Encoder Issues?

[JGoodish]

I recently had the biennial static/altimeter/transponder inspections completed on my Cherokee, with no problems noted.

Subsequently, I was inside Class B airspace when I discovered that my Mode C was reporting an altitude 300 feet higher than what was actually being flown. I had the encoder re-calibrated in the aircraft, and flight testing afterward resulted in a dead-on Mode C altitude within 15 minutes, but off by 300 feet after 1 hour (I might expect this to be reversed). Encoder was pulled from aircraft, calibrated on the bench, and permitted to run on the bench for a few hours with no drift noted. Reinstalled in the aircraft, encoder now reports anywhere from 150 to 300 feet high when compared with aircraft altimeter set to verified altimeter setting.

The encoder is a Narco AR850, mated to a King KT78 transponder. I had the Narco encoder installed a few years ago to replace an aging and inaccurate unit from another manufacturer.

Does anyone have thoughts on what may be the issue, or is it likely a faulty encoder? If the latter, why would it run fine on the bench but be off when in the aircraft?


Thanks,
JKG

 

[PilotAlan]

Remember that the encoder reports altitude based on standard pressure (29.92). The encoder has no adjustment for local altimeter setting (it's a safeguard against someone having the wrong altimeter setting dialed in, al encoders always are reporting the error from actual altitude).

The altimeter reading with local altimeter setting is almost always off from encoder altitude, unless the local altimeter setting is 29.92.
I would run the test again against standard pressure dialed into the altimeter.

 

[BillTIZ]

Remember that the encoder reports altitude based on standard pressure (29.92). The encoder has no adjustment for local altimeter setting (it's a safeguard against someone having the wrong altimeter setting dialed in, al encoders always are reporting the error from actual altitude).

The altimeter reading with local altimeter setting is almost always off from encoder altitude, unless the local altimeter setting is 29.92.
I would run the test again against standard pressure dialed into the altimeter.

If the error is reported inflight by ATC, they have already corrected the mode-c reporting from 29.92 to the corrected altimeter setting.

 

[kontiki]

If Gillham Code is involved (10 wire parrallel bus), maybe a wire could have a loose connection. That might give you a intermittant error of some significant altitude. I don't know if this is the best reference, it was a the subject of an AD in the late 90s.

http://en.wikipedia.org/wiki/Gillham_code

I believe Gilham is still used in GA for Mode C.

 

[Skylane81E]

Another option is how is the encoder plumbed into the static system?

 

[JGoodish]

Another option is how is the encoder plumbed into the static system?

That is a good question. I need to check the static plumbing and the wiring, which I haven't had an opportunity to do yet. If the avionics shop aligned the encoder on the bench and tested it from cold to hot repeatedly with good results as they have claimed, I'm not sure why it would suddenly act up when reinstalled in the aircraft. I will have to do some verification work and see what I can find.


Thanks,
JKG

 

[kgruber]

Spending money on an ancient Narco AR850 is just pouring money down the drain. They were junk when brand new.

Ditto for a KT78, except they usually worked for a decade.

 

[JGoodish]

Spending money on an ancient Narco AR850 is just pouring money down the drain. They were junk when brand new.

Ditto for a KT78, except they usually worked for a decade.

Well, my KT78 has worked just fine for 30+ years.

My local avionics shop has had good luck with the AR850, which is why I elected to have it installed a few years ago to replace my second or third ACK in as many years. However, the AR850 is obviously a throwaway piece of equipment at this point, as Narco no longer exists. It doesn't appear that anyone makes a decent blind encoder that will last.


JKG

 

[Capt. Geoffrey Thorpe]

Is it hot where the encoder is installed? They are temperature sensitive and have a heater installed, but if it is getting overheated... That would be consistent with drifting over time in the aircraft but not on the bench.

The static system would have to be totally hosed to be off 300 feet compared to the altimeter that is connected to the same source - and it wouldn't explain why it changes over time.

Or, your altimeter could be frelled. Did you try giving it a couple sharp raps with your knuckle to shake the indicator off the stuck position?

 

[MarkG]

As mentioned, Narco is gone so messing around with this unit is probably a waste of time. They can be slightly heat sensitive and they are definately voltage sensitive. Generally if they are set on a bench at 28v and you install them in a 14 volt system they are off by around 100 feet. Conversely it works the other way as well. Same with almost all encoders.

The AR 850 came with a 25 pin connector and a 15 pin connector. If you have the 25 pin, you'll have to rewire the the transponder/encoder harness. However you mention you had this transponder 30 years and ACK encoders before that so most likely you will have the 15 pin encoder connector. My advice is chuck the 850, buy a nice Trans Cal unit and you will probably be good for years.

 

[MarkG]

I just re-read this series. How exactly do you know you encoder is off by 300 feet. Maybe it is okay, but your altimeter is off?

