Testing Tuesday: Instrument Interpretation (HSI)

The correct answer is c: 19

The first thing to understand about an HSI is that is really a combination of a gyroscopic heading indicator (HI) – a.k.a. “Directional Gyro” (DG) and a VOR indicator. It becomes easy, at a glance, to see whether you are flying toward or away from whichever VOR radial you have selected on the instrument (and tuned in on your navigation radio).

Answer Trick #1: Sometimes, you can eliminate an option simply by noticing the heading shown in the example, and eliminating an answer option that is not flying that heading. In the above example the aircraft is flying a heading of 135 degrees, according to the HSI. In this case, all the answers are also flying the same heading, so we cannot eliminate any of them based solely on that criteria, but it DOES work on some other questions, and it means that, even if you have to guess, your chances go from 1 of 3 (about 33%) to 1 of 2 (50%). That’s a big jump if you are truly stumped.

The VOR portion of the HSI works similarly to a “standard” VOR receiver. You turn the pointy end of the needle to the VOR radial you want to use, and the “flat” end of the needle points to the reciprocal radial.

REMEMBER: Radials always “radiate” outward FROM the VOR, so if you are flying toward the VOR, you are actually on the reciprocal radial until you have crossed the VOR itself and are flying away from it. On a standard VOR receiver, you have a “TO/FROM” indication telling you whether you are flying generally TO the VOR or away FROM it.

On an HSI, you have a “carrot” that points toward the head (pointy end) of the needle if you are flying generally TO the VOR, and the carrot points toward the tail (flat end) of the needle if you are flying generally away FROM the the VOR. An HSI gives you the benefit of being able to see, in relation to your actual airplane, whether you are ON the radial (if the needle is centered), or whether the radial is ahead of you, behind you, or to your left or right (all depending on the where the needle is offset).

So knowing these things, can we look at the answers and eliminate one (or two) of the answers? We know we are flying away FROM the 270 radial of the VOR, according to the indications on the HSI, because the needle is deflected behind and to the left of our miniature aircraft. The 270 radial would give similar indications to its reciprocal, the 090 radial, and of the answers offered, 13 and 19 are flying away FROM the 270/090 radials, but 9 is flying TO the 270/090 radial, so we can eliminate 9 from the possible answers.

Now, we just need to determine whether we are on the east or west side of the VOR, and we have our answer! So, we know that the triangle “carrot” points toward the VOR, generally. In our example, the carrot is pointing toward the tail of the VOR, telling us we are on the “FROM” side of the VOR. In other words, if we were on the 270 radial, the VOR would be east of us (on the tail side of the needle – the reciprocal side – or in this case, the 090 side). If the VOR is east of us, then, we cannot be in aircraft 13, so we must be in aircraft 19!

Whew! That is most definitely a mental workout to figure all of those things out and arrive at the correct answer.

So let’s make it even easier.

The beauty of the HSI is that, if you can mentally superimpose the HSI over a 4-quadrant map (imagine the VOR at the center, with lines running north and south through the VOR (the 360 and 180 radials), and another line running east and west through the VOR (the 270 and 090 radials)), you can rotate the HSI so that north is up, just like over a map, and line up the selected radial with the deflected part of the needle, and the “carrot” of the HSI should be pointing toward the VOR. Our example is below: