The HSI is an improvement on the standalone CDI instrument in that it takes into consideration your actual heading. The old CDI would just have a radial set in the OBS and show your deviation (which could have reverse sensing depending on which radial you tuned) in relation to that radial.
HSI’s can also be very adaptable to both legacy navigational technology (like VORs) and even newer navigational technology (GPS). For example, I have an Aspen Evolution 1000 (pictured below) in my Cessna 182 that contains a digital (versus analog which is pictured above) HSI.
HSI Horizontal Selection Indicator does exactly the same thing but the design of the instrument is different like in a Beechcraft Baron. OBS button on a GPS unit Omnidirectional Bearing Selector turns the GPS into a VOR tracker just like above but you do not have to tune the radio as it works off of the GPS.
Note that the VOR/LOC CDI is to the left of the instrument as it should be. But, now that the indicator is slaved to heading, it has rotated so that left needle deflection is relatively to our right. So, we can still fly toward the needle to correct. Pretty Simple, Eh?
The HSI has a fixed aircraft symbol and the course deviation bar displays the aircraft's position relative to the selected course. The TO/FROM indicator is a triangular pointer: When the indicator points to the head of the course select pointer, the arrow shows the course selected.
A standalone CDI (Course Deviation Indicator) is an instrument that shows your deviation from a VOR radial that is selected using the OBS (Omnibearing Selector) knob located on the instrument itself. An HSI (Horizontal Situation Indicator) is an entirely separate instrument that combines a CDI and a heading indicator.
A course deviation indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a course to or from a radio navigation beacon. If the location of the aircraft is to the left of this course, the needle deflects to the right, and vice versa.
Reverse sensing occurs when the current heading is on the bottom half of the VOR. Taking this theory to the HSI where the currently flown heading is always on the top half and you can never have reverse sensing.
Like other cockpit instruments, HSIs can vary in appearance and operation by brand. Fundamentally, however, all HSIs indicate an aircraft's horizontal position relative to a selected VOR or ILS. The HSI works by combining a VOR/DME receiver and an electrically-slaved heading indicator.
Each division (dot) represents 1.25 degrees deviation from the ideal ILS flightpath approach.
OBS vs CDI The OBS (Omni-Bearing Selector) is the knob that the pilot adjusts. On the other hand, the CDI (Course Deviation Indicator) is the needle in the window that shows your desired course. So the pilot turns and sets the OBS but sees their course by looking at the CDI needle.
On what course should the VOR receiver (OBS) be set in order to navigate direct from Majors Airport (area 1) to Quitman VORTAC (area 2)? 101°.
1. When the course deviation indicator (CDI) needle is centered during an omnireceiver check using a VOR test signal (VOT), the omnibearing selector (OBS) and the To/From indicator should read: a. 0 degrees TO or 180 degrees FROM, regardless of the pilot's position from the VOT.
What Is A "Back Course" Approach? When you fly a localizer back course approach (LOC BC), you're navigating to the runway using horizontal guidance off of a localizer system, but in the opposite direction that you'd normally use the localizer.
HSIs can be driven by iron gyros or by electronic (digital) gyros. If iron gyros, those gyros can be spun by electricity or by vacuum. The displays can be electronic (EHSIs) or mechanical. Finally, the HSI compass card can be slaved to a magnetic sensor or non-slaved like your basic heading indicator.
The horizontal situation indicator (commonly called the HSI) is an aircraft flight instrument normally mounted below the artificial horizon in place of a conventional heading indicator. It combines a heading indicator with a VHF omnidirectional range-instrument landing system (VOR-ILS) display.
If you think they are, just run out and try a back course with a VOR without a backcourse switch.#N#The reason an HSI reads "correctly" on a back course when set to the localizer heading is that the "arrow" is actually pointing towards where the localiser will be when you cross it, whereas in the case of a VOR, you are "from" a station that doesn't have a "from" side, and you are going "to" it so the needle has no option but to sense backwards.
Technically, w/o engaging the autopilot, you don't! But have you ever tried to apply a crab angle when the CDI is 90 degrees off. You have, say, the CDI set at 090 on an ILS to runway 18. It will work fine except try making a 3 degree correction while looking at the CDI off 90 degrees.
The glide slope deviation pointer indicates the relationship of the aircraft to the glide slope. When the pointer is below the center position, the aircraft is above the glide slope, and an increased rate of descent is required.
The HSI is a combination of two instruments: the heading indicator and the VOR. By bringing these two instruments together you are able to get a much clearer picture in a quicker amount of time. This instrument is particularly handy when flying on instruments or under an IFR flight plan. For me it just makes the job of flying much easier, but you first need to completely understand how to read and gather information from the HSI.
To orient where the aircraft is in relation to the facility, first determine which radial is selected (look at the arrowhead). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator) to find which hemisphere the aircraft is in. Next, determine how far from the selected course the aircraft is (look at the deviation bar) to find which quadrant the aircraft is in. Finally, consider the aircraft heading (under the lubber line) to determine the aircraft’s position within the quadrant.
The TO/FROM indicator is a triangular-shaped pointer. When this indicator points to the head of the course arrow, it indicates that the course selected, if properly intercepted and flown, will take the aircraft TO the selected facility, and vice versa.
