Sometime during the 1990s-we cant say when-we noticed an expectation shift among owners and avionics buyers. Until then, HSIs were considered standard in twins and jets, relative rarities in low-end singles.
Frankly, we found it amusing when a Skyhawk or Archer owner inquired about HSI retrofits. (Ho-hum…another tire kicker.)
All thats changed. Owners of all sorts of airplanes-from four-place cruisers on up-want and will pay for HSIs and many who already have them want a better one. With two all-glass designs out there-from Sandel and soon, Bendix/King-the market doesnt lack an upper end.
Not that it would matter much if it did or even if the top-of-the-line pushed $25,000. With high-end GPS and MFDs in such demand, owners no longer balk at avionics upgrades that represent a third of the airframes value.
In this article, well examine and compare the major HSI offerings; systems we consider affordable and practical for owners of light singles and twins.
Well admit upfront that were ignoring one model, the Collins PN101 system. Its a proven design but one more suited to heavier iron than a Skylane or a Bonanza.
Why Have One?
As the name implies, horizontal situation indicators present the aircrafts position in two-dimensional space in pictorial form, although theyre hardly as pictorial as the new primary flight displays now on the drawing board. If you consider that HSIs incorporate glideslope flags, they have a combined three-dimensional aspect, too.
HSIs combine the directional gyro, nav indicator and glideslope indication on one instrument so they both reduce workload by centralizing the scan and they make it much easier to interpret the aircraft position relative to a selected course.
Heading, course deviation and glideslope information-including nav flags-are presented on the display in either a conventional mechanical meter movement or electronic display. (More on the electronic display later.)
The HSI also contains the autopilot and/or flight director heading command control-the heading bug. For aircraft not equipped with an autopilot-and most aircraft equipped with an HSI usually have one-the bug is still there and can serve as a shorthand reminder for an assigned heading.
Simple to Complex
At its heart, every HSI is nothing but a directional gyro. Some models-the Century designs, for instance-have an internal gyro powered by vacuum or electricity while others-the Bendix/King variants-use a remote gyro that feeds heading information to the HSI display electrically.
The most basic HSIs are non-slaved and rely on the user to set the gyro, just as with a plain-vanilla DG. These are few and far between and most HSIs these days use remote-mounted flux sensors or flux gates to automatically set the gyro display to the correct magnetic heading; no pilot input required and no precession.
Well, almost; the pilot inputs money to pay for the slave system, which isnt cheap nor especially easy for a shop thats never done one, given the importance of flux gate location and the wiring.
Get your shop to pencil out the price difference between a slaved and non-slaved system. Dont be surprised if the price delta is significant-as much as 15 percent. We think its worth it, however, because if youre fooling around chasing a precessing gyro in the clouds, you lose the HSIs considerable advantage as a workload reducer.
Century Flight Systems
Century has nailed the market for as long as we can remember with its popular NSD360A series HSI. This model has a self-contained vacuum-driven gyro that has proven reliable in the field, subject to the vagaries of the basic vacuum system which are, admittedly, considerable.
With the exception of some earlier versions, the NSD360 is easily serviced by most instrument shops and is common enough to be economically repairable. (Relatively, at least.)
This system is available in both slaved and non-slaved configurations. The non-slaved unit is quite straightforward, requiring no remote components at all. In fact, except for the electrical connections to the nav system, a non-slaved NSD isnt any more involved to install than a plane-Jane vacuum DG.
The non-slaved NSD360A version is a popular player because of its relatively low cost-about $9000 installed-ease of installation and reliability.
Bargain shoppers who might otherwise sink into shock when quoted for a full-slaved system with remote gyro generally find the non-slaved NSD to be an attractive bargain.
Like the entire line of Century HSIs, the NSD360A has an easy-to-interpret display. The course pointer is bright yellow, nav flags are red and white candy stripes with a yellow heading bug. And the autopilot heading command will interface with most autopilots without the need for adapters.
