So Mosquito Netting Won't Work, Eh?

I do have some December flying stories to write up and post, but I am still reading and thinking about the US Airways ditching, so they can wait a few days more.

People ask why they couldn't just put screens in front of the engines to keep the birds out. I suppose people are envisioning the sort of things you staple around your eaves to keep the birds from flying in and nesting. One letter to the editor of the New York Times compared the concept to that of cowcatchers. Let's compare. A cow weighs about 500 kg. A locomotive tops out around 80 mph, or 36 m/s. The kinetic energy of a moving object is 1/2 m v², so for the locomotive-cow collision that's 0.5 x 500 x 36 x 36 = 324 kilojoules, if I have my units right. I'm told the A320 is going to be climbing out at about 250 knots (corrected from a conservative or 210) or 128 m/s. A Canada goose weighs in at around 5 kg. Put those two together and we have a kinetic energy of 41 kJ. So the energy that must be dissipated when a slowly climbing airplane hits a single goose hits is about an eighth that of a speeding train hitting a cow. Could a structure be built over engine intakes so as to absorb that energy?

The structure would have to span what is about a two metre diameter across the intake, be strong enough to withstand bird strikes and never itself shed foreign objects into the engine. If you're having trouble believing that eight birds in the air equals one cow on the tracks, type "birdstrike" into Google's picture search for images proving that birds penetrate windshields, radomes and wings. This giant screen would have to have small enough holes not to let birds pass through, yet large enough holes to not impede airflow to the engine. That combination is an impossibility, as anything in front of the engine impedes airflow, so either the engine would have to be bigger or the performance of the airplane less, in order to compensate. I don't have the flow dynamics knowledge to begin to construct equations for this, but LTV Aerospace Corp. figures it's feasible, and here's a 2001 patent for a retractable bird deflector grille. I do know that tiny changes to an airframe, such as an angled piece of metal the size of half a Ritz cracker, can make huge differences in handling and controllability, so that even if such a structure were built, the whole airplane might have to be redesigned around it.

A reporter tried to answer the screens question live on CNN while I was watching, and he first admitted he didn't know, and then speculated that accumulated debris would block airflow into the engines. Not a bad speculation. The screens would have to have some way of preventing ice formation, probably by being electrically heated, so that chunks of ice neither blocked the intake nor fell through the engines. Heating the intake air would decrease engine performance, however, and heating a screen that size and weight would take a lot of power.

A lot of thought has gone into birds damaging engines. A Wall Street Journal article pointed me to the lengthy FAA rules governing the testing of engines against birdstrikes. Here are some excerpts.

All ingestion tests must be conducted with the engine stabilized at no less than 100-percent takeoff power or thrust, for test day ambient conditions prior to the ingestion. In addition, the demonstration of compliance must account for engine operation at sea level takeoff conditions on the hottest day that a minimum engine can achieve maximum rated takeoff thrust or power.

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.

Medium bird engine tests shall be conducted so as to simulate a flock encounter, and will use the bird weights and quantities specified in Table 2. When only one bird is specified, that bird will be aimed at the engine core primary flow path; the other critical locations on the engine face area must be addressed, as necessary, by appropriate tests or analysis, or both. When two or more birds are specified in Table 2, the largest of those birds must be aimed at the engine core primary flow path, and a second bird must be aimed at the most critical exposed location on the first stage rotor blades. Any remaining birds must be evenly distributed over the engine face area.

The specs include tables of bird weights and numbers and expected performance that any engine must meet before certification. And it has to be said, because this is the internet and someone else is going to say it if I don't: "chicken cannon." Yes, they actually fire (already dead) poultry at airplane components to test them. And no, it isn't always necessary to thaw the chicken first.