OBI, RMI, HSI… What combination do GA aircraft w/o glass typically have?

• Automatic Direction Finder (ADF) - Provides basic bearing information to practically any radio source the pilot can tune to. Designed for use with the older NDB system developed during WWII, still in use but about two generations redundant in the face of VOR and GPS. Later models include a manual compass card to reduce mental math.
• Radio-Magnetic Indicator (RMI) - A two-channel ADF with compass card slaved to the magnetic compass (or in some cases using an internal gyro like the Heading Indicator).
• Omni Bearing Indicator (OBI) - Used for VOR navigation and the cheapest way to do so, has a manual compass card that selects the desired VOR radial and a CDI needle to provide a steering aid to place the aircraft on that radial, with a "TO/FROM" indicator to resolve directional ambiguity.
• Horizontal Situation Indicator (HSI) - Combines the OBI with the aircraft's gyroscopic direction indicator to provide a simplified "moving map", with the aircraft's current heading, relative direction of the selected VOR radial, course deviation and glide slope all in one instrument. Can replace the Heading Indicator as well as an OBI to reduce instrument count and/or allow for redundant backups like a second artificial horizon. Biggest downside is cost (about \$13k for the display unit itself).
• Distance Measuring Equipment (DME) - essentially IFF in reverse; the aircraft interrogates a ground station with radio pulse pairs, and the station sends a matching pulse pair after a specially-timed delay; total round-trip time of the interrogation and response, minus the preprogrammed delay and halved, is the time the signal took to reach the station at the speed of light through air (about one nmi in 12.36 microseconds), which can then be used to calculate distance. These transponders are often colocated with NDB or VOR stations on a corresponding frequency allowing aircraft to get an accurate fix from a single ground station.

My understanding is that, for an aircraft to be suitable for IFR flight, the aircraft requires all VFR instruments (airspeed, altitude, compass, tach, oil pressure, oil or coolant temp, fuel gauge) plus a timepiece, gyroscopic direction indicator, Turn Coordinator or Turn/Slip Indicator, adjustable pressure altimeter, artificial horizon, and "two-way radio communication and navigation equipment suitable for the route to be flown", that last requirement being just wonderfully ambiguous regarding what you as a pilot may want or need from the above listed equipment.

The standard configuration for the 172 and Cherokee seems to be two OBIs, one with glide slope and one without, plus a single-channel ADF. This seems like a fairly economical gauge cluster for modern radionavigation, but the ADF seems old-fashioned in the face of the newer RMI that allows triangulation from an ADF and a VOR or two VORs more or less at a glance.

Obviously the very newest and most expensive aircraft have full glass cockpits, but for the rest of us, what displays are typically used? Do people tend to upgrade the ADI to an RMI, or is that the display that's considered redundant with NDBs on their way to retirement? How many people spring for the HSI? Do you keep the non-GS OBI if you have an HSI, or do you replace that with something else too? Is DME standard equipment, can't fly without it, or do pilots more often rely on multi-station triangulation for a position fix?

• First, I think this is a good question that should be kept. That said, it sure does split into a whole lot of little questions at the end there. – Steve V. Jul 7 '15 at 2:13
• This is an excellent resource question for those of us who are non-pilots. It helps explain in one simple step what many of the acronyms are. If you'd spell out HSI and DME it would be ideal (for the ones you've included). – FreeMan Jul 7 '15 at 13:23
• @SteveV. I admit there are a few additional questions but they're all really clarifiers to the main question, specifying the type of information I'm after. – KeithS Jul 7 '15 at 14:44

"two-way radio communication and navigation equipment suitable for the route to be flown", that last requirement being just wonderfully ambiguous regarding what you as a pilot may want or need …

That statement isn't ambiguous at all. You can file and fly IFR with a transponder and a VHF comm radio if you can get controllers to issue you radar vectors for the route -- just be making a visual approach at your destination (or something like an ASR or PAR approach where you get your guidance from a controller).

Of course since your question was not "How do I piss off ATC?" that's neither here nor there - you're asking about typical instrument configurations in GA aircraft (for purposes of this answer I'm going to assume you mean the usual "Light GA / trainer" sort of aircraft - typical Pipers, Cessnas, Beechcraft, Mooneys, etc.)

It's hard to nail down "typical" because it's a rapidly moving target these days. When I started flight training two Nav/Com radios and an ADF was a pretty sweet IFR airplane, even if the ADF was a little sketchy. Now even a six-pack cockpit has a good chunk of glass in the panel in the form of a GPS (Garmin 430 or similar), and people are ripping out ADFs as the FAA decommissions them, but I poked around at some of the rental fleets near me and it seems to shake out like this as of right now:

• Most IFR aircraft have dual OBIs. If there's a GPS they tend to both have glideslope capability (either connected to two GPS Nav/Coms or one to the GPS and one to a traditional Nav/Com radio that has a glideslope receiver).

