HSI- the yellow course bar arrow head?

[magneticflip1]

Hi guys,

Just wondering, what funtion does the arrow head on the course bar have (found on a HSI)? It doesnt seem to matter whether I set the arrow facing downwards or pointing upwards while intercepting VORs.

Are there any rules how to set it? And what function does the arrow have anyway?

Thanks!

[DBate]

It's been quite a while since my last flight with an HSI equipped AC, but as far as I remember, you have to set the arrow pointing up or down depending wether you want to fly outbound or inbound on the radial.

So long, DBate

[keithl]

The arrow has no function when working with a VOR, the Beam Bar will always show the correct "picture". But it is vital to set the inbound track (QDM) on the arrow when using the HSI for an ILS, or you will get reversed demands - and quickly become confused!

[DFC]

Sorry, Keithl, but that is back to front.

When using the HSI to track a localizer, the to/from flags do not apply and it makes no difference what the arrow is set to because the indication of horizontal position is relative to the centerline (which is fixed). However, it is good practice to align the arrow with the QDM of the localiser so that the picture presented is intuitive.

When using the HSI to display inforation based on VOR signals, the arrow head must be set to the required track (since in theory, there are an infinite number of possible tracks to/from a VOR station) for the display to show the correct representation of the actual horizontal situation. The to/from flag will tell you if the chosen track will take you to or from the station.

In both cases, when displayed correctly, the pilot must track towards the bar in order to follow the desired course displayed.

Hope that helps.

Regards,

DFC

[keithl]

DFC - I've just sat in my simulator and proved what I said was correct. The only way we can both be right is if there are different kinds of HSI, but I didn't think that was the case.
[Edited to add:] When on the LOC allow a Beam Bar displacement to develop. Wind the arrow round to the reciprocal of the ILS. The BB will travel with it and NOT swap sides as it would if you did the same thing on a Radial.

[ft]

DFC,
I think what he's saying is that if you change the course setting 180 degrees when flying a VOR radial, you'll still use the HSI the same way. Fly towards the bar to get back on the chosen radial. The to and from flags will change, but not the way you use the instrument. Do the same thing on an ILS approach and you'll have to fly away from the bar to get back on the localizer, as the HSI in ILS mode does not consider the course setting.

Cheers,
Fred

[DFC]

Ta Fred.

To put things in a slightly simpler note;

The makers of the HSI have designed the instrument in such a way that the arrow is pointed in the direction that one wishes to travel in. Consequently, the information displayed is only representative of the true horizontal situation when the arrow is set to the desired track.

However, the statement of "The arrow has no function when working with a VOR, the Beam Bar will always show the correct "picture". But it is vital to set the inbound track (QDM) on the arrow when using the HSI for an ILS, or you will get reversed demands - and quickly become confused!" is incorrect because if one fails to set the required track to or from the VOR, one will never establish on the correct radial.


Example.....ILS 28 has only one course.....280deg. Regardless of the direction the arrow is pointing, working on the basis that the arrow "points at the runway", the aircraft at the centre of the instrument will be displayed in the correct position relative to the beam bar. Thus if the aircraft is right of centerline, this will be displayed as such regardless of the position of the arrow provided that one remembers that the arrow points at the runway. In order to make that idea easier to understand, one must ignore the compass card...otherwise one gets confused.

Note we are talking about the aircraft position. Not the direction in which it is pointing.

Now if one wants to establish inbound to a VOR on a course of 280deg, unless the arrow is pointed to 280, the indicator will never directly tell where the required track is relative to the aircraft. Example......if the aircraft is SSE of the VOR and say the arrow just happens to point to 300, the beam bar will be out to the right and the flag will show TO. When the aircraft crosses the 310 inbound track, the beam bar will start to move towards the centre, reaching the centre as the aircraft crosses the 300deg inbound track and then move out to the left reaching full left deflection as the aircraft crosses the 290 inbound track. From that point on, no further information is provided and at no time has any information been provided relevant to the 280deg inbound track. That is why it is essential to set the required track when using the HSI to track a VOR.



To try the above, fail the slaved compas system i.e. the compass card driver and use the magnetic compass to find direction. Having done that, place the arrow straight up regardless of direction and track a few localisers.....the info provided will always be correct regardless of what number is behind the arrow on the compass card.

