Narrative:

We originated an aircraft that had a writeup from the previous day that made me wonder. It was an info only overspeed writeup that said 'ACARS reports overspeed 332.7 cas .658 mach ...' I remarked to my first officer that the writeup made no sense because neither of those numbers should have triggered an overspeed. Later that day we were descending into [the airport] and the aircraft was struggling to stay on VNAV path without going fast. We had asked for (and had been relieved) of the STAR speed restriction; so I let the aircraft run. It got up to .813 so I deselected VNAV and for momentary use of cws (control wheel steering) pitch to reset the attitude and control the speed. I slowed it down to about .785. When we were down around FL230 and about to enter a layer; I reached up to turn on the engine anti-ice when I heard the clacker. When I looked down I saw .84! I quickly raised the nose as we entered the layer and started reducing indicated speed. When we landed we received an ACARS message for overspeed of .84 mach. I called dispatch and maintenance control was brought into the conversation. I described the overspeed and pointed out the writeup from the day previous. I told maintenance control center that something did not make sense. The previous day's writeup could not have been an overspeed; and ours seemed that way also. I had not seen any trend in increasing mach prior to looking up the glance at the anti-ice switches and it happened almost instantaneously. Maintenance control center put me on hold; and after a very long time returned and said they'd looked at the data and the aircraft had gone to .84; therefore; it would have to be taken out of service for an inspection. I felt really bad because there is only contract maintenance [at this airport] and this would cause a long delay. I apologized to the crew taking the aircraft and headed towards hotel. The whole thing bugged me; because the previous day's writeup made no sense and I doubted our overspeed was accurate also. I do a lot of test flight work for other aviation organizations; so I refreshed my memory with some resources I had and went online to confirm my suspicion. After doing some math on my cr-2 computer (yes; I still carry one of those!) it was clear that something was wrong. We all know that vmo equals mmo at FL260 that's where 340 KIAS equals .82 mach. Therefore; at FL230 and 337 cas; you could not achieve .84 mach. Mach number is strictly a function of temperature. In order to reach .84 mach; at FL230 and 337 kcas; the temperature would have to be plus 35C! For an indicated airspeed of 337 knots; FL230 and isa would result in typical mach of .75. Something is wrong.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737-700 Captain reported an aircraft overspeed; but suspected a faulty air data computer.

Narrative: We originated an aircraft that had a writeup from the previous day that made me wonder. It was an Info Only overspeed writeup that said 'ACARS reports overspeed 332.7 CAS .658 Mach ...' I remarked to my FO that the writeup made no sense because neither of those numbers should have triggered an overspeed. Later that day we were descending into [the airport] and the aircraft was struggling to stay on VNAV PATH without going fast. We had asked for (and had been relieved) of the STAR speed restriction; so I let the aircraft run. It got up to .813 so I deselected VNAV and for momentary use of CWS (Control Wheel Steering) Pitch to reset the attitude and control the speed. I slowed it down to about .785. When we were down around FL230 and about to enter a layer; I reached up to turn on the engine anti-ice when I heard the clacker. When I looked down I saw .84! I quickly raised the nose as we entered the layer and started reducing indicated speed. When we landed we received an ACARS message for overspeed of .84 Mach. I called Dispatch and Maintenance Control was brought into the conversation. I described the overspeed and pointed out the writeup from the day previous. I told Maintenance Control Center that something did not make sense. The previous day's writeup could not have been an overspeed; and ours seemed that way also. I had not seen any trend in increasing Mach prior to looking up the glance at the anti-ice switches and it happened almost instantaneously. Maintenance Control Center put me on hold; and after a very long time returned and said they'd looked at the data and the aircraft had gone to .84; therefore; it would have to be taken out of service for an inspection. I felt really bad because there is only Contract Maintenance [at this airport] and this would cause a long delay. I apologized to the Crew taking the aircraft and headed towards hotel. The whole thing bugged me; because the previous day's writeup made no sense and I doubted our overspeed was accurate also. I do a lot of test flight work for other aviation organizations; so I refreshed my memory with some resources I had and went online to confirm my suspicion. After doing some math on my CR-2 computer (Yes; I still carry one of those!) it was clear that something was wrong. We all know that VMO equals MMO at FL260 that's where 340 KIAS equals .82 Mach. Therefore; at FL230 and 337 CAS; you could not achieve .84 Mach. Mach number is strictly a function of temperature. In order to reach .84 Mach; at FL230 and 337 KCAS; the temperature would have to be plus 35C! For an indicated airspeed of 337 knots; FL230 and ISA would result in typical Mach of .75. Something is wrong.

Data retrieved from NASA's ASRS site 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.