Final Rule, Engine Bird Ingestion Airworthiness Standards

Wed, Nov 28, 2007 — David Evans


17 October 2007
FR Doc E7-20407 – Docket No. FAA-2006-25375

Changes announced by the FAA better address the threat of flocking birds to turbine engine ingestion. The action also harmonizes U.S. standards with those of the European Aviation Safety Agency (EASA). The new standards, though, “do not go far enough,” according to the National Transportation Safety Board (NTSB) in its comment of September 2006 on the draft standards.

In this respect, the FAA may have “harmonized down” to a European standard the NTSB, by implication, deems inadequate.

The FAA has decreed that:

“The impact to the front of the engine from the large single bird, the single largest medium bird which can enter the inlet, and the large flocking bird must be evaluated …
“The bird must be targeted on the first rotating stage or stages at a blade airfoil height of not less that 50 percent measured at the leading edge.
“Ingestion of a large flocking bird under the conditions described in this paragraph must not cause … a sustained reduction of power or thrust to less than 50 percent of maximum rated takeoff power or thrust …”

The NTSB is concerned that the size of the bird is too small, based on an 8-pound snow goose, and that a Canada goose carcass, weighing up to 24 pounds, would more appropriately represent the threat. Moreover, the test should include pre-existing service-acceptable blade damage to ensure sufficient margin against preexisting damage effects.

According to a special report produced this month by Richard Dolbeer and John Seubert of the wildlife services branch of the U.S. Department of Agriculture, there were 9 Canada goose strikes with engine damage in 2006, and the Canada goose population is continuing to multiply, totaling some 5.5 million birds in 2006. This marks a fourfold increase from 1970. Separately, Dolbeer advises that the NTSB’s estimated 24-lb. weight for the Canada goose is awfully high; a male goose, heavier than females, typically weights about 9 pounds (compared to 7.6 pounds for a male snow goose). Perhaps the NTSB meant to refer to adult male swans, which weigh 25-26 pounds.

The NTSB is also concerned about the intent to conduct the test at less than the full power of the engine:

“The Safety Board is … concerned about the FAA’s proposal to conduct the flocking bird ingestion test at only 90 percent of an engine’s rated thrust. The Board concurs with the FAA’s assessment that the world’s jetliner fleet uses de-rated power for most takeoffs. However, the Board also notes that all other engine certification tests … are conducted at maximum power. The level of engine unbalance that occurs if a blade breaks off after a bird strike increases exponentially with the rotor’s speed. Thus, the level of engine unbalance from a fractured fan blade would be significantly higher if the engine were operating at maximum rpm than if the engine were operating at 90 percent power.

“Although the Safety Board recognizes that conducting the flocking bird ingestion with a Canada goose and with the engine operating at maximum rpm would require a more robust design, the Board believes that this robust design would be more likely to survive a bird ingestion event.”

The FAA has dismissed the NTSB concerns an all counts. The proposed rule, the FAA said, “was not intended to encompass the worst possible combination of factors.” Engine type certification tests, for example, “are intended for and applied to undamaged products as a baseline.”

The problem here is that the FAA’s pristine baseline of engine condition, power setting, and size of birds ingested also biases testing towards results that could be misleading in the real world.

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