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.

:sad:

[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!

[Captain Stable]

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.And therein lies the crux of basic airmanship - being prepared for any foreseeable eventuality. And instrument failure is, basically, foreseable. Best train your studes to be prepared for it and to know how to use all other aids to their maximum usefulness. Otherwise, IMO, you are failing them.

[DFC]

OK. At last I think that I know where the question originates and where Keithl, is basing his answer on.

Let's look first at the LOC........a single LOC with a backcourse and say a front course of 270deg serving runway 27.

The 150Hz lobe will over lie the runway and to the north of the runway and appraoches from both ends.

The 90Hz lobe will overlie the runway and to the south of the runway and approaches.

The LOC indication produced in the cockpit is not an indication of aircraft position or of heading required to intercept or of direction to turn or of anything else. The LOC indication is a simple indication of which lobe is being received the strongest.

In this case, the arrow head represents an east-west line through the aircraft with the arrow pointing 270deg. The beam bar simply tells the pilot if they are in this case NORTH of the centerline (150Hz dominant) or SOUTH of the centreline (90Hz dominant).

Regardless of where the arrow is pointing on the HSI, the arrow points 270 and the indication of being N or S of the centerline is correct. Thus it is not necessary to set the arrow to any particular direction to deduce this information.

However, provided the arrow is set to the inbound FRONT course, situational awareness will be improved because the display will be intuitive regardless of where the aircraft is and regardless of the desired tracking inbound/outbound/front/back.

So using the HSI or for that matter, the OBS, it does not matter what the arrow is set to for LOC tracking, the indication will be the same (confusing as that may be to the pilot).

Moving on to the VOR.

First of all, the VOR compares the current radial from the VOR station and after checking the "desired" course TO or FROM the station displaces the beam bar appropriately. Unless the arrow is set to an appropriate number then this comparison is meaningless.

While Keithl is incorrect to make a sweeping statement saying that it does not matter where the arrow is pointing, He is correct to say that when using a HSI, provided that either the arrow head OR the arrow tail is set to the required track, the beam bar will show the correct position of the aircraft relative to the desired radial.

Replacing the runway above with a VOR station and making our desired radial the 090/270 through the VOR. Our aircraft is N of the 090/270 radial. NW or NE it matters not.

Once again, the arrow and tail represent an east west line through the aircraft and the beam bar tells the pilot the position of the desired radial in relation to the arrow line.

Let's look at the position with the arrow set to 270. The beam bar will show that the desired radial is SOUTH of the aircraft. This is the true position.

With the arrow set to 090, the beam bar still shows that the desired radial is SOUTH of the aircraft.

However, without the TO/FROM flag, it is IMPOSSIBLE to know if the aircraft is NE or NW of the VOR.

Consequently, without the TO/FROM flag, a further instrument must be looked at and the information absorbed in order to maintain situational awareness......extra workload.

Now we can see that whan making a VOR approach by tracking outbound anlong the final approach track and then completing a procedure turn to establish inbound, Keithl is technically correct to say that the final approach inbound course can be set from the very start on a HSI.

However, the system Keithl relies on is that the slaved compass system MUST BE OPERATING.

Failure of the slaved compass throws the idea out the window.

Furthermore, if the approach is made using an OBS then what Keithl suggests will not work.

Also, some automatics will have problems with the procedure Keithl describes.

So to be safe........when using the VOR always point the arrow in the desired direction!.......it's "failsafe" :D

Regards,

DFC

[keithl]

OK, DFC acknowledges I have some basis for my argument, Mono recognises I have some experience. I’m glad we’ve moved away from the exclamation marks and have a rational atmosphere. I’m going to try one more time to make my case, then quit before this gets obsessive. Even taking the time to write a reply as long as this shows it’s getting a bit out of proportion…

I think we’re all happy about the ILS case. As I said originally, here it does matter which way round the arrow goes. Setting it to the Inbound, Front Course, ILS QDM gives you a correct Horizontal picture whether you are going Outbound on LLZ, Inbound on the ILS, or even using a Backbeam (Back Course) signal.

Now the more contentious VOR. Flight Systems vary. Some have gimmicks like a “Reciprocal” switch, some don’t, and anyway the original question was about the HSI alone. So lets set the System aside. It’s just you looking at your HSI. Fully serviceable. Not an OBS. Not an RBI. Nor yet a bl**dy Astrolabe! Sorry. Exclamation marks, sorry. Regains poise and continues… The To/From flag always points to the VOR. The CDI may be aligned either way without affecting that. All agree? Ah, someone doesn’t. Captain S. Very well, gentlemen, everyone get out their SID Plates and look up the Grice 4D from Edinburgh. You don’t need to book a sim slot, try this on RANT, Jepp FlitePro or even (if your eyes are good enough) MS Flight Sim. Approaching the TLA 351R from the direction of St Abbs ( from the East, if you don’t have the chart ), we set the CDI to 351 or to 171. Either way, the to/from flag points to the left, towards Talla, and the BB is away from us, towards the top of the instrument. Why, I hear you ask, would I want to set 171? I’m going to Grice. Well, maybe you have a personal convention that the arrow always points to the station. That would be consistent with the ILS case and you like to remind yourself that you are going outbound by having the yellow arrow pointing behind you. It also means that whether you are flying a CDI or an RMI, Outbound is always up the tail of the needle. Others may not like such a convention – that’s fine, I leave it to you. My case is that it doesn’t matter. So you turn right, towards Grice, putting the To arrow behind you. If the APs confused, use the heading mode, but YOU have no reason to be confused.

Capt S, I have no quarrel with the view that you use everything you’ve got to maintain situational awareness. I just don’t think that meets the terms of the original question, which I took as an academic one about the HSI alone. I do disagree with your contention that “when the BS hits the ACD, SA goes out the window”, but we won’t (for space) take it up here.

Finally, I may have caused those exclamation marks by saying that something in aviation “doesn’t matter”. We all know that “flying is inherently safe, but very unforgiving of any carelessness or error”. So I agree, everything matters in that sense. When I use the phrase “doesn’t matter” in this context I mean, “will not cause disorientation”. If you still think it will, then we must agree to differ.

“Keithl Out.”

[alf5071h]

On entering this scrum from an offside position; a significant issues is that equipment should always be used as the designer intended. The VOR beam bar evolved from a needle display and through international agreement the TO/FROM facility was required (RTCA). In more recent avionics, manufacturers use the TO/FROM information to ensure that the flight guidance maneuvers the aircraft in the correct direction. Modern auto flight and EFIS’s also use all of the VOR information (LEFT/RIGHT and TO/FROM) to correctly position course lines or intersecting radials.

Thus the student pilot who is just beginning to use VOR must be aware of all of the indications and uses of the VOR functions. Just because one function appears not to be appropriate in one scenario does not mean that the system should be taught that way.

A critical issue appearing in some accidents is that of negative transfer of information/understanding; where something learnt for one situation is erroneously used in an inappropriate situation. Navigation is #2 on the do list (Aviate, Navigate, etc); CFIT with lack of position awareness is #1 on the do not do list.

It is very difficult to unlearn concepts; - “first taught, best remembered”