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Dolbeer's Rule

Air Safety Week, Nov 7, 2005

Dr. Richard Dolbeer, an ornithologist with the Department of Agriculture at the Wildlife Services in Sandusky, Ohio, is one of the leading researchers into bird strikes on aircraft. He has a paper appearing in the Journal of Wildlife Management in early 2006, which isn't likely to be read by many pilots or airport managers, so relevant extracts are printed here with his permission. As he relates, "My analysis of a large number of bird strike reports, coming independently from thousands of pilots, revealed a remarkably consistent pattern of bird distribution by height above ground level."

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"The number of bird strikes (and thus birds) declined by precisely 32 percent for every 1,000 foot gain in height from 500 feet to 20,500 feet. A negative exponential equation with height as the only variable explained 99 percent of the variation in bird strikes! I call it 'Dolbeer's Rule,' " he said.

His paper presents some very practical implications for reducing strikes, which are indicated in the box. Dolbeer, by the way, last month received the FAA's "Excellence in Aviation Research Award." (Contact: Dolbeer, (419) 625- 0242; e-mail Richard.A.Dolbeer@aphis.usda.gov. Dolbeer has appeared earlier in ASW, see Aug. 16, 2004)

Height Distribution of Birds Recorded by Collisions With Civil Aircraft

By Richard Dolbeer (extracts)

My objective was to examine bird-strike reports for civil aircraft in the USA to determine the distribution of strikes by height in relation to season of year and time of day (daylight vs. dark). This analysis may prove useful to ornithologists interested in the height distribution of migrating and soaring birds and to people in the aviation industry and wildlife management profession attempting to reduce the probability of bird strikes in the airport environment and off-airports at heights above 500 feet.

Data came from the FAA's National Wildlife Strike Database for Civil Aviation. I used strike reports from January 1990-December 2004, excluding strikes with non-bird species [e.g., deer on runways] and bird strikes in which the height was not reported.

From 1990-2004, 38,961 bird strikes were reported in which height was provided; 74 percent of these strikes occurred within 500 feet of the ground, 19 percent from 501-3,500 feet, and 7 percent above 3,500 feet.

Waterfowl comprised 53 percent of the identified birds struck above 3,500 feet.

Overall, almost 3 times more strikes were reported during daylight hours than at night. However, because an estimated 4.5 times more aircraft movements occurred during day than at night, about 1.8 times more strikes per aircraft movement occurred at night than in day. This overall higher strike rate at night was due to the pronounced proportion of strikes above 500 feet occuring at night.

This analysis documented that 74 percent of all strikes and 66 percent of strikes causing substantial damage to civil aircraft occurred at ? 500 feet. The months of July-November, especially August, were the months with the highest strike rates below 500 feet. With the addition of large numbers of recently fledged birds, populations of most bird species in North America are at their highest in late summer. In addition, these young birds are less adept than older birds at avoiding aircraft. During this pre fall-migration period, most strikes (84 percent) at ? 500 feet occurred during day, probably when birds were undertaking localized movements for foraging and roosting.

This study confirmed that management programs to reduce bird strikes with civil aircraft should focus on the airport environment ... These findings were also supported by an analysis of data on the 25 large-transport aircraft (> 5,700 kg takeoff weight) crashes caused by bird strikes worldwide since 1960. These data indicate that the bird strike occurred at less than 400 feet in 23 of the 25 (92 percent) accidents. The months of July-November, especially August, are the months in which management efforts to disperse birds from airports should be most intense because these months have peak populations of recently fledged birds and the highest strike rates below 500 feet. Most strikes (84 percent) at ? 500 feet occurred during day, so management efforts at airports should focus on disbursing birds during daylight hours.

Number of reported bird strikes with civil aircraft, 1990-2004, as a function of height above ground level (AGL). The number of reported strikes declined consistently by 32 percent for each 1,000-foot gain in height.

There were 28,806 strikes reported from 0-500 feet and 12 strikes reported from 20,501 to 32,500 feet not included in this analysis. One of those was the Falcon 50 that crashed off the end of the runway at Lorain County, Ohio. The loss was reported in this publication (see ASW, Sept. 12). The event occurred after sunset, when the aircraft hit a flock of mourning doves, causing it to lose engine power. The pilots were taken to a hospital for observation and then released. The aircraft was deemed beyond economical repair. According to Dolbeer's inspection of the aircraft and report of the accident, "We found major damage in #1 engine ... There was abundant evidence of birds ? blood smears and feathers ? in all parts of the engine ... At least 2 birds were ingested in core and 2 through bypass ... #2 engine had ... evidence of 5 birds being ingested, 2 in core and 3 in bypass, based on blood smears and small remnants of feathers on fan blades and compressor blades ... Swept about 10-20 fresh mourning dove carcasses/remains off runway into grass right after the strike ... at spot where pilot said he hit birds on rotation."

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Dolbeer's Rule

Air Safety Week, Nov 7, 2005

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