Mode C issue

[Onewinglo]

Guys I flew through the Gulfport TRSA yesterday at 5500' but ATC said my transponder reported 1500'. I recycled the transponder with no change. This was the first time I used my transponder - just bought the airplane in November. The vacuum driven artificial horizon failed yesterday also. The transponder is a Narco AT-150 which rides in a 1951 170A with a Venturi. Any ideas what could be the problem?
Thanks, JP White

[GAHorn]

JP... I LOVE your username "Onewinglo" !! That's a GOOD one!

OK, your transponder is a good one. IT has an external little avionic box connected to it called an "encoder" which is either bad.... or has a bad connection to your transponder. The "encoder" samples static air (or cabin air) and measures the barometric pressure available, then signals that barometric pressure to your transponder thru a set of 6-8 wires (depending on which model encoder) using what is known as "gray code". Either your encoder is failed or the wiring harness to your transponder has failed.
Either way, it's off to the avionics shop to get it troubleshot and repaired or replaced. (You're looking at $300-$500 plus a couple hours labor if it's the encoder and an hour labor if it's just the wiring harness.)
Get underneath your panel and look for an obvious wiring issue before you run to the avionics shop. You might get lucky and find a broken wire or connector, but if you do...then buy a lottery ticket also because if it's that easy for you... you are one lucky guy and we're ALL your BEST FRIENDS if when that lottery ticket comes in for you.)

[Onewinglo]

Thanks George. I'll stick my head under the panel Friday to look for an obvious problem. If I get lucky I'll buy a couple lottery tickets! If not, I'll make an appointment with the avionics shop.
JP

[cessna170bdriver]

The last time that happened to me, it was a loose connector at the encoder.

[c170b53]

I just sent my Altimeter and Airspeed out to Quality Aircraft Instruments, for overhaul. Likely the last time they were overhauled was in the seventies. Flying to last year's convention my Altimeter was out by various amounts depending on my cruise altitude. At times the error approached 400 ft ATC knew there was a problem at times and politely pointed it out.
It would have been interesting to know whether they saw you at different altitudes or whether the blind encoder just reported 1500 ft all the time. Digital devices need good grounds otherwise their outputs can be random. I would also look at the static line from the Altimeter to the encoder, just to make sure something silly isn't involved.

[Onewinglo]

Miles / Jim, thanks for the pointers. I look forward to giving the panel underside a good long look Friday. I'm feeling lucky...

[GAHorn]

Jim... how do you know that it was your altimeter that was "out" of tolerance...and not your encoder (which might need calibration?)

My KLN-90B GPS has gray code input from my encoder, so I know at all times what it is reporting to ATC and can compare it to my altimeter reading. (Just an FYI for those of you with VNAV-enabled GPS units which might be helpful if you have gray-code input capability to hook it up so you can enjoy that feature.)

[c170b53]

Correct George the best way to identify the culprit is with a pitot static test kit. Although it's one or the other,I'm thinking its the altimeter as it has not been bench checked for years, the error seemed to magnify with altitude and the encoder is fairly new.

[c170b53]

And the incorrect way, was sneaking a peek at my handheld GPS Altitude

[cessna170bdriver]

Jim... how do you know that it was your altimeter that was "out" of tolerance...and not your encoder (which might need calibration?)

My KLN-90B GPS has gray code input from my encoder, so I know at all times what it is reporting to ATC and can compare it to my altimeter reading. (Just an FYI for those of you with VNAV-enabled GPS units which might be helpful if you have gray-code input capability to hook it up so you can enjoy that feature.)

When you're off by a few hundred feet, it's probably calibration. When it's off by 4000 feet, something's broken (encoder, wire, connection, etc.)

[GAHorn]

GPS altitude can be off by hundreds of feet. Do not rely upon that unless the GPS is corrected for local barometric pressure (such as via an accurate encoder).
GPS determines altitude by using MOA (minute of angle) to the constellation coupled with WAAS (if your GPS is so enabled). If the average constellation is low to the horizon the error is greater than if it is well dispersed.

The barometric altimeter in our airplanes depends upon having an accurate local barometer input. The encoder is pre-set (permanently) to the standard 29.92 in Hg, (and the ATC computer makes the appropriate correction for the ATC-specialist to observe) ...so just because ATC tells you that your txdr is reporting 8300 MSL (when your altimeter is showing 8500) doesn't mean that your altimeter is wrong..... it could be that the local altimeter setting is wrong...OR that ATC is providing you an altimeter setting which is only .2" different than what you are actually experiencing locally. This can happen near fronts, near mountain waves, near TRWs, etc etc. In other words, unless you have good reason to believe your altimeter is damaged/worn-out/etc..... (BTW, a good check to to set local altimeter while on the ground and compare with the actual field elevation).... then it's not necessarily indicative of the need to repair your altimeter.

