Narrative:

After level off at cruise altitude FL290, the PF became distracted and did not reduce power from climb to cruise thrust. Consequently, airspeed accelerated to mmo, about .81 mach. The 'maximum airspeed' limiting feature of the autoplt then engaged itself, disengaging altitude hold, and the airplane began a slow climb. I noticed this between 29200 and 29300, and immediately slowed and descended the aircraft back to FL290. The ATC controller said nothing so I guess there was no conflict. Further to this, I called the area manager at lax center and he said he had no reports. This was a pilot error. It would have been helpful if the autoplt was designed to give the pilots an aural warning when disengaging itself from altitude hold to maintain vmo or mmo. There is an mmo clacker but the autoplt speed limiter took over too soon!

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Original NASA ASRS Text

Title: ACR CREW EXCEEDS MAX AIRSPD. AUTOPLT DISENGAGES, ACFT CLBS.

Narrative: AFTER LEVEL OFF AT CRUISE ALT FL290, THE PF BECAME DISTRACTED AND DID NOT REDUCE PWR FROM CLB TO CRUISE THRUST. CONSEQUENTLY, AIRSPD ACCELERATED TO MMO, ABOUT .81 MACH. THE 'MAX AIRSPD' LIMITING FEATURE OF THE AUTOPLT THEN ENGAGED ITSELF, DISENGAGING ALT HOLD, AND THE AIRPLANE BEGAN A SLOW CLB. I NOTICED THIS BTWN 29200 AND 29300, AND IMMEDIATELY SLOWED AND DSNDED THE ACFT BACK TO FL290. THE ATC CTLR SAID NOTHING SO I GUESS THERE WAS NO CONFLICT. FURTHER TO THIS, I CALLED THE AREA MGR AT LAX CENTER AND HE SAID HE HAD NO RPTS. THIS WAS A PLT ERROR. IT WOULD HAVE BEEN HELPFUL IF THE AUTOPLT WAS DESIGNED TO GIVE THE PLTS AN AURAL WARNING WHEN DISENGAGING ITSELF FROM ALT HOLD TO MAINTAIN VMO OR MMO. THERE IS AN MMO CLACKER BUT THE AUTOPLT SPD LIMITER TOOK OVER TOO SOON!

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.