schone

So, on a couple occasions I've had a chance of observing white needles vs. green needles and am trying to figure out how the FMS on a CRJ determines the magnetic course it's going to fly to a station.

On one occasion, the FMs was tracking what it claimed to be a 066 Mag Course, while the VOR was showing the radial being tracked was 062 or so....

The astonishing part was the heading was NOT 066Mag. It was something different on it's own.

Suggestions?

detpilot

Probably best suited for tech/ops.

andy171773

Well the heading being different was probably for wind..........

the MC is probably determined in the FMS by the variation for wherever you are stored in the database...the VOR course difference is probably attributed to a slight error in the VOR equipment in the airplane.

ExperimentalAB

VOR courses vary and bend between stations and through the atmosphere...fly green needles on a jet-route, and you can see that most of the time when the courses change at the change-over-point, you'll actually be centered on both courses, though there can be 5 degrees or more course change...I believe the FMS also flies a great-circle route...

AirWillie

This is the reason why the FAA wants to come down on regional pilots. Some of us don't even know how a GPS works, while at the controls.

rickair7777

Also VOR courses may not exactly match actual magnetic because the earth's magnetic field changes over time. They don't want to move/rename all of the radials and have to change all airways and charts every time it shifts by one degree.

Diver Driver

True. This may account for the slight difference.... but it only does so if you go direct... it obviously doesn't skew an airway to keep with GCR.

schone

Alright, I might have not made my self clear. I've noticed this on several occeasions, and the numbers I've put here are completely out of nowhere.

AirWillie: Before you tell us how we don't know anything, you might want to share something meaningful next time.

Now let's clear some more info:

Variation it's not because it was and I remember checking it, something greater than variation. For instance we flew in a 3W area and the diff between VOR and FMS MC was more than 3 degrees.

So I started thinking alright. Maybe the FMS chooses the wind correction as an artificial course, suppose you track 062MC to VOR and the wind correction is 066, so the FMS shows that as course and then you keep the same heading and viola... you get to the same results - that is the reason why I also threw that remark in about the heading. I've flown a couple days in my life to know what wind correction is all about.

Now, as for the stored course from a database.... on all occeasions we were flying direct to a VOR.

I urge you to look into it on your airplane (CRJ that is). It's quite interesting.

MD11

My understanding of the difference in degrees is due to the FMS plots a course based upon a great circle regardless of a lat/long,vor or waypoint. Where as the VOR is strictly line of sight. Also, whether a Honeywell, Smith or Collins FMS,,, they all do the same calc. Also, any changes in mag variation are corrected each 28 days at 9z. If my understanding is incorrect... somebody please correct me.

JetPipeOverht

All an FMS ever wants to do is maintain a zero degree off mentality of solving equations to line up the needle dead center. Mag variation and other such arguements being brought into the conversation only confuse things. Autopilots/FMS are designed as ' difference engines ' who's sole reason for being is to avoid ' deviations '. Albeit some have a hard time being perfect ( which, I understand is impossible IE: CRJ200 joining up final on an ILS quite erractly ) , for the most part they do their job well and as long as you understand the ' garbage-in/garbage-out mentality- it's all quite easy to understand.