Loran is OUT... GPS is IN.... well.....

[GAHorn]

Loran is OUT... GPS is IN.... well..... I'm working on it, anyway.

I've decided to remove my BE-LOVED Loran...my KLN-88....

...and replace it with a look-alike.... a KLN-90B GPS unit I've bought from a local avionics shop. This will likely take a week or two to complete as I've got other honey-do's scheduled in-parallel, ....and it's not as simple a swap as might appear at first-glance.... but thought I might as well bring it up in discussion.

Firstly, things the two units actually have in common besides mfr'r is their size, appearance, and many features of operation. They also have a mutal requirement for cooling of their CRT displays, and I will re-use the existing set-up of the KLN-88 installation, a small air-scoop on the left side, just aft of the static-port, similar to later C-172's of the mid '70's vintage.

The installation manual of both units state that ram-air cooling does not meet the requirements of warranty, that their KA-33 cooling fan or similar must be used. But I did/do not wish to make unnecessarily-complicated additions to this class of airplane and the cooling scoop is so nicely in the prop-blast, and with no plans to operate the unit more than a few minutes without the engine running, I had no concerns regarding adequacy of cooling air. (4 cu.ft/min are recommended, and the 10 years of operation of the KLN-88 has never given any indication of lack of cooling adequacy.)

I installed a small piece of screen material between the scoop and the scat hose to prevent the entry of insects, and I routed the scat forward/upward from the scoop to prevent water ingress, even in rain. There is no evidence of water ever entering the system despite numerous washings and countless flights thru moderate and heavier rainshowers.

The things that will require changes are:

1. The rack. They do not actually utilize the same rack despite their other similarities. The 88 used a 37-pin connector and the 90B uses a 50 pin connector, so their racks have different sized cut-outs for their connectors.

2, The antenna. This is an obvious matter, especially since the 88's antenna was mounted on the belly in order to avoid making holes in the cabin roof of my airplane, as during it's previous restoration that cabin-roof skin was completely replaced to get rid of numerous patches from previous years of avioncs/antenna changeouts. I also am in favor of as few holes as necessary in any roof, but directly overhead appears to be the best location for a GPS antenna.

I'd like to place the antenna along the centerline, but I am thinking about the possibility of the occasional avgas that might exit the gooseneck and flow back over the antenna, and also the fact that my comm antenna is already on the centerline and above the baggage compartment. The GPS antenna should ideally not be closer than 3 feet from any comm antenna and the centerline over the cabin will violate that by about a foot or so. (The KLN 90B does make claims to be more tolerant of comm-interferance than most other units, however I'd hate to cut a hole and then find out I've got a problem. This antenna-placement consideration is my most prevalent concern at the moment. Any/all input from others is solicited on this matter.)

3. Altitude input data. The KLN 90B requires altitude data, usually from an encoder, in order to provide accuracy for approaches. Although I do not plan to IFR-certify this installation, I would like to preserve the unit's capability in that regard. No other peripheral equipment such as OBS/CDI indicator, annunciators, or external switching is planned. This will be a stand-alone unit, but it will require patching the 90B harness into the existing harness between my Narco AT-150A and my Narco AR-850 encoder. The KLN-88 had no such input requirements.

I will begin removal of the KLN-88 this afternoon and will post updates as this changeout advances. Please post any comments/advice/laughter as you may feel inclined.

PS: I have three KLN-88 units available for purchase by any interested parties. This includes the one pictured, as well as the two "spares" I'd purchased in anticipation of the future high-desireability of these model units. This includes the one John Hess unloade... errr... sold me at the Tehachapi Convention-auction,... doubtless contributing significantly to the assets of the Assoc'n. Anyone wishing to make a similar donation to our wonderful Assoc'n may purchase any/all of these units at a remarkable discount from their original $5,000.00 sales-prices. All proceeds will go to TIC170A.

PSS: The CRT in the 88's are very expensive should it have failed, I figured I might as well have cheap spares, another reason I wanted the extra units. This CRT should also be a swap with the same CRT in any 89/90A/90B series GPS, so I'll keep my 4th KLN-88 as a spare.) (I wonder if perhaps I may have ever over anticipated the scarcity of KLN-88's.... ???)

[blueldr]

George,
Concerning your worry about av gas exiting the gooseneck and flowing back over the new antenna, now is the perfect time for you to switch over to mogas.
I have it on reliable authority that mogas absolutely will not hurt an antenna.

[GAHorn]

George,
Concerning your worry about av gas exiting the gooseneck and flowing back over the new antenna, now is the perfect time for you to switch over to mogas.
I have it on reliable authority that mogas absolutely will not hurt an antenna.

Yes, but it's hell on the fuel system (and fuselage when it gets old.)

I am more concerned that the antenna gasket might someday dry out and leak from the exposure, although I have no evidence that my airplane spits fuel from the gooseneck, I've seen others that did.

[Bruce Fenstermacher]

George before I cut any antenna holes I'd hook everything else up and temporarily place the antenna and see if you have any com interference. When I installed my Garmin 155 GPS antenna I was oblivious of the interference issue and therefor bravely used an existing hole for the GPS antenna. It was about 2ft from my comm antenna.

After testing the GPS showed interference on one obscure com frequency. Keying the mic on that frequency knocked the satellite reception to nil in a second or so. Releasing it they popped right back. A knock filter improved that but didn't eliminate it as I recall. I decided to restrict GPS use while transmitting on that frequency by noting it in the FM supplement required as my installation was IFR certified.

I was lucky as I'd have had to move the antenna and redo a really nice Proseal job I had done around the antenna to stop leaks though my Rat Plane would have had another really nice patch to add to it's ratty patina.

