Airports & Engines Need to Account for Large Flocking Birds

The USAir A320 was downed 15 January by birds being ingested into both engines, according to preliminary accounts of the water landing on the Hudson River of the stricken jet, in which all aboard miraculously survived and the crew is being hailed for its professionalism.

 

The USAir jet is fished from the Hudson River. Note that the left engine was torn away in the water impact.

The airplane encountered a flying flock of Canada geese shortly after liftoff from New York’s LaGuardia airport, some of which were sucked into the two engines, which choked on the feathers, bone and blood, and ceased generating thrust.

A recent study of the threat to aviation posed by large flocking birds, conducted by the Safety Regulation Group of the UK’s Civil Aviation Authority (CAA), found:

“The risk of a catastrophic accident owing to multiple engine thrust loss after a bird strike encounter with a flock of geese is rising dramatically. The risk of such a strike will be approximately 2.5 times higher in 2010 than 2000, and around 6 times higher than 1990. There are no natural forces acting to limit this population growth [in particular, of Canada geese].”

According to the study, adult Canada geese can weigh up to 16 lbs, but the average weight is 8 lbs. Although the geese usually fly at altitudes below 5,000 ft, they have been encountered as high as 20,000 feet. The USAir jet never got above 3,200 feet, so it flew into the flock at an altitude the birds favor.

Engines are tested to make sure they can ingest some birds and continue generating thrust. The Federal Aviation Regulations (FARs) are explicit about how this test should be conducted. Extracts follow:

Ҥ 33.76 Bird Ingestion
“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. Applicants must show that the associated components when struck under the conditions prescribed … will not affect the engine to the extent that the engine cannot comply with [blade containment and rotor unbalance] …
“(b) Large single bird. Compliance with the large bird ingestion requirements shall be in accordance with the following:
“(1) The large bird ingestion test shall be conducted using one bird determined from Table 1 aimed at the most critical exposed location on the first stage rotor blades and ingested at a bird speed of 200-knots for engines to be installed on airplanes …

Inlet Throat Area (square inches)       Bird weight (lb)

2,092                                                      4.07

2,092 – 6,045                                           6.05

6,045                                                      8.03

“(d) Large flocking bird. An engine test will be performed as follows:
“(1) Large flocking bird engine tests will be performed using the bird mass and weights in Table 4 [which features smaller weights, not exceeding 5.51 lbs] and ingested at a bird speed of 200 knots.
“(2) Prior to ingestion, the engine must be stabilized at no less than the mechanical rotor speed of the first exposed stage or stages that, on a standard day, would produce 90 percent of the sea level static maximum rated takeoff power or thrust.
“(3) The bird must be targeted on the first exposed rotating stage or stages at an airfoil height of not less than 50 percent measured at the leading edge..
“(4) Ingestion of a large flocking bird under the conditions prescribed in this paragraph must not cause any of the following:
“(i) A sustained reduction of power or thrust to less than 50 percent of maximum rated takeoff power or thrust …”

The full test provisions are explained in three pages of dense text, but the abridged version above captures essential details. Note first of all that for the single-bird ingestion test, the heaviest bird is the average weight of the Canada goose (8 lbs), not the maximum weight (16 lbs). For the multiple bird strike, only one “bird quantity” is required, and that carcass can weigh less, at 5.51 lbs. Thus, the multiple bird strike test does not assume the average weight of Canada geese nor must the test account for more than one bird sucked into the engine inlet.

The CAA report said:

“Geese fly in v-shaped ‘skeins’, diagonal formations with birds spaced about 10 to 12 feet apart. Thus, they must be considered as flocking birds since the same skein could strike multiple engines.”

Again, note that we are addressing only one bird in each engine; investigators will doubtless try to determine if the USAir jet’s engines were clogged by more than one bird.

It is clear that the bird ingestion standards need to be revisited, and improved to account for heavier birds flying in formations in which more than one bird is ingested into a single engine. However, once an improved standard is incorporated into the regulations, it will only apply to new types of engines.

For the flying public, there may be a better way to assess the hazard. Reported bird strikes per aircraft movements could serve as a basis for rating airports in terms of the effectiveness of their bird control programs. Airports could be rated on a scale of, say, 1, 2 or 3, or green, yellow, red, and the rates could be posted on the Federal Aviation Administration’s Web site, accessible both to the aviation industry and the public.

Such a rating system for airport wildlife management programs would certainly bring the potential risk of bird strikes into plain view.