted wildlife refuges and other natural areas contain unusually high local concentration of birds which may create a hazard to aircraft.”  FAA AIP 23rd Edition Amendment 3 (May 26, 2016)

Being aware of bird hazards is one thing, but avoiding them in flight is another. In this case, investigators think the setting sun may have been in the pilot’s eyes, making avoidance difficult if not impossible. When pilots do see birds, particularly in flocks, the AIP recommends climbing to avoid a collision “because birds in flocks generally distribute themselves downward, with lead birds being at the highest altitude.” It also recommends putting landing lights on in areas “where flocks of birds may be expected”

Even if the pilot had expected to encounter griffon vultures in the area, did he understand the certification criteria for his aircraft with respect to bird strikes or the relationship between his cruise speed and impact energy? 

Possibly not. 

The Spanish safety board notes that the TB-20, like all Part 23 light aircraft, does not have to undergo bird impact analysis or testing as part of certification. For commuter aircraft certified under Part 23, the aircraft must handle (without penetration) the impact 2 lb. bird at the aircraft’s maximum approach speed with flaps extended (Part 25 aircraft – airliners – have more rigorous requirements).

Investigators note that the kinetic energy (KE) of a 2 lb. bird at the maximum flaps extended approach speed for a commuter like the Fairchild SA-227 Metroliner is 5,566 Joules.

Based on the size of a mature griffin vulture, the aircraft would have had to handle a KE of 15,561-23,342 Joules, depending on the bird’s weight (13-20 lb). “Therefore, normal, utility, acrobatic and commuter category airplanes are not certified to withstand impacts with a kinetic energy as high as that delivered by this impact with the vulture,” the final report states. Click HERE for an excellent NTSB analysis of how to minimize bird-strike damage.

However, if the pilot had throttled down for a cruise speed to 100 kt., the velocity-squared effect would have lowered the KE to 7,803 – 11,704 Joules, a hefty decrease that may have helped here. 

My condolences to anyone who knew this family. We can only hope that their story might help others avoid the same fate.

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