HSI Questions
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We all hopefully know what the Horizontal Situation Indicator (HSI) does. If not a glance at the manual for a refresher is in order.
So it’s a no brainer that it shows you how far to your TACAN station, along with station bearing/heading, your current magnetic heading, your course/radial, amount of deviation from that radial, and the heading you wanted to have.In this thread I don’t want to re-hash all that, but I’d like to add in some real world problems. In my current case I’d like to add in wind. You can see its effects on your jet when it pushes your FPM to the side of the HUD on landing. And it can kick your iron bombs off target. You have it indicated in the DED when you’re on the base page, the one showing both com channels and tacan channel, by pressing the 4 way switch right/seq. But this is merely the first step in understanding what it will do to you.
Your FPM in the HUD has this information already implemented from the ground portion of your flight. Notice the Drift/CO switch in the middle position? You know right off the bat where the wind is going to push you. If you never look away from your HUD for information you may not care about anything more. But what happens if you loose your HUD? Well, sure, you have your MFD page with the HSD showing relevent information about (at the very least) your steerpoints in relation to your bird. And yes, I know that “bird” has another meaning, but when I use it I mean Aircraft/Airframe. If you have a datalink available and running you can also see who in your package is where, and what targets they may be sending you. Those who are worried more about ground threats may even have known emmiters and the FEB displayed.
Point at my steerpoint on the HSD and that’s all I need to do to get there, right? WRONG!!! The wind can and will kick you off course, if I understand mother nature correctly.
So to start things off, here is my (hopefully) easier question: How far is 1 dot of deflection at 10, 30, 50 and 100 miles from the source? And in the same vein, how far is 2 dots at the same distances? I’ve been looking through the manual, the only thing I can find is in the first edition, chapter 17. It said that the HSI dots were 2.5* of deviation apiece. Is this still the case?
Read the remainder of my post at your own risk.
What I’ve been trying to figure out lately is how, without the HUD giving me the answer, do I navigate from point a to b. Well, I need to know my flightplan. In it is specified IMPORTANT information, like distances, arrival time at steerpoint, heading from previous steerpoint to next, airspeed, and altitude. With that information I can set up my Joker and Bingo settings, set my HSI to the correct radial (or as I’m using it bearing), know what altitude to hit when, and know how fast I’m expected to move. Pretty useful.
So what happens when I’ve passed steerpoint X and intercept the radial/bearing to my next steerpoint and just put my nose right on it? Well, O.K… I put the upper Lubber line on it to be acurate. I’m still moving towards my next dot on the map and on last check my CDI arrow had no deflection. Some time passes (lets say 2 mins) and while I’ve been fighting my height and VVI to stay roughly where I’m supposed to, my CDI has opened up to the first dot. WHY?
Obviously, if I kept my heading the same in a no wind situation this wouldn’t have happened. But there is wind and so it did happen.
What went wrong? I didn’t take the wind value into account. In my most recent run I was at 15k’ (alt was set for 29.92"HG) and had turned to meet the heading required for 7 from 6, 291* magnetic. On takeoff I had seen some wind around 4 knots. It was a sunny day, so temps at 09:00 should have been 30C - I don’t think it was dawn. Anyway, the worst wind I should see on a sunny day is 5knots, while it may have a gust of 2 knots. Upon reaching steerpoint 6 I was still showing 4 knots in my DED, at 319. This is where I tend to get messed up - so if the wind is heading 319*, or NW, it is comming from the SE, or 139*. Did I read that off the DED correctly?
So at 09:30:36 + 45s or so I’ve flown 9 miles, from stp 6, and have 43 miles to go. My HSI CDI shows about 1 dot of deflection, and when I had it centered my HSD didn’t show my steerpoint connector line as straight up and down, but from center to some right. I had my course set for the HSI correctly, so I want to know how far off course I got, and I haven’t yet determined the wind corrections I should have applied. Still getting there.
-Babite
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… so if the wind is heading 319*, or NW, it is comming from the SE, or 139*. Did I read that off the DED correctly?
You know where the wind comes from. But you don’t know where it is heading to if I remember correct. I was always wrong on that question.
Quote by google -> Wikipedia:
“Wind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees.”If you read 319 it is coming from there, not heading there.
By the way, do we have a deviation tape on the compass? and does the HSI have one too?
When I learned for the PPL I had this nice little sheet calculator (those 3 layers you can turn around to define your course, your airspeed, windspeed, and wind direction.
What you get is heading and ground speed.(There are tons of cornographip things you can calculate with that thing, fuel consumption, time, weight, speed and different units and so on, just by turning some layers :D)
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SnowSky - I don’t see any adjustment for the compass, period. Nor the HSI in this case. But I think that we are led to believe BMS keeps track of itself in magnetic alignment, code wise, so we won’t have any added errors from that, if that’s what you mean.
