HSI/REMOTE COMPASS INTERFERENCE

[capt beeky]

Any one with any ideas?

HSI regularly reads 30 to 180 degrees out. It tends to be worse at certain locations just after take off. The local rescue chopper has noticed the same thing.

Could mobile phone relays or microwave Tx be the cause?

[redtail]

This is like being at work. “XXXXX is inop”.

Either it happens all of the time, anywhere, or it happens at certain locations. Which is it? What were you and the rescue chopper flying next to? What did the emergency wet magnetic compass have to say during all this?

[capt beeky]

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).]

[redtail]

Thanks for expanding on the problem. Have you talked with anyone who has been is the same situation and not had the problem? Unless this is a design defect of your system (Makes a great sign off : "Design characteristic of airplane") I would inspect the HSI and related shielding of the wiring, to see if it is actually still connected as originally designed. Shielding doesn't get very much respect or attention in the field, but it can lead to write-ups such as this. Shielding is designed to keep your wiring from acting as an antenna and prevent interference and cross-talk between systems.
This could also be a 1960's system suffering from 1990's problems.

[dqcolony]

I believe I understand this to be occuring in one area at one airport. If so, do the affected aircraft have a remote compass system using flux valves (remote compass transmitters) in the wing tips? Would the compass error reverse if the other side of the aircraft was flown past the anomalie? I do know that B 707's, 727's and L-1011's will show an unpredictable and often LARGE compass spread if vehicles, etc. are under or passing by wingtips. I suppose a large iron transmitting tower could affect aircraft passing by. Also, how long, or to what distance from the anomalie does the compass error remain?

[capt beeky]

Had it again today. This time I turned right after T/O. The error was less - 90 degrees. I'm sure with a left turnout it would have been 180. I took the time to scan the HSI during the ground acceleration. It rotated 30 degrees on the ground despite a backtrack and two mins at the threshold to let everything stabilise. By 900' it was out 90 degrees.

The worst of it was that at no point did the heading flag appear - well not untill I unslaved and started to realign, then it came up.

Its just a theory, but I'm sure its environmental. The kit itself has been given a clean bill of health. At first I thought of iron deposits but the geology is wrong. New phone masts went up 6 months ago. I'm told that the Emb 145 (not my aircraft type) gets 'comparator' warning captions (10+ degrees difference between HSI/compass)accelerating over buried power cables at a big regional airport. There are no such features effecting my T/O.

I will explore it further (if VFR) in a few days when I go back there, I will also speak again to the ASR team.

Any gen on electromagnetic interference appreciated, or other pet theories.