Menu Sign In Contact FAQ
Banner
Welcome to our forums

Upgrading the GNS530 – 530W or GTN750?

Apologies if this choice has already been exhaustively worked through on the forum – if it has my search didn’t find it! The main reason for wanting to upgrade is to get the full procedural path of typical ‘teardrop’ ILS and LOC/DME approaches depicted on the screen rather than just the final approach course. IIUC both will do this, so choosing which to do depends on other factors. The aircraft is an N reg TB20.

I’ve got two queries. First, for the regulation specialists – I have an AFMS approving my GNS530 for instrument approaches. Would an upgrade to the 530W need a new AFMS or would the existing one for the 530 remain valid?

Second, for the avionics specialists – I’d like to know what I’m taking on in rewiring to the various boxes currently linked to the 530. At the moment the main links are these: GPS NAV and LOC/ILS display on a Sandel 3308; in NAV mode the 530 drives the KFC150 autopilot, through the Sandel; the Stormscope (Wx500) and the Traffic Advisory System (Bendix/King 850) both display on the 530 screen. Would upgrading to the 530W need any rewiring for this set up or is it more ‘plug and play’? And if I went down the route of upgrading to the GTN 750 – is this set of ancilliary boxes compatible with the 750 at all? And if so, would it need an extensive rewiring?

Any enlightenment gratefully received!

TJ
Cambridge EGSC

A GN530W is more or less plug and play to replace the GNS530. However you will need to replace the current GPS antenne (likely the GA-56) to be replaced by a new GPS WAAS antenna (likely to be GA-35). This would also require to upgrade the coax cable to a high quality cable, if not currently installed (RG-400 or RG-142). If your GPS antenna currently uses black coax cable it is likely to be lower quality RG-58, which is quite common. If you have to renew the coax cable I would recommend using RG-400 as it has a stranded core, while the RG-142 has a solid core. The solid core is in my opinion, altough allowed, not suiteable for use on aircraft.

An upgrade to GTN-750 would a new rack, an instrument panel and rack mount with sufficient thickness and require extensive rewiring. It also doesn’t take a parallel altitude source, so you need to receive in a serial format, this might be possible to receive from other equipment, suchs as your transponder. If your transponder is not capable, you might need another altitude source. You also would need to upgrade to a WAAS/GPS antenna, as with the GNS-530W.

Last Edited by Jesse at 15 Dec 08:44
JP-Avionics
EHMZ

The existing AFMS will not apply to the updated GNS530W. If you install the upgrade using the Garmin STC for the GNS530 to GNS530W upgrade, you will get a new standardized AFMS that is printed out, is already FAA approved, and only a few blank fields need to be filled in or boxes checked. I don’t think the Sandel 3308 supports GPSS internally, so if you don’t have a GPSS, you will need to add one to support having the KFC150 follow the curved paths. The DAC or Icarus SAM GPSS units will work with your equipment.

At software level 2.3 of the Sandel 3308, there is support for the required annunciators and vertical guidance on RNAV GPS approaches. Depending on how you interface to the GNS530, you may end up rewiring the Sandel 3308 to GNS530W interface.

Last Edited by NCYankee at 15 Dec 14:16
KUZA, United States

The Sandel GPSS (a.k.a. roll steering) feature is AFAIK available only on the 3500, same out a year or two ago, and needs the GPS-EHSI connection to be ARINC429 (RS232 won’t do). It is a $1000 option (this just buys you a software unlock code). Also this feature requires a specific autopilot wiring: the AP needs to be analog-deviation (not ARINC429) controlled, from the EHSI’s “FCS” outputs, not from say the GPS directly.

It is thus a super feature for non-ARINC429 autopilots (i.e. all the pre-KFC225 ones) where somebody has an ARINC429-outputting GPS (GNS or GTN box) because the autopilot is guaranteed to be following the same deviations which the EHSI is showing. The traditional alternative is a roll steering converter like the DAC one, which is a bit crude because it bypasses the HSI/EHSI, converts ARINC429 into a faked heading bug, and makes the autopilot do “stuff” which is not indicated anywhere else in the aircraft and you don’t know what it’s going to do until the aircraft starts to turn one way or the other – or not (IMHO, having flown in a few of them, that is really not good).

