ADF vs VOR vs HSI

ADF simplified


An ADF tells you how much you must turn to be pointed to a station. That’s it!

To find the Magnetic Heading You need, simply add what the ADF shows to your current heading.

Simple?! Yes but the above is correct only on fixed-card ADF, where the 0 is always pointed the heading that the nose is pointing to.

The Complication


So why do they making a movable card? The addition you did above can be eliminated if you rotate the card so that your current magnetic heading is pointed up on the card. When you do that, the needle points to the magnetic heading to the station, i stead of how much you must turn to point to the station. But caution! If you change your heading later, then the ADF is no longer valid. You must turn the card such that your heading is pointed up on the card, for the needle to be valid

Staying Simple


My preference is to simply keep 0 pointed up on the ADF even if it has a movable card

VOR


The VOR, like the ADF is NOT coupled to the magnetic compass or DG

However, while an ADF is usable even if your current heading is Not pointed up, a VOR is hard to use if your current heading doesn’t match the “up” direction on the VOR

So for a VOR, but only when you intend to head to or from the station directly, always change your heading to match the reading on the VOR

There are times when you MUST NOT head in the direction of the VOR. The most common case is when the VOR is to the side of your course

RMI


Wouldn’t it be nice if the up position on the ADF always pointed your current heading?

That’s exactly what an RMI does! An RMI’s card automatically points to your current heading. Ok ou might be asking but that’s what a DG does too. But there are two big differences. First, the DG has to be manually adjusted to show the correct direction periodically. The RMI automatically figures magnetic north. Secondly the needle on the RMI can be slaved to an ADF or VOR. In other words, it’s like superimposing the needle of an ADF on your DG. You can do the same with a VOR, but then, you won’t get the CDI of the VOR to display on the RMI.

Most RMI have two needles each of which can be slaved to an ADF or VOR

Full scale deflection


On VOR …
5 dots
Each dot is 2 degrees
1 dot at one mile from VOR = 200 ft off-course
(distance off course = num dots x miles from VOR x 200 ft)

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