Narrative:

Descending from cruise altitude, the captain and I realized there was a stuck microphone on frequency. Because I could hear side tone, I knew the stuck microphone was ours. We quickly checked all 4 mikes (2 hand held plus 2 yoke microphone transmitters). At times, we got it fixed only to have it stick again. In the process, and not knowing which switch it was we blocked several frequencys on the way into bwi. We also had to squawk 7600 for lost communication, however, the captain inadvertently squawked 7500 (hijack) briefly. The bad switch was the captain's side yoke transmitting switch. The captain unplugged his headset and I used radio #2 and landed the airplane at bwi. In the process we ended up blocking several frequencys until we arrived at the gate, then figured out why. Although the captain had unplugged his headset, the microphone selector was still transmitting on radio #1. 3 things we learned about stuck mikes: 1) if you hear side tone similar to when one pilot is transmitting normally and the frequency has a stuck microphone, the stuck microphone is yours. 2) don't FLIP too many switches or buttons at first. Stop, and try to remember who transmitted last. This will make it easier to find which microphone on which side is hot. 3) even if you unplug the hot side headset, you will still be transmitting until the microphone selector switch is deselected from communication #1 and communication #2. A good place to select would be flight interphone.

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Original NASA ASRS Text

Title: BLOCKED FREQS BY A STUCK MIKE ON A B737-700 GOING INTO BWI, MD.

Narrative: DSNDING FROM CRUISE ALT, THE CAPT AND I REALIZED THERE WAS A STUCK MIKE ON FREQ. BECAUSE I COULD HEAR SIDE TONE, I KNEW THE STUCK MIKE WAS OURS. WE QUICKLY CHKED ALL 4 MIKES (2 HAND HELD PLUS 2 YOKE MIKE XMITTERS). AT TIMES, WE GOT IT FIXED ONLY TO HAVE IT STICK AGAIN. IN THE PROCESS, AND NOT KNOWING WHICH SWITCH IT WAS WE BLOCKED SEVERAL FREQS ON THE WAY INTO BWI. WE ALSO HAD TO SQUAWK 7600 FOR LOST COM, HOWEVER, THE CAPT INADVERTENTLY SQUAWKED 7500 (HIJACK) BRIEFLY. THE BAD SWITCH WAS THE CAPT'S SIDE YOKE XMITTING SWITCH. THE CAPT UNPLUGGED HIS HEADSET AND I USED RADIO #2 AND LANDED THE AIRPLANE AT BWI. IN THE PROCESS WE ENDED UP BLOCKING SEVERAL FREQS UNTIL WE ARRIVED AT THE GATE, THEN FIGURED OUT WHY. ALTHOUGH THE CAPT HAD UNPLUGGED HIS HEADSET, THE MIKE SELECTOR WAS STILL XMITTING ON RADIO #1. 3 THINGS WE LEARNED ABOUT STUCK MIKES: 1) IF YOU HEAR SIDE TONE SIMILAR TO WHEN ONE PLT IS XMITTING NORMALLY AND THE FREQ HAS A STUCK MIKE, THE STUCK MIKE IS YOURS. 2) DON'T FLIP TOO MANY SWITCHES OR BUTTONS AT FIRST. STOP, AND TRY TO REMEMBER WHO XMITTED LAST. THIS WILL MAKE IT EASIER TO FIND WHICH MIKE ON WHICH SIDE IS HOT. 3) EVEN IF YOU UNPLUG THE HOT SIDE HEADSET, YOU WILL STILL BE XMITTING UNTIL THE MIKE SELECTOR SWITCH IS DESELECTED FROM COM #1 AND COM #2. A GOOD PLACE TO SELECT WOULD BE FLT INTERPHONE.

Data retrieved from NASA's ASRS site as of July 2007 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.