If the static port get blocked for any reasons and the aircraft is not fitted with an alternate static source while in Mode C with SSR, will the ATC still get the height information from my transponder?

  • $\begingroup$ how does it know that the port is blocked? So I'd say: yes.... $\endgroup$ Mar 12, 2018 at 13:48
  • $\begingroup$ I remember something about transponder with mode C which is mechanically actuated by the altimeter's aneroid capsule so I guess it will give erroneous information. But I will wait an explanation from someone who knows more about this. That's all I've got. $\endgroup$ Mar 12, 2018 at 15:13
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    $\begingroup$ Related: en.wikipedia.org/wiki/Aeroper%C3%BA_Flight_603 $\endgroup$ Mar 12, 2018 at 18:33

2 Answers 2


It will stop reporting correctly as it is connected to the same static air line as the altimeter

The transponder depends on an external altitude encoder to provide the digitized pressure altitude that the transponder needs for its Mode C replies. The encoder is connected the same static air line as the altimeter and is wired electrically to the transponder

However the transponder may not know the port is blocked and will still report the erroneous information it is receiving. If the port is blocked it’s pertinent to shut off altitude reporting manually.

  • $\begingroup$ yes if you see the malfunction then shut it off. however in most cases ATC will ask for your altitude when the mode-c altitude does not match your assigned altitude. but you will also have trouble with the main altimeter and airspeed indicator. It may be most noticed with the behavior of the vertical speed indicator. Also most GPSS units have an option to show geometric altitude, which will show gross error in the altimeter, but not reliable for small errors as it is not compensated for pressure variations. $\endgroup$
    – Max Power
    Mar 20 at 23:36

If the static ports were blocked the mode c transponder would likely still report an altitude based on the static (atmospheric) pressure that was in the system when it became blocked. But as soon as the aircraft changed altitude or other pressure variations occurred, the mode c altitude report (displayed to ATC), would be erroneous.

From the FAA's AMT Airframe Handbook Volume 2 (full version) (FAA-H-8083-31), volume 10 (link is here)

(emphasis is mine)

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