-Babite -
On my phone right now, so quite hard to write a long reply. If it’s not answered tomorrow (after 0900Z probably) I’ll see what I can provide you with.
In the meantime, another question is how valid the real deviation formulas really are to dead-reckoning in BMS? BMS doesn’t use the spheroid globe, but flattens it all out, so I’d expect some (minor) errors, especially when using it for longer distances?
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The question of what heading to take to achieve a desired course track is known in my education as the Wind Correction Angle (WCA). My usual tool was to use an E6-B to do the vector addition. My E-6B only has speeds marked to 260 so doubling or tripling the scale mentally is required for F-16 speeds (I just realized that the plate in the middle of the tool is reversible and is marked up to 650 on the back side). The bonus is if you desire a specific ground speed for timing purposes you can find the TAS that achieves it using the same tool.
Here’s an ASA video using the tool:
As far as I know variation (difference between true and magnetic north) and deviation (difference between magnetic and compass north) are not a factor in Falcon.
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For a HUD out situation, why not use the HSI NAV mode for current STP?
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On every aircraft I have flown one dot on the HSI is 5 degrees from the inbound course. I am not sure about the F-16, but just make sure the reference you have is referring to when the HSI is in TACAN mode, as the scale will change depending on the navigation source (IRS/ILS etc). At any rate you will be able to figure out in game by comparing your needle to your course bar when you are 1 dot off track.
Regarding distance, well that will change depending on your range, because the HSI shows the error in degrees (radials). Luckily though, the nautical mile system is based of degrees so at 60NM one radial = 1nm. This will increase as you get further away and decrease as you get closer. So if you are at 120nm from the station and one radial off then you will be 2nm off track. This is also useful for bullseye calls eg: you are bullseye 010 90nm, bandit is 030, 90nm. At 90nm, 1 radial = 1.5 nm. bandit is 20 radials away = 30nm. This is not exact as the distance assumes the distance via the 90nm arc, when in actual fact the distance should be a straight line.
A rough way to figure out drift angle for wind is to divide the total crosswind by the ground speed you are doing in miles per minute. Sounds complicated but once you have done it a few time itis no problem. So for example:
You want to fly the 360 track. You are flying 300 knots and the wind is 030 at 20 knots.
Using the sin rule you can calculate that the total crosswind is 10 knots (sin30 x 20) google it if you have not heard of this.
Now convert your speed into miles/minute. To do this you divide your speed by 60, 300/60 so speed = 5miles/min.
Now divide you crosswind by your speed in miles/minute 10knots / 5 miles/min = 2
You always need to offset your heading into wind, so the wind is from the right, we must add our offset. 360 + 2= 002, so to fly 360 track you need to fly 002 heading.
As you can see from the formula the faster you go, the less you are affected by wind.
Think of the huey in the same wind. They will cruise roughly 90 knots = 1.5nm/minute. 10/1.5 = 6.6, so they would have to offset ~7 degrees!
This is all fairly academic though, as caper says, I would just put the HSI in nav, offset a little into wind and fly the bar.
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Hi Kaiza
Thanks for a nice practical explanation -
buying a whiz-wheel is more fun and lets you do the same thing, plus with a handy circular slide rule : )
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To all - I AM using the NAV setting on the HSI, it appears I need to point that out better, my appologies. I’ve got a E6-B8, which only does wind crossing calc.s to 270 knots. Here’s what I determined last night, before I nodded off. For my indicated speed I used what the GUI recommended speed was, since I couldn’t remember. So IAS was 225 knots. My altitude was 15,000 feet. It should have been 30C, since I don’t know if Falcon changes temps and at what amount for height above surface, on a sunny day. The wind speed and direction were 4 knots @ 319. Once I started using my slider I determined my true airspeed to be 308 knots, ground speed to be 304 knots. How valid that is I’m not sure, since I was near lala land. And I’m not sure how important this is in Falcon, but my density alt. should have been 20k’. At present I haven’t determined how far off track I got there, my head had been bugging me all afternoon (weather related light sensitivity - cloudy then BRIGHT cause me light headaches) so I stopped there. Now I’m back to my normal state and will plug away some more.
@ Frederf - I’m jealous now. Yes, WCA is correct
@ caper - I am using the HSI in nav mode, that’s how I get the cdi to move for each steerpoint. Used like this when I set the course knob to the heading given for the steerpoint in the GUI flight brief the CDI pointer will be in the same direction as the HSD connecter lines, and the deviator will move relative to which side my bird is of that line/course, and roughly the same amount (when HSD is in the 8nm scale).
