Certain locations, some of the times!
Its definately worst below 500' close to two antenna arrays - mobile phone and microwave links. However, it was noticed at FL80 as a 20/30 degree error, a long way from the TX sites, but on a direct track between them. It does occur elsewhere occasionaly, but there have to be other less obvious TX sites we haven't spotted.
Is the irregularity of the error due to the sites transmitting/dormant.
The liquid compass is swung to tell the truth with a double geny failure and all systems off. It doesn't really help but I will shut down the geny's if I don't have pax. The error was detected by several pilots by cunningly noticing that they were flying south ref terrain, but indicating north bound on the HSI (and vv).
Sometimes you get a HDG flag, sometimes you don't. Manual slaving doesn't help, it just reverts. It is always sorted by return to base - our engineer loves that as it can't be replicated on the ground. NB we can't use a mobile phone at the airport due to adjacent hills - therefore interference masked as well???
The snag has been spotted on two fixed wing, three different HSI & sensor units plus the ASR chopper.
Of course, it might be nothing do do with phones.
[This message has been edited by capt beeky (edited 28 May 2000).]