Does the radius (shown below) take into account smaller objects (FOD), or only people? How is it determined?

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Source: Facebook

  • 1
    $\begingroup$ The ramp/gate area of a typical commercial airport has a rich supply of zipper tags from luggage. That's probably the biggest FOD risk besides birds. How to calculate it? Who knows. $\endgroup$
    – John K
    Commented Sep 3, 2021 at 22:33
  • $\begingroup$ Restrictions on coming to close to the engine area of the Image is the printscreen from Facebook, relation of image to my question is red colored half of this circle in front of the compressor blade. I assume that the max distance is calculated to human not things that are prone to get into the engine. $\endgroup$
    – George Geo
    Commented Sep 4, 2021 at 7:38
  • $\begingroup$ I assume you do not mean probability? You are asking about how the distance is defined? This would include calculation or measurement of airflow towards the engine and area & mass of the object, a human for example. $\endgroup$
    – Jpe61
    Commented Sep 4, 2021 at 7:53
  • $\begingroup$ That's got to be a screen shot from a video game of some sort. $\endgroup$
    – FreeMan
    Commented Oct 22, 2021 at 16:24

1 Answer 1


The calculation is based on experimental data and research on better understanding the role of the vortex action. For example:

Scaling laws are derived to relate model and full-scale tests end experimental evidence is cited to demonstrate their validity. For a given inlet velocity the strength of the vortex, and thus the maximum size of particle that can be lifted, is very much a function of the ambient vorticity, end of the strength of the wind blowing on to the intake [...]

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— Glenny, D. E., and N. G. T. E. Pyestock. "Ingestion of debris into intakes by vortex action." (1970).

Manufacturer data also comes into play. The F-16 in your photo – based on a US military manual I stumbled on – its ground marking is optional, and the manual doesn't state why the F-16's intake hazard is unique to warrant a marking while the other jets are not mentioned, but based on available literature, it's likely due to its intake diameter to height ratio.

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(Click to view)


15' [4.6M] IDLE (SHOWN)



In an effort to reduce the hazard caused by F-16 engine inlet suction, F-16 parking positions are optionally marked in accordance with Figure 8-13; however, aircraft maintenance personnel determine whether the radius is established at 15 feet (4.6 meters) for idle thrust or 25 feet (7.6 meters) for mil-thrust.


"Hazard caused by" here should be referring to hazard to maintenance personnel. Logical practice dictates keeping the area clear of any items as well. The engine manufacturer would have provided the data, similar to civilian aircraft: How long to clear the 'suck zone' of a turbofan after start is initiated?


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