OBI, RMI, HSI... What combination do GA aircraft w/o glass typically have?
"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 &amp; 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.