Altitude encoder

[azure]

Among the many mx issues keeping me from going anywhere interesting this weekend: my altitude encoder is (intermittently) reading 1600 feet low. Unfortunately I'm based under a Mode C veil so this is pretty much a showstopper by itself (yes, I know, I can call ATC and get permission to operate without it, but some facilities don't have a number to call in advance, and the ones that do might say no). I'm wondering about several things:

1. When it is reading low, the pressure altitude readout on my panel-mounted SL70 still follows any changes in altitude, i.e. it's not stuck at the erroneous value. Once I saw it read +008 when powered on, only to switch to -008 and then back and forth a couple of times before settling on -008. I looked over the gray code table and this doesn't seem to fit a problem with a single bit. Is this even a possible encoder failure mode? If so, does it point to something possibly fixable or is my encoder likely to be hosed?

2. I have two transponders, both SL70s, one panel mounted and the other in the tailcone, controlled by the 480. So if I need a new encoder, I need one that can connect to both to preserver transponder redundancy. My current encoder is apparently a Transcal SSD120-30-RS232 (from the logs), which costs somewhere in the $300-$400 range. Is dual connectivity a common feature of encoders or does my setup pretty much force me into buying an expensive one?

3. Actually, for full redundancy I would prefer a setup with two separate encoders. However, I also like to see what pressure altitude my transponder is squawking in Mode C, and the 480 doesn't seem to have a way to display this info -- nothing I can find in the manual anyway.

 

[wabower]

Several portable grey-code display boxes are available and relatively inexpensive. They also serve as altitude alerter/reminders and various other functions. Mine is an Airsport that has worked flawlessly (using a power adaptor rather than the internal battery) for 15 years. Just dial in current altimeter setting and read the altitude being reported to ATC.

Among the many mx issues keeping me from going anywhere interesting this weekend: my altitude encoder is (intermittently) reading 1600 feet low. Unfortunately I'm based under a Mode C veil so this is pretty much a showstopper by itself (yes, I know, I can call ATC and get permission to operate without it, but some facilities don't have a number to call in advance, and the ones that do might say no). I'm wondering about several things:

1. When it is reading low, the pressure altitude readout on my panel-mounted SL70 still follows any changes in altitude, i.e. it's not stuck at the erroneous value. Once I saw it read +008 when powered on, only to switch to -008 and then back and forth a couple of times before settling on -008. I looked over the gray code table and this doesn't seem to fit a problem with a single bit. Is this even a possible encoder failure mode? If so, does it point to something possibly fixable or is my encoder likely to be hosed?

2. I have two transponders, both SL70s, one panel mounted and the other in the tailcone, controlled by the 480. So if I need a new encoder, I need one that can connect to both to preserver transponder redundancy. My current encoder is apparently a Transcal SSD120-30-RS232 (from the logs), which costs somewhere in the $300-$400 range. Is dual connectivity a common feature of encoders or does my setup pretty much force me into buying an expensive one?

3. Actually, for full redundancy I would prefer a setup with two separate encoders. However, I also like to see what pressure altitude my transponder is squawking in Mode C, and the 480 doesn't seem to have a way to display this info -- nothing I can find in the manual anyway.

 

[Silvaire]

Not sure how you are connected but the first thing I would try is to disconnect the 232 cable to the rear transponder and see if that makes any difference.

 

[azure]

Thanks. I'm not sure what the results of that experiment would tell me though. I normally use the rear transponder (since it's controlled by the 480) and monitor the pressure altitude readout on the panel mounted one. There is 100% correlation between the readout (corrected for altimeter setting) and what altitude ATC says they're seeing me at.

 

[chemgeek]

This is almost certainly an encoder failure. Your model is the basic Trans-Cal RS-232 with dual output. The TransCal units are highly regarded by the aviation consumer rags, and is what I put in my plane when my encoding altimeter died. I'd bite the bullet and replace it with the same--plug and play, no re-wiring.

 

[azure]

I looked inside the tailcone today to see whether the encoder was back there or not. It isn't, it must be behind the panel. I couldn't easily budge the serial connector on the rear SL70, and the leads are all exposed and it wouldn't be hard to break one off if I really tried to remove the connector, so I gave up.

After looking at the installation manual for my encoder, I'm guessing Silvaire's suggestion was to eliminate the possibility of crosstalk between devices?

