Narrative:

9/88, I was PIC of an experimental mdt aircraft, conducting VNAV testing on a FMS per type inspection authorization D-8-292. In an effort to determine the suitability of the VNAV system (which at that time was capable of a 9 degree descent angled) several dscnts were performed on that flight. On the particular descent in question, I asked for a descent from 35000 to 15000' and programmed the VNAV to perform the descent. During the descent, with the throttles at idle and the air brakes out, the airplane was coming down at about 6000 FPM or greater and reached vmo (370 KIAS). At vmo (by design), through a combination of the autoplt/EFIS/FMS, the vertical mode reverted from VNAV to airspeed hold. At that point, the autoplt pulled nose up to keep the airplane from exceeding vmo and the airplane's altitude went from about 17500 to about 18000', then in airspeed hold mode to 15000'. In the meantime, ZME made the comment to the effect that I had been cleared to 15000'. I apologized and said that the autoplt acted up. In fact, it was a VNAV problem. ZME did not suggest that any other traffic was involved. After the flight, it was decided by corp and navigation corp that the VNAV system's software would be changed to eliminate the possibility of programming the system in a descent angle that would permit the airplane to reach vmo. That change is due to be completed about 10/88 (change for maximum descent angle of 9 to 6 degrees). Callback conversation with reporter revealed the following: reporter called in to report that the FAA is investigating this incident. During conversation he said he had his head down recording aircraft performance and did not think to tell the ARTCC radar controller that they had levelled off momentarily until the controller questioned aircraft altitude.

Related reports:

Google
 

Original NASA ASRS Text

Title: GA MDT ON TEST FLT ALT DEVIATION.

Narrative: 9/88, I WAS PIC OF AN EXPERIMENTAL MDT ACFT, CONDUCTING VNAV TESTING ON A FMS PER TYPE INSPECTION AUTHORIZATION D-8-292. IN AN EFFORT TO DETERMINE THE SUITABILITY OF THE VNAV SYS (WHICH AT THAT TIME WAS CAPABLE OF A 9 DEG DSCNT ANGLED) SEVERAL DSCNTS WERE PERFORMED ON THAT FLT. ON THE PARTICULAR DSCNT IN QUESTION, I ASKED FOR A DSCNT FROM 35000 TO 15000' AND PROGRAMMED THE VNAV TO PERFORM THE DSCNT. DURING THE DSCNT, WITH THE THROTTLES AT IDLE AND THE AIR BRAKES OUT, THE AIRPLANE WAS COMING DOWN AT ABOUT 6000 FPM OR GREATER AND REACHED VMO (370 KIAS). AT VMO (BY DESIGN), THROUGH A COMBINATION OF THE AUTOPLT/EFIS/FMS, THE VERT MODE REVERTED FROM VNAV TO AIRSPD HOLD. AT THAT POINT, THE AUTOPLT PULLED NOSE UP TO KEEP THE AIRPLANE FROM EXCEEDING VMO AND THE AIRPLANE'S ALT WENT FROM ABOUT 17500 TO ABOUT 18000', THEN IN AIRSPD HOLD MODE TO 15000'. IN THE MEANTIME, ZME MADE THE COMMENT TO THE EFFECT THAT I HAD BEEN CLRED TO 15000'. I APOLOGIZED AND SAID THAT THE AUTOPLT ACTED UP. IN FACT, IT WAS A VNAV PROB. ZME DID NOT SUGGEST THAT ANY OTHER TFC WAS INVOLVED. AFTER THE FLT, IT WAS DECIDED BY CORP AND NAV CORP THAT THE VNAV SYS'S SOFTWARE WOULD BE CHANGED TO ELIMINATE THE POSSIBILITY OF PROGRAMMING THE SYS IN A DSCNT ANGLE THAT WOULD PERMIT THE AIRPLANE TO REACH VMO. THAT CHANGE IS DUE TO BE COMPLETED ABOUT 10/88 (CHANGE FOR MAX DSCNT ANGLE OF 9 TO 6 DEGS). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR CALLED IN TO RPT THAT THE FAA IS INVESTIGATING THIS INCIDENT. DURING CONVERSATION HE SAID HE HAD HIS HEAD DOWN RECORDING ACFT PERFORMANCE AND DID NOT THINK TO TELL THE ARTCC RADAR CTLR THAT THEY HAD LEVELLED OFF MOMENTARILY UNTIL THE CTLR QUESTIONED ACFT ALT.

Data retrieved from NASA's ASRS site as of August 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.