Narrative:

After switching from a backup aircraft into our primary aircraft; we flew back to our base. During takeoff tower reported that they were not receiving our mode C encoding; I re-entered the transponder code and recycled the transponder. Tower approved my frequency change to denver area CTAF. It was a dual nurse crew with one of the nurses orienting for his second shift. After crossing through boulder and making all of the appropriate calls; we were discussing the importance of maintaining eyes out whenever possible; especially through this area. About 3 miles east; on an easterly heading; the veteran nurse on the left side called 'traffic! Break right!' I immediately broke right and lowered the collective. As I straightened out I said; 'that's odd it didn't show up on the TCAS;' and as I looked back out there was another airplane about 1/2 mile at the 12 o'clock level position moving left to right. I called traffic and broke left. That airplane was on about a 5 mile final landing at the erie airport. I had switched from boulder CTAF to 123.025 just prior to the first near miss and switched to erie CTAF just after the second close call. The first airplane never reported and the second was on the correct frequency. After clearing the area I scrolled to the TCAS page and it read 'failed' which explained why we didn't receive any traffic alerts. After an uneventful landing; I went up to the circuit breaker panel and saw the altitude encode circuit breaker was popped. I reset the breaker and the TCAS and mode C both began operating normal. During the post-flight debrief the nurse reported the first target was between 50-100 feet above us. I never saw the first target because of the blind spot. Following the debrief with program management; it was determined to take the crew oos for the final 2 hours of the shift for them. I consulted with the lead mechanic to try to determine why the circuit breaker popped. I was unable to duplicate the event during ground testing; and a 10 minute maintenance flight. I could not 100% verify that the circuit breaker was in during my start up; although I did my normal physical and visual check of the breaker panel. It's possible that the breaker was bumped during the move from the back up aircraft or it popped during the start up. I didn't correlate the mode C not working with the circuit breaker or the TCAS failure until after we landed. Having had 2 flights since then; I have changed that portion of the start and spend an extra moment to visually and physically ensure all breakers are in.

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Original NASA ASRS Text

Title: Helicopter pilot is informed during a VFR departure that his Mode C transponder is not working and attempts to recycle are ineffective. A few minutes later an NMAC occurs and the reporter cannot initially understand why the TCAS did not alert. Post flight reveals a tripped Altitude Encode circuit breaker which had caused both failures.

Narrative: After switching from a backup aircraft into our primary aircraft; we flew back to our base. During takeoff Tower reported that they were not receiving our Mode C encoding; I re-entered the transponder code and recycled the transponder. Tower approved my frequency change to Denver area CTAF. It was a dual nurse crew with one of the nurses orienting for his second shift. After crossing through Boulder and making all of the appropriate calls; we were discussing the importance of maintaining eyes out whenever possible; especially through this area. About 3 miles east; on an easterly heading; the veteran nurse on the left side called 'Traffic! Break Right!' I immediately broke right and lowered the collective. As I straightened out I said; 'That's odd it didn't show up on the TCAS;' and as I looked back out there was another airplane about 1/2 mile at the 12 o'clock level position moving left to right. I called traffic and broke left. That airplane was on about a 5 mile final landing at the Erie airport. I had switched from Boulder CTAF to 123.025 just prior to the first near miss and switched to Erie CTAF just after the second close call. The first airplane never reported and the second was on the correct frequency. After clearing the area I scrolled to the TCAS page and it read 'Failed' which explained why we didn't receive any traffic alerts. After an uneventful landing; I went up to the circuit breaker panel and saw the Altitude Encode circuit breaker was popped. I reset the breaker and the TCAS and Mode C both began operating normal. During the post-flight debrief the nurse reported the first target was between 50-100 feet above us. I never saw the first target because of the blind spot. Following the debrief with program management; it was determined to take the crew OOS for the final 2 hours of the shift for them. I consulted with the Lead Mechanic to try to determine why the circuit breaker popped. I was unable to duplicate the event during ground testing; and a 10 minute maintenance flight. I could not 100% verify that the circuit breaker was in during my start up; although I did my normal physical and visual check of the breaker panel. It's possible that the breaker was bumped during the move from the back up aircraft or it popped during the start up. I didn't correlate the Mode C not working with the circuit breaker or the TCAS failure until after we landed. Having had 2 flights since then; I have changed that portion of the start and spend an extra moment to visually and physically ensure all breakers are in.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.