Date: 201307
Local time of day: 1801-2400
Location: CLT.TRACON, NC
Light: Daylight
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach; Descent
Airspace: Class B CLT
ATC / advisory: Center ZTL; TRACON CLT
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Training / Qualification
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Atlanta Center issued 'maintain 250 KTS during the descent' while on the Johns Arrival into CLT.  We were subsequently vectored off the arrival then handed off to CLT Approach.  At 28 NM from CLT; I (Pilot Flying) initiated slow down to 210 KTS while the Pilot Monitoring was off the radio receiving new ATIS via radio as one MCDU had failed during ACARS use causing us to avoid using ACARS on the remaining MCDU.  When Pilot Monitoring came back into the loop I [was] informed that I had slowed.  We were now approximately 20 NM from CLT and CLT Approach queried our assigned speed.  PM reply was none assigned; and ATC stated 'Center usually gives 250 there' then he sped us up to 230 presumably to maintain arrival spacing. As flying pilot; my interpretation of the speed clearance was that we were to maintain it 'during descent' while on the Johns STAR.  When we were being vectored for the approach I was beginning to configure the aircraft for the approach as per our standard practices within 30 NM and 10;000 FT. I think the confusion or possible misinterpretation was exacerbated when we lost MCDU on pilot flying side during descent.  This increased the workload most likely caused miscommunication and confusion as to what phase of flight we were in and the corresponding requirement to adhere to ATC issued speed 'during descent' as opposed to during the approach.  This seems more evident by PM's response of no speed issued when PM was the one that originally replied to the maintain 250 KTS during the descent.  In retrospect; we clearly believed that we were no longer in the descent but rather on the approach. Task saturation with an equipment failure close in to a high density traffic airport can very easily lead to miscommunication and confusion.   It would have been more prudent to ask ATC what speed they wanted us to maintain during vectors for the approach.  This is particularly true of Johns arrivals landing south at CLT as the 'slam dunk' seems to be common with CLT Approach.  More interface with ATC and pilots may help alleviate this as it seems that since 9/11 controllers don't get to see things from our perspective and thus don't understand many operational requirements we have such as energy management and our need to slow down to go down or for the approach.  It seems we have been increasingly told via clearances to push such limits simply out of lack of understanding of our operational requirements - and we pilots have most likely encouraged this by blindly complying.




Synopsis: Air carrier Captain reports being assigned 250 KTS during descent then slowing as he nears the airport without advising ATC. Approach advises that 250 KTS was assigned and should be maintained.