Now try tracking arround a VOR and one will find that in this case, the aircraft will only ever establish on the track indicated on the compass card.

Hope that is a better explanation!!!

Regards,

DFC

[keithl]

DFC. Can I take you back to the original question. Magflip says it doesn't seem to matter whether the arrow is pointing up or down while intercepting VORs. He is therefore using an HSI with a serviceable, slaved card. In this situation, the BB will show a correct demand, and enable interception of a VOR radial whether the arrow points at QDM or QDR.
In the case of the ILS, if you have the arrow pointing away from the runway, it WILL show a reversed demand. We use this fact to fool the HSI into giving correct demands for a BackBeam ILS.
In the ILS case you say you are concerned only with the a/c position, not the way it is pointing. I am describing the real life situation where one is concerned with both parameters.

Perhaps if we pursued this long enough we might find that we agree, but are expressing it different ways. But whereas you say bluntly "That's the wrong way round!", I say CRM-style, "consider another way of looking at it."

[Captain Stable]

Pardon me for appearing dense, but how can the system know what you radial you want to intercept if you don't select it?

Put it another way:-

It will attempt to intercept whatever radial you have selected. If that results in a nonsense (e.g. you are West of the beacon tracking north, wanting to intercept 315 outbound, but you have the yellow pointer on 360, it will never intercept. Similarly, if you are crossing the 270 radial, select 045 and hit "VOR" on your FMCS, it will carry on tracking north until the tanks run dry.)

You must always have the radial you want to intercept selected, with the yellow arrow pointing in the direction you want to go.

Consider another situation. You are on a bearing of 190 from the beacon, tracking north. If you select 270, then when you arrive a few miles west of the beacon, you will turn left. If you point the arrow the other way (090), you will turn right.

According to my logic (feel free to point out any errors) it most certainly does matter which way the arrow points.

[mono]

I actually can't believe I'm reading this!!

Of course it matters which way the arrow is pointing!!

I will say 3 words - TO/FROM FLAG.

Yes the beam bar deflection will operate in the correct sense but the TO/FROM flag will be pointing in the wrong direction. Now imagine that the RMI or the auto VOR function is U/S. How do you know where the station is? You know because the HSI has set on it the 090 deg radial (for example) along which you are flying, and the beacon is either ahead of or behind you depending on the position of the TO/FROM arrow. Therefore it is an important part of your situational awareness.

I blame GPS myself. basic navigation skills are not what they used to be.

[bookworm]

Consider another situation. You are on a bearing of 190 from the beacon, tracking north. If you select 270, then when you arrive a few miles west of the beacon, you will turn left. If you point the arrow the other way (090), you will turn right.

This may seem trite but... only if you choose to turn in the direction of the arrow! There's nothing in the behaviour of the beambar in those circumstances to direct you to fly left or right -- it just moves down the instrument towards you.

I could envisage a model of use whereby you always use the arrowhead to select the radial on which you're flying (i.e. you always have the FROM flag showing). The beacon is always at the "blunt" end of the arrow. I think it's self-consistent (for VORs) but unconventional, compared with the usual practice of having the arrowhead more closely aligned with heading. Of course autopilots would get horribly confused...

[keithl]

Capt Stable. To take your example: We are West of the beacon heading North. We wish to intcp the 315R o/b. You can set the yellow arrow to 315 or to 135 and the BB will show the right picture. As you come onto the radial you know (because you know you want to go o/b) that you turn onto 315. I notice you refer to "the system" knowing which way to turn, and to the FMCS. Well that's different. Our aircraft mostly dont have those. They have pilots whose situational awareness is rather good.

(Edited to add: Thanks Bookworm, that's what I'm trying to say - and I needed some support!

[forget]

I’m with Mono on this. As a simple avionics man I can't believe I'm reading this.

For the system to tell you what you want to know - you need to first tell it what you want to know. You do this by positiong the course arrow. Note the word ‘selected’ below.

The To-From arrows indicate whether the selected course is going toward or away from the selected beacon station. The course deviation dots indicate course deviation in degrees relative to the course deviation bar position. The COURSE SET knob allows the operator to set both the course arrow to a desired course and the COURSE indicator to the desired course readout when in any mode other than TAC NAV.

