Narrative:

Air carrier aircraft on a revenue flight. Aircraft assigned airway routing at FL350. After passing etp en route (1000 mi to travel to destination) as captain of this flight I noticed an aircraft on TCAS; approximately 1000 ft above my altitude approaching from the rear slowly overtaking my flight. As the aircraft flew over my aircraft I noticed that my indicated mach speed was lower than my assigned mach number so I reset in the FMC my planned cruise mach number. My aircraft accelerated and was slightly behind and left of the aircraft 1000 ft above my aircraft. While not unusual; it seemed odd to me that the B767 aircraft 1000 ft above my altitude was not pulling away from my aircraft. I rechked my indicated mach number to find that it was now above the target speed of mach .79. I reset my cruise mach to the correct speed and the aircraft finally stabilized at the assigned speed. A contributing factor in the speed control dilemma was that there was light turbulence immediately preceding and during the time that the aircraft passed above me which caused the indicated mach speed to wander without 'timely' autothrottle inputs. I got a call from commercial radio requesting my mach number and I reported mach .79. There were no further communications regarding my ETA or speed. The aircraft crew above me was apparently uncomfortable with my close proximity to their aircraft and turned left again over the top of my flight to an offset course to what looked like on my TCAS to be 1 mi. I later determined that the aircraft's destination was the same as ours. While monitoring the HF radio and waiting to pass my position report; I heard this aircraft ahead of me pass its position report. Comparing my position and estimates mine were within 2 mins of the aircraft ahead for the next 2 position and my planned and actual position estimates remained steady. The aircraft above me checked in at a coastal waypoint with center on VHF while approximately 18 mi ahead of my flight. An issue in this speed dilemma was the fact that on the air to air communication frequency; there was an excessive amount of chatter as well as interference on the frequency at the time which distraction me being able to receive any radio calls from the aircraft above me.

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Original NASA ASRS Text

Title: AN OCEANIC B757-200 FLYING AN INCORRECT MACH SPD WAS APCHED BY A RVSM ACFT 1000 FT ABOVE. DUE TO CONTINUED PROX ACFT OFFSET PARALLEL TRACK.

Narrative: ACR ACFT ON A REVENUE FLT. ACFT ASSIGNED AIRWAY ROUTING AT FL350. AFTER PASSING ETP ENRTE (1000 MI TO TRAVEL TO DEST) AS CAPT OF THIS FLT I NOTICED AN ACFT ON TCAS; APPROX 1000 FT ABOVE MY ALT APCHING FROM THE REAR SLOWLY OVERTAKING MY FLT. AS THE ACFT FLEW OVER MY ACFT I NOTICED THAT MY INDICATED MACH SPD WAS LOWER THAN MY ASSIGNED MACH NUMBER SO I RESET IN THE FMC MY PLANNED CRUISE MACH NUMBER. MY ACFT ACCELERATED AND WAS SLIGHTLY BEHIND AND L OF THE ACFT 1000 FT ABOVE MY ACFT. WHILE NOT UNUSUAL; IT SEEMED ODD TO ME THAT THE B767 ACFT 1000 FT ABOVE MY ALT WAS NOT PULLING AWAY FROM MY ACFT. I RECHKED MY INDICATED MACH NUMBER TO FIND THAT IT WAS NOW ABOVE THE TARGET SPD OF MACH .79. I RESET MY CRUISE MACH TO THE CORRECT SPD AND THE ACFT FINALLY STABILIZED AT THE ASSIGNED SPD. A CONTRIBUTING FACTOR IN THE SPD CTL DILEMMA WAS THAT THERE WAS LIGHT TURB IMMEDIATELY PRECEDING AND DURING THE TIME THAT THE ACFT PASSED ABOVE ME WHICH CAUSED THE INDICATED MACH SPD TO WANDER WITHOUT 'TIMELY' AUTOTHROTTLE INPUTS. I GOT A CALL FROM COMMERCIAL RADIO REQUESTING MY MACH NUMBER AND I RPTED MACH .79. THERE WERE NO FURTHER COMS REGARDING MY ETA OR SPD. THE ACFT CREW ABOVE ME WAS APPARENTLY UNCOMFORTABLE WITH MY CLOSE PROX TO THEIR ACFT AND TURNED L AGAIN OVER THE TOP OF MY FLT TO AN OFFSET COURSE TO WHAT LOOKED LIKE ON MY TCAS TO BE 1 MI. I LATER DETERMINED THAT THE ACFT'S DEST WAS THE SAME AS OURS. WHILE MONITORING THE HF RADIO AND WAITING TO PASS MY POS RPT; I HEARD THIS ACFT AHEAD OF ME PASS ITS POS RPT. COMPARING MY POS AND ESTIMATES MINE WERE WITHIN 2 MINS OF THE ACFT AHEAD FOR THE NEXT 2 POS AND MY PLANNED AND ACTUAL POS ESTIMATES REMAINED STEADY. THE ACFT ABOVE ME CHKED IN AT A COASTAL WAYPOINT WITH CTR ON VHF WHILE APPROX 18 MI AHEAD OF MY FLT. AN ISSUE IN THIS SPD DILEMMA WAS THE FACT THAT ON THE AIR TO AIR COM FREQ; THERE WAS AN EXCESSIVE AMOUNT OF CHATTER AS WELL AS INTERFERENCE ON THE FREQ AT THE TIME WHICH DISTR ME BEING ABLE TO RECEIVE ANY RADIO CALLS FROM THE ACFT ABOVE ME.

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.