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I live next to HTDA Julius Nyerere International Airport and I do occasionally see Airborne debris, mostly plastic bags flying around at 1000 or more feet.

How dangerous can this be?

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The FAA's advisory circular on Airport Foreign Object Debris (FOD) Managament mentions plastic bags and other trash as a type of FOD and says these are the dangers:

FOD Hazards. FOD can severely injure airport or air carrier personnel or damage equipment. Types of potential damage include: cutting aircraft tires; being ingested into engines; or becoming lodged in mechanisms affecting flight operations. Personnel injuries or even death can occur when jet blast propels FOD through the airport environment at high velocities.

Since you're asking specifically about airborne items, that would mean the most likely dangers from that list are engine ingestion or "becoming lodged in mechanisms affecting flight operations".

The main issue with a plastic bag seems to be that it damages the engine, rather than stopping it entirely. This paper recommends an inspection if any "soft material" (including plastic bags) are ingested:

  • Remains on impellers and/or LP diffuser exit ducts confirm a bird or soft material has passed through the Engine and possibly contaminated the secondary air system, fuel nozzle and HP turbine vane cooling air passages.
  • If contamination occurs, hot section components deteriorate rapidly, increasing the cost of refurbishment. Therefore, operators are recommended to carry out a hot section inspection (HSI) as soon as possible after a bird strike or ingestion of soft material is confirmed

As for affecting flight instruments, the main danger is probably that the bag covers the pitot tube, resulting in false airspeed readings. That can be catastrophic if the pilots don't realize what's going on, but many larger aircraft have multiple pitot tubes so it would probably need a lot of plastic bags (or a lot of bad luck) to cause an accident. And a bag stuck on the windscreen could block a pilot's view but if there are two pilots then that shouldn't be critical.

Of course, all of the above applies to airliners. If you fly a light piston single through airborne debris the results could be worse because of the lack of technical (and perhaps crew) redundancy.

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  • $\begingroup$ With a light piston single, though, you don't really have to worry about the bag being ingested into the engine and it would get shredded by the prop before it would have a chance to get stuck on the windshield. Blocking the pitot tube would certainly still be unhelpful, but, when operating that close to the field, light aircraft will usually have the ground in sight and can be flown without the instruments if needed. $\endgroup$ – reirab Jul 20 '15 at 14:18
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    $\begingroup$ @reirab there's a (small) chance that a plastic bag that gets past the propeller could get blown into the engine cooling inlets (where it would probably melt and smell awful, and might start to smoke). It also has a (small) chance of blocking the engine air intake, which can choke out the engine. I can't recall reading about either of those in an accident report's Probable Cause though :) $\endgroup$ – voretaq7 Jul 20 '15 at 15:55
  • $\begingroup$ @voretaq7 True, but that would take one serious plastic bag to survive an encounter with a prop. Kevlar bag, maybe? $\endgroup$ – reirab Jul 20 '15 at 16:19
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    $\begingroup$ @reirab: The bag can be simply lucky and miss the blades. At 80 knots the plane moves about 1 m during the time it takes a prop at 2400 rpm to make a complete revolution. With 4 blades, it still leaves 25 cm between the blades and that's enough space for noticeable piece of plastic to get through. $\endgroup$ – Jan Hudec Jul 20 '15 at 18:46
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I would say not much. Aircraft are designed to survive collision with small bird and plastic bags are lighter than that. A plastic bag probably won't even visibly damage the skin and if ingested in an engine, it will burn quickly and likely without noticeable impact on the engine performance. The only issue I can think of is if it gets stuck on some important sensor.

There has been an incident where aircraft collided with some kind of banner hanging from a hot air balloon which clogged pitot tubes and caused airspeed indicators, and auto-flight systems that need them, to fail.

Unreliable airspeed should not be a big deal with proper crew training, but the way that crew mismanaged the situation shows there is some risk. However plastic bags are smaller than the banner in that incident, so the chance of one hitting one of the three critical spots on the aircraft is quite low.

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    $\begingroup$ Not all aircraft are rated for bird strikes. $\endgroup$ – KeithS Jul 20 '15 at 14:40

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