I have flown in TJ’s TB20 (a very nice one, same model/era as mine but with better paint) and his 3308 install appears to have been done relatively competently but I would go to a shop familiar with Sandel products… there is only about 1 in the UK that comes to mind. I don’t know which way his autopilot and GPS have been wired up. His 3308 is wired in the proper “Master” mode i.e. his NAV/GPS switch is gone and the selection is done on the EHSI. I didn’t do that on mine because it is a lot more rewiring, most of which requires the centre stack out and thus requires a heated hangar with a nice bench to work on.

Last Edited by Peter at 15 Dec 15:44
Administrator
Shoreham EGKA, United Kingdom

It is thus a super feature for non-ARINC429 autopilots (i.e. all the pre-KFC225 ones) where somebody has an ARINC429-outputting GPS (GNS or GTN box) because the autopilot is guaranteed to be following the same deviations which the EHSI is showing. The traditional alternative is a roll steering converter like the DAC one, which is a bit crude because it bypasses the HSI/EHSI, converts ARINC429 into a faked heading bug, and makes the autopilot do “stuff” which is not indicated anywhere else in the aircraft and you don’t know what it’s going to do until the aircraft starts to turn one way or the other – or not (IMHO, having flown in a few of them, that is really not good).

The only time an autopilot flies the CDI is on a straight line course. All the curved courses are not related to the CDI deviation. Even the Sandel 3500 with the GPSS feature converts the GPS ARINC 429 Label 121 (Horizontal command to autopilot) which consists of a precise bank angle to fly to a heading error signal for the autopilot and I seriously doubt there is any direct connection to the EHSI depiction of the curved course, although the GPS provides the graphic information to be displayed as well as the autopilot label via the ARINC 429.

BTW, the map page on the GNS/GTN series displays what the autopilot is going to do, quite clearly in my opinion, including the projected turn anticipation. My opinion also differs with your stated one regarding the lateral guidance coming from the GPS/FMS regardless of what the HSI is displaying aids in the autopilot following the selected flightplan. In particular, the GNS430/530/480 continue to output the roll steering label when an ILS is selected and inside the FAF. The GNS430W/530W/GTN do not. All these navigators continue to provide the roll steering label when VLOC is selected and either outside the FAF or anytime Back CRS or VOR are selected. Not all autopilots have a NAV course intercept capability nor do they all provide vertical guidance for a GS. The roll steering job does a much better job of flying the ILS path than a CDI tracker. Garmin has taken your position and wants the GPSS output that they continue to provide not to be used except when the NAV source is GPS (G500 example). But other vendors have taken the other view, Aspen leaves it up to the pilot to choose. In the fully integrated G1000/GFC700 system, Nav tracking is by roll steering except for on an ILS. Flying a back to back ILS and LPV on a G1000 in strong crosswinds can be very instructive in how much better the roll steering method is. On the latter, wind correction is near instant and continuously adjusts as needed all the way to the pavement.

If I were designing an autopilot ILS tracker today, at a minimum, I would take advantage of the track information when it is available to adjust the WCA and only default back to the current method of CDI tracking when the GPS data was unavailable.

KUZA, United States

I don’t disagree wth you, NCYankee. Roll steering will always be better than driving the autopilot from the HSI deviation bar deflection. What I was getting at is that using the SN3500’s roll steering output is an inherently “safer” way to do it than using a roll steering converter, whose failure will not be apparent until too late. But I would not suggest anybody buying a 3500 – $5k-9k depending on whether used or new.