• Older planes with dual Nav/Comm radios often have glideslope capability on the #1 Nav/Com & not the #2 radio, which is why you often see one OBI with a glideslope needle and one without.
• Older IFR trainers tend to have ADFs still. Some are placarded as INOP and are basically heavy, drag-producing AM Radios (the Mets won).

• Aircraft with a GPS rarely have an ADF, because the GPS can usually display the ADF indications using a "normal" OBI. One less thing for the pilot to have to remember how to operate.
• I've never seen a light GA aircraft with an RMI.
They're probably out there but I doubt they're common. Slaving them to the magnetic compass requires equipment which is not cheap, and as ADFs are dying out there's not much benefit to installing one now.

• None have HSIs, but I have seen a few planes around with them.
My sense here is it tends to be an owner-installed upgrade because the pilot wants it and they have the money to spend on it. Like you noted they're still pretty expensive, mainly because they're rather complicated bits of equipment combining a vacuum-powered DG, a bunch of sensors, and a VOR/LOC/GS presentation.

• Every HSI installation I've seen takes the place of the Nav 1 OBI.
What gets put into the now-vacant hole varies from aircraft to aircraft. This Cessna has an ADF card there (thanks to CFC for the photo)
• Very few have DME.
If you're flying on Victor airways you're pretty much always flying toward some VOR (so you have one needle that won't move), and you can easily get a cross-bearing from a VOR that would be roughly perpendicular to your route to find your current position on the chart.
The DME gives you this in Distance/Direction form without needing to tune a second VOR or draw on your chart, and it can do nice things like give you an approximate groundspeed, but that doesn't make it indispensable.

• A reason many IFR trainers lack a DME seems to be because on the IFR checkride you can't be tested on equipment that's not installed: If you have no DME you can't be asked to fly a DME arc.
• DME is another technology that's being made redundant by GPS: The GPS can give you distance and bearing to any arbitrary point, and that point can easily be a VOR. It can also compute groundspeed.
• An ever-increasing number have large chunks of glass somewhere in the panel.
As GPS installations become more common it's increasingly rare to find a plane without a moving map display next to the six pack. The GPS provides the magic "Magenta Line" and also typically feeds an OBI (or HSI).
If the OBI or HSI fails the GPS can usually also display a digital OBI on its display.

• Excellent answer, very informative. Re: the ADF, I didn't realize it was a separate radio from the Comm/Nav unit. I thought you tuned NAV 1/2 to the ADF station and it pointed the way. As far as usefulness, if it'll pick up and direction-find commercial AM radio, then if you know where the transmitting tower is it could have some remaining NAV use once the NDBs are gone, albeit at shorter range. – KeithS Jul 7 '15 at 6:50
• @KeithS Yeah the ADF is a separate box in every installation I've ever seen - the underlying radio principles are fundamentally different from a VOR, as is the frequency range, so it doesn't make much sense to combine them. An ADF will find AM radio towers quite well, but unfortunately those tend to move around a little as station owners look for cheaper leases for their gear (though they seem to be less volatile than FM transmitter locations). – voretaq7 Jul 7 '15 at 15:58
• Many higher performance singles have HSIs and DME bc they are often used for long XC IFR. – rbp Jul 7 '15 at 22:01
• @rbp Yeah, if you step up to the Cessna 210 & such you've probably got a better change of finding a DME (unless it's been replaced by a GPS). I haven't seen many HSIs though (the only plane on the ramp by me that I know has one is a CAP 182) - we're talking mostly owner-flown older aircraft (1960s-1980s) though, HSIs may be more common in newer (1990s/2000s) stuff, especially if it was a factory option. – voretaq7 Jul 8 '15 at 4:21
• If you mark the ADF as INOP, the DPE can't make you do an NDB approach on a checkride; I suspect many of them actually work just fine. Actually taking it out would save some weight but costs real money. The few HSIs I've seen in rental trainers replaced the DG, and the second OBI (usually the one without GS) was also removed. – StephenS Jan 7 at 17:24

GPS equipped planes, especially with newer GPS/NAV/COM Flight Management Systems (FMS), may have an older NAV/COM radio with CDI for ILS/LOC/VOR approaches, and then a DG/HSI connected to the FMS. Avidyne IFD540 & Garmin G5 DG/HSI would be one combination, with autopilot as well.