How does HSI decide to give a left or right CDI indication?

Since there is no reverse sensing in HSI, it tells you to turn left or right, So Is the interpretation correct that it will tell you to turn to the direction to that direction, which requires less turning - to intercept that radial.?

For example, Our present radial is 070 degrees, course selected is 90 degrees ,

So if if the heading is 355 degrees, would it tell to turn left since it requires less turning ?

• AFAIK, the HSI does not tell you which direction to turn. Do you maybe mean a flight director instead? Commented Dec 2, 2021 at 18:58

Good question. But first understand that an HSI (Horizontal Situation Indicator) will not tell or direct you to turn in any direction. You have to decide what direction (left or right) to turn to best achieve your needs. There are other instruments, such as a Flight Director, as part of an integrated navigation system that can direct/command a turning direction.

Are you saying in your question that the aircraft is on the 070 degree radial (in the NE quadrant of the VOR) and you want to track outbound (from the VOR) on the 090 radial and your current heading is 355 (and you want to intercept the 090 radial at a perpendicular angle using the least amount of heading change)?

If so, as some HSI tutorials use for positional awareness training, the most direct/shortest distance to the 090 radial would be turning to intercept it (the 090 radial) from a perpendicular course/heading (heading 180 degrees in your example).

Consequently, if your current heading is 355 degrees turning left would indeed require less turning (175 degrees heading change vs turning right 185 degrees heading change) to intercept the 090 radial exactly perpendicular (no wind considered).

Of course, this would just be a tutorial exercise for the purpose of learning to visualize joining a radial at a perpendicular angle and turning the shortest number of degrees while doing so.

Based on your example (assuming I understand your question correctly) if you wanted to track outbound on the 090 degree radial and your heading now is 355 degrees and you are currently on the 070 degree radial, turning left to a heading of 180 degrees and intercepting the 090 degree radial at a perpendicular angle would not be the most efficient technique.

A more efficient technique would likely be a right turn to a heading of 120 or 130 degrees (a turn of 125 to 135 degrees) thereby intercepting the 090 degree radial outbound at a 30 to 40 degree intercept angle.

• I'm trying to ask would the needle be left or right of the aircraft in this case in HSI? Commented Dec 3, 2021 at 4:37
• If you know that your aircraft is on the 070 radial and you have set the course needle to 090 then, no matter what your current heading is, the cdi needle will be to the right and the to/from indicator will be showing from the station. It does not matter what the shortest direction of turn to intercept the 090 radial at a perpendicular angle is.
– user22445
Commented Dec 3, 2021 at 17:01
• The "HSI" does CONSIDER the actual heading of the aircraft to factor in left/right indication. It would not be always to the right. Commented Dec 3, 2021 at 19:59
• @SachinChaudhary If you are physically located on the 070 radial and set a course of 090 the needle will deflect to the right relative to the needle head of 090 (meaning you have to go south to center the needle). If you're in a helicopter and spin around on the 070 radial (stay in the same spot) the needle will always stay deflected in the same direction (to the right using your example). It won't always be off of the aircraft's right just deflected right in relation to the head of the HSI's needle (set in your example to 090). Unless I'm totally misunderstanding your question.
– user22445
Commented Dec 3, 2021 at 20:59
• @SachinChaudhary, do you have a source for your claim that an HSI considers the heading of the aircraft when determining which direction to “tell” a pilot to turn? Because it seems you are stubbornly contradicting a credible source that 757toga linked to in his answer. Commented Dec 5, 2021 at 17:41

First, an ordinary HSI doesn’t contain digital logic to decide anything. Unlike an FMS or flight director, it no more tells a pilot where to go than a \$5 compass recommends a direction to a disoriented hiker. Both simply present a display that informs the user of the situation. Even the full name - Horizontal Situation Indicator - implies those same limitations.

Next, if you think that in your scenario a left turn is the best method to get established on an easterly course of 090 from a navaid, then I propose we have a friendly imaginary race to demonstrate why that might not be the best choice.

We will run 2 heats. You will turn left while I turn right. We agree to perform standard rate turns, and fly at precisely the same airspeed.

Here are the objectives:

• Heat 1: Be the first to cross the 090 radial.
• Heat 2: Be the first to reach the 090 at 20 DME. (I must make the initial turn at least 180 degrees to mirror your turn)

I will describe the results, and offer an illustration. Ready? OK, here goes…

Heat #1: You have already figured yourself correctly to be the winner because you have less turn to perform. Not 5 degrees less, but 10 degrees – you turn 175 degrees left to 180, I turn 185 degrees right to the same heading. Your margin of victory, however, is miniscule. A standard rate turn is 3 degrees per second, so I would spend just over three seconds more than you turning. But because we are heading in generally the same direction as we both initiate and complete our turns, the actual margin is much less. (the angular change doesn’t equal forward speed) Anyway, the victory is well within the margin of error and exceeds either of our abilities to control flight parameters well enough for consistent results. (as well as my ability to actually compute it!)

Heat #2: I’m providing an illustration for this one because this is where it really makes a difference. Because our objective is never just to get to a radial, we must consider our actions in the context of where we intend to go after we intercept. As you can see, I beat you soundly by a margin of 4 times the radius of turn. We could calculate a distance, but we don’t need to – at a standard rate of turn I will have beaten you by 2 full minutes.

Note: I just realized that 4xRadius is correct, but not 2 minutes. Because the 2 minutes spent turning in opposite directions was along an arc. Pi is probably needed here, but it’s getting too late to think on it and make a correction right now.

Reality: I would either make a right turn and proceed direct to the fix, or set up for an angle that will get me on the radial expeditiously, while not sacrificing downrange travel towards the destination. Because the real goal is to get to your destination in the most direct and efficient manner, you should generally always plan a turn in the direction of travel. Intercepting a radial is just one means to the end, and turning the opposite way to save a few seconds of turn will cost you way more in distance traveled in the wrong direction.

Angles between 20 and 45 degrees seem to be the best compromise for intercepting.

HSI with regards to VOR information is a positional instrument so you can strike "heading" out of any question. For the given position on the 070 radial with an OBS setting of 090 then you will have a CDI deflection RIGHT twenty degrees and a FROM flag shown. This is the displayed state regardless of the heading of the airplane. Be aware that because an HSI will spin in the dashboard according to airplane heading. For example if heading is 270 with an OBS setting of 090 the HSI will be displayed upside down. The rightward CDI deflection will be toward the left wingtip but is still considered a right deflection.

Considering all possible positions given an OBS setting of 090 the CDI will have the following indications. For positions north of the 090 radial (this line running east-west through the station), the CDI will deflect right. You can imagine the CDI as the "radial road" which would be to the right of someone traveling a course 090. You'd be parallel to the desired radial displaced north (left) of it.

For positions east of the station (all radials from 360 through 180) the flag would show FROM. This can be thought of as any motion along the OBS 090 course would be increasing distance from the station.

The deflection can be anticipated by answering the question: "From a particular position, facing the azimuth of the set OBS, would the selected radial by right or left?" The TO/FROM flag can be anticipated by answering the question: "From a particular position, if I traveled in the direction of the set OBS, would I be decreasing or increasing distance to the station?"