Originally Posted by
3Greens
The main reason was the Americans were broadcasting on 243 (military guard HF)and not 121.5(civilian). Consequently, the Iranian civilian airliner wasn’t monitoring 253 and didn’t hear the calls.
Well yes and no. Like most such events, it was a combination of circumstances.
The USS Vincennes made 10 attempts to contact IR655.
Seven of those attempts were on UHF (243.0 MHz), which the Iran Air couldn't receive as it didn't have UHF, and three were on VHF (121.5 MHz), which IR655 did receive but disregarded because it cited an unknown aircraft flying with a 50 kt speed difference (GS vs TAS) and failed to quote IR655's Mode A squawk, which would have unambiguously identified it.