Narrative:

Flight XXX, an ftr, was operating between nrt and sin. The airway being used was R471 between hcn and nobok intxns. The infraction being reported occurred after passing kadet intersection swbound en route to the gupil intersection. Radio communications were being conducted on HF frequency 3942 P and 98903 south with manila as the controling agency. Our flight had requested FL350 in the approximately area of hcn VOR from taipei control. Our request was denied at that time for reasons unknown. After passing kadet intersection, an area where altitude changes are normally not allowed, we received a clearance from manila to climb to FL350. All 3 pilots initially, even though severe static and other agencies such as singapore, hong kong, ho chi minh, bangkok and vientiane caused considerable frequency congestion, were able to make out what we all felt was a clearance to FL350. The copilot was flying at the time and left FL310 for FL350. As we left 310, I began to express doubts as to the validity of the clearance and requested the copilot to immediately return to FL310. He did so after reaching a high altitude of FL325. Many xmissions were required to reach manila to get the proper clearance and to advise them of the fact that we were maintaining FL310. The correct clearance was an advisory to expect FL350 after passing nobok intersection. No traffic conflict was involved in this incident. The reason it occurred was because tremendous over use of a single frequency over the south china sea. This type of frequency over use is a potential safety problem and should be corrected immediately. HF radios with their static problems are difficult enough as an operational problem but to have the multiple agencies all cutting each other out all the time compounds the situation. The use of additional HF frequency over the south china sea is greatly needed.

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Original NASA ASRS Text

Title: FLC OF ACR WDB ACFT DEV FROM ASSIGNED ALT WHEN THEY MISUNDERSTOOD AN EXPECTED CLRNC FOR AN ACTUAL ONE THROUGH THE HF RADIO FREQ.

Narrative: FLT XXX, AN FTR, WAS OPERATING BTWN NRT AND SIN. THE AIRWAY BEING USED WAS R471 BTWN HCN AND NOBOK INTXNS. THE INFRACTION BEING RPTED OCCURRED AFTER PASSING KADET INTXN SWBOUND ENRTE TO THE GUPIL INTXN. RADIO COMS WERE BEING CONDUCTED ON HF FREQ 3942 P AND 98903 S WITH MANILA AS THE CTLING AGENCY. OUR FLT HAD REQUESTED FL350 IN THE APPROX AREA OF HCN VOR FROM TAIPEI CTL. OUR REQUEST WAS DENIED AT THAT TIME FOR REASONS UNKNOWN. AFTER PASSING KADET INTXN, AN AREA WHERE ALT CHANGES ARE NORMALLY NOT ALLOWED, WE RECEIVED A CLRNC FROM MANILA TO CLB TO FL350. ALL 3 PLTS INITIALLY, EVEN THOUGH SEVERE STATIC AND OTHER AGENCIES SUCH AS SINGAPORE, HONG KONG, HO CHI MINH, BANGKOK AND VIENTIANE CAUSED CONSIDERABLE FREQ CONGESTION, WERE ABLE TO MAKE OUT WHAT WE ALL FELT WAS A CLRNC TO FL350. THE COPLT WAS FLYING AT THE TIME AND LEFT FL310 FOR FL350. AS WE LEFT 310, I BEGAN TO EXPRESS DOUBTS AS TO THE VALIDITY OF THE CLRNC AND REQUESTED THE COPLT TO IMMEDIATELY RETURN TO FL310. HE DID SO AFTER REACHING A HIGH ALT OF FL325. MANY XMISSIONS WERE REQUIRED TO REACH MANILA TO GET THE PROPER CLRNC AND TO ADVISE THEM OF THE FACT THAT WE WERE MAINTAINING FL310. THE CORRECT CLRNC WAS AN ADVISORY TO EXPECT FL350 AFTER PASSING NOBOK INTXN. NO TFC CONFLICT WAS INVOLVED IN THIS INCIDENT. THE REASON IT OCCURRED WAS BECAUSE TREMENDOUS OVER USE OF A SINGLE FREQ OVER THE S CHINA SEA. THIS TYPE OF FREQ OVER USE IS A POTENTIAL SAFETY PROB AND SHOULD BE CORRECTED IMMEDIATELY. HF RADIOS WITH THEIR STATIC PROBS ARE DIFFICULT ENOUGH AS AN OPERATIONAL PROB BUT TO HAVE THE MULTIPLE AGENCIES ALL CUTTING EACH OTHER OUT ALL THE TIME COMPOUNDS THE SIT. THE USE OF ADDITIONAL HF FREQ OVER THE S CHINA SEA IS GREATLY NEEDED.

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.