See http://www.professionalpilot.ca/help/tuning_help.htm

[Captain Stable]

This may seem trite but... only if you choose to turn in the direction of the arrow! There's nothing in the behaviour of the beambar in those circumstances to direct you to fly left or right -- it just moves down the instrument towards you.

Trouble is you're only considering half the information here. As has already been pointed out, you're not considering the information that the TO/FROM pointer is providing. And of course, there's nothing to tell you which way to turn at any time - you can go anywhere you please. But the question concerns how to use part of the information telling you that. If you don't want to use it, why bother?

To return to the question - consider you're on a bearing 190 from the station, tracking north.

Select 045 on the HSI. The needle will show fly left, with a TO indication. When you approach the 225 radial it will gradually come in until you're on the radial.

Go back to somewhere out 190 from the station. Now select 225 instead. You will now get a fly right on the needle and FROM pointer showing.

Now tell me it doesn't make any difference.

keithl, when the brown solids encounter the air conditioning device, situational awareness goes out the window. Give yourself a chance, and use all the tools you can to improve and maintain situational awareness. Because sooner or later, you'll wish you had. But by then it will be too late. And that includes setting up your avionics correctly.

[bookworm]

To return to the question - consider you're on a bearing 190 from the station, tracking north.

Select 045 on the HSI. The needle will show fly left, with a TO indication. When you approach the 225 radial it will gradually come in until you're on the radial.

Go back to somewhere out 190 from the station. Now select 225 instead. You will now get a fly right on the needle and FROM pointer showing.

But what's a "fly right" on an HSI? The beambar is displaced to the (top) left side of the instrument, just as it was in the previous case. As I hold my track, the beambar moves down and right and comes in when I'm on the radial, just as it did in the previous case.

If I were to hide the arrowhead and the TO/FROM flag I'd have no way of distingushing between the cases with a VOR. Whichever way round it is, it allows me to establish northeastbound on a line running through the VOR in the orientation 225-045, with the same actions in response to the same beambar movements. What it doesn't tell me, unless I look at both the arrowhead and the flag, is which side of the VOR I am.

Do I use an HSI that way? No. Is it easier to fly like that? No. Is it possible to fly like that? Yes.

[FE Hoppy]

I'm with Keithl on this.
Also some Electronic systems will intercept bc ILS with front course set as a function of system situational awareness. So the answer is navigate with reference to more than a single indication or instrument.

[Captain Stable]

Hoppy, your final sentence is my point precisely.

You don't use an HSI by using solely the beam bar. You use that and the TO/FROM indication. Anything else is handicapping yourself by ignoring most of the capability of your istruments and degrading your situational awareness.

[keithl]

Hmm.. I think we’ve reached the point of entrenched attitudes and to go further would create more heat than light. Believe it or not, Mono, I have survived a 32 year flying career without ever considering the TO/FROM flag to be very important. Sure, in some of the failure cases used in the arguments above I would have regarded it as important, but if the RMI needles work and the compass card works I have used the situational awareness those things give me, to which the TO/FROM has added nothing.

In my present job, I see many people flying outbound (up the Final Approach Track) for Procedure Turn to come inbound. They like to have the arrow on the Inbound QDM rather than the way they’re going. I can’t see that that matters much. And that is why, in answer to the original question I maintain that it doesn’t matter.

The other reason we will never all agree is the word “matters”. What matters to one will be trivial to another. But if, therefore, it is a question of your personal definition of the word, then it can’t “matter” in a fundamental sense. I say that which way round you have the needle is not going to kill, or even disorientate you, unless you have some other failures to go with it.

[mono]

OK Keith,

I don't doubt your airborne longevity or personal airmanship. I also agree that for a purely instrument type approach (no FD or AP), it does not matter as long as you know where you are at any given time and you know that you want to either track inbound or outbound, then ignoring the arrow head will get you there.

My point is that you are establishing a bad habit. As an instructor (as it appears you are from your profile) you should set up your students with techniques which put them in good stead for the remainder of their aviation career. By allowing student pilots to fly VOR approaches when the arrow head does not point in the direction of travel will require them to UNLEARN the technique when they convert to a/c with flight guidance and autopilot systems.

Also as Capt Stable has stated if by some mistake you are the wrong side of the VOR at capture you could end up flying along the reciprocal of ones intended track as a result.

[keithl]

By the way - Magflip, have we answered your question? Having lobbed this grenade you've disappeared!