Engine Certification: Meet the Flockers

Washington DC
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A new analysis under way in the USA and Europe will determine whether "large" flocking birds - those weighing more than 1.13kg (2.5lb) each - are a growing threat to narrowbody aircraft and, as such, should be part of the engine certification criteria.

Heightening the expectations of the work, which began in 2009, are several recommendations by the US National Transportation Safety Board from its wrap-up of the January 2009 ditching of US Airways Flight 1549 in the Hudson river.

In that accident, both of the A320's CFM International CFM56-5B engines simultaneously ingested several Canada geese on the climb out from New York's LaGuardia airport. The birds, which weigh on average 3.6kg each, were ingested into the engines and cores, causing a near-total loss of thrust.

damage caused by birdstrike to a320

Click for full size image

Wildlife experts say the population growth among such large birds is cause for concern. "We have a lot more large birds out there today than we used to," says Richard Dolbeer, a retired US Department of Agriculture (USDA) scientist and wildlife hazard consultant. "Almost all of the large birds over 1.81kg have increased, many of them dramatically."

Data compiled by the European Aviation Safety Agency shows that worldwide, from 1999 to 2008, large flocking birds accounted for a full 45% of all reported birdstrike accidents, by far the largest contributor across all sizes of bird.

Dolbeer says the Canada goose is the most problematic of the large birds in the USA, with the resident year-round population increasing from 1 million birds in 1990 to 3.5 million now. Similarly, the snow goose population in central and eastern North America has gone from 1 million birds in 1970 to as many as 4 million now. "What makes it so dangerous is that when we certify, it's a single large bird going into one engine," he says. "We don't talk about flocks going into both engines." The bird ingestion analysis under way by the government/industry bird-ingestion rulemaking database group comes nearly a decade after the same group delivered a report that led to new Federal Aviation Administration "large flocking bird" engine certification requirements in 2007.

Among other new criteria, the rules required manufacturers of larger engines - those with 2.5m2 (27ft2) inlet area or larger, or about a 178cm (70in) fan diameter - to ingest a single 1.81-3.6kg bird (the larger the engine, the larger the bird) and continue to maintain at least 50% thrust for 20min, including one go-around on return to the airport. For comparison, a snow goose typically weighs about 2.5kg. The test did not, however, apply to narrowbody engines such as the CFM56, which has an inlet diameter of about 165cm.

hudson river a320, rex features
© Rex Features

Both of Flight 1549's CFM56-5B engines simultaneously ingested several Canada geese

By way of certification, narrowbody engines have to survive a battery of bird collision tests - typically with much smaller birds than geese, however.

Engines like the CFM56 or Engine Alliance V2500 have two specific tests - a "small and medium flocking birds" test and a single "large bird" test. The flocking bird test requires the engine to continue providing at least 75% thrust for 20 minutes after ingesting a mix of 0.68kg and 1.81kg birds, some of which must be directed into the core. A herring gull typically weighs about 1.13kg.

For the large bird test, which involves shooting a 1.81-2.5kg bird into the fan spinning at full power, the engine is not required to continue performing, but it must not detach from the aircraft or catch fire and pilots must be able to shut it down.

An engine the size of a CFM56 is required to withstand a hit by one 2.72kg bird in the large bird test. Larger widebody engines, like the GE90, are subject to the small and medium flocking bird test as well as the large single bird test, but also the large flocking bird test for which the engine must continue to develop 50% thrust for 20min. For each test, the size or number of birds varies upward with the size of the engine.

BIRDSTRIKE DATA

The NTSB, after its May 2010 final meeting on the Hudson river accident, asked the FAA to have the bird-ingestion rulemaking advisory group determine if the 2007 large-flocking-bird engine requirement should apply to smaller engines such as the CFM56, with inlet areas less than 2.5m2.

CFM's analysis of the Hudson river A320 engines shows that large portions of the geese not only damaged the engine fans, which provide 80% or more of an engine's thrust, but also significantly damaged core components in the low- and high-pressure compressor, combustor and turbines (see infographic). The bird-ingestion group, assembled under the auspices of the Aerospace Industries Association, plans to complete its work and report back to the FAA by summer, but officials are not saying what their initial look at the data from thousands of birdstrikes is showing.

flight phase of birdstrikes

"The working group has agreed that the core ingestion element of the overall bird ingestion threat needs closer evaluation against the safety objective of the rule, and against our standard practices for conducting such tests," says the FAA. "At this time no final conclusion has been reached."

Included with the EASA, the FAA, Airbus and Boeing in the rulemaking committee are engine makers General Electric, Honeywell, Pratt & Whitney and Rolls-Royce.

The FAA will in turn respond to the NTSB recommendation. Engine manufacturers developing new-generation engines are spending significant resources on new fan designs, a process complicated by comprehensive bird-ingestion survival criteria, even with today's criteria.

To handle a larger bird, a fan blade must be able to withstand higher forces, which in turn may require a stronger, heavier material, which requires a stronger, heavier hub mechanism; a thicker, heavier blade-out containment case; and sturdier bearings, shafts and engine mounts.

TAKE-OFF POWER

"A Pratt & Whitney internal study showed that adding a tiny amount of material to a fan blade, equivalent to the thickness of two sheets of copy paper, could result in more than 45kg of extra engine weight due to the required structural compensation," says P&W flight safety investigations lead Christopher Demers in a 2009 paper presented at the Bird Strike North America conference.

A fan blade at take-off power can travel as fast as 1,400ft/s (427m/s) at the tip, which can result in bird impact velocities of 850kt (1,570km/h), says Demers. "This imposes a significant impact load on the fan blade, an impact force which is roughly equivalent to dropping a men's bowling ball on to the fan blade from about 10ft."

Improvements to engines over time, some driven by the bird threat, include increased spacing between the fan and the core as bypass ratio increases (making it less likely that a bird enters the core), wide-chord fan blades with greater flexibility and mass per blade, new vane-retention methods that hold the aerofoils in place during and after a birdstrike, fan-blade leading-edge geometry, new fan-spinner construction methods, engine-bleed architecture and foreign object damage engine control logic, says Demers.

Except for a few isolated events such as US Airways Flight 1549, aircraft have made it back to the airport after birdstrikes, although costs have not been insubstantial. A list of incidents compiled by the USDA reveals damage that Flight International estimates to total more than $200 million during a 10-year period, just for bird hits to engines - the USDA estimates only about 40% of birdstrikes are reported. Regardless of what the rulemaking committee and the FAA decide, engine makers say their options are limited for making engines more robust while maintaining the operational economy increasingly demanded by airlines.

That reality is putting the prevention focus on technology for trying to keep birds away from an aircraft - for instance new lighting systems that scare the birds - and on airports, using habitat measures and scare tactics, to keep the animals away from the location where they are most likely to meet with an engine.