This article discusses the significance of bird ingestion in jet engines. Bird strikes occur at various aircraft locations, however, usually inflict most damage to the engines. Airplane damage and effect on flight from bird strikes are closely correlated to kinetic energy, derived from the mass (determined by the bird species) and the velocity of collision squared. The US statistics described in the article show that bird ingestion in commercial jet engines is significant and even more so, when combined with records from the rest of the flight world. Current statistics show that incidents of commercial aircraft jet engine bird ingestion are increasing, and are a continuing challenge over the next decade. Recently, European Aviation Safety Agency, working with other authorities, is proposing additional original equipment manufacturer bird ingestion testing requirements for an engine operating under climb conditions, following the ingestion of a medium sized bird into the engine core. It is also emphasized in the article that there is a need of a new systems approach to adequately mitigate the risks of aircraft and bird occupying the same air space at the same time.
#34 May 2018
For those of us who attended last year's ASME TURBO EXPO '17 in Charlotte, North Carolina, there was an opportunity to visit a unique museum jet airliner display. The largely intact wreck of the Airbus A320 that landed safely in the Hudson River is on display at Charlotte's Carolinas Aviation Museum, complete with its two bird-ingested disabled jet engines.
As you may recall, this was an airline bird strike incident that has come to be called “Miracle on the Hudson”. On January 15, 2009, US Airways flight 1549, this Airbus 320 with 150 passengers was taking off from La Guardia Airport bound for Charlotte. About 3 minutes from takeoff and at about 2800 feet altitude, it struck a flock of Canada geese just northeast of the George Washington Bridge. Each CFM56 engine ingested at least two geese (weighing about 8 pounds each), one of which was ingested into each engine core. This caused mechanical damage, which prevented both engines from maintaining thrust for sustained flight. The crew then successfully ditched the aircraft in the Hudson River with no loss of life.
Dual bird ingestion engine incidents in twin engine jets are proving not to be a rare occurrence. Three other such incidents occurred in 2009 (a Boeing 737 destroyed at Rome's Ciampino Airport, an A320 on takeoff at Bourgas, Bulgaria, and a Boeing 737 in Ireland).
Aviation regulators are now proposing new bird ingestion tests for aircraft engine certification to address these growing flight safety issues. I'll give a short review of all of this in what follows.
Bird Ingestion Engine Damage
As Boeing points out , bird strikes occur at various aircraft locations (see Fig. 1) but usually inflict most damage to the engines. Airplane damage and effect on flight from bird strikes are closely correlated to kinetic energy, derived from the mass (determined by the bird species) and the velocity of collision squared. (A 20% increase in speed raises the kinetic energy by 44%.)
Just how many engine bird strikes occur for civil aviation? Dolbeer, et al  report on statistics for the U.S. (which includes U.S. registered aircraft in foreign countries) for the recent quarter century, 1990-2015. During these 25 years, 17,494 jet engines were struck in 16,694 bird strike events. Out of these, 4516 engines were damaged in 4370 bird strike events (4227 events with one engine damaged, 141 with two engines damaged (such as the Hudson landing event), 1 with three engines damaged, and 1 with four engines damaged).
These U.S. statistics indeed show that bird ingestion in commercial jet engines is significant and even more so, when combined with records from the rest of the flight world.
Current statistics show that incidents of commercial aircraft jet engine bird ingestion are increasing, and are considered to be a continuing challenge over the next decade. Factors that contribute to this avian threat are increasing populations of large birds and increased air traffic by quieter, turbofan powered aircraft . These and other factors have been discussed in two past issues of this column .
Bird Strike Certification Testing
All commercial jet engines must comply with bird ingestion regulations established by regulatory authorities such as the U.S. Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA). These regulations involve certification testing of commercial jet engines for bird ingestion, calling for demonstrations of an engine's ability to ingest birds in small, medium and large categories at takeoff power and still maintain a specified level of performance. I refer the reader to a concise and a fascinating GGTN article by Robert Mazzawy  on details of current bird strike certification testing by OEMs.
Recently, EASA, working with other authorities, is proposing additional OEM bird ingestion testing requirements for an engine operating under climb conditions, following the ingestion of a medium sized bird into the engine core. The test engine must continue to operate with a fan speed representative of climb conditions, and then approach conditions for a safe landing. (If the test engine includes features that prevent bird material from entering the core, the engine should continue to operate at approach conditions, after ingestion.)
It seems that the major findings in the EASA led proposal, is that current tests don't result in enough bird mass reaching the engine core where it can lead to significant power loss. They want OEMs to increase the threat mass (bird size and/or number) and to adjust the engine RPM and bird velocity to increase the chance for a bird to get through fan blades and reach the core (as happened with the Hudson landing incident). They also are placing priority on doing run-on tests of the engine after the strike to validate its ability to safely land. Fan blade design has advanced to where blade failure is not the issue (although proving this is still part of the certification testing) so any new testing is focused on the core intrusion by the bird(s) along with run-on requirements.
A Few Comments
As I commented in an earlier column , approaches to solve birdstrike issues by the civil aviation community are rather fragmented. Engine companies are repeatedly called upon to make their engines strong enough to endure bird ingestion, rather than regulations being enacted to prevent the bird strikes themselves.
As Capt. Paul Eschenfelder, a retired Delta airlines pilot has told me, the proposed regulations discussed above are a big and best change, in that they require an engine robust enough to last until a successful turn back can be made. But what is really needed is a new systems approach to adequately mitigate the risks of aircraft and bird occupying the same air space at the same time.