@ Kaiza - Thats one vote for 5* per dot, and the original vote for 2.5* per dot. I’d love one of the DEVs to chime in and set the record straight for BMS. Without that information the CDI will only be useful to me for intercepting nav. points, and not for determining how far off I really am. Your explenation for wind correction sounds right, it’s just been so long since I’ve done it …
@ Blu3wolf - Whiz wheel? That’s what I say when I mean a cutoff wheel! Can’t fit one in the A/C, and it does very bad things to body parts while spinning. NowIcan’tusespacebecauseIcutoffmythumbswiththewhizwheel!!! Wink wink.
-Babite
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To all - I AM using the NAV setting on the HSI, it appears I need to point that out better, my appologies. I’ve got a E6-B8, which only does wind crossing calc.s to 270 knots. Here’s what I determined last night, before I nodded off. For my indicated speed I used what the GUI recommended speed was, since I couldn’t remember. So IAS was 225 knots. My altitude was 15,000 feet. It should have been 30C, since I don’t know if Falcon changes temps and at what amount for height above surface, on a sunny day. The wind speed and direction were 4 knots @ 319. Once I started using my slider I determined my true airspeed to be 308 knots, ground speed to be 304 knots. How valid that is I’m not sure, since I was near lala land. And I’m not sure how important this is in Falcon, but my density alt. should have been 20k’. At present I haven’t determined how far off track I got there, my head had been bugging me all afternoon (weather related light sensitivity - cloudy then BRIGHT cause me light headaches) so I stopped there. Now I’m back to my normal state and will plug away some more.
@ Frederf - I’m jealous now. Yes, WCA is correct
@ caper - I am using the HSI in nav mode, that’s how I get the cdi to move for each steerpoint. Used like this when I set the course knob to the heading given for the steerpoint in the GUI flight brief the CDI pointer will be in the same direction as the HSD connecter lines, and the deviator will move relative to which side my bird is of that line/course, and roughly the same amount (when HSD is in the 8nm scale).
@ Kaiza - Thats one vote for 5* per dot, and the original vote for 2.5* per dot. I’d love one of the DEVs to chime in and set the record straight for BMS. Without that information the CDI will only be useful to me for intercepting nav. points, and not for determining how far off I really am. Your explenation for wind correction sounds right, it’s just been so long since I’ve done it …
@ Blu3wolf - Whiz wheel? That’s what I say when I mean a cutoff wheel! Can’t fit one in the A/C, and it does very bad things to body parts while spinning. NowIcan’tusespacebecauseIcutoffmythumbswiththewhizwheel!!! Wink wink.
-Babite
you can calculate crosswinds at ANY speed using an E-6B, you just need to apply a sensible multiplier… for instance, if you multiply all the values by 10, or by 2…
for the same stunt to work for the opposite side, on the circ slide rule side, you need to multiply all values by 10 (or some power of 10, ie 10^2, 10^-1…)
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So I’ve re-flown my TE to get a look at how the #s change in relation to eachother. When I check my airspeed indicator, I show ~ 228knots. At this point I’m pausing the sim to see what it claims to have for a true speed and a ground speed, VIA the hud. I have been flying by instruments, remember. So when I flick the speed switch to TAS I’m shown 287 knots, and when I flick it to GS I see 283 knots. Now remember this is on a 30C SUNNY DAY. Now I set the pressure altitude already to 29.92, since my height is supposed to be at 15.0M, or FL15. As I punch the needed numbers into my E6-B8 (yes, it’s not the fanciest, yet…) I need my pressure altitude, which is 15020, rounding down to 15000, set to the air temp of 30C. Then I read from my IAS on the inner circle to get my TAS from the outer circle. So 228 on the inside gives 287 TRUE for this height and temp. So the TEMPERATURE of the air doesn’t change with altitude in BMS then, in my case here. Anybody else wish to confirm?
-Babite -
@ Kaiza - Thats one vote for 5* per dot, and the original vote for 2.5* per dot. I’d love one of the DEVs to chime in and set the record straight for BMS. Without that information the CDI will only be useful to me for intercepting nav. points, and not for determining how far off I really am. Your explenation for wind correction sounds right, it’s just been so long since I’ve done it …
You dont need a dev to answer that one, you can work it out in 5 secs in the pit. Just dial the course bar around till it is 1 dot off and see the difference between the course bar and the needle.
I use the modified CCIP pit which has a modified HSI, so I can’t confirm. When I did it, 5 degrees was just over one dot, so a dot was about ~4 degrees, maybe just a bit over. This is probably just an error with the HSI scaling of my modified pit though.
