Transponder Altitude Comparison After Static System Work
After a static-system leak check and replacement of a static port on a twin-engine turboprop, the AME connects a transponder ramp tester. Field elevation is 1,200 ft and QNH is 996 hPa. The cockpit altimeter, set to 996 hPa, reads 1,200 ft. The tester displays the transponder reply as 1,480 ft. The pilot says the altimeter and the ADS-B altitude display in the cockpit agree, and suggests the encoder is '280 ft high' and can be adjusted. The aircraft has ADS-B Out and is due for a check flight. The maintenance manual gives an encoder calibration procedure and a tolerance for altitude reporting; the AME also has a pitot-static test set capable of applying a known pressure altitude. The operator wants the aircraft released for a 90-minute flight. The AME must decide whether to adjust, test fly, or troubleshoot.
1. Is the pilot's 280 ft comparison valid? Explain pressure altitude versus indicated altitude and calculate the expected pressure altitude.
Read the worked answer
No. The pilot has compared the transponder reply with the cockpit altimeter, but the two instruments are referenced differently. The altimeter is set to the local QNH of 996 hPa, so it displays height above the altimeter setting datum. The transponder encoder reports pressure altitude, which is referenced to 1013.25 hPa.
Using the approximate low-altitude conversion of 27 ft per hPa:
Pressure altitude = field elevation + (1013 - QNH) x 27 Pressure altitude = 1,200 + (1013 - 996) x 27 Pressure altitude = 1,200 + 17 x 27 Pressure altitude = about 1,659 ft
The tester shows 1,480 ft, which is about 179 ft below the expected pressure altitude, not 280 ft above the altimeter reading. The 280 ft figure comes from comparing 1,480 ft with the altimeter's 1,200 ft, which mixes two different references. The exact conversion should be taken from the test set or the manual, but the direction of the error is clear: the transponder is low relative to pressure altitude, not high relative to indicated altitude. Adjusting the encoder to make it read 1,200 ft would introduce a large error.
2. What tests would you perform to isolate the fault, and why is a check flight not the first step?
Read the worked answer
I would use the pitot-static test set to apply known pressure altitudes to the static system and compare the transponder reply with the applied pressure altitude, not with the cockpit altimeter. This separates the static system from the encoder and transponder.
Checks, in order:
- Confirm the static system is leak-free after the port replacement and that the static lines are correctly connected, not crossed or restricted.
- Confirm the altimeter is set to 1013 hPa for the comparison, or calculate the pressure-altitude equivalent correctly.
- Apply several pressure altitudes, for example field elevation, 5,000 ft, and 10,000 ft, and record the transponder reply at each point. Check against the manual tolerance.
- If the error is consistent, check the encoder calibration and the air-data source selection. If the error changes with altitude or is erratic, look for a leak, a partially blocked line, or a wiring/connector fault.
- Check the ADS-B altitude source configuration. ADS-B Out may use the same encoder or air-data computer, and a configuration error can make the cockpit display agree while the transmitted value is wrong.
A check flight is not the first step because it exposes the crew and aircraft to an unknown altitude-reporting error and does not isolate the fault. A ground test with known pressure is more controlled and repeatable. A flight check may be required after repair, but it should follow correction and ground verification.
3. What return-to-service decision and records would you make?
Read the worked answer
I would not release the aircraft for the 90-minute flight while the altitude reporting error is unresolved. The discrepancy is significant relative to the manual tolerance and affects both transponder replies and ADS-B Out, which other aircraft and air traffic services may use for separation.
After correcting the fault, I would repeat the pressure-altitude test at multiple points, confirm the static system leak check is satisfactory, and verify the ADS-B altitude source and configuration. If the manual requires a flight check or an operational check after the repair, I would complete or arrange it before release. I would not adjust the encoder simply to match the cockpit altimeter.
The maintenance record should include the original discrepancy, the QNH and field elevation used, the calculated pressure altitude, the test-set values applied, the transponder replies before and after, any encoder or static-system correction, the ADS-B configuration check, and the manual references. The release should state that the altitude reporting system was tested and found within the approved tolerance.