I have flown a GPS which drove a mechanical HSI and required a lighted switch to ensure you knew whether the CDI was navigating either with GPS or VOR/LOC. This was before RNAV GNSS approaches, and this GPS did not have the capability for an approach.
I have since flown RNAV GNSS approaches but with glass (G1000 or Honeywell). Here you have auto slew, approach sequencing and visual confirmation of terminal and approach sensitivity.
How does an IFR GPS linked to say a plain VOR head type CDI, non HSI or glass, work an approach? I missed this part of the tech evolution. Do you manually adjust the track on the CDI as you fly by the waypoints? Or do you use a CDI type page on the GPS? Would there be a VNAV capability?
There may be a forumite with this set up, so details or photos would be appreciated.
RobertL18C wrote:
How does an IFR GPS linked to say a plain VOR head type CDI, non HSI or glass, work an approach?
Exactly as ever before with mechanical instruments.
RobertL18C wrote:
Do you manually adjust the track on the CDI as you fly by the waypoints?
Yes. The Garmin units 430 and 530 for example will briefly show you what CDI course will be required for the next leg in the lower left right corner of the screen. If you miss that, just make sure that one of the data fields is set to “DTK” (desired track) and keep your course selector in sync with that.
RobertL18C wrote:
Or do you use a CDI type page on the GPS?
No. (There may be a technique by which you use that, but I know nothing about it).
RobertL18C wrote:
Would there be a VNAV capability?
Yes, if GPS unit and (electromechanic) HSI are properly wired together, the GS needle will work with GPS derived VNAV just like it would with normal glideslope signals.
Just make sure that the lower left button of the Garmin 430/530 is switched to “GPS” (will light up in green on the screen above) and not in “VLOC” (displayed in white for easy distinguishing).
w_n many thanks.
How marketable is a 530W, GNC 255A, DME and Mode S – no glass, no HSI, simple two axis AP?
Potential estate sale SEP retractable aircraft which would need a panel overhaul, currently with old 80’s gear.
RobertL18C wrote:
How marketable is a 530W, GNC 255A, DME and Mode S – no glass, no HSI, simple two axis AP?
I would say that this is a lot better than what you will find in most flying school IFR trainer aircraft. Around here one would still need an ADF though to make it useful as a trainer.
Do you manually adjust the track on the CDI as you fly by the waypoints?
The one with which I am familiar functions just like a LOC receiver; you can set the OBS (you meant OBS, didn’t you?) as you wish or not, but it’s not connected to the GPS receiver which simply drives the CDI to indicate cross track distance according to the scaling operative at the time: FSD of 5nm, 1nm and 0.3nm or whatever it is.
In the same vein it’s up to you to twist the OBS On the HSI as you pass each waypoint. But if you don’t, the visual presentation is really confusing and unhelpful.
We need input here from some of the avionics installers who post here…
However a GPS drives a CDI just like a VOR receiver drives a CDI, with a difference that while a VOR receiver uses the rotating scale as the VOR radial, a GPS ignores it (except in the OBS mode) and just drives the deviation bar.
If you are driving an autopilot, in NAV mode, the rotating scale of the CDI (or the course pointer of an HSI) goes to the AP and gives it a rough heading to fly, while the deviation bar gives it a fine adjustment.
Ultimately what happens will depend on what the GPS is outputting, which depends on what it has in the database. If you are flying a GPS approach, the rotating CDI scale is ignored (just like it is ignored when flying an ILS – this can be confusing to the pilot). All that works is the CDI deviation bar.
Terminology: the rotating scale bit of a VOR indicator is called the Omni-Bearing Selector or OBS. The CDI is the Course Deviation Indicator, the wiggly needle in the middle.
A typical Garmin installation will nag you if the OBS is out by more than a few degrees when following a track in GPS mode
To perform an RNAV approach, the approach is required to be loaded as a part of the flight plan in the unit (e.g. GNS530). Also the flight plan has to be active and in LEG mode (as opposed to the OBS mode). The LEG mode implies that the navigation command is based on deviations from the straight lines going from WPT to WPT in the approach. The OBS on the (mechanical) CDI has no influence on the indication, just like when flying an ILS. However, just as when flying the ILS, it is advised to set the OBS to the actual track course to avoid confusion. As Cobalt writes, the unit will detect if that is not the case and tell you: “Set OBS to nnn”.
To sum up, you do not HAVE TO manually adjust the OBS to fly the approach, but you are adviced to do so, by the unit and probably also by your instructor, to avoid confusing yourself.
