Why Did a U.S. Spy Plane Disguise Itself as a Philippine Aircraft? 

On September 22, a U.S. spy plane was detected over the Yellow Sea but it was emitting a signal that was reserved for Philippine aircraft. 

The aircraft is a United States Air Force RC-135S reconnaissance plane typically used for espionage and counter-espionage missions. It was separately detected by the South China Sea Strategic Situation Probing Initiative (SCSPI) and aircraft movement monitoring agency Aircraft Spots.

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In a tweet, Aircraft Spots flagged the spy plane which was “miscoded.” 

USAF RC-135S 61-2663 GASP47 conducted a mission over the Yellow Sea on a miscoded Mode-S code 75C75C. pic.twitter.com/XMDw18Vcp1

— Aircraft Spots (@AircraftSpots) September 22, 2020

As you can see, its flight path took a back-and-forth route over the center of the Yellow Sea, a hotly contested area between China, Korea, and Japan. After completing its mission, the U.S. spy plane dropped the Philippine hex code and reverted back to its assigned signal. 

Hex codes are used to identify different aircraft and their countries of origin as they fly over different airspaces in the world. 

But it seems like military planes around the world regularly pose as commercial aircraft when conducting missions. Just a day earlier before the U.S. spy plane was detected using a Philippine-assigned hex code, another one was detected using a different code while flying over the West Philippine Sea on September 21.

USAF RC-135W 62-4134 FOLD83 operating over the South China Sea, on a miscoded Mode-S code 249249. pic.twitter.com/u0hToBVqa4

— Aircraft Spots (@AircraftSpots) September 21, 2020

Common Practice for Military Planes

The U.S. is not the only country that does these “miscodes” when using military aircraft. In the last decade, the United Kingdom, the Netherlands, Turkey, France, Greece, Norway, Portugal, and Luxembourg—which are incidentally all members of NATO—have all used miscoded military hex codes to appear as civilian aircraft on radars, according to this index. 

While it seems common practice for military aircraft to disguise their hex codes, doing so creates risk for civilian aircraft. 

In 1983, Russia shot down a Korean Airlines KLA007 passenger flight in its airspace because they mistook it for a U.S. spy plane.