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GTN650 installation considerations

I’m looking into replacing a KLN94 GPS, a KX155A NAV/COM and a KT76C mode C transponder with a Garmin GTN650 and a Trig TT31 transponder.
Are there any particular things that I should to point out to the avionics shop?

The aircraft has the following avionics:
GPS KLN94
GPS Antenn KA92
GPS/AP switch assy MD41-528
2 x KX-155A NAV/COM
HSI KI 525A
KT76C mode C transponder
DME KN62A
GPS Annuncation Control Unit MD42 Mid-Continent
Autopilot KAP140
Audiopanel PM 1000 II PS Engineering
Marker KMA28
Engine Monitor VM1000 Vision Microsystems

ESTL

This looks a fairly standard job.

They will need to change the antenna, so make sure they do it neatly and change the entire length of the antenna cable, preferably for RG400.

Same with the transponder, I guess.

Make sure they connect the HSI course pointer back to the GPS, so that when the GPS is in the OBS mode, you can use the HSI course pointer to set the OBS in the GPS. A lot of installers don’t make this connection, even though it is really basic stuff.

How will the autopilot be driven? The KAP140 can’t take ARINC429 digital commands from the GPS, so you have a choice of either fully analog connections, or install a roll steering converter which takes the ARINC429 and fakes a heading bug. Whether this is of value to you depends on what sort of flying you do. If you fly a lot of holds then roll steering is worth having. Otherwise, analog control will work fine. My KLN94 drives the KFC225 perfectly well enough.

Do also verify how exactly LPV is going to work – this involves the GPS generating a fake ILS glideslope, and will probably involve some curious button sequence which you have to press just before GS intercept. I would 100% definitely make sure you get fully working LPV because that will become operationally relevant in years to come. I was at Bergerac this week and the controller told me they will be removing the ILS in 2008 and will be going to LPV.

Administrator
Shoreham EGKA, United Kingdom

Same with the transponder, I guess.

Why should the transponder coax cable and antenna be renewed? If they are serviceable, this would just add up quite some labour. No need to do that if the coax cable is fine.

Make sure they connect the HSI course pointer back to the GPS, so that when the GPS is in the OBS mode, you can use the HSI course pointer to set the OBS in the GPS. A lot of installers don’t make this connection, even though it is really basic stuff.

I can not imagine that they wouldn’t. It is part of the installation and is needed for proper VOR mode indication. Without it only LOC mode would work correctly. It would fail the post installation check. Part of the checkout is the calibration of this signal.

JP-Avionics
EHMZ

Are there any particular things that I should to point out to the avionics shop?

You MD41-528 switch annunciator box is not approved for the GTN and should be removed. The TSO approval is also void if it is used with any other GPS then a KLN89B or KLN94. This switching will be done on the GTN so will the annunciations.

As Peter indicated the GPS antenna will need replacement, and likely the coaxcable to be upgraded to better quality.

The GTN is not approved for all audiopanels. It is approved to use the KMA-28 so this is not an issue for you.

Depending on the approval it could be possible that the GTN outputs position to the Trig TT-31 for ADS-B OUT. Some approvals include this options, most later ones don’t.

JP-Avionics
EHMZ

Why should the transponder coax cable and antenna be renewed? If they are serviceable, this would just add up quite some labour. No need to do that if the coax cable is fine.

If the antenna is the right type then I would leave it. Regards the cable, IME most installers (here in the UK) use cheap and dirty cable and any opportunity to replace it is worth taking

I see loads of GA planes show up on my TCAS only at a very short range e.g. 1-2 miles, or sometimes much less, and they are at a similar level so this is not airframe masking. Their transponder installation is clearly shagged. I see some GA planes, and airliners, show up at up to 15nm which is as claimed in the TAS605 spec.

It is part of the installation and is needed for proper VOR mode indication. Without it only LOC mode would work correctly. It would fail the post installation check. Part of the checkout is the calibration of this signal.

