Narrative:

About 1 1/2 hours into the flight we took our first restroom break. As per the FARS we each used our oxygen mask while the other left the flight deck. Following the restroom break; we started having radio problems. One of many mikes was intermittently transmitting leaving us with a 'stuck microphone' situation. It took us a good 200 mi and several ATC frequencys before we were able to narrow down our problems in order to retain our communications with ATC. Captain and myself tried numerous procedures including: switching radio control units; and disconnecting the handheld mikes. What we were left with was a single radio control unit or a single VHF radio to work with. This was 1 out of 3 radios to work with. Captain and I were not able to ascertain precisely what caused the radio malfunctions; but the problems could have easily been with the oxygen mask because only after using the oxygen mask our 'stuck microphone' problems started. This in fact was discussed between captain and myself as the possible culprit. About 2-3 hours into the flight; the captain had to use the restroom again. This time while he left his station I did not don the oxygen mask because of our degraded state and instead placed it on my lap. The FAA inspector questioned this and I explained about our state and how remote the chance of having a decompression would be versus further complicating our already poor radio situation. I also pointed out that I had the oxygen mask on my lap in case I did need it I could access it quickly and easily. When the captain returned from the restroom break we discussed the situation and he concurred that it was a reasonable decision on my part. In fact; as we approached iad; we briefed ATC on our radio difficulties and established a contingency plan in case of additional communications problems. This was a decision made by myself in the best interest of safety. I reasoned that with 4 sources of air; 4 controllers; 2 out-flow valves; and in a stable state with our pressurization; that the risk of a rapid decompression was very slight; but that using the oxygen mask might again jeopardize our only radio and in that state would create a very significant safety problem. In summary; I was well aware of the oxygen mask rule. In fact; I used it the first break. The state of the aircraft was not in a normal working state and the cause could have been due to the oxygen mask. I had my mask on my lap to use if I needed it. I used the best reasoning for a safe operation for the given situation. I have recently received a letter from the FAA advising me that this incident is under investigation.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B747-400 EXPERIENCED STUCK MIKE PROBS IN CRUISE FLT THAT MAY HAVE BEEN CAUSED BY OXYGEN MASK USE. FOR THAT REASON; THE FO DECIDED TO NOT DON HIS MASK WHEN THE CAPT LEFT THE COCKPIT FOR A BREAK; BUT TO LAY THE MASK ON HIS LAP.

Narrative: ABOUT 1 1/2 HRS INTO THE FLT WE TOOK OUR FIRST RESTROOM BREAK. AS PER THE FARS WE EACH USED OUR OXYGEN MASK WHILE THE OTHER LEFT THE FLT DECK. FOLLOWING THE RESTROOM BREAK; WE STARTED HAVING RADIO PROBS. ONE OF MANY MIKES WAS INTERMITTENTLY XMITTING LEAVING US WITH A 'STUCK MIKE' SITUATION. IT TOOK US A GOOD 200 MI AND SEVERAL ATC FREQS BEFORE WE WERE ABLE TO NARROW DOWN OUR PROBS IN ORDER TO RETAIN OUR COMS WITH ATC. CAPT AND MYSELF TRIED NUMEROUS PROCS INCLUDING: SWITCHING RADIO CTL UNITS; AND DISCONNECTING THE HANDHELD MIKES. WHAT WE WERE LEFT WITH WAS A SINGLE RADIO CTL UNIT OR A SINGLE VHF RADIO TO WORK WITH. THIS WAS 1 OUT OF 3 RADIOS TO WORK WITH. CAPT AND I WERE NOT ABLE TO ASCERTAIN PRECISELY WHAT CAUSED THE RADIO MALFUNCTIONS; BUT THE PROBS COULD HAVE EASILY BEEN WITH THE OXYGEN MASK BECAUSE ONLY AFTER USING THE OXYGEN MASK OUR 'STUCK MIKE' PROBS STARTED. THIS IN FACT WAS DISCUSSED BTWN CAPT AND MYSELF AS THE POSSIBLE CULPRIT. ABOUT 2-3 HRS INTO THE FLT; THE CAPT HAD TO USE THE RESTROOM AGAIN. THIS TIME WHILE HE LEFT HIS STATION I DID NOT DON THE OXYGEN MASK BECAUSE OF OUR DEGRADED STATE AND INSTEAD PLACED IT ON MY LAP. THE FAA INSPECTOR QUESTIONED THIS AND I EXPLAINED ABOUT OUR STATE AND HOW REMOTE THE CHANCE OF HAVING A DECOMPRESSION WOULD BE VERSUS FURTHER COMPLICATING OUR ALREADY POOR RADIO SITUATION. I ALSO POINTED OUT THAT I HAD THE OXYGEN MASK ON MY LAP IN CASE I DID NEED IT I COULD ACCESS IT QUICKLY AND EASILY. WHEN THE CAPT RETURNED FROM THE RESTROOM BREAK WE DISCUSSED THE SITUATION AND HE CONCURRED THAT IT WAS A REASONABLE DECISION ON MY PART. IN FACT; AS WE APCHED IAD; WE BRIEFED ATC ON OUR RADIO DIFFICULTIES AND ESTABLISHED A CONTINGENCY PLAN IN CASE OF ADDITIONAL COMS PROBS. THIS WAS A DECISION MADE BY MYSELF IN THE BEST INTEREST OF SAFETY. I REASONED THAT WITH 4 SOURCES OF AIR; 4 CTLRS; 2 OUT-FLOW VALVES; AND IN A STABLE STATE WITH OUR PRESSURIZATION; THAT THE RISK OF A RAPID DECOMPRESSION WAS VERY SLIGHT; BUT THAT USING THE OXYGEN MASK MIGHT AGAIN JEOPARDIZE OUR ONLY RADIO AND IN THAT STATE WOULD CREATE A VERY SIGNIFICANT SAFETY PROB. IN SUMMARY; I WAS WELL AWARE OF THE OXYGEN MASK RULE. IN FACT; I USED IT THE FIRST BREAK. THE STATE OF THE ACFT WAS NOT IN A NORMAL WORKING STATE AND THE CAUSE COULD HAVE BEEN DUE TO THE OXYGEN MASK. I HAD MY MASK ON MY LAP TO USE IF I NEEDED IT. I USED THE BEST REASONING FOR A SAFE OP FOR THE GIVEN SITUATION. I HAVE RECENTLY RECEIVED A LETTER FROM THE FAA ADVISING ME THAT THIS INCIDENT IS UNDER INVESTIGATION.

Data retrieved from NASA's ASRS site as of January 2009 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.