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From a related question, I got to thinking about bird strikes again and the fact that they are sucked into the engine at great velocity.

If a pilot had 5 to 10 seconds of warning time, would throttling down the engines to idle be a good idea? Would it save those engines even if a bird hit the intake fan? This would of course be a temporary measure, intended to prevent the birds being sucked in harder or faster than otherwise. Once the birds passed, the engines would be throttled up again.

I'm thinking about large commercial jets like the Airbus A320.

EDIT: one concern I have is that at high throttle, the engine will suck in objects from a wider area than just the engine inlet. Is this assumption true?

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    $\begingroup$ Another thing is that an engine at high power will take more severe fan damage from ingested foreign matter, but is more likely to centrifuge that foreign matter into the bypass duct instead of ingesting it into the core, where it can really break stuff! (This was discovered during the TACA 110 investigation.) $\endgroup$ – UnrecognizedFallingObject Aug 13 '15 at 2:45
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A goose can easily be flying at 40mph and a commercial jet will be landing at about 150mph, giving a closing speed of about 190mph (305km/h). That means that, to spot a goose with 10s warning, you would need to spot it when it's more than half a mile away.

Not only would you have to spot that tiny speck of a goose half a mile away during the busiest phase of the whole flight, but you'd also have to immediately work out whether the tiny speck was even coming towards you. If it was, you'd have to estimate whether it would even hit the plane.

Then, supposing you'd immediately and accurately recognized the danger to your plane from that tiny speck, you'd have to immediately reduce power to the engines, while being very careful not to hit the ground the bad way.

In short, regardless of how much better the outcome of a bird hitting an idling engine would be than one hitting an engine under power, it's just not feasible to try to power down the engines. Once you take into account the false alarms, which would vastly outnumber the actual bird strikes, trying to power down the engines would probably be far more dangerous than just hitting the bird.

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    $\begingroup$ It should probably also be noted that final approach is not a great time to be fiddling with engine power. Better to just go around if you see something that might compromise the landing. $\endgroup$ – reirab Oct 19 '15 at 15:23
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In general, greater the engine thrust, greater the damage caused due to the bird strike. This is to both the higher speed of the compressor blades and the pressure ratio.

So, it makes sense if the pilot can throttle down the engines if bird strike is imminent. In fact, approaching in idle condition is on of the strategies for reducing the possibility of bird strikes. See http://www.boeing.com/commercial/aeromagazine/articles/2011_q3/4/ and http://www.skybrary.aero/index.php/Bird_Strike_on_Final_Approach:_Guidance_for_Flight_Crews.

However, there are other things to consider. For example, most of the bird strikes happen during take off and landing and it is not advisable to throttle back the engines at that point of time (it is one thing to descend in idle power and another to reduce power during landing).

Also, given the speed of the aircraft and the size of the birds, it is unlikely that the pilot will have enough warning unless he's approaching a huge flock.

Another thing is, given that the velocity of bird strike is a function of both aircraft speed and compressor speed, reducing to idle power won't prevent damage due to bird strike as the compressor rotates at a very high speed even in idle condition.

In any case, modern engines are designed to operate (or fail safely) in the event of bird ingestion and it is far better to continue the flight and take appropriate action like go around and land.

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"would throttling down the engines to idle be a good idea?"

Probably not, 3 reasons I can think of.

Landing speeds for an Airbus A320 (your example) will typically be in the region of 130kt to 140kt faster then the cruise speed of many GA aircraft. Even at those speeds, a bird strike is likely to have a significant impact on anything it hits.

The published minimum speed of the high pressure compressor/turbine spool is 66% of the rated maximum. Since the max turbine speed is VERY fast think 20-30k RPM, 66% of it is probably enough to cause any damage that would occur with a normal strike.

In the unlikely even you see the birds with more than 2-3s to react (there very hard to spot at high speeds), you would more likely want to INCREASE thrust to A) try to outmaneuver the bird and B) in case of an engine failure the other would already be running near the power required for continued flight on one engine (essential since most bird strikes are at lower altitudes where you may not have enough altitude to reduce power for 10s or so then power back up to max thrust taking another few seconds)

EDIT: an argument could be made for not touching the throttles at all until you can total verify one or more engines have failed based on the number of pilots, who in a panic manage to turn off the wrong engine during an emergency.

Note. referenced speeds are approximate I would welcome edits with more specific data.

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  • $\begingroup$ Out manoeuvre a bird? I think not. $\endgroup$ – Simon Aug 13 '15 at 7:17
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    $\begingroup$ The question talks about 5-10 seconds. That is barely enough for the engines to spool down. The speed won't change significantly in that time and does not seem to be the point. $\endgroup$ – Jan Hudec Aug 13 '15 at 7:21
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    $\begingroup$ I couldn’t find any quotes on how long it takes from cruse to flight idle, but it is a moot point as flight idle is still a few thousand RPM. as to out manoeuvre a bird. "5 to 10 seconds of warning time" [improbable I know] might be enough to clime an extra 40 feet. $\endgroup$ – wanna-beCanadianPilot Aug 13 '15 at 8:21
  • $\begingroup$ Since the pilot is not expecting to see a flock of geese ahead of him, there will be some delay in his response as his brain processes this unfamiliar object(s) in front of him, and formulates a plan to deviate/slow down. This reaction time could easily be 5 seconds. Five to ten seconds warning time is therefore 0 to five seconds, clearly not enough time for an effective aircraft diversion. $\endgroup$ – Skip Miller Sep 1 '15 at 1:03
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To answer your question "Can they be dealt with", I think throttling down is nowhere near a total solution, although it might have a small effect.

The big problem is that the aircraft is typically flying at a speed of between 130 and 250 knots towards the birds. The engines don't suck the birds in (because there is relatively little time for this to have an effect) the birds just get in the way of the engines and the impact forces of a 10 lb bird doing between 130 and 250 knots (relatively) do the damage.

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