Perhaps a small distinction…

In reality, I find that driving the autopilot from the EHSI deviation bar is good enough for every practical purpose in European IFR

Currently, due to near-impossible access to parts of the wiring, I have my stuff wired so that the KFC225 is driven from the SN3500’s FCS output during “NAV” (VOR/LOC) but is driven from the KLN94 GPS’s deviation output during “GPS” (GPS). LOC intercepts are great (the 3500 decodes the NAV1 composite signal in software – the KN72 is gone) and GPS/RNAV approach intercepts are plenty good enough and well inside RNP1.0 requirements. Consequently I would not pay for roll steering unless it came as a package with some really necessary stuff.

In Europe, it’s hard to know what will be the main “drivers”. In the USA it’s clearly LPV, and I guess ADS-B out. Here it is 8.33, the completely pointless PRNAV may be next (but nobody knows), LPV is operationally irrelevant, and ADS-B is off the horizon.

What is IMHO definitely relevant is LNAV+V (the fake glideslope coded by Jepp for many GPS/RNAV approaches, which can be flown with the autopilot) and for that you need to do everything that you need to do for LPV.

Last Edited by Peter at 15 Dec 19:56
Administrator
Shoreham EGKA, United Kingdom

Quote What I was getting at is that using the SN3500’s roll steering output is an inherently “safer” way to do it than using a roll steering converter, whose failure will not be apparent until too late.

I guess I don’t understand your point as the SN3500 has a built in roll steering converter rather than an external one. When an Aspen or G500 is installed, they also have a built in roll steering converter. If an individual already had an external roll steering converter, I recommend they keep it. That way, if the glass goes dark, they can still control the autopilot laterally with the external converter. They are just wired in series with the glass passing the heading error signal thru the external converter and on to the autopilot.

KUZA, United States

OK – I see your reasoning. The EHSI is not likely to be displaying anything different to what it would be displaying if one had a direct GPS-autopilot ARINC429 connection or via an external roll steering converter.

Administrator
Shoreham EGKA, United Kingdom

I know I am drifting in a different direction here, but why would anyone buy new Sandel kit?

The HSI is about $13,000 and the AI about $20,000, so you could put Aspen or Garmin in for half the cost, both much more capable, and with reversionary capability giving redundancy.

Darley Moor, Gamston (UK)

why would anyone buy new Sandel kit?

It’s a question many ask

  • The Sandel AI is not applicable to most light GA due to certification issues, so one would not be replacing that, unless the existing AI is not driving an autopilot in which case you can stick almost any old AI in there. I don’t think Sandel have an AI which is STCd to directly replace a KI256. The SA4550 might be but that’s a King Air / TBM size instrument.
  • The build quality of Sandel stuff is way above anything else in GA
  • The sunlight readability is way above all other cockpit instruments currently available

The Aspen is good value for the features but it has suffered a very high failure rate – probably of the order of tens of percent of installations and people I know personally who have it certainly support that %. Some of the stuff that went on was totally comical like fixing a misreading OAT probe with a firmware update. It appears to be much improved now, however. So I’d put it in the “Thielert” bracket, but opinions will differ, as always…

Which Garmin product is “half the cost”? The UK install cost of a G500 is something in the region of £30-40k depending on which GPS you put in to drive it and what else is involved. I have one quote of €48k plus VAT for a G500+GNS430W. But that’s a different thing – the G500 is a PFD. The Sandel EHSI is just an EHSI. A GNS530W is just a GPS.

I installed the original SN3500 for reasons given here. Then 2 years later I got a chance to buy a “used but never installed” another SN3500 plus the SG102 AHRS gyro at a very good price so put those in. I don’t think I would have paid the full price for these had that been the only option, because there is nothing really wrong with the KG102A+KMT112 and they are cheap to overhaul.

You can get a used but freshly Sandel-rebuilt SN3500 on US Ebay for about $5k and it’s great value for that, for replacing a KI525 which is on its way out, and you get the duplicate display of stuff like NAV, DME, stormscope, TCAS, etc.

Administrator
Shoreham EGKA, United Kingdom
26 Posts
Sign in to add your message

Back to Top