How does an EHSI work?

[maverick_fp00]

I know it's electronic, but does it work off of a vacuum like a normal DG or what?

 

[flint4xx]

Since an ehsi displays multiple bits of info, I'm not sure if I'm answering what it is you want to know. I think you are asking about the direction information it displays. That comes from inertial reference systems or other similar apparatus. (Laser ring gyros and the like)

I'm sure one of the resident techno weenies can give you a better answer.

 

[maverick_fp00]

Well I'm not familiar with any type of EFIS. There is a C-210 that has an EHSI and I was just wondering where it gets it heading info from. Also, what other kind of stuff does it display?

Thanks!

 

[skeezer]

Let me give this a shot, it has been a while since I flew a C182 with a Sandell EHSI so my memory may be shaky but...

I believe that the unit had a gyro within it that was slaved using a remote indicating compass in the wing.

As far as info that it displayed, waypoints, course, airspace, RMI from nav 1 or 2 (many both at once but I cannot remember), GS, and probably comething else. It is a lot of stuff but fit well.

Hope that helps.

Skeezer

 

[KC-10 Driver]

flint4xx hit upon the crux of your question, I believe. I myself, however, am not specifically familiar with the C-120.

In my experience, though, an EHSI typically gets it's basic heading data from the aircraft's inertial reference systems (IRS). In some cases, this data may be augmented by a gyro system. [After the fact edit: I meant to say it could be augmented by a flux system -- that's what happens when you type posts at 0100.]

As far as additional data, the manufacturer can provide all sorts of information. Bearing to radio-navaids using bearing pointers, selected course referenced to FMS/GPS or radio-nav, VOR/waypoint identification, and any sort of other data can be displayed.

For more information about electronic information systems and FMS's, go to the www.bluecoat.org site.

 

[maverick_fp00]

Thanks for the reply's. Of course, any other comments are greatly appreciated.

Thanks Again

 

[Timebuilder]

Since it is unlikely that anything smaller than a transport category aircraft will have an inertial system on board to feed data to an electronic HSI, there is a more likely explanation.

Most of the smaller aircraft will likely use a flux valve out in the wing, which is isolated from the plane's various electrical fields. This little marvel senses the change in its alignment with the earth's magnetic field, and creates an output signal that represents that relationship. Most of the time, this device is used to correct heading gyros on mechanical HSI's, but there is no reason that its data could not be passed through the necessary circuitry to drive an image of an HSI on the face of a tube or LCD screen. The latter screens are small, light, and produce very little heat, which makes them ideal for new or retrofits in small aircraft.

As mentioned above, you can feed these screens almost any info you like, such as approach charts, radar, TCAS, speed, altitude, text weather, engine data, etc. The possibilities are almost endless. When the virtual highway system comes along, a screen like this will display the image you will attempt to follow, much like a flight director shows the pitch and roll inputs necessary to let you take the place of the autopilot servos when you hand fly an aircraft.

Could an EHSI system contain an actual gyro? Sure. Like slicing a tomato, there are many ways to end up with the same result. Why not visit you local avionics shop and ask for an in-person explanation?

 

[RJlifer]

The EFIS eqipped A/C I've flown have 2 black boxes on the avionics rack called AHRS. (Attitude Heading Refernce System) There's two for redundancy. They get their heading info from a device called a flux gate which is usually located in the wings. Look for an area with decals warning against using mettalic screws or magnetic tools. I believe nowadays they can also crosscheck themselves using GPS as well. The AHRS also contains the gyros (ring laser) which give you your EADI signal as well. As far as I know, there is no vacuum type device anywhere in the system.

That's hella cool that there's EFIS available for GA airplanes. NOTHING I ever flew had anything nearly that nice. I just about passed out from joy the first time I got to fly a 172 with an HSI.

 

[maverick_fp00]

if there was an avionics shop near here then I would go ask them, but there isn't. next time I'm in Mobile I'll drop in by Southern Avionics and ask them.. they do a lot of maintenance for the planes here in Panama City.

thanks