Keep in mind that your altimeter is allowed to indicate up to 70' of error in the above check, (and that error can be easily calibrated out of the instrument with a simple screwdriver....all you have to do is set field elevation indications, then remove the guarded cap next to the knob, pull out on the knob and rotate it to the correct barometric reading, then push the knob back in and reinstall the cap. While technically this will not repair an altimeter which has a hysteresis* problem, it nonetheless will return the altimeter to the correct field-elevation-vs-Kollsman-reading relationship.)
Of course, any altimeter used for IFR must meet the 2 year recertification requirement...but so does your altimeter/encoder/txdr reporting, and the latter can also be the problem if ATC regularly reports encoder issues. (The altimeter is not as likely to be the problem unless it's decades old.)

*Hysteresis is the term used to describe a long term error caused by loss of elasticity of the bellows inside the sensitive altimeter. Think of it like this: A new toy balloon is only 2" in diameter before it is inflated for the very first time. Now, blow it up to it's maximum... and let it remain fully-inflated for a few minutes...then release the air back out and re-measure the balloon, and you will find it has stretched and is now slightly larger than originally. This is similar to the hysteresis-effect which the bellows eventually experience inside the altimeter.... the altimeter now being slightly out of calibration.
However, our little airplanes rarely experience the maximum altitudes for which our altimeters are designed, and therefore they are quite durable and less likely to experience early-hysteresis problems than if we had the same altimeter in a P-51 or Ron's Fearcat that regularly leaves sea-level and climbs up and plays at 40,000 feet and then returns to sea level. The fighter altimeter will suffer hysteresis sooner than the 170 altimeter.
Of course, the fleet of 170's likely are operating with altimeters which haven't seen an instrument shop bench in 25 years or more and are way past due for overhaul. Every altimeter should be checked for hysteresis about ten years or so, and if operated IFR, every 2 years.

[sfarringer]

Another reason that an aircraft altimeter may not agree with GPS altitude: The aircraft altimeter assumes a standard lapse rate. It is not uncommon for the actual atmosphere to have a non-standard lapse rate. This is not a problem, as long as everyone is using aircraft altimeters.

So, the GPS altitude could actually be more accurate, but the aircraft altimeter should be your primary altitude reference.

[GAHorn]

Hmmmn... well, yes...sorta.
When I wrote that GPS altitude can be off by several hundred feet... that was in reference to the standard we all use when flying...which is barometric altitude. Which is why it's always best to use the altimeter rather than the GPS.
Lapse rate can refer to temperature lapse rate...or atmospheric-density lapse rate... the latter also being why we use local barometer when flying below the flight-levels, and 29.92 when flying above/in the flight levels (where speed and distances covered make a standard atmosphere more convenient, if not more or as accurate.

My main concern in commenting was to deter anyone from automatically sending their altimeters off for overhaul simply because of a disagreement with "reported" altitude, if the encoder has not recently been checked. (But I didn't intend to discourage that if your altimeter hasn't been recertified in recent years it might be a good idea to have it done.)

[jetguy]

And then there's this.....my baro window, set to the field AWOS, never produced the correct indication on the needles (and vice-versa, obviously); they mismatched by a couple hundred feet. I finally got tired of it so I undid the little screw, pulled and pushed and jiggled the knob but couldn't get it to let go. Gave up, yanked the altimeter and schlepped it over to the avionics shop. The first guy, John, said "oh this is a Garwin....it's a piece of sh*t." So he takes out the front screw, then removes the other, little hidden side screw, and proceeds to jam a small ice pick in the hole, where he finally breaks off the whole adjustment knob. About that time, the other avionics guy shows up, looks at the broken altimeter, and says "yea, this is a Garwin.....it's a piece of sh*t." I now have a nice near-new United Instruments altimeter which works perfectly.....and it ain't a P.O.S.

[voorheesh]

The airworthiness and reliability of pitot static systems including altimeters and Mode C altitude reporting is an important safety issue. ATC relies on this information for air traffic separation under IFR and for traffic advisories involving VFR operators both using the system and many times just flying along minding their own business. I recommend that any work involving an altimeter or transponder be accomplished by a rated repair station, bearing in mind that 91 requires the system to be tested I/A/W 43 Appendix E & F anytime it is opened up if the aircraft operates under IFR or if there is a possibility of inducing error in data correspondence between altimeter and transponder. Transponders require inspections every 24 months and it is recommended that pitot/static systems be tested concurrently to ensure integrity of the system. This 24 month inspection is occasionally overlooked and should be verified on each annual inspection of a transponder equipped aircraft. This type of work is not appropriate for non certificated personnel and there are few independent technicians who meet the rating requirements of Part 43. My experience with a few avionics shops (rated 145 repair stations) here in California has proved that these tests and occasional corrective actions are not that expensive and well worth the effort in reducing the risk of my aircraft broadcasting inaccurate altitude data. No criticism intended, just some thoughts/reminders on aviation safety.