An "on course" condition is indicated when the course pointer, the course deviation bar, and the course reciprocal are all "in line."
The HSI provides a basic horizontal view of the aircraft's navigation picture. In the F-15E, it can provide navigation data to selected ground navigation facilities (TACAN or Instrument Landing System (ILS)) or to onboard navigation systems like the Inertial Navigation System (INS). Instrument flying without an HSI requires a fair amount ...
The card is mechanically coupled to the compass card set knob and, at the start of each flight, must be set by the pilot to agree with the magnetic compass heading. As the flight progresses and headings change, the directional gyro rotates the card to indicate the current heading.
This red warning flag indicates inadequate VOR or LOC signal, or loss of power to meter circuits. Under these conditions, course information is unusable, however, all heading information remains valid.
This orange line, located at the top of the display, indicates the aircraft's magnetic heading on the compass card. The lubber line is "in line" with the reference symbol to reinforce this association. This card, located beneath the lubber line, indicates the aircraft's current heading.
The selected heading is marked by an orange heading bug which can be moved to any point about the perimeter of the compass card. As the aircraft's heading changes, the bug rotates with the compass card, thus alerting the pilot to the difference between the selected heading (located under the bug) and the actual aircraft heading. The heading bug may also be coupled to an autopilot or flight director system. When coupled, "off heading" signals will be generated causing the autopilot to fly the aircraft so as to maintain the selected heading.
As the aircraft's heading changes, the bug rotates with the compass card, thu s alerting the pilot to the difference between the selected heading (located under the bug) and the actual aircraft heading. The heading bug may also be coupled to an autopilot or flight director system.
On AIS charts it still takes a little mental energy to determine exactly what the inbound front course is. But, if you remember Rule #2, it’s easy. Just orient yourself on the chart so you’re centered on the localizer and the charted feather shading is on your right. What’s your heading? That’s the inbound front course that you set in the OBS on your HSI.
If you’re off course on a localizer or a localizer back course approach, and you’re on the side of the localizer feather on the chart that has the shading or other differentiation, the course deviation indicator (CDI) will swing to the left. It doesn’t matter where you’re headed or what you’re doing—if you’re on the shaded side of the feather, the CDI is to the left. Period.
So, the difference between a rather challenging NDB approach and one that you could fly with relative ease was a fixed or manually adjustable compass card behind the bearing pointer versus a compass card that turned with your heading. So it is also with VHF navigation.
If you were lucky—or wealthy—enough to have an RMI (remote magnetic indicator), which was really nothing more than that bearing pointer on a compass card that, instead of merely being fixed or manually adjustable, was slaved to your heading indicator, life got a whole lot easier.
The venerable and once-ubiquitous King KR-86—when it worked—provided an ADF bearing pointer on a user-adjustable compass card. The bearing pointer was useful for navigation, but that compass card was a challenge because the numbers on it never corresponded to your actual heading on the heading indicator unless you were really diligent at manually adjusting it. Consequently, most of us struggled to precisely fly an NDB approach. (I’m told it could be done, but I’ve never witnessed it myself.)
The horizontal situation indicator is a rather busy instrument, displaying lots of useful information. For our purposes, just as an RMI combines a bearing pointer with a heading indicator, let’s think of the HSI as a single instrument that combines a gyro heading indicator with a VOR/LOC navigation instrument. Let’s look at one of many reasons that depiction is meaningful.
If you’re inbound on the back course or outbound on the front course, the shaded part of the feather is on your left, so you correct by steering away from the CDI.
The center portion of the course pointer needle moves to indicate deviation from selected course. A series of "dots" provides a linear indication of how far the aircraft is "off course." In VOR use, each dot represents 5 degrees; when being used to fly the localizer, it shows 1 1/4 degrees per dot; for RNAV "APPR" mode, 0.625 nm per dot; and for RNAV "Enroute" mode it indicates 1.25 nm per dot. An "on course" condition is indicated when the course pointer, the course deviation bar, and the course reciprocal are all "in line."
An HSI, or Horizontal Situation Indicator, is a combination of two familiar cockpit instruments: the directional gyro with a heading bug and a VOR/ILS indicator. WHAT DOES AN HSI DO FOR THE PILOT?
This red warning flag indicates inadequate VOR or LOC signal, or loss of power to meter circuits. Under these conditions, course information is unusable, however, all heading information remains valid.
Lubber Line. This orange line, located at the top of the display, indicates the aircraft's magnetic heading on the compass card. The lubber line is "in line" with the reference symbol to reinforce this association.
As the aircraft's heading changes, the bug rotates with the compass card, thus alerting the pilot to the difference between the selected heading (located under the bug) and the actual aircraft heading. The heading bug may also be coupled to an autopilot or flight director system. When coupled, "off heading" signals will be generated causing the autopilot to fly the aircraft so as to maintain the selected heading.
This yellow wedge relates the vertical glide path centerline to the aircraft's position. The aircraft is "on glide slope", when the wedge covers the horizontal index. Each dot on this vertical scale represents approximately four-tenths degrees vertical deviation from the centerline.
This red warning flag indicates loss of electrical power to the gyro. Heading information is then unusable but all course information (comparable to a standard VOR/ILS) remains valid.