AAIB BA38 B777 Initial Report Update 23 January 2008

[Chris Scott]

Known RFI versus Hostile RFI

Quote from Whatisthematrix (today, 15:19):
Aircraft such as the B777 are tested against HIRF (High Intensity Radio Frequency) and critical areas such as the Main Equipment Centre of this aircraft is shielded against it (you only have to look at the SRM for this).
[Unquote]

Thanks, Whatisthematrix, those of us waiting to fly it were naturally very concerned about this when the A320 was being certificated, 20 years ago, in 1987/8. But I'm not sure that hostile RFI was on the engineering agenda in those days, or even the B777's.

Just to correct your (unintentional?) slip, the signal strength is quartered, not halved, at double the distance. [As you say, signal strength is inversely proportional to the distance SQUARED.] That's why my proposed transmission would need to be so powerful.

I've experienced small high-frequency control oscillations from the AP during my own routine HF R/T transmissions on a big jet in the 1980s (pre-FBW technology). Typically, the omni-directional antenna is in/on the fin (vertical stabiliser).

A legitimate HF antenna (for comms) on an aeroplane may be as little as, say, 3 metres from a rudder control servo. But the nearest digital computer of the FADEC/EEC variety might be as much as 30 metres away. Any shielding has comfortably to cope with that. To replicate that signal strength from a ground antenna 300 metres away, it would appear that 100 times the transmission power would be required, but this applies only if the illumination is omni-directional.

So the transmission has to save power by being directional. ATC surveillance radar is directional (in azimuth, at least), as you point out. But, if I remember rightly, radar operates on SHF between (roughly) 3GHz (10cm) and 10 GHz (3 cm), at moderate power, with a BIG receiving dish.

I'm thinking more at the other end of the scale: LF (30KHz - 300KHz) or VLF (< 30KHz). I am minded that the military have used some powerful transmitters in the past, for long-range (directional?) comms. Perhaps someone could enlighten us?