Date: 201304
Local time of day: 1201-1800
Location: ZDC.ARTCC, VA
Flight conditions: IMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight phase: Climb
Airspace: Class A ZDC
ATC / advisory: Center ZDC
Second aircraft: A320
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: ATC Equipment / Nav Facility / Buildings; Human Factors
Primary problem: Ambiguous

Narrative: While on ZDC frequency; we were cleared to climb to FL340. We were then switched to a new frequency. When we checked in; there was no answer. We waited a moment to call again. While passing FL290; the Controller called us to say we were only cleared to FL280 and to expedite through FL300. We informed him we were cleared to FL340 on the previous frequency. He said they would check tapes. When we were switched to a new frequency 10 minutes later; we were told the tapes had not been checked yet; but not to worry about it.


Second narrative: I was working the R-Side during the afternoon push. Traffic volume had been steadily increasing and I was in the process of getting a D-Side. I observed Aircraft X being flashed at me; east of LDN proceeding westbound; from sector 04 with a T altitude of FL280 in the data block and the Mode-C indicating the aircraft was climbing out of FL243. I did a traffic scan and accepted the handoff observing a limited data block; outside of my airspace; proceeding north at FL290. Shortly thereafter; Aircraft X (approximately 5 miles east of LDN) checked on climbing out of FL260 for FL280. I acknowledged the aircraft and said I would have higher shortly. The pilot did not acknowledge that transmission. I continued working until; during my normal scan; I observed Aircraft X's Mode-C indicating the aircraft was climbing through FL283. I queried the pilot; with no response; 3 separate times. Prior to the second query; Aircraft Y began flashing a conflict alert with Aircraft X when they were approximately 8 miles apart and closing fast. Given that other aircraft had checked on and acknowledged other clearances after the first query of Aircraft X; I surmised that my transmitter was working properly. Unsure of whether or not Aircraft X could hear me; I issued 'Aircraft X if you can hear this; expedite your climb through FL300; traffic off your left wing; less than 5 miles; northbound; level FL290.' This was the first transmission the pilot acknowledged. I notified the Supervisor. At this point the sector was very busy; when I noticed Aircraft X and Aircraft Y were no longer a factor; I continued working. Approximately 2 minutes later; during a somewhat quiet period; Aircraft X questioned me about what happened. He explained that the only transmission he heard from me was the expedite clearance and that he had received the climb to FL340 (requested altitude) on the previous frequency. I advised him he had checked on climbing to FL280 however we had begun the process of investigating and I thanked him for the information. My Supervisor indicated that the Mode-C indicated 1;000 FT vertical separation at 5.0 miles apart. I have not heard anything since. It is known that the area around LDN; where the transmitter/receiver for the Moorefield sector is located; is the 'cone of silence' where usually the Controller can hear the aircraft; but the pilots cannot here the Controller. This is unsafe and was demonstrated by the incident above. Changing between main and standby sometimes has an effect; however switching to BUEC rarely helps with the aircraft being able to hear us. At the same time; using the frequency for the Bryce sector makes it difficult for both the Controller and pilot to hear each other in the same area...or anywhere east of the sector boundary. For a very busy; high transition/crossing traffic; sector it is dangerous to have areas where the frequencies do not provide adequate coverage. I have no solid recommendations; but figured filing a report was a good place to start.


Synopsis: A B737 Captain and the ZDC Controller involved describe an altitude deviation and loss of communication in ZDC Airspace.