On the display, 45-degree tic marcs and an orange reference aircraft marking give the NSD a high-class look beyond its modest price. The instrument weighs approximately 4.5 pounds and takes some 9 inches of depth behind the panel.
On the downside, if you lose your vacuum source, you also lose your HSI and if the departing pump squirts carbon powder back into the system, its possible youll trash the HSI, too.
For owners paranoid of vacuum systems in general, Century introduced the NSD1000 all-electric HSI. As with the NSD360A, the NSD1000 is a self-contained gyro but is powered by a 14- or 28-volt bus.
Since its introduction in the early 1990s, the NSD1000 is considered Centurys flagship model, although not a very popular seller from the retrofit point of view, probably due to its higher price. The unit stands alone in that its the only electric HSI that has a self-contained gyro.
This is one reason why Cirrus picked the NSD1000 for its SR-20. Cirrus and owners, however, tell us this choice hasnt yielded the perfect panel. Numerous failures have been reported-one owner had four failures.
This is consistent with owner reports weve received. A few NSD1000 owners we know report a fair amount of maintenance associated with this instrument.
A Baron owner with a low serial NSD1000 told us that his HSI was back at the factory three times with less than 1000 hours of service. The gyro was overhauled, meter assemblies replaced and repairs made to a sticky glideslope flag. The factory provided loaners when appropriate but clearly this incidence of repair is an inconvenience any owner would like to avoid.
The NSD1000 is offered in two versions, slaved or slaved with RMI bootstrap. (Sorry, no non-slaved cheapie with this one. Its a premimum instrument. )
Many pilots are stumped by the term bootstrap, by the way. In short, bootstrapping provides an electronic heading output to properly orient a moving map display or Stormscope with heading sychronization.
Whether bootstrapping is worth the expense is a personal choice. We know owners who swear by it, while others seem blas about this feature. Unless you have specific plans to use it, dont bother.
All things considered, if youre looking for a budget HSI, a non-slaved NSD360A will do the job and eat both the least amount of useful load and upgrade budget.
A basic non-slaved NSD360A installation will cost about $9000 while the top-of-the-line NSD1000 with bootstrap will easily come to $12,000 or more.
The biggest problem with this system is what to call it. Now, its Honeywell/Bendix/King but will it soon be General Electric/Honeywell/Bendix/King? We give up. Whatever, the KCS55A is a helluva good HSI.
Its earned a reputation as the industry standard system for general aviation and even corporate aircraft and it has excellent field service history.
The KCS55A is offered in one basic version: Slaved with bootstrap output. It has a remotely mounted electric gyro, the KG102A. This is the main gyro reference for the system and its entirely electric, running on either 14 or 28 volts DC.
Other components include the KMT112 magnetic flux detector and KA51B slaving control/compensator unit thats mounted on the instrument panel and provides the pilot with options for free gyro mode, which eliminates the slaving operation. It also contains a slaving meter which indicates proper operation or any slaving errors.
The panel display, the KI525A, is a familiar face with a rugged look and feel. The 7-inch deep HSI display features yellow dual glideslope pointers on the left and right side of the display, which only come into view during a valid glideslope condition.
This type of presentation means less clutter and eliminates a separate glideslope warning flag and one less delicate meter movement.
Nav and heading-valid flags are bright red and easy to read. The course select pointer and CDI is yellow and is a nice contrast with the white lubber line. To/From indications are shown as a simple white triangle.
Installation hardware is, of course, Bendix/King traditional; high quality and very reliable, thanks to years in the field. If your plans include interface with a Century or Cessna/Sperry autopilot system, expect additional expense as your shop gets technical and describes the required autopilot adapter youll need to buy.
The KA52 and KA57 autopilot adapter modules are behind-the-scenes happy boxes that turn heading output signals from the HSI into proper signals required by some autopilots other than Bendix/King. Century Flight Systems also manufactures such adapters for some interfaces with their systems.
