Dangerous Antennas

[Sparweb]

I've recently had a bad experience with a type of aircraft antenna that I want to pass on to others. I've been debating for a while wether it's wise to specify the exact name, for liability reasons, but for now I'll describe the antenna in sufficient detail that anyone who's seen it will know what I mean. A search on Google put it at the top of the list.

A type of antenna is currently on the market that is touted as an ORBCOMM/GPS combination antenna, which has the shape of a very thin fin. Its cross-section is that of an airfoil, at its tip is 3/4" in chord, tapering to about 5" at its base. It looks very much like a little wing with a teardrop footprint. The manufacturer claims that its maximum speed is 350 knots True Air Speed.

Actually the antenna was tested in a wind tunnel only capable of about 210 knots at sea level. At the wind-tunnel's maximum speed, vibration at the tip of the antenna was easily visible. The erroneous 350 KTAS maximum speed is still on the website, despite having the error rubbed in their face for over a year now.

The consequence of putting the antenna on any aircraft faster than 220 knots or so is that vibrations are severe enough to tear the skin away within less than 100 hours. I have personally seen the damage on 3 aircraft now. Through the grapevine, I know of many, many more installations that have failed, on King Air's, Beech 1900's, Hawker's, and even a Dash-8.

I have found other antennas that function the same, but are dome-shaped, so they will not suffer from vibration. Another alternative is to use the IRIDIUM system. More expensive, but you can get more data through.

Hope this ruffles some feathers out there. Just because the manufacturer claims something, doesn't mean it's true.

STF

 

[RGRG]

I only can add that I personally know of 40 plus failures of this antenna as described and these failures were not only confined to high speed aircraft. (They failed on Twin Otters too !) The manufacturer insists on blaming the installer or installation designer to exculpate themselves from potential liabilities.

 

[Sparweb]

Yes, there's been a fair bit of blame to go around on our troubles, too. Twin Otter failures tended to be only on the old-style antennas, which would just snap off in heavy icing about half-way up. No destrutive vibration occurred in those cases, for me at least.
Perhaps I should emphasize that installers of ORBCOMM data tracking systems have other options for antennas. Trak/Tecom comes to mind, and I think AeroAntenna has a model that works on ORBCOMM frequencies, too (been a while since I checked, now). Their antennas don't seem to have the same inherent problems.

Too bad that the odds of an avionics tech reading this are small.


STF

 

[jstolp]

We have installed a slew of these "combo" antennas recently on various rotorcraft. I hope that the typical 130-150 knot rotorcraft Vne will avoid any trouble. Stray parts under the rotor system and in front of the engine(s) make for a bad combination.

Jstolp