 

[JGoodish]

As mentioned, Narco is gone so messing around with this unit is probably a waste of time. They can be slightly heat sensitive and they are definately voltage sensitive. Generally if they are set on a bench at 28v and you install them in a 14 volt system they are off by around 100 feet. Conversely it works the other way as well. Same with almost all encoders.

The AR 850 came with a 25 pin connector and a 15 pin connector. If you have the 25 pin, you'll have to rewire the the transponder/encoder harness. However you mention you had this transponder 30 years and ACK encoders before that so most likely you will have the 15 pin encoder connector. My advice is chuck the 850, buy a nice Trans Cal unit and you will probably be good for years.

TransCal is the avionics shop's suggested replacement, but are these units really any better? I seem to remember reading somewhere that these units had an expected life of 5-10 years, which is longer than the AR850 has lasted but nowhere near the 30+ years that my King stack has lasted. Makes me wonder if these encoders aren't all pretty much junk.

I believe that you are correct regarding the 15-pin connector. Is the TransCal unit pin compatible?

I have no issue chucking the AR850 as long as I can be reasonably certain that the encoder is the issue, and it would be great to replace it with something that it much more reliable. Apart from possibly a dirty or intermittent connection between the encoder and transponder, it appears that not much else could cause the type of behavior that I am experiencing.


Thanks,
JKG

 

[MarkG]

Trans Cal's are pretty close to bullet proof. They are more money, but last a long time. See this link http://www.trans-cal.com/prodServ/Nano.html
Install manual, Ch 9.6 shows pin for pin compatability with your AR850 assuming it's the 15 pin model.
Something else to think about - you mentioned the transponder was close to 30 years. Is the harness the same age? You might want to check that wiring very closely from the rack at where the transponder slides in to the encoder plug. Wiring issues could be causing this grief. Also it does not sound likely, but it could be your transponder. Your avioncis shop probably has a harness to run your transponder and encoder together. Since they found no fault on the bench with just the encoder, you should really try the encoder and transponder together on the bench to rule out the transponder.

Don't be too quick to rule out your altimeter and maybe your encoder is okay. During the IFR cert, the altimeter gets to altitudes it does not see in normal operation (like 20K feet in your case). Just because it is okay when you are on the ground, does not mean it is not off at the altitude you are flying. Generally the higher you are flying the more it will be off. Did they give you an altitude correction card when they certified your altimeter. Also, if the adapter falls off and dumps the altitude too quickly when doing the altimeter test and can easily cause the altimeter to be off. Even though you got a fresh cert on the altimeter don't rule it out if you cannot prove the encoder or transponder are bad.

 

[gismo]

TransCal is the avionics shop's suggested replacement, but are these units really any better? I seem to remember reading somewhere that these units had an expected life of 5-10 years, which is longer than the AR850 has lasted but nowhere near the 30+ years that my King stack has lasted. Makes me wonder if these encoders aren't all pretty much junk.

I believe that you are correct regarding the 15-pin connector. Is the TransCal unit pin compatible?

I have no issue chucking the AR850 as long as I can be reasonably certain that the encoder is the issue, and it would be great to replace it with something that it much more reliable. Apart from possibly a dirty or intermittent connection between the encoder and transponder, it appears that not much else could cause the type of behavior that I am experiencing.


Thanks,
JKG

IIRC they are indeed pin compatible. BTW I think I have a nice working Trans-Cal encoder sitting around somewhere in my hangar. I'd be happy to part with it for around half what a new one costs with a guarantee that it works when you get it.

 

[Piloto]

One way to verify your altimeter and your encoder in-flight follows:

1. Fly the plane above 8,000 feet and level.

2. Using the altitude reading from a WAAS GPS navigator maintain constant GPS altitude.

3. Set the baro altimeter to the local pressure setting and read the indicated altitude.

4. Using an E6B calculator compute True Altitude. The result should equal your GPS altitude.

5. For the encoder check you will need something to read the transponder altitude data like the Monroy ATD-300.

6. While flying at the above altitude set the baro altimeter to 29.92".

7. The transponder altitude should match the baro altimeter.

While doing the above open and close cabin ventilation outlets. If there is baro altitude or transponder altitude changes you have a static line leak or either the encoder or the altimeter line is not connected to a static port. You can verify this on the ground by connecting the altimeter test set to the static port instead of directly to the altimeter.

José

 

[JGoodish]

Thanks to everyone for the input.

I see no indication that the altimeter gets stuck at any altitude; it appears to move freely with changes in aircraft altitude.

I would think that a static system problem would be consistent at the same altitude. What I am experiencing are varying errors at the same altitude, on the same flight. The encoder has been dead-on, but then will be off by 200 or 300 feet (at the same indicated altitude) later in the flight.

I have a call in to the avionics shop to see if they inspected and/or cleaned up the wiring between the transponder and encoder.

The avionics shop did say that the transponder itself is not to blame, since it is simply transmitting the altitude value sent to it by the encoder.

It would appear that if the wiring isn't an issue, the most likely culprit may be the encoder itself. I'll have to talk with the avionics shop again to get their feedback.


Thanks,
JKG