One nice and high-class feature we like about the KCS55A is the automatic fast slaving. At initial power up, the system will automatically slave itself to the proper aircraft heading without any input from the operator. By contrast, the Century systems use slow slaving. They get there, but not as fast.
As an added bonus, Honeywell offers several optional vacuum horizon and flight director gyros with the same casing as the KI525A HSI, if a matched set is your wont.
While the KCS55A has generally enjoyed a first-class reputation, its also tends to be the most expensive, depending on options. The combination of its higher acquisition cost, installation difficulty and the autopilot adapters put this system in the $13,000-plus range in many cases. The end result is a reliable system thats easy to maintain but youll pay the price.
This HSI remains an integral part of Bendix/Kings newer autopilot systems, part of KFC225 flight control STC requirements. In other words, if you plan to install a KFC225 system in your newer Malibu, for instance, you might need a KCS55A if you dont already have one. Not a bad proposition, just more dollars.
Frankly, weve seen more S-TEC ST180 HSI/compass systems in the past six months than we have in the many years this system has been offered, perhaps because of Meggitts marketing efforts.
The S-TEC system seems to have been overshadowed by the Bendix/King products because Bendix/King had such a lock on the OEM market. Now that S-TEC has gained market share in autopilots, the ST180 is gaining favor, too.The ST180 has an all-electric remotely mounted slaved gyro that looks similar in appearance to the Bendix/King KG102A gyro, as does the remote flux detector and slaving accessory in comparison. (Prices quoted include the remote gyro, by the way.)
The S-TEC HSI differs from most other units as its course selector knob is on the lower right of the unit and the heading select knob is on the lower left.
In fact, we have heard comments from some pilots that after many hours of flying, they still cant get used to having the heading select knob on the left side. Picky, picky, picky. But reaching through the center of the control yoke on some airplanes to get to the heading bug is somewhat awkward.
In any case, the ST180s controls and markings are well labeled, although this unit has only one glideslope indicator, as does the Century design.Two unique features found on the ST180s HSI are the GPS annunciator light and the AEM annunciator light. When displaying GPS course information from a remote GPS, the green light illuminates indicating GPS is active. Helpful, indeed.
The AEM or automatic emergency mode light illuminates if the compass card does not agree with the remote flux detector. The pilot pushes the reset button at the top of the instrument and the unit enters this test mode, triggering the light within three minutes if a fault exists.
While flagged in this mode, the system can be operated non-slaved, like an ordinary directional gyro. Clearly, operation of any HSI system with faulty slaving information can be hazardous; this warning system helps avoid that.
In general, this system is ergonomically well designed. It might be the design of the case, but the unit appears smaller than the others in this group, although its a true 3-inch ATI cut.
We find the ST180 to have fast slave operation similar to the Bendix/King system, finding the proper heading right out of the gate with little adjustment needed from the pilot.
Reliability of the gyro and other system components has been excellent. This system is generally standard in new aircraft that have an S-TEC System 55 autopilot and it needs no autopilot adapter to work with the S-TEC stuff. Pricewise, expect the installation to be similar if not a bit cheaper than a Bendix/King KCS55A.
The S-TEC bootstrap output interface is done a little differently than the Bendix/King system but this is an issue for the installer, not the owner. And like the KCS55A, the ST180 is offered in one basic although well-featured version.
Component size and weights are similar to the Bendix/King system and installation should take about the same amount of time. Expect to see more of the ST180 as the popularity of the S-TEC equipment continues to grow.
Sandel SN3308 EHSI
Well admit that lumping the Sandel all-glass HSI with conventional electro-mechanical instruments is like comparing apples and oranges. Or maybe apples and bowling balls. Theyre that different.
While conventional HSIs are stuck with cards and needles, the Sandel projects the display electronically on a screen, thus its hugely flexible, with the ability to combine moving map features-waypoints, courses, bearing information-with the HSIs traditional CDI and heading indications.
