ATC Reports Loss Of Altitude
DISCREPANCY - ATC informs pilot they have lost his altitude indication. Pilot previously noticed altitude indication dashed out on the transponder during various phases of flight (takeoff’s, touch and goes & turns).
AIRCRAFT EQUIPMENT - Piper PA-28 is equipped with a GARMIN GNX 375 GPS/Transponder and Trans Cal SSD120-20 Gray Code Blind Encoder.
SYSTEM DESCRIPTION - The blind encoder is connected to the static system tubing. The blind encoder analyzes the pressure altitude and converts it to a digital altitude. The digital altitude is then reported to the transponder via Gray Code or RS-232. When applicable transponder is installed, the transponder displays it for the pilot and also sends an altitude signal to ATC via the Mode C function of the transponder.
GUIDANCE BEFORE TROUBLESHOOTING - The Blind Encoder is in charge of converting the pressure altitude into indication altitude. The encoder is most likely the culprit causing the issue because blind encoders usually do not shut down causing the transponder to display dashes when they first start to fail. The first sign of a blind encoder failing is the reported altitude is higher or lower than the actual altitude. This is usually reported by ATC, or it can also be noticed on the transponder display.
TROUBLESHOOTING STEPS
Step 1 - Disconnect the blind encoder wiring harness.
Step 2 - With a multimeter check for 14V or 28V at the blind encoder as applicable per the aircraft and blind encoder wiring.
Step 3 - With a multimeter check ground continuity. Always check for resistance during this check. A good ground is usually less than 1 ohm.
FINDINGS
Upon checking the resistance with a multimeter, the blind encoder ground was reading high at 39 ohms. Reminder: A good ground is usually less than 1 ohm. The ground wire was chased to the ground stud and the ground screw was found loose causing the high resistance. The screw in this situation was striped and could not be tightened.
Corrective action: The ground screw was replaced by a ground stud using a brass screw and nuts for better conductivity. After the replacement of the ground screw the ground wire was checked for resistance and was found to be within serviceable limits of 0.3 ohms.
After putting the system back together a transponder test was accomplished in accordance with 14 CFR 91.411 and 91.413. This is because any time work is accomplished on the altitude reporting system 91.411 (a)(3) and 91.413 (b) state that testing must be accomplished “Following the installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected and found to comply with paragraph (c), Appendix E, of part 43”.
Aircraft has flown multiple flights with no altitude issues reported.
If no discrepancies are found after performing the troubleshooting steps mentioned above, the next step would be to check wiring integrity from the blind encoder to transponder. If no issues are found, then a replacement of the blind encoder should be considered.