Indeed, but then you work for a shop which does it right I know one installer who has done maybe 100 GNS430 boxes and who never did that part. When I showed it to him he said “wow I never knew anything from the HSI went back to the GPS!”. Loads of people get avionics work done and find bits are not working. And to be honest lots of people would never notice if this bit was missing because

  • they never use the OBS mode, and most IR holding pilots don’t even know what it does
  • they never track VORs
  • the OBS bearing can be set on the GPS itself

In fact it can be quite subtle because e.g. in my installation the HSI is coupled to the GPS only when

  • the GPS is in the OBS mode, and
  • the GPS/NAV switch is in the GPS mode

Once the GPS/NAV switch is in the NAV mode (which it normally would be for VOR/LOC tracking) the HSI CP is no longer coupled to the GPS. So I have a checklist for this setup sequence.

Administrator
Shoreham EGKA, United Kingdom

If the antenna is the right type then I would leave it. Regards the cable, IME most installers (here in the UK) use cheap and dirty cable and any opportunity to replace it is worth taking

Most older installations use RG-58 coax cable. It is true that this cable is considerd unsuitable for use above 30 MHz. Aircraft manufacturers used it widely over 1 GHz. A 3dB loss is allowed on the cable. This means is allowed to loose half the transmitter power on the coaxcable. A 250 Watt transponder should therefor radiate at least 125 Watt antenna.

Under the UK CAP 766 the annual transponder test doesn’t seem to require power output or sensitivity testing. In other countries such as Netherlands and Germany this power output testing is part of the test (I do them on UK aircraft as well, it is just part of the automated test sequince). An indication with low receiver sensitivity AND low power output is caused 99,9% by a poor coax cable.

In Germany and the Netherlands this would be very easily detected during the test, so shouldn’t be a problem.

Indeed, but then you work for a shop which does it right I know one installer who has done maybe 100 GNS430 boxes and who never did that part.

That is really poor quality! Clearly they don’t have knowledge, and DON’T perform testing if that are the results. Testing is important to find out correct operation, including some items you can’t normally test during flight such as flag circuits, but also receiver sensitivity and power output.

JP-Avionics
EHMZ

RG400 (spec) is about 0.15db/m attenuation at 1GHz which is going to need 20m to reach 3db.

Last time I paid about 7 quid per metre for this stuff so it’s clearly worth it. In fact I have about 30m left over from the TCAS job – the installer (one of UK’s biggest 145 firms) was going to use some cheap cable so I bought 50m and free issued it to him.

I can’t see the point in having a transponder if people (especially those who have invested money in traffic detection) can’t see you. And since visual lookout works poorly, being visible and enabling others to “see” you will help you also even if you can’t see them.

Actually I wonder how many “altitude unknown” targets reported by radar controllers are actually Mode C but are bodged installations.

Last Edited by Peter at 28 Jun 09:26
Administrator
Shoreham EGKA, United Kingdom

RG-400 is in common use now on new GA aircraft and with modification. It also has better screening specifications as it has dual screens. It is less likely to introduce crosstalking.

The encoder problems is also mandatory display of the flight level in A-C-S mode on mode S transponders. This brings up another point, the GTN requires a 25 ft incrediment serial altitude encoder for maximum performance. This is something Anders might want to have a look at, as his KT-76C transponder will use a Gillham (parallel) encoder.
Parallel to serial is possible, but gives a lower resolution, which is not acceptable if you want highest performance of the GTN.

Most altitude encoders use a heater element to make them temperature independ. First it will take longer to get to the correct temperature, (and it takes longer before it displays altitude) when it fails, the encoder won’t come online anymore, as the required temperature isn’t reached.

JP-Avionics
EHMZ

What does the GTN use the altitude for – apart from the old part of the RAIM check (which may not be used in “W” boxes?)?

I know the KLN94 uses pressure altitude just for the RAIM check, to substitute for 1 satellite when only 3 (?) are being received.

Administrator
Shoreham EGKA, United Kingdom
What does the GTN use the altitude for – apart from the old part of the RAIM check (which may not be used in “W” boxes?)?

I know the KLN94 uses pressure altitude just for the RAIM check, to substitute for 1 satellite when only 3 (?) are being received.

It uses it for the same purpose. Although RAIM is not required when in an SBAS service volume, it is used anytime SBAS is down or outside of the SV.

KUZA
57 Posts
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