Moreover, the Sandel can overlay Stormscope indications or present multiple nav course and bearing information in a 360-degree or arc view.
Where a mechanical HSI has but two controls, the Sandel has 11, which seems overwhelming at first blush. But keep in mind that this device is both an HSI and a moving map. Operationally, its more difficult to master than a plain HSI but we wouldnt call it difficult to learn.
Weve been very impressed with this device but many buyers are disappointed to learn that beyond the $9495 price for the Sandel itself, theyll have to purchase and integrate other components to complete the package. This can easily double the cost of the typical installation.
While mechanical HSI system configurations are strapped at the interface connectors (via wire) the Sandel takes care of all set-up parameters through operating software. Welcome to the age of smart avionics.
The first few Sandel installations by a new installer will result in a steep learning curve and as Sandel introduces yet more features and interfaces, shops have to pedal to keep up. Have patience with your installer.
The quality of the SN3308 has been excellent and product support continues to be first rate. The technical support engineers at Sandel are helpful and troubleshooting over the phone seems to work every time; they clearly know this box inside and out.
Being an all-glass design, the Sandel has a rear-projection display system that uses a bright bulb to project the HSI depiction on a flat, dark screen. The display is highly readable straight on; less so when viewed at high angles. When the system was first fielded, there were concerns about bulb life and although there have been failures, this hasnt proven to be a significant worry thus far.
Excluding the Sandel, shopping the HSI market is definitely more a question of price than of features, since all do essentially the same thing. Including Sandels EHSI-and soon Bendix/Kings KI 825-the HSI has vastly more capability.
When getting a quote, however, keep this in mind: Virtually all HSI systems require a remote VOR/LOC converter, although newer boxes such as Garmins GNS 430/530 system have an integral converter.
An old KX155 or KX170, however, will need a remote converter, such as the Bendix/King KN72 or equivalent Century. A KX165 has a built-in converter but if you have one of those, you may already have an HSI.
As far as were concerned, the Sandel occupies the top of the heap at the moment, both in terms of capability and overall value. But its not a slam-dunk first choice for this reason: Its not right for every aircraft.
The Sandel makes perfect sense in a Malibu or Saratoga, less so in an Archer or older Bonanza, unless youre sure youll want to display external nav, traffic or storm information your HSI, there might be better map choices. Sandel pro: Lots of room to upgrade. Sandel con: Like wearing a tux to a tractor pull.
For a low-end HSI in a low-end airplane, then-a Skyhawk, Skylane or older Mooney-the Century NSD360A is the hands- down best value. Typically, the vacuum NSD360A series have less time between overhauls than the remote electric units offered by Bendix/King and S-TEC.
Average overhaul of the NSD360A instrument/gyro is in the $2000 range and this costly routine maintenance ritual sometimes disappoints owners. But its cheaper to overhaul than the Bendix/King unit and cheaper to install. Well admit, were not crazy about installing new instrument requiring vacuum but you probably already have a pump anyway and perhaps a vacuum back-up, so these instruments still have their place in the panel.
If you prefer an electrically driven slaved system, we think its a draw between the Bendix/King and S-TEC systems. The Bendix/King seems easier to service in the field with more loaners available due to its large population. But S-TEC/Meggitt may be the real comer in the eletromechanical HSI market. And if youre not sure about any of this, patience, patience, patience. As more competition emerges, the prices on EHSIs may decline. Or, the emerging primary flight display technology might render HSIs obsolete. Were not at that point yet, but with new product development coming fast and furious, we soon could be.
Also With This Article
Click here to view the HSI Checklist.
Click here to view the HSI Profiles.
Click here to view “Mid-Continent’s New Remote Gyro.”
Click here to view “Bendix/King’s New Glass.”
Click here to view “The Anti-HSI Argument.”
Click here to view the Contacts and Addresses.
-by Larry Anglisano
Larry Anglisano is a consultant and test pilot at Exxel Avionics in Hartford, Connecticut. Hes an Aviation Consumer contributing editor.