The panel mounted SL70 now reads -008 on power up, so it's no longer an intermittent problem (it seems).

Chemgeek, that's what I will probably do, especially since dual output encoders are generally more expensive anyway. But I want to be sure it's the encoder before springing for a $0.35 AMU (with shipping, from Spruce) unit. Why do you say "almost certainly"?

 

[N801BH]

I looked inside the tailcone today to see whether the encoder was back there or not. It isn't, it must be behind the panel. I couldn't easily budge the serial connector on the rear SL70, and the leads are all exposed and it wouldn't be hard to break one off if I really tried to remove the connector, so I gave up.

After looking at the installation manual for my encoder, I'm guessing Silvaire's suggestion was to eliminate the possibility of crosstalk between devices?

The panel mounted SL70 now reads -008 on power up, so it's no longer an intermittent problem (it seems).

Chemgeek, that's what I will probably do, especially since dual output encoders are generally more expensive anyway. But I want to be sure it's the encoder before springing for a $0.35 AMU (with shipping, from Spruce) unit. Why do you say "almost certainly"?

If this is a certified ship I bet the avioncs shop will charge at least that for labor alone......

 

[Silvaire]

...I'm guessing Silvaire's suggestion was to eliminate the possibility of crosstalk between devices?...

I'm just suggesting you have a very long interface cable between the encoder and the rear xponder so it's possible it may have been compromised at some point - maybe broken shielding or perhaps it's tie-wrapped to a coax somewhere. I would disconnect the rear xponder from the encoder and see if you get any results. If you are going to troubleshoot with the shotgun method the key is to shoot the cheap stuff first.

 

[azure]

If this is a certified ship I bet the avioncs shop will charge at least that for labor alone......

He charges $82/hr. Do you really think it will take him 4+ hours to change a Mode C encoder?

I'm just suggesting you have a very long interface cable between the encoder and the rear xponder so it's possible it may have been compromised at some point - maybe broken shielding or perhaps it's tie-wrapped to a coax somewhere. I would disconnect the rear xponder from the encoder and see if you get any results. If you are going to troubleshoot with the shotgun method the key is to shoot the cheap stuff first.

But how would that affect the pressure altitude readout on the OTHER transponder? Maybe I'm missing something here.

Anyway I talked with my avionics guy today and he agrees that the likeliest cause is a faulty encoder. On his advice, I ordered a replacement TransCal from Spruce and had them ship it to me overnight. My only concern is the mounting tray, since the unit I have is no longer made and the replacement is a "nano" version that requires an adapter plate to attach it to the mounting tray used for the older unit. Bill said he thought the nano encoder came with its own mounting tray, but looking at the "what's in the box" list in the installation guide, that doesn't seem to be the case. Unfortunately I placed the order before checking.

Hopefully he'll be able to improvise something, though now I'm starting to think THAT might stretch the job to 4+ hours.

 

[kontiki]

If the transponder gets altitude data on a serial bus, RS232, ARINC 429, altitude is an all or nothing proposition.

Some transponders have a Gilham code interface. Multiple wires where each wire represents some number of 100 ft or 500ft increments. I have this ancient doc, it may or may not help. But it could be just a couple broken wires.

 

[Silvaire]

...But how would that affect the pressure altitude readout on the OTHER transponder? Maybe I'm missing something here....

That's a very good question and I'm not saying it does but one thing I've learned in my years as an avionics tech is to not theorize agonizingly over something that takes twenty five seconds (and no cash) to physically verify. The fact that you were back in the tailcone messing around and your fault has now gone from intermittent to hard is suspicious in itself.

But go ahead and put in a new encoder. It'll probably fix your problem. If it doesn't at least you'll know that that wasn't it.

 

[chemgeek]

Well, there is no full certainty, but I've had a blind encoder and an encoding altimeter have the same failure mode: an intermittent jumping of the pressure altitude readout by some fixed value, like +/- 3000 feet. Seemed like it could be a bad wire dropping a gray code bit, but it was in both cases a failing encoder unit. YMMV. I'd bet on the encoder.

 

[azure]

That's a very good question and I'm not saying it does but one thing I've learned in my years as an avionics tech is to not theorize agonizingly over something that takes twenty five seconds (and no cash) to physically verify. The fact that you were back in the tailcone messing around and your fault has now gone from intermittent to hard is suspicious in itself.

