HSI- the yellow course bar arrow head?
Join Date: May 2002
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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.
Join Date: Mar 2002
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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"
Regards,
DFC
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"
Regards,
DFC
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.”
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.”
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”
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”