Remember though, the scale will change depending what mode you are in, so this is only valid for TACAN.
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Or you could just go to page 3-9 of the manual that states 5 degrees for tacan/nav and 2.5 for ILS…
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Yes, but what about NAV? NAV defaults to 2 dots = 4 NM but the airplane should have an option (I’ll have to check BMS) for 2 dots = 3 degrees for most waypoint types (not STP 21-30).
Babite, if the temperature doesn’t change with altitude in BMS that could be a very significant finding and would lead to some rather odd results.
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Just reporting the information I have. If anyone else has either the know how or tools to run their own test on this and would like to report their own findings, I’m all ears. To an extent it should be warmer the higher up you go, since hot air rises and cold air falls, but after a certain point you would start to cool down the higher you went. Funny contradiction there.
Kaiza - Thanks for catching that, I new I must have missed something. That’s in the old brick manual, b.t.w. (glad the old resources still exhist). My question there still stands though. Did that change for BMS or does the old ruling still apply? Although your HSI is so much easier to read than stock … much clearer, but how are you changing it? I don’t see the HDG or CRS knobs, are they still in the same place painted black or do you manipulate them elsewhere?
Frederf - 2 dots = 4 nm? at what distance from station/steerpoint? As Kaiza states from the good old brick 5* is for TACAN and NAV, while 2.5* is for ILS TACAN & ILS NAV. 3* isn’t mentioned anywher, I’m guessing you’re rounding? Still would like to know how BMS handled this - my math skills in the pit are S…L…O…W. I never was superfast with big numbers, without a calculator doing trig problems would be hours, looking up coefs. for angles and x^2 roots & such, might be days. And I still wouldn’t have an exact fix on my location, which keeps changing unless I’m on the ground, and even then it changes. Per the temp., what issues did you have in mind? I.e. Worse engine performance due to hoter high alt. air? Funny handling characteristics?
To update the vote, since I misunderstood Kaiza the first time - 2 votes for how the brick states it, possibly 3 depending on Frederfs’ response.
Maybe I should have titled the thread “AIRMASS CHARACTERISTICS, MATH, AND BMS” due to the way things are playing out at the moment?
-Babite -
Hot air does rise but it also expands. The hotter the more buoyant and tends to find a place at low pressures which causes it to spread out and have less energy per volume and be colder. Higher, lower pressure, lower temperature is the overriding trend. Initial temperature more determines the destination than what to expect when it arrives. Depending on how things are calculated things could be very weird or already accounted for, but yeah, low air density, poor engine performance, that sort of thing.
2dots/4nm at all distances and that’s straight out of the book. Cross range error per dot would be constant no matter if the STPT was 10nm away or 1000nm. The computer takes your destination and desired course and determines what path (great circle no less I think) travels through that destination on that course. This imaginary pencil line on the map is compared against your current position and a measurement is taken how far you are from that line. The distance is displayed by deflecting the CDI.
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You oughtta figure for a 2 degree drop in temperature
per thousand feet of altitude gained. -
You don’t need to figure out Wind Correction angle with an HSI. Set the OBS and keep the needle centered you have corrected for the wind. In real life you would slave it to the GPS or the INS. I guess it depends on what you’re trying to figure out.
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To ALL - As I read this post I see that it may come across as a HOSTILE ATTACK. That is not my intent. I’m trying to bridge the gap in my mind between REALITY and the BMS world. What I’m noticing is REALITY isn’t always happening in BMS. Test data #s from others would be a real help if you can provide it.
Frederf - The “brick” manual states on pp. 3-8, “Each dot represents either 5* or 2.5*, depending on which mode the HSI is in. When the needle is fully deflected, the aircraft is 10* or geater from the selected course.” Which book are you refering to, then? As I’ve stated before I haven’t seen ANY information in the newer documents/manuals on changes to the HSI. PLEASE LIST supporting reference.
Blu3wolf - That’s not what my math has shown. I wouldn’t be suprised to find that I did something wrong, but my answers line up with what is shown on the HUD depending on speed mode for 30C. I ASSUME that the 30C is the ground level temp., since I’m not positive on the weather implementation in BMS. Having said that, the data I have supports that the temperature has NOT changed from ground level up to FL15. I reiterate that I’m in STOCK KOREA, BMS v4.32 + u7. If you have the #'s to back this up for BMS I’m all ears.
Capt.Stubing - by OBS I assume you mean On Board Sensor. If that were doable in BMS things might be easier. But you’d still be off-heading to keep the needle centered. So you’d still want to know what heading would be the correction for wind to stay on line.
-Babite