Well actually, no. The fault had stopped being intermittent before I opened up the tailcone. The first thing I did was power up the avionics bus to see if the transponder would briefly display the correct altitude as it did Saturday. It showed -008 right away, and waiting a minute or so and then power cycling the thing made no difference. However the wrinkle in that is that my mechanic tried reseating the transponder in the panel at some point and he didn't say when that was. Stupid me, I didn't think to ask when, and knowing him, my next chance to ask him will be sometime after I've been to the avionics shop.

It would definitely cost me some cash if I broke one of the leads trying to get that connector off with a pair of pliers.

But go ahead and put in a new encoder. It'll probably fix your problem. If it doesn't at least you'll know that that wasn't it.

I'm planning to carry the new encoder along with me on Thursday. If it's not needed, I'll return it.

 

[Silvaire]

....I'm planning to carry the new encoder along with me on Thursday. If it's not needed, I'll return it.

Will they let you return an encoder that you have used for troubleshooting?

 

[azure]

Will they let you return an encoder that you have used for troubleshooting?

I'm not sure, but why would I need to use the new encoder for troubleshooting? Can't he remove the old one and bench test it?

My plan was to not even open the package until Bill says, it's definitely the encoder.

 

[Silvaire]

You'll be okay with that. Good luck, hope you get it fixed.

 

[azure]

I'm sitting right now in the avionics shop. Bill ended up using the new encoder for troubleshooting as removing the old one proved to be a bear. But it worked from the get-go, so no worries there. Now the only problem will be physically mounting the new one. Hoping this doesn't rack up too many labor hours... Thanks for all the helpful replies guys.

 

[somorris]

All's well that ends well . It sounds like you have the problem corrected. I had to do a similar thing (replace the encoder) two years ago.

 

[azure]

Yep, it's fixed. If I had left it at just replacing the encoder, it would have been just 2 hours of labor. But since they had to recertify the static system as well, I had them do the full IFR certs now - an extra $350 - so that I wouldn't have to come back in the cold of winter (I was due 2/14). So it was more expensive than I had anticipated, but worth it.

 

[N801BH]

;

Yep, it's fixed. If I had left it at just replacing the encoder, it would have been just 2 hours of labor. But since they had to recertify the static system as well, I had them do the full IFR certs now - an extra $350 - so that I wouldn't have to come back in the cold of winter (I was due 2/14). So it was more expensive than I had anticipated, but worth it.

 

[steingar]

I spent $2K fixing the encoder on my transponder only to discover it still broken when I tried to fly into DC airspace after Wings. For the next certification the aircraft gets a new transponder. With luck it'll keep working until 2020.

 

[azure]

I spent $2K fixing the encoder on my transponder only to discover it still broken when I tried to fly into DC airspace after Wings. For the next certification the aircraft gets a new transponder. With luck it'll keep working until 2020.

I remember you posting about that unfortunate debacle. In my case I have two transponders, and they were both putting out the same, incorrect altitude.

I'm just curious though, was your problem intermittent or a total failure? It's hard to believe a decent avionics shop would return your altitude reporting system to service without testing it out thoroughly. My shop said they had to do a complete static system certification as well, after installing the new encoder.

 

[poadeleted21]

I remember you posting about that unfortunate debacle. In my case I have two transponders, and they were both putting out the same, incorrect altitude.

I'm just curious though, was your problem intermittent or a total failure? It's hard to believe a decent avionics shop would return your altitude reporting system to service without testing it out thoroughly. My shop said they had to do a complete static system certification as well, after installing the new encoder.

Why do you have 2 transponders? I've seen dual transponders in planes before and scratched my head.

 

[kgruber]

Why do you have 2 transponders?

Redundancy.

 

[azure]

Redundancy.

Yep -- Tom's doing, not my idea. I can think of at least a couple things I'd place higher on the priority list for installing backups for, but I have to admit that being able to control my transponder through the GPS is convenient. That arrangement also has enough points of possible failure to justify having a backup -- for example if the GPS unseats itself, it's very possible for it to lose the ability to command the transponder (this actually happened to me once). I could live with having only the transponder in the panel, but since I already have the other one, I can't think of a good reason to rip it out.