[170C]

George, on my plane I have two com antenna's like the one you have. They are on the left & right sides of my cabin roof so they are pretty close to one another. They are back of the door posts. My old (1994 vintage) Magellan panel mounted GPS (5000) antenna is in the middle of the cabin roof but back pretty close to the hat shelf and I have never had any problem with it (GPS) working like gang busters. Only problem with it is its data base is no longer updatable. So, I don't think you will have a problem mounting your GPS antenna on the cabin roof. However, I agree with Bruce that you should hook it up temporarily to see if there are any apparent interferances. While the antenna gaskets are supposed to prevent water, air, etc. from entering under the antenna, I have added some non-corrosive silicon sealer to my radio antenna's as a precaution in case the gaskets should leak. As a side note I removed one of my radio antenna's a year or so ago to see if it was making contact with the cabin roof and discovered that whomever installed that antenna apparently didn't have the proper gasket and made one from a plastic oil container However, the antenna had worked OK all the years since I bought the plane and did not appear to have leaked water. You never know

[HA]

I wouldn't worry about gas on your antenna, but I would put the antenna off to one side and toward the front anyway. and don't use the gasket for an antenna, you want the best, cleanest grounding surface between the skin and antenna that you can get. proseal around the base after installation.

except for the IFR certification, antenna placement isn't really that important anyway (and for FR cert, there is likely some guidance in the installation manual that you have to go by to get the 337 approved). for instance, we use cheap gps units in our dataloggers in our work airplanes and we generally just toss the antenna up on the glareshield. or look at all the handhelds with antennas inside the airplane, like my fancy Garmin 295 on the yoke that works just fine. or for that matter, my kid's gps running watch that he uses when he's running. I told him he should tape it to the top of his head but he won't listen

[Bruce Fenstermacher]

HA there isn't any hard rule but guidance for the antenna installation. IFR certification requires actual testing of the system in flight regardless were the antenna ends up. I flew the certification test required and certified the GPS system worked as advertised. The safety inspector never questioned or in fact even looked at the installation.

[blueldr]

My experience with the gooseneck spewing fuel isthat it will occur with real full tanks and a steep descent when the inter tank cross cabin vent line floods
with fuel.

[GAHorn]

... don't use the gasket for an antenna, you want the best, cleanest grounding surface between the skin and antenna that you can get. proseal around the base after installation....

According to the Honeywell Installation Manual, the antenna gasket is a special, conductive material that should be used to guarantee a proper bond between the antenna and aircraft skin. (Although the genuine article may look like rubber inner-tube material...it is not. Do not use other material than the proper part-number antenna gasket.)

[Bruce Fenstermacher]

The obvious solution to your problem hit me early this AM as I finished my first coffee. Mount the antenna on the belly in the loaran antenna location then roll you airplane upside down when you want to receive GPS signals.

Alternately you could mount a mirror under the antenna angled to give it a view of the sky. This alternate methon could only be considered a minor alteration and would blend in nicely with all those other "almost original" modifications you seem fond of doing lately.

BTW I just have to ask. Now that you'll be using "modern" GPS will you be arriving at fly-in events in the day light rendering your mostly original dual light mod less useful?

[GAHorn]

I realize this may diminish the opportunity for a good "ribbing" and perhaps disappoint those rat-plane owners disdainful of lesser airplanes.... ....but I'd point out my motto has never been "most original" airplane.
It has also never been "mostly original" airplane.... ....not even "original (mostly) airplane".

One may re-peruse my motto at leisure and ponder it's truths.

[hilltop170]

....but I'd point out my motto has never been "most original" airplane.
It has also never been "mostly original" airplane.... ....not even "original (mostly) airplane".

One may re-peruse my motto at leisure and ponder it's truths.

An originality nut (mostly) for the right reasons.

Yep, George never said it applies to airplanes!

[Green Bean]

George - Is your instrument panel color a shade of pink??

[GAHorn]

George - Is your instrument panel color a shade of pink??

That's an oddity of the camera-flash I was using back when I took that pic. It does look sorta "original", heh?

(It appears as a medium-brown in natural daylight...and is actually a close-version of the original "Trooper Tan" Cessna used.)


UPDATE: I've got the KLN-88 and rack removed and have been matching up the harness to the KLN 90B. For those of you who get into re-wiring avionics, ... you will find a neat little tool that will save a lot of time by allowing you to remove wired-pins from one connector and insert them into another. In my case, I've removed the PWR-IN, GROUND, and LIGHTING circuits from the old 37-pin connector the 88 used...and simply inserted them into the appropriate sockets of the 50-pin connector of the 90B. This saves a lot of time and makes for a more pleasureable project.

The tool for the task is known as a "D-connector insertion/extraction" tool and you can pay $40 for it from avionics tool supply shops...or $3.99 from Radio Shack.



http://www.radioshack.com/product/index ... Id=2102600

These tools are sort of like large hypodermic needles with one-fourth of their circumference removed. (Viewed from their end they are shaped like a "C".) By sliding the extractor down/alongside the wire and into the connector-socket, the teeth of the pin are captured, allowing you to pull the pin and wire completely out of the connector. Now the wire and it's pin may be inserted into the new avionics connector and ....VOILA No soldering/de-soldering/crimping/etc..

[170C]

My avionics guy made an extractor tool out of a hacksaw blade. The advantage of it is you can remove a pin with the avionics chassie still installed in the rack w/o having to either remove it or removing the connector block from the chassie. He came by my hangar yesterday and used his device to remove the inter-